HOMOCYCLIC DIVISION Organic compounds may be classified on the basis of molecular structure into two main divisions : open-chain, aliphatic or acyclic compounds, and (2) closed-chain, or cyclic compounds. The latter division presents a great diversity of types : oxygen, nitrogen, sulphur and other elements may participate with carbon in manifold ways to form cyclic nuclei; the resulting ring-systems exhibit characters closely resembling open-chain compounds, in so far as they yield substitution derivatives and behave as com pound radicals. In classifying closed-chain compounds, the first step consists in dividing them into (i.) carbocyclic, in which the closed chain, ring, or nucleus is composed solely of carbon atoms —these are known also as homocyclic or isocyclic on account of the identity of the members of the ring—and (ii.) heterocyclic, in which different elements go to make up the ring. A molecule containing two or more ring-systems, e.g., naphthalene or camphor, is said to be polycyclic.
Homocyclic compounds may be conveniently subdivided into (a) benzene derivatives, benzenoid compounds or aromatic com pounds proper, and (b) alicyclic compounds. The homocyclic nucleus of benzene, C,H,, and its derivatives invariably contain six carbon atoms associated with fewer hydrogen atoms (or their substituents) than suffice to satisfy the carbon valencies, the deficit being in all instances the equivalent of three double link ages; this peculiarity of structure is associated with the "benzen oid" or "aromatic" character which is shown also by certain similarly constituted heterocyclic compounds, such as pyridine, CI,HO,N. Alicyclic compounds do not exhibit the "aromatic" character, but, as indicated by their name, bear a general chemical resemblance to the aliphatic compounds.
The alicyclic subdivision embraces the important class of cyclo paraffins or polymethylenes (CH,),, and their derivatives, in which the nucleus contains three or more carbon atoms, all of which are fully saturated. In addition, it comprises a similar range of cyclo-olefines and their derivatives, in which the carbon nucleus contains one or more olefinic linkages (double bonds). The most important members of these two classes, like the aromatic com pounds, contain a nucleus composed of six carbon atoms : it is convenient to classify these six-membered-ring compounds under the special heading of hydroaromatic compounds. A particular significance thus attaches to all homocyclic compounds containing six carbon atoms in the ring; i.e., to the aromatic compounds and the hydroaromatic compounds. A summarized scheme of classifi cation is appended : Aromatic Compounds.—The presence of a closed chain, or ring, in an organic molecule was first suggested for benzene by Kekule, in 1865. The representation of this hydrocarbon and its innumerable derivatives as closed-chain compounds has been termed the crowning achievement of the doctrine of the linking of carbon atoms (Japp, Chemical Society's Memorial Lectures, 1901). The benzene derivatives, or aromatic compounds, form the most important and most numerous class of homocyclic com pounds; it has been stated, indeed, that three-quarters of organic chemistry is concerned with benzene and its derivatives, and that the benzene ring is the most familiar emblem in scientific litera ture. Most of these substances are purely synthetic in origin, and became known as a result of the enormous volume of research upon coal-tar components which followed the synthesis of the first coal-tar dye by W. H. Perkin, Sr., in 1856. Organic chemical research for more than 5o years after Perkin's discovery centred mainly around the synthetic production of aromatic compounds from benzene and related coal-tar components. These derivatives are of great scientific and industrial interest; but although the benzene nucleus is found in most of the natural organic colouring matters and in numerous other plant and animal products, it does not possess the biochemical importance of the open-chain and other non-benzenoid structures of carbohydrates, fats and proteins.
The treatment of aromatic compounds will be restricted mainly to a general consideration of their structure and fundamental chemical characteristics ; specific information concerning impor tant individual compounds should be sought under their names and under such headings as AZO-COMPOUNDS and DIAZO-COM POUNDS ; NITRO-COMPOUNDS, QUINONES and DYES, SYNTHETIC.
Sources of Aromatic Compounds.—The origin of the name "aromatic" may be traced to the fact that many of the aromatic spices and fragrant oils of plants contain benzene derivatives; only a limited number of these derivatives, however, are charac terized by an aromatic odour. Benzene and homologous hydro carbons occur also in natural petroleums, particularly in those of Borneo and Papua. By far the most important source of aromatic compounds is coal-tar, of which coal yields from 4 to 7% when destructively distilled in horizontal retorts at 900-1100° C, as in the manufacture of coal-gas. Coke-oven tar is very similar in composition to gasworks tar, but the carbonization of coal at lower temperatures, or in vertical retorts, furnishes tars in which the aromatic substances are replaced largely by paraffins and naphthenes (vide infra). Gasworks coal-tar contains some 300 substances, but of these only about ten are isolated for common use as "primaries," in the synthesis of coal-tar dyes, medicinal chemicals, perfumes, high explosives and other aromatic com pounds of industrial importance. When submitted to nitration, sulphonation, chlorination, oxidation and other chemical processes, the "primaries" give rise to "intermediates" (of which several hundreds are manufactured) and these by further treatment, which often includes an interaction between two intermediates or their derivatives, yield the finished synthetic dyestuffs (q.v.) and related "coal-tar chemicals." The most important primaries are the four aromatic hydrocarbons, benzene, toluene, naphthalene and anthracene, together with phenol (carbolic acid) or hydroxy benzene. These are separated from the tar by repeated fractional distillation, supplemented by appropriate chemical treatment. Coal yields only about 0.5% of the five chief primaries; i.e., about r 'lb. per ton, of which naphthalene forms approximately two-thirds, benzene together with toluene one-fifth, and phenol and anthracene each one-fifteenth.
Benzene.—Benzene (q.v.), is theoretically the funda mental substance from which all the aromatic compounds are derived; it is also the most economical practical source of many of them : these two facts combine to render benzene one of the most outstanding of all organic compounds. Much study has accordingly been devoted to its physical and chemical properties and to the satisfactory representation of these properties by means of a structural or constitutional formula. Benzene was discovered in 1825 by Michael Faraday, in an illuminating gas prepared from fish oils, and its presence in coal-tar was established by A. W. Hofmann, in 1845 ; but it was not until 1865 (seven years after the publication of the theory of molecular structure) that Kekule arrived at his conception of the benzene ring, or closed chain, of six carbon atoms. This delay was due to the difficulty of giving an adequate structural interpretation of the fundamental chemical differences which exist between aromatic and aliphatic com pounds; differences which it is essential to include in a general review of the chemical behaviour of benzene and its derivatives before proceeding to a discussion of its molecular structure.
Chemical Behaviour of Aromatic Compounds.—In comparison with hexane, the molecule of benzene lacks eight hydrogen atoms; but in spite of the high unsaturation indicated by its molecular formula, benzene is a particularly stable substance. It is affected only very slowly by oxidizing agents, and is indifferent towards hot dilute acids and alkalis, bromine water or the ordi nary reducing agents. In these respects it differs profoundly from the isomeric open-chain compound, dipropargyl, which has all the chemical properties of a highly unsaturated substance. More striking still, the typical reactions of benzene are substitution reactions. Benzene derivatives may be regarded as formed through the replacement of the hydrogen atoms of benzene by other elements, groups or radicals; and in this respect they resemble the substitution products of saturated aliphatic hydrocarbons. They are often produced, however, by reactions peculiar to the aromatic series. Ability to undergo direct nitration and sulphonation is especially characteristic of aromatic com pounds. Thus, benzene and its homologues react readily with (a) a mixture of concentrated nitric and sulphuric acids ("nitrat ing mixture" or "nitrating acid") and (b) fuming sulphuric acid, to yield nitro-derivatives and sulphonic acids, respectively; the nitration and sulphonation of benzene are represented by the following equations : The corresponding aliphatic derivatives usually have to be pre pared by indirect processes. In the presence of certain catalytic agents (halogen carriers), benzene and its homologues may also be chlorinated and brominated, the process being again one of substitution: Evidence of a certain type of unsaturation in the aromatic molecule is provided by the capacity of benzene and its deriva tives to form substitution products under special conditions. When exposed to bright sunlight (in the absence of a halogen carrier), benzene combines with a maximum proportion of six atoms of chlorine or bromine per molecule, to form benzene hexa chloride and hexabromide, and it also forms a triozonide, ozobenzene, j but it is unable to combine, like the olefines, with hydrogen chloride, hypochlorous acid, etc. Benzene also undergoes catalytic hydrogenation (q.v.) in presence of finely divided nickel, forming cyclohexane, but this substance reverts to benzene upon oxidation. Thus benzene differs profoundly from ethylene and other unsaturated open-chain com pounds in possessing a greater stability than its reduction product.
Besides exhibiting distinctive reactions—in particular, ready nitration and sulphonation—aromatic compounds differ from ali phatic compounds by virtue of their higher proportion of carbon and their greater tendency to solidify. No aromatic compound proper contains fewer than six carbon atoms in the molecule, and the more complex members tend to break down thus far when submitted to oxidation and other disintegrative processes ; further oxidation of the resulting then usually disrupts the molecule at a stride to carbon dioxide and other simple substances. Thus, anthranilic acid, yields aniline, and carbon dioxide when heated; and naphthalene, upon oxidation gives phthalic acid, C$H804, which when heated with lime is degraded further to benzene. Such changes point to the presence in the molecules of benzene and its derivatives of a stable nucleus containing six carbon atoms.
Equivalence of the Six Hydrogen Atoms in Benzene.—That the six hydrogen atoms in the molecule of benzene are similarly situ ated may be inferred from the fact that isomerism does not occur among monosubstituted benzenes of the general formula C,H,X ; thus only one bromobenzene, one nitrobenzene, C,H,,•NO.,, and one benzoic acid, C,H,•COOH, are known. This equivalence, which was assumed by Kekule, was first demon strated by Ladenburg in 1874. It is unnecessary to detail the for mal experimental proof, but the underlying principle may be illustrated by referring to the isomeric nitrobenzoic acids, NO2•CRH,.COOH, of which three are known. When the nitro group is replaced experimentally by a hydrogen atom, all three isomerides yield one and the same substance, viz., benzoic acid; thus in all three cases the carboxyl group,—COOH, may be assumed to have taken the position originally occupied by a particular hydrogen atom (a) of the molecule of benzene. The existence of three distinct nitrobenzoic acids shows that after the introduction of the carboxyl group the remaining five hydrogen atoms of the original benzene molecule are not equivalent ; denot ing these atoms by the letters (b) to (f), it may be assumed further that hydrogen atoms (b), (c) and (d), respectively, have been replaced by the nitro-group in the three nitrobenzoic acids, which may now be represented as follows : Each of these acids when distilled with lime yields nitrobenzene, and since the product is identical in all three cases it follows that three of the hydrogen atoms of the original molecule of benzene—namely, (b), (c) and (d)—are equivalent. An extension of this principle has shown that the replacement of any one of the six hydrogen atoms by the same atom or group leads to an identical product.
Isomerism of Benzene Derivatives.—A large volume of experi mental evidence indicates that disubstitution products of benzene, whether of the type or can exist in three iso meric forms, as indicated above for the nitrobenzoic acids, in which X and Y = COOH. Trisubstitution products of the type are found to be capable of existence in three isomeric forms, as are also tetra-substituted benzenes, in which the four substituents are identical ; only one derivative is known for each of the types C6IX, and The Benzene Ring.—"I was sitting, writing at my text-book; but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gambolling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by repeated visions of the kind, could now distinguish larger structures, of manifold conformation : long rows, some times more closely fitted together; all twining and twisting in snake-like motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke; and this time also I spent the rest of the night in working out the conse quences of the hypothesis." These are the words in which Kekule described the way in which the conception of the benzene ring came to him (v. Japp, loc. cit.) . The equivalence of the six hydro gen atoms and the isomerism of the substituted benzenes had not been established at that time (1865) ; but both of these fundamental characteristics of the benzene structural assemblage are at once accounted for when the molecule is represented as a closed chain, or ring, of six carbon atoms, each of which is associated with one hydrogen atom. The ring may be represented by a circle, or, more conveniently, in one of the following ways: The disposal of the fourth valency of each carbon atom, which is not indicated in these symbols, will be discussed later; for the time being it is sufficient to state that the benzene "ring," how ever it may be represented, stands as an expression of a com pletely symmetrical structure in which all six hydrogen atoms are equivalent. The hexagon is in general use as an abbreviated symbol for the benzene molecule ; it is assumed that at each corner there is a CH group; the substitution of a hydrogen atom is shown by attaching the replacing atom or group (i.e., the substituent) to the corner concerned, as shown below. For purposes of systematic nomenclature the six carbon atoms are numbered consecutively : From the second of these formulae it is apparent that the in troduction of a substituent in place of any one of the six equiva lent hydrogen atoms at once destroys the equivalence of the remaining five, and creates three kinds of positions in the new molecule. The two similar positions (2, 6) adjacent to the substit uent are known as ortho-positions, with respect to the substituent ; the single position (4) diametrically opposite the substituent is called the para-position; and the two remaining positions (3, 5), which are similar, are distinguished by the prefix meta-. It is obvious, therefore, that disubstituted benzenes should exist in three isomeric forms ; and this consequence of Kekule's theory is in accordance with the facts already outlined. The three nitro benzoic acids may thus be represented and named as follows, the prefixes o-, in-, and p- being the recognized contractions for ortho-, meta-, and para-, respectively : Di- and poly-substituted benzenes related in this way are known as position isomerides. It will be seen that I, 2-nitrobenzoic acid is identical with I, 6-nitrobenzoic acid, and i, 3-nitrobenzoic acid with 1, 5-nitrobenzoic acid. It may be added that the closed chain of six carbon atoms, known as the benzene nucleus, accounts for the observed isomeric relationships of all types of substitution products of benzene.
The number of isomerides of various types when all the sub stituent groups are alike has been indicated above. When the groups are unlike a greater number is possible, except in the case of di-derivatives. When two of three substituents are alike, six isomerides are possible, as in the case of the diaminobenzoic acids, When all three are unlike, ten isomerides are possible; thus ten hydroxytoluic acids, are known. In the case of tetra-substituted compounds, 3o iso merides are possible when all the groups are different. Of the tri substitution derivatives, the I, 2, 3-compounds are known as "adjacent" or "vicinal" (v), the 1, 2, 4- as "asymmetrical" (as), and the 1, 3, 5- as "symmetrical" (s). The same three terms are applied to tetra-substituted derivatives having the groups in the I, 2, 3, I, 3, and I, 2, 4, 5-positions, respectively.
Disposal of the Fourth Valency.—Although, very rarely, the carbon atom may exhibit tervalency, as in triphenylmethyl (vide infra) , there are no grounds for assigning this exceptional valency to the carbon atoms of benzene. Kekule's original formula (1866) showed the six carbon atoms attached to one another by alternate single and double bonds, as shown in formula (I.) : It may be said at once that despite much criticism this original formula is quite as convenient as any of its modifications, espe cially in the consideration of syntheses and decompositions of the benzene nucleus. The physical properties are also in keeping with Kekule's formula, and although benzene is not visibly coloured it exerts a strong selective absorption in the ultra-violet region of the spectrum. The lack of resemblance between benzene and tri-ole fines may be ascribed to some extent to the fundamental difference in structure between a symmetrical ring and an open chain; whilst to meet the further objection that isomeric I, 2-disubsti tution products corresponding to formulae (A) and (B) do not exist, Kekule (1872) suggested a dynamic formula (II.) , which represents a rapid oscillation between two tautomeric forms and is curiously reminiscent of his original vision of a structure "twin ing and twisting in snake-like motion": Meanwhile Dewar (1866) proposed an unsymmetrical formula (III.) . This has an interest at the present time, since it illustrates a relationship of benzene to the important group of quinonoid colours ; it appears also to have a bearing upon the puckering of the benzene ring which has been indicated by the X-ray analysis of benzene crystals. Dewar's formula, moreover, finds support, as representing a possible phase of the benzene molecule, in J. F. Thorpe and C. K. Ingold's recent work on intra-annular tautomer ism. A "diagonal" formula was suggested by Claus (1867) and a "prism" formula by Ladenburg (1869) ; but more importance attaches to the so-called "centric" formula (IV.) of H. E. Arm strong and A. Baeyer (1887): Spatial Configuration of Benzene.—According to the space theory of Le Bel and van 't Hoff (see STEREOCHEMISTRY), the six hydrogen atoms of benzene must either be permanently co planar or capable of easy passage through a co-planar phase. If this were not the case, substitution derivatives (in which none of the substituents contain asymmetric atoms) should exist in enan tiomorphous forms, differing in their action on polarized light : such optical isomerism has not been established, in spite of ex haustive experiments. Most of the formulae given above fulfil this condition when interpreted in terms of the theory of the tetra hedral environment of the carbon atom ; but Ladenburg's prism configuration, which requires optical isomerism for ortho-disub stituted benzenes, is thereby ruled out. The modern tendency is to regard the molecule of benzene as a vibratory system passing through a number of phases, some of which are represented closely by the formulae of Kekule, Armstrong–Baeyer, and others. This view was elaborated by Collie (1897), who suggested a spatial configuration consisting of a symmetrical grouping of two triplets of CH groups, each placed at the apex of a regular octahedron ; the successive vibration-phases of the molecule, obtained by simple rotation of the space-model, are then represented in plane pro jection as follows: A strong argument against the ethylene linkages of Kekule's formula is provided by the remarkable stability of all benzenoid compounds towards oxidizing and reducing agents and towards halogens and haloid acids. The widely different behaviour of hydro aromatic compounds, containing either one or two ethylene link ages (which behave towards these reagents exactly as unsaturated aliphatic compounds) suggests that in benzenoid compounds the fourth valencies are symmetrically distributed in such a manner as to induce a peculiar stability in the molecule. The centric formula assumes that the fourth valencies are simply directed towards the centre of the ring, thereby undergoing a mutual satu ration ; the formula is thus less rigid and precise than most of the others.
More recently (1899), Thiele's theory of partial valencies (see VALENCY) has been applied to the benzene structure with con siderable success. Thiele's formula (V.) represents the benzene molecule as a particular combination of the so-called conjugated systems which were first studied in open-chain compounds; the centric formula, on the other hand, has been criticized as an ad hoc representation calling for a novel system of valencies and a unique distribution of affinity. According to Thiele, benzene is to be represented as a closed series of conjugated double bonds, which, since it possesses no free partial valencies, is devoid of un saturated characteristics. "As by the neutralization of the partial valencies the original three double bonds vanish, no distinction can be drawn between them and the secondary (conjugated) double bonds. Benzene contains six inactive double bonds. Thus, the difficulty presented by the two ortho-positions, which Kekule attempted to meet by the aid of his dynamic hypothesis, disap pears" (Thiele). Moreover, the appearance of partial valencies in di- and tetra-hydrobenzene, and hydroaromatic compounds in general, explains the resemblance of these compounds to unsatu rated open-chain compounds: Electronic representations of the benzene molecule, based on the modern ideas of atomic structure and valency, have also been advanced. For all ordinary purposes the original representation of Kekule remains quite as convenient as any of the proposed modi fications ; and, aromatic characteristics of homocyclic compounds are thus to be associated with cyclic structures of six atoms which are capable of assuming a symmetrical arrangement of alternate single and double bonds. Cyclo-octatetrene, which conforms to the last requirement, but possesses a ring of eight carbon atoms, is not aromatic in character (R. Willstatter, 191I ). Since, apart from the consideration of additive reactions, it is usually possible to ignore the disposition of the fourth valencies, the simple hexagon provides a convenient symbol for the benzene molecule. It may be added that the purely physical researches of Sir W. H. Bragg and W. L. Bragg have demonstrated conclusively the pres ence of a six-carbon-atom ring not only in benzenoid molecules but also in crystallized carbon (diamond and graphite) itself. "When we consider the diamond construction we cannot but notice the striking appearance, in every part of the model, of an arrange ment of the carbon atoms in a ring of hexagonal form. If we take one of these rings out from the model, it has the appearance of a perfect hexagon when viewed from above, but not a flat ring. There are layers in graphite as in the diamond structure ... from above it presents the same appearance of a hexagonal network. Recent experiments seem to show that the layers have been flat tened out, so that each carbon is now surrounded by three atoms in its own plane.... Whether the benzene ring is actually puck ered under all circumstances, or is sometimes flat, we find it diffi cult at present to say." (W. H. Bragg.) Orientation of Substituent Groups in the Benzene Ring.—The determination of the relative positions of the substituents in the molecule of a benzene derivative constitutes an important factor in the general investigation of such compounds ; the process is known as orientation (oriente, situated). Since, as a rule, if high temperatures are avoided, a replacing group enters the nucleus in the position formerly occupied by the replaced group, any corn pound which can be obtained from or converted into a standard substance, for which the relative positions of the substituents are known, may be definitely orientated. Benzene derivatives of known constitution are now so numerous that this method may be readily applied ; but originally the number of standard compounds was very small. Among the earliest derivatives to be orientated in this way were the benzenedicarboxylic acids (phthalic acids), C,H, (COOH The ortho-acid was orientated by Graebe (1869), owing to its production from naphthalene by oxidation (vide in f ra) . The meta-acid was correlated with mesitylene, which by reason of its formation from acetone (q.v.) by condensation must be regarded as I, 3, 5-trimethylbenzene (Ladenburg, 1875). Upon oxidation, mesitylene yields the monobasic mesitylenic acid ; this when distilled with lime loses carbon dioxide and gives a dimethyl benzene (xylene), which must obviously be the meta-compound; and this in turn yields meta-phthalic acid when oxidized: The third isomeric phthalic acid is thus, by elimination, the compound. The orientation of the two remaining xylenes also lows from their oxidation to o- and p-phthalic acid, respectively.
Korner's "absolute method" (r 874) renders it unnecessary to make any assumptions respecting the constitution of standard com pounds, such as o-phthalic acid and mesitylene. Starting with the three isomeric dibromobenzenes, he found that when converted into tribromobenzenes, the first isomeride gave only one tri-derivative ; the second gave two ; and the third gave three : from the following formulae, in which equivalent positions for the third bromine atom are denoted by identical numerals, it is obvious that the first dibromobenzene was the para-compound, the second the ortho-, and the third the meta- : The method is generally applicable, but in actual practice it is often difficult to isolate all the various isomerides, since some may be produced in very small proportions. It will be seen that a simultaneous orientation of the tri-derivatives is accomplished; thus, the sole tri-derivative yielded by para-dibromobenzene is I, 2, 4-tribromobenzene. The same fundamental idea may be utilized by eliminating one and the same group from each of a complete series of isomeric tri-derivatives; thus, when the car boxyl group is eliminated from each of the six possible diamino benzoic acids, the resulting diamino-ben zene, from one only of these acids is the para compound; from two, the ortho- ; and from three, the meta (Griess, 1874). The orientation of higher substitution derivatives is determined by considering the di- and tri-substitution corn pounds into which they may be transformed.
Orientating Influence of Substituents in the Benzene Ri ig. When a second substituent is attached to the nucleus of a non substituted benzene it may enter in one of three positions to form the ortho-, para- or meta-compound. A priori, it might be imagined, for example, that upon nitrating toluene (methyl benzene), with one equivalent proportion of nitrating mixture, an equi-molecular mixture of o-, m- and p-nitrotoluene, would be produced. As a matter of fact, the prod uct consists mainly of o- and p-nitrotoluene, with only about 3% of m-nitrotoluene. Thus the substituent originally present in the molecule, i.e., the methyl group, has exerted a pronounced direc tive action, or orientating influence, upon the entering substit uent. Again, when nitrobenzene, is nitrated further it yields m-dinitrobenzene, to the almost entire ex clusion of the o- and p-derivatives. Thus while the methyl group favours entry to the o- and p-positions, the nitro-group dominates the m-position ; or, in other words, the methyl group exerts the ortho-para orientating influence while the nitro-group exerts the meta orientating influence. Such observations indicate a close relationship between the ortho- and para-positions, which are clearly differentiated from the meta-position. The result is inde pendent of the nature of the second group introduced. Experience has shown that as a general rule the ortho-para influence is exerted by hydrocarbon radicals, halogen atoms, and saturated groups such as -OH and while the meta influence is associated with unsaturated groups, such as -COON, -CHO, -CN and As a rule, the former groups facilitate further substitu tion in the benzene nucleus, while members of the second class hinder further action. A working guide to orientating influence is also provided by the empirical rule of Crum Brown and Gibson (1892) : if the hydrogen compound, HX, of the substituent, X, already present in the benzene nucleus can be directly oxidized to HXO, then meta-derivatives predominate in further substitu tion; if not, ortho- and para-derivatives predominate. For exam ple, since hydrobromic acid cannot be directly oxidized to hypo bromous acid, bromobenzene yields mainly ortho- and para-disub stituted benzenes. It follows from what has been said that the order of introducing substituents into the benzene nucleus is of importance; e.g., the bromination of nitrobenzene yields almost exclusively m-bromonitrobenzene, whilst the nitration of bromo benzene leads to the formation of the isomeric o- and p-bromo nitrobenzenes, the product consisting of 35% of the former and 65% of the latter. When a third substituent is introduced into a disubstituted nucleus, the orientating influence of the two groups already present may either be conjoined or opposed, depending upon their nature and relative positions.
No completely satisfactory mechanism has yet been devised to account for the complex phenomena of orientation. Hypotheses have been advanced by J. N. Collie, B. Flurscheim, A. F. Holle man and others; and more recently attempts have been made to base explanations upon the electronic theory of valency. The phenomena are closely associated with the mutual influence ex erted between substituents in the benzene nucleus. Thus, the re activity of nuclear-substituted halogen is greatly enhanced by the presence of a nitro-group in the ortho- or para-position to it ; no influence of the kind, however, is exerted by a nitro-group in the meta-position. An ortho-substituent often diminishes the reactiv ity of the neighbouring substituent, but this effect is due largely to steric hindrance, contingent upon the spatial proximity of the groups: for example, di-ortho-substituted benzoic acids of the type (i) COOH (2) Y (3) cannot usually be esterified with alcohol and hydrogen chloride (V. Meyer, 1894).
Synthesis and Fission of the Benzene Ring.—(i) The char acteristic distinctions which exist between aliphatic and benzenoid compounds make the transformations of one class into the other particularly interesting. In the first place, many aliphatic corn pounds, including simple substances like methane and tetra chloromethane, show a tendency to yield aromatic compounds when subjected to a high temperature, in so-called pyrogenetic reactions ; the predominance of aromatic compounds in ordinary coal-tar is probably to be associated with the occurrence of similar pyro-condensations. The termolecular polymerization (q.v.) of numerous acetylene compounds to form derivatives of benzene is of considerable interest. M. P. E. Berthelot (187o) first synthe sized benzene by passing acetylene through tubes heated to dull redness; at higher temperatures the action becomes reversible, the benzene yielding diphenyl, diphenylbenzene and acetylene. The condensation of acetylene to benzene is also possible at ordinary temperatures in the presence of pyrophoric iron, finely divided nickel, spongy platinum and other catalysts (P. Sabatier and J. B.
Senderens). The homologues of acetylene condense more readily; thus allylene yields trimethylbenzene merely under the influence of sulphuric acid: The "potassium carbon monoxide" obtained by Liebig through the action of carbon monoxide on heated potassium was later (1885) found to yield hexahydroxybenzene, when treated with dilute acid ; further investigation of this compound brought to light a number of highly interesting derivatives (see QUINONES). Mellitic acid, or benzene-hexacarboxylic acid, first obtained from the mineral honeystone (alumin ium mellitate) by Klaproth, in 1799, was afterwards (1883) pre pared by the oxidation of graphite or charcoal with alkaline per manganate ; this result is of particular interest when correlated with recent work on the crystalline structure of graphite (vide supra). A large and important series of condensations may be effected by eliminating the elements of water between carbonyl (CO) and methylene groups. A fundamental and historic example is that of the condensation of three molecules of acetone, in the presence of sulphuric acid, to one molecule of mesitylene (s-trimethylbenzene) and three molecules of water; this reaction, which was first observed by Kane, in 1837, is represented below : The condensation of geranial (citral) to cymene (p= isopropyl methylbenzene) is noteworthy : C, i,RO = The foregoing syntheses lead directly to a true benzene ring; but there are many reactions by which aliphatic compounds yield cyclic compounds with reduced benzene rings, from which true benzenoid compounds may then be prepared. An important example is the condensation, upon heating, of sodiomalonic ester, to phloroglucinoltricarboxylic ester, a sub stance which gives phloroglucinol (s-trihydroxybenzene) when fused with alkalis.
(2) The relative stability of the benzene nucleus invests the question of its fission, or disruption, with a peculiar interest. Kekule's formula indicates the possibility of a successive fission of the three double linkages, with the production of open-chain compounds. Fission usually occurs at more than one point ; so that the six carbon atoms of the nucleus are rarely preserved as an unbroken open chain.
Certain aromatic compounds withstand ring decomposition more strongly than others ; thus, benzene and its homologues, and also carboxylic acids and nitro-compounds, are much more stable towards oxidizing agents than amino- and hydroxy-benzenes, aminophenols, quinones and hydroxycarboxylic acids.
Decompositions by strong oxidation to carbon dioxide, formic acid, oxalic acid, etc., are of little interest. More important is the hydrolysis of benzene triozonide (ozobenzene), to glyoxal, CHO-CHO, since this disruption, suggestive of three double bonds in the ring, has been advanced as evidence in favour of Kekule's formula: Tartaric acid, containing a chain of four carbon atoms in its mole cule, has been obtained by oxidizing 1, 2-dihydroxybenzene (catechol) and hydroxybenzene (phenol) with nitrous acid and dilute potassium permanganate, respectively. 1, 3-dihydroxyben zene (resorcinol) when successively reduced and oxidized yields glutaric acid, with five carbon atoms in the chain. An open-chain compound with six carbon atoms may be had, for example, from I, 4-dihydroxybenzene (hydro quinone) ; whilst salicylic acid, or 1, 2-hydroxybenzoic acid, CRH,(OH)•COOH, when reduced by sodium in amyl alcohol yields n-pimelic acid, HOOC•(CH2),•COOH, an aliphatic com pound whose molecule contains, in the form of a seven-membered open chain, the one substituent and six nuclear carbon atoms of the original aromatic molecule.
Chemical Characteristics of Nucleus and Side-chain in Aro matic Compounds.—Aliphatic chains, and also single groups con taining carbon, when attached to a nuclear atom of a ring com pound, are known as side-chains. Thus the molecules of toluene (methylbenzene) and cumene (isopropylbenzene) each contain one side-chain, while that of mesitylene (s-trimethylbenzene) con tains three. The substituent radical, –NH-CO-CH, of acetanilide, also is called a side-chain; but the term does not usually comprise simple groups which do not contain carbon, such as –NO2,, –OH and Side-chains attached to the nucleus by carbon atoms may be oxidized to carboxyl, --COON: thus upon oxidation yields suc cessively o-toluic acid, and o-phthalic acid, while the isomeric ethylbenzene, yields only benzoic acid, The basicity of the final aromatic acid obtained upon oxidation thus indicates the number of carbon-linked side-chains in the original molecule. Moreover, halogen atoms present in the side-chains are removed in the oxida tion, while nuclear halogen atoms remain; for example, benzyl chloride, oxidizes to benzoic acid, while the iso meric o-chlorotoluene, yields o-chlorobenzoic acid, In mixed aliphatic-aromatic compounds, like toluene, the aliphatic side-chains retain the characters of their class, while the benzenoid nuclei retain their aromatic properties. Thus toluene is readily oxidized to benzoic acid, as stated above, the aromatic residue remaining unattacked. Nitric and sulphuric acids, on the other hand, affect only the nucleus, producing o- and p-nitrotoluenes, and o- and p-toluene-sulphonic acids, by the usual processes of substitution. Chlorination may result in the formation of derivatives substi tuted either in the aromatic nucleus or in the aliphatic side-chain. The former substitution occurs more readily, o- and p-chloro toluenes, being produced; while the latter substitu tion, which needs an elevation in temperature or other auxiliary, yields benzyl chloride, benzal chloride, and benzotrichloride, Halogen atoms in the side-chain are very reactive, resembling in this respect the halogen atoms of alkyl halides; nuclear halogen atoms, on the contrary, are normally unreactive. Thus, benzyl chloride, is readily hydrolyzed by aqueous alkali, forming benzyl alcohol, but the isomeric chloro toluenes, are unaffected by this treatment. Further differences become apparent when various other typical aliphatic and aromatic derivatives are compared. The introduction of hy droxyl groups into the benzene nucleus gives rise to compounds generically named phenols, which, although resembling the ali phatic alcohols in their origin, differ from these substances in their increased chemical activity and their acid nature. Structurally, the phenols resemble the tertiary alcohols, since the hydroxyl group is linked to a carbon atom which is united to other carbon atoms by its three remaining valencies ; hence on oxidation, instead of yield ing corresponding aldehydes, ketones or acids, they undergo nu clear fission, as indicated above.
The amines, or amino-compounds, also exhibit striking differ ences. In the aliphatic series these compounds may be formed directly from the alkyl halides and ammonia, but in the benzene series this reaction is impossible unless the nuclear halogen atom be weakened by the presence of other substituents; e.g., nitro groups. Thus, primary aromatic amines are usually prepared by reducing nitro-compounds, nitrobenzene, for example, yielding ani line, in this way; intermediate reduction products, known as azoxy- and azo-compounds (q.v.), may also be prepared from aromatic nitro-compounds. Primary aromatic amines differ fundamentally from primary aliphatic amines in giving the impor tant diazo-reaction with nitrous acid (see DIAZO-COMPOUNDS). Moreover, whilst methylamine, dimethylamine and trimethylamine increase in basicity with the introduction of successive methyl groups, the opposite effect is observed upon introducing succes sive aromatic groups, such as the phenyl group, For in stance, aniline (phenylamine), diphenylamine, and triphenylamine, are in decreasing order of basicity, the last-named substance being insoluble even in strong mineral acids. Mixed aromatic—aliphatic amines, both secondary and tertiary, are more strongly basic than the pure aromatic amines, and less basic than the true aliphatic compounds; e.g., aniline, mono methylaniline and di-methylaniline, are in increasing order of basicity. These observations may be summarized by saying that the benzene nucleus is more negative in character than the alkyl or other aliphatic radicals. In agree ment with this conclusion, benzoic acid, is a much stronger acid than acetic acid, As a general rule, homologues and mono-derivatives of benzene react more readily than the parent hydrocarbon with substituting agents; for example, phenol is converted into tribromophenol by the action of bromine water, and into nitrophenols by dilute nitric acid. Similar activity characterizes aniline. As already pointed out, not only does the substituent group modify the readiness with which the derivative is attacked, but it exerts also an orien tating influence which determines the position of the further substitution.
Homologues of Benzene.—The following homologous formulae are obtained by the progressive addition of the usual increment, to the molecular formula for benzene. The isomerism here indicated may readily be deduced from the foregoing discussion: Benzene (b.p. 80.4° C).
Toluene or methylbenzene (b.p. Xylenes or dimethylbenzenes (3) ; ethylbenzene.
Trimethylbenzenes (3) ; methylethylbenzenes (3) ; pro pylbenzene ; isopropylbenzene (cymene) .
Tetramethylbenzenes (3) ; p-methylisopropylbenzene (cymene) ; etc.
All four isomerides of the formula occur, like benzene and toluene, in coal-tar; o-, m- and p-xylene boil at 142°, 139° and 138°, respectively, and ethylbenzene at 136°. Mesitylene or s-trimethylbenzene (b.p. 165°), also occurs in coal-tar. Cymene (b.p. 175°) is found in the essential oils of various species of eucalyptus and other plants; hexahydrocymene (menthane or ter pane), is the parent compound of the majority of the terpenes (q.v.) and camphors (q.v.) , and thus cymene is formed when ordinary camphor is dehydrated with phosphorus pentoxide and when pinene is heated with iodine. All these homologous hydrocarbons resemble benzene and toluene in their general character, and readily undergo nitration, sulphonation and halo genation.
Aromatic Radicals.—Among the aromatic radicals which occur commonly in the molecules of mononuclear aromatic compounds are the following : Phenyl, often abbreviated to Ph ; ben zoyl (Bz), benzyl, C0H5 ; benzylidene or benzal, COHS•CH = ; phenylene, —C„1-1,—. Generically, aromatic radicals, or aryl radicals, are denoted by the symbol Ar, and aliphatic radicals by R. Mixed aromatic—aliphatic radicals, like benzyl, are sometimes called alphyl radicals.
Distinct Nuclei.—The simplest compound of the first type is diphenyl, a crystalline hydrocarbon (m.p. 75° C) which may be prepared from sodium and iodobenzene in ether (Fittig's reaction). Diphenylmethane (m.p. 26°), and tri phenylmethane (m.p. 92°), are formed when benzene interacts with methylene chloride and chloroform, respectively, in presence of aluminium chloride (Friedel and Crafts' reaction) ; tetraphenylrethane, also is known.
Condensed Nuclei.—The three important coal-tar hydrocar bons, naphthalene, anthracene and phenanthrene, have been as signed molecular structures which result from the fusion of ben zene rings; although they are not homologues of benzene, they show the general aromatic behaviour characteristic of the ben zene nucleus. The molecule of naphthalene, is usually represented as consisting of two benzene rings having a pair of adjacent carbon atoms in common; while the molecular structures of the pair of isomeric hydrocarbons, anthracene and phenan threne, are formed from three benzene nuclei. The discus sion will be confined in this place mainly to the structure of these compounds ; further information concerning their properties, reac tions and derivatives will be found under the individual headings.
Naphthalene occurs in coal-tar to the extent of about 5%, and is the most abundant of the important primaries. It is the parent compound of a large number of purely synthetic dyes, and also of synthetic indigotin. It was isolated from coal-tar by Kidd, in 182o, and its composition was established by Faraday, in 1826. In 1866, the year following the publication of Kekule's theory of the benzene ring, E. Erlenmeyer, Sr., proposed for naphthalene a structure (I.) consisting of two "ortho-condensed" benzene nuclei, with two common carbon atoms. Since naphthalene yields o-phthalic acid upon oxidation, its molecule must contain at least one benzene ring ; and according to Erlenmeyer's formula, either of the benzene rings (a) or (b), may undergo oxidative fission to yield this product : Shortly after Erlenmeyer's suggestion had been made, Graebe established the symmetry of the naphthalene nucleus, and showed that whichever half of the molecule be oxidized o-phthalic acid results. Similarly, a-nitronaphthalene, formed by the action of strong nitric acid on the hydrocarbon, yields nitro-o phthalic acid upon oxidation. The nitro-group, entering either of the rings (a) or (b), serves to distinguish that ring from the other; assuming the group to enter (a), it is obviously (b) which then undergoes disruptive oxidation. If, however, the nitro-group be reduced to the amino-group prior to this oxidation, the result ing a-naphthylamine, yields o-phthalic acid when oxidized; in this instance it is the second nucleus (b), distin guished by its non-substitution, which survives in the product. Thus the naphthalene molecule contains two potential benzene nuclei, having two adjacent carbon atoms in common: The fundamental character of the naphthalene nucleus has been confirmed by a number of syntheses. Thus, the vapour of For all ordinary purposes it is sufficient to represent the naph thalene molecule by the double hexagon (II.) ; in order to facili tate nomenclature, the carbon atoms carrying hydrogen atoms are then numbered and lettered as shown below : Mono-substitution derivatives of naphthalene can thus exist in two isomeric modifications, depending upon whether or not sub stitution occurs on an atom adjacent to one of the two carbon atoms common to both rings : a-derivatives are produced by substitution in one of the equivalent positions, i, 4, 5 or 8; 0-derivatives by substitution in one of the second set of equiva lent positions, 2, 3, 6 or 7. Orientation of such derivatives can often be accomplished by oxidation to the corresponding sub stituted o-phthalic acid. Substituted naphthalenes of the types and may exist in i o and 14 isomeric modifica tions, respectively. If the two substituents are attached to adja cent carbon atoms in the same ring, the derivative bears a general chemical resemblance to ortho-disubstituted benzenes. Some of the characteristics of the latter compounds, particularly the ability to participate in further ring-closures, are shown also by the so-called peri-derivatives, which contain substituents in the I, 8- or 4, 5-positions.
Since, as already indicated, Kekule's formula affords an inade quate interpretation of the benzene molecule, it is not surprising that Erlenmeyer's derived formula fails to express the complete chemical behaviour of naphthalene. Indeed, the naphthalene molecule, like that of benzene, is probably capable of passing through a series of closely related structural phases. Erlenmeyer's formula explains the decompositions and syntheses of naphthalene derivatives, and accounts satisfactorily for the observed isomer ism ; but although naphthalene is more reactive than benzene, its chemical character is not in keeping with the presence of five double linkings in the molecule. According to Bamberger, the molecule is aromatic but not benzenoid ; however, by the reduc tion of one ring, the other assumes a true benzenoid character. Thus, 3-naphthol yields alicyclic tetrahydro-f3-naphthol upon re duction; and this substance, in which the hydroxyl group is at tached to the reduced ring, closely resembles the alcoholic i-0-hy droxyethyl-2-ethylbenzene. Upon reducing a-naphthol, the hy drogen atoms enter the non-substituted ring, and the resulting aromatic tetrahydro-a-naphthol resembles the phenolic r-hy droxy-2,3-diethylbenzene : Bamberger's formula represents the molecule as containing a monocyclic nucleus of ten carbon atoms, which, however, is capable of facile transition into the ortho-condensed bicyclic system. When, as in the formation of naphthalene tetrachloride or I, 2, 3, 4-tetrahydronaphthalene (tetralin), for example, the one ring becomes saturated, the other might be expected to assume the normal centric form and become truly benzenoid in character. This is so; thus, tetrahydronaphthalene has the character of an alkyl benzene, and ar-tetrahydro-a-naphthol that of a phenol, as is shown in the diagram above. Bamberger's observations on re duced quinoline derivatives likewise point this out, that condensed nuclei of this type are not benzenoid, but possess an individual character; this disappears, however, when the molecule is reduced. Armstrong's formula (IV.) obviates the necessity of postulating the existence of a ten-membered ring, by assuming that one of the affinities of each of the two central carbon atoms common to the two rings acts into both rings. This assumption represents a deviation from the ordinary views of valency and affinity, but the symbol harmonizes with the fact that the two rings are in complete sympathy, the one responding to every change made in the other.
Other structural representations have been based upon Dewar's formula for benzene. Thiele's theory of partial valencies applies to the naphthalene molecule with a considerable degree of suc cess. According to this theory, the molecule of naphthalene, un like that of benzene, does not necessarily present a closed con jugated system. Thiele's formula (Va) indicates open partial valencies in two of the four peri-positions. The formula thus gives expression to the fact that naphthalene differs notably from ben zene in the ease with which it forms additive compounds ; more over, the additive process sets in at the a-positions. Another phase of the molecule is possibly represented by an alternative formula (Vb), also based upon Thiele's theory: When naphthalene is reduced with sodium in boiling alcoholic solution it yields r, 4-dihydronaphthalene, a substance which pos sesses all the unsaturated characteristics of ethylene. This re sult is completely in accord with formula Va. The constitution of the dihydronaphthalene is shown by its oxidation to o-phenylenediacetic acid: Analogous relationships hold for the corresponding amino-com pounds.
In order to account for these peculiarities, formulae derived Further reduction yields i, 2, 3, 4-tetrahydronaphthalene, and finally decahydronaphthalene (decalin, q.v.), a fully satu rated hydrocarbon, whose molecule contains two ortho-condensed cyclohexane rings.
As a rule, naphthalene yields mainly a-derivatives in substitu tion reactions; e.g., when it is chlorinated or nitrated; when it is sulphonated, however, the a-derivative predominates at 8o° C and the )3-derivative at 16o° C. Substitution in the aromatic nucleus of I, 2, 3, 4-tetrahydronaphthalene, on the contrary, fur nishes a mixture of the a- and f3-derivatives in which the latter often predominate. The derivatives of naphthalene resemble broadly those of benzene in their general behaviour. Thus, the naphthols exhibit the acidic character of phenols; the naphthyl amines may be diazotized like aniline, etc. ; and the naphtha quinones are very similar to the benzoquinones. Moreover, halo gen atoms attached directly to the nucleus, although more reactive than in the benzene series, are stable towards hot aqueous alkalis.
Anthracene, C14H,0, forms from about o.25-0.45% of coal-tar. It is the basis of many artificial dyes and also of synthetic ali zarin. According to Graebe and Liebermann's formula (I.), the anthracene molecule is composed of three ortho-condensed six membered nuclei, the medial ring (which contains a para-linkage) being fatty in character, while the lateral rings are benzenoid. This formula represents adequately the syntheses, decompositions and general chemical behaviour of anthracene, but it should probably be regarded as one phase of a dynamic system which embraces formulae (II.) and (III.) . The last of these is some times called the "ortho-quinonoid" formula. A centric formula also has been advanced by Bamberger.
According to Armstrong, one lateral ring is centric, while the remaining lateral ring and the medial ring are ethenoid. The fundamental nature of the anthracene nucleus is evident from the synthesis of dihydroanthracene ; this compound, which results when o-bromobenzyl bromide is condensed in presence of sodium, may be oxidized to anthracene: The abbreviated representation of anthracene, with the accepted numbering and lettering of the carbon atoms, is indicated below. These representations show that three isomeric monosubstitution compounds are possible, together with a considerable variety of disubstitution products: When anthracene is reduced with sodium and alcohol, the medial ring is attacked with the formation of dihydroanthracene, men tioned above. Nitric acid attacks the molecule at the same points, yielding anthraquinone, the parent compound of alizarin and many other dyes.
Phenanthrene, is an isomeride of anthracene. It is not oxidized so readily as anthracene, but with chromic acid it yields phenanthraquinone and then diphenic acid: An alicyclic ring is often denoted by the prefix ac-.
It was long supposed that the simplest ring obtainable contained six atoms of carbon; so that the discovery by Freund, in 1882, of cyclopropane aroused an interest which was enhanced by the novel chemical properties of the new hydrocarbon. With bromine, it combines less readily than the isomeric propylene, : but it is easily converted by hydrogen bromide into n-propyl bromide, The separation in this manner of car bon atoms united by single affinities was unprecedented at that time, but a similar behaviour has since been noticed in derivatives of cyclopropane, the ring being very susceptible to fission. Cyclo butane and its derivatives are rather more stable, and the nuclei containing five and six carbon atoms are very stable, showing little tendency to form open-chain compounds under ordinary conditions.
Baeyer has explained these variations in stability with consider able success by a purely mechanical hypothesis known as the "strain" or "Spannungs" theory (1885) . Assuming the four valencies of the carbon atom to be directed from the centre of a regular tetrahedron towards its four corners, the angle between any two valencies is 109° 28'. According to the strain theory, the valencies undergo deflection in the formation of carbon rings, and the tension thus introduced may be deduced from a comparison of the tetrahedral angle with the new angle. The amount of the deflection is regarded as a measure of the reactivity, or instability, of the ring. Thus, the extraordinary reactivity of ethylene is attributed to the circumstance that the distortion is here a maximum, since, if deflected into parallelism, each valency will be drawn out of its original position through 44'. The values in other cases are calculable from the formula .(a-109° 28'), where a is the internal angle of the regular polygon contained by sides equal in number to the number of carbon atoms in the ring. The displacements for various polymethylenes are given below : The general behaviour of the several types of saturated homo cyclic systems is certainly in accordance with this conception. It is found, for example, that the most stable of the above ring systems are those containing five or six carbon atoms ; a slight inward displacement is indicated for the five-membered ring and a somewhat larger outward displacement for the six-membered ring. It is, therefore, noteworthy that when benzene is reduced with hydriodic acid it is converted into a mixture of cyclohexane and methylcyclopentane; many other conversions of six-carbon rings into five-carbon rings have also been recorded. Similar con siderations apply to heterocyclic rings; thus con taining four carbon atoms and one oxygen atom in the ring, are more stable than the 5-lactones, which contain an additional carbon atom in the ring.
The strain hypothesis has been modified by J. F. Thorpe and C. K. Ingold. "It has, for instance, been calculated that whilst the natural inclination of two (carbon to carbon) valencies of a carbon atom carrying two hydrogen atoms is 115.3°, this inclina tion becomes depressed to 109.5° when the hydrogen atoms are replaced by groups such as methyl, which require a larger share of the space surrounding the carbon nucleus. This attachment of two methyl groups to carbon atoms involved in a 3-, 4-, or 5-membered ring considerably augments the stability of the struc ture, whilst if the ring contained six or seven carbon atoms a diminution in stability would result." (Thorpe, Chemistry in the Twentieth Century, London, 1924, p. 96.) The limiting number of atoms which can be associated in a carbocyclic system by single valencies has not yet been deter mined. Large rings, containing up to about 3o carbon atoms, are difficult of access; but once formed they are just as stable as the and C,-systems. This striking fact finds an explanation in the Sachse-Mohr theory of strainless rings, according to which the strain in large polymethylene rings may be relieved by a twisting into a multiplanar configuration. Thus, according to Mohr (1918), an indication of the existence of two strairnless forms of the cyclo hexane ring is to be found in the occurrence of two stable isomeric modifications of decahydronaphthalene, formed by the fusion of two such rings (Annual Reports on the Progress of Chemistry, 1926).
No isomerism occurs in the monosubstitution derivatives, but ordinary position isomerism exists in the di- and poly-substitution compounds. Stereoisomerism (see STEREOCHEMISTRY) also may occur, since the two atoms or groups attached to each saturated nuclear carbon atom are disposed, respectively, above and below the plane of the ring. The simplest examples are the dibasic acids: these yield a cis- (malenoid) form and a trans- (fumaroid) form. Such stereoisomerides may be depicted (Aschan) by representing the plane of the carbon atoms of the ring as a straight line. Thus, for the dicarboxylic acids (X = COOH) the possibilities are repre sented as follows: The trans-compound, being usually asymmetric, is in such in stances capable of existing in enantiomorphous forms, as indicated in the diagram. For example, trans-hexahydrophthalic acid, has been resolved into dextro- and laevo-rotatory modifications; the cis-acid is symmetrical and unresolvable.
Hydrocarbons may be obtained from the dihalogen paraffins by the action of sodium or zinc dust, provided that the halogen atoms are not attached to the same or to adjacent carbon atoms (Freund, 1882; W. H. Perkin, Jr., 1888) : They may be prepared also by the action of hydriodic acid on benzene hydrocarbons (vide supra) ; by passing the vapour of benzene hydrocarbons over finely divided nickel at 18o-25o° C (Sabatier and Senderens) ; from hydrazines of the type by oxidation with alkaline potassium ferricya nide (Kijner) ; and from semicarbazones of the corresponding cyclic ketones, by reduction. Unsaturated hydrocarbons of the series may be prepared by the elimination of water or halogen acid from the corresponding alcohols or halogen derivatives. Pure cyclobutene has been made by distilling the quaternary ammonium hydroxide of aminocyclobutane.
The boiling points of successive cycloparaffins, from cyclopro pane to cyclononane, are —35°, 1I °, 49°, 81°, 117°, 146° and 1 j I ° C. The boiling points and specific gravities of these hydro carbons are higher than the constants for the corresponding open chain olefines or paraffins. Cyclohexane, is an important constituent of Caucasian petroleum; it may be prepared by the catalytic hydrogenation of pure benzene. It is oxidized by nitric acid to adipic acid, and is converted by chlorine to monochloro cyclohexane ; the latter derivative yields tetrahydrobenzene (b.p. 84°) when treated with alkalis. Aminocyclohexane is funda mentally similar to the primary hexylamines. Cyclopentadecane, C,,,,,H30, and cycloheptadecane, C,711., prepared by reducing the semicarbazones of the corresponding cyclic ketones, melt at 61° and 65°, respectively.
Of the cyclo-olefines, cyclopentadiene, (b.p. 40), ), occurs in crude coal-tar benzene. Owing to the peculiar reactivity of its methylene group, this substance condenses readily with aldehydes and ketones, acetone, for example, yielding the intensely coloured hydrocarbon, dimethylfulvene :- Cyclohexene or tetrahydrobenzene, boils at 84° (vide supra), while 01.3- and Al (the isomeric dihy drobenzenes, boil at 81-82° C.
Cycloheptatriene (tropilidene), may be prepared from cycloheptanone ; it is also a degradation product of cocaine and atropine (see ALKALOIDS). has been prepared from the alkaloid, 1'-pelletierine, by exhaustive methyla tion. Cyclo-octatetrene, is interesting by reason of the fact that it displays no aromatic characteristics, although the nucleus possesses a system of alternating single and double bonds; this evidence was adduced by Willstatter (191I) in favour of the centric constitution for benzene. Mention may also be made of C. Harries' unsubstantiated suggestion of a cyclo-octadiene ring as the possible structural basis of the molecule of caoutchouc, the hydrocarbon of rubber, whose properties show that its molecular structure is more complex than its empirical formula indicates.
Alcohols are obtained from the corresponding halogen com pounds by the action of moist silver oxide or silver acetate and acetic acid; by the reduction of cyclic ketones with metallic sodium ; by passing the vapours of monohydric phenols, in admix ture with hydrogen, over finely divided nickel; by the reduction of cyclic esters with sodium and alcohol; and by the addition of the elements of water to unsaturated cyclic hydrocarbons, on boiling with dilute acids.
Cyclohexanol or hexahydrophenol, (b.p. 16o.5° C), is readily prepared by the catalytic hydrogenation of phenol in presence of finely divided nickel at i 5o° C. It reacts with hydrogen bromide to yield monobromocyclohexane (b.p. 162°), and under goes dehydration to tetrahydrobenzene when heated with oxalic acid. Quinitol (cyclohexane-l:4-diol or hexahydroquinone) results when cyclohexane-l:4-dione (p-diketohexa methylene) is reduced with sodium amalgam ; it exists in cis- and trans- forms (m.p. 102° and 139°) . Quercitol, or cyclohexanepen tol, occurs in acorns; it is optically active, forms colourless prisms (m.p. 234°), and has a sweet taste. Inositol, or cyclohexanehexol, is widely distributed in plants and animals. Like the structurally isomeric hexoses, it has a sweet taste. Although it possesses no carbon atom which is asymmetric in the ordinary sense, it is capable of displaying optical activity in the amorphous condition. Anhydrous d (or l)-inositol melts at 246°, dl-inositol at 253°, and an internally compensated form at 225°.
The aldehydes, of which comparatively few have been studied in this series, are prepared in the usual manner from primary alcohols and acids : hexahydrobenzaldehyde, C6H,1.CHO, and hexa hydro-m-toluic aldehyde, CHO, boil at 1J9° and 176°, respectively.
The ketones are obtained by the dry distillation of the calcium salts of dibasic saturated aliphatic acids (Wislicenus, 1893), and by synthetic methods from malonic ester, acetoacetic ester, etc. (see ALIPHATIC section). The first method affords satisfactory yields of the saturated members up to cycloheptanone; and cyclo octanone can be obtained in a similar way from thorium azelate in a yield of 25%. Owing to their accessibility, these ketones have been utilized largely in the preparation of other derivatives in this group. Cyclobutanone, (CH,>) boils at i oo°, and cyclopen tanone, at 129°. Cyclohexanone, or ketohexamethylene, is prepared by distilling calcium pimelate or by oxidiz ing cyclohexanol, to which it is readily reduced. It oxidizes to adipic acid, and may be acetylated, owing to its tautomeric rela tionship to Cyclohexanone boils at and cycloheptanone (suberone) at 18o° C.
Apart from their existence in a few naturally occurring sub stances (v. inf.), alicyclic rings with from 10 to about 3o carbon atoms are accessible only by vacuum distillation of the thorium salts of the above-mentioned acids (Ruzicka, 1926). The resulting ketones, from cyclododecanone, to cyclo-octadeca none (CH2),7CO, are solids, the melting-points of the successive members being 59°, 32°,52°, 63°, 56°, 63° and 71° C. These ketones possess characteristic odours, and cyclopentadecanone ("exaltone"), is used as a synthetic substitute for musk. Muscone, C18H300, the main odoriferous constituent of natural musk, is a 2-methylcyclopentadecanone, and civetone of the civet cat is 0 C„1-1.O (Ruzicka).
A great impetus was given the study of polymethylene deriva tives by the important and unexpected observation made by W. H. Perkin, Jr., in 1883, that ethylene and trimethylene bromides are able to react with sodio-acetoacetic ester to form tri- and tetra methylene rings : Cyclopropanecarboxylic acid is a colourless oil. The truxillic acids, are polymerides of cinnamic acid, and they result in the hydrolysis of truxilline. Structurally, they are phenyl derivatives of cyclobutane. Derivatives of the cyclopentane group occur commonly as break-down products of the terpenes and camphors (q.v.) ; for example, campholic acid, obtained from ordi nary camphor by hydrolysis, is 1,2,2,3-tetramethylcyclopentane 3-carboxylic acid. Hexahydrobenzoic acid, or cyclohexanecarb oxylic acid, (m.p. 30° C), may be prepared by the vigorous reduction of benzoic acid, or by the action of carbon dioxide on cyclohexyl magnesium iodide (see GIUGNARD REAGENT). The naphthenic acids of Caucasian petroleum are probably homo logues of this substance. l-Quinic acid, or hexahydrotetrahydroxy benzoic acid, m.p. 162°, is an important sub stance occurring in cinchona bark, coffee beans, sugar beet and many other plants. Discovered by Hofmann in cinchona bark, in I 79o, it is a by-product in the manufacture of quinine, and is used in pharmacy. At 25o° C, it yields the lactone, quinide, which on warming with baryta water furnishes optically inactive quinic acid. Three hexahydrophthalic acids, or cyclohexanedicarboxylic acids, are known, containing the carboxyl groups in the 1, 2-, I, 3-, and I, 4-positions, and each exists in cis- and trans-stereoisomeric forms. The reduced phthalic acids were sub mitted to a detailed examination by Baeyer in his prolonged researches (1887-92) on the constitution of benzene, and numer ous acids of the kind are known. Thus, from o-phthalic acid (benzene-1, 2-dicarboxylic acid) alone, it has been possible to prepare two hexahydrophthalic acids, four tetrahydrophthalic acids and eleven dihydrophthalic acids.
Spirocyclanes and their derivatives possess two alicyclic rings having only one carbon atom in common. Such compounds, although not of natural occurrence, are interesting stereochemi cally, since the two rings occupy two planes intersecting at right angles through the common carbon atom. A single example is afforded by spiroheptanedicarboxylic acid (Fecht, 19o7) :