AROMATIC COMPOUNDS, a numer ous and exceedingly important class of sub stances, fundamentally differing from the fatty compounds in constitution, and named from the fact that the earliest known representatives of the class were resins, oils and balsams, distin guished by a marked aromatic odor. The name is now applied to all substances containing a °benzene nucleus° (presently to be described). Benzene itself is the simplest example of an aromatic body. Its formula, expressed in the simplest way, is GIL, but when the attempt was made to represent the composition of ben zene by a °structural formula," numerous diffi culties were encountered. For example, benzene behaves like a saturated compound in most re spects, yet it contains eight atoms less of hydrogen than the saturated paraffin "hexane* (GK.) containing the same number of carbon atoms. Again, any or all of the hydrogen atoms in benzene can be replaced by other monovalent elements (or radicals) ; and the persistence of the group C. in the derivations of benzene, even when all the original hydrogen atoms have been replaced by other elements or radicals, indicates that the carbon atoms in that body are inti mately related to one another, in some manner. Furthermore, it has been proved by experiment that the hydrogen atoms in benzene are "of equal value," so that it makes no difference, in forming a substitution compound, which atom of hydrogen is replaced, and this fact indicates that the hydrogen atoms should occur in the structural formula symmetrically. To reconcile these considerations (and many others) Kelcule, in 1865, proposed for benzene the structural formula preceding.
The symmetry of the body with respect to hydrogen is here evident, and the persistence of the group C. is explained by assuming the six carbon atoms to be united to one another in the form of a closed chain, supposed to possess sufficient chemical strength to maintain its own integrity, save under exceptional circumstances. The closed ring of six carbon atoms is the "benzene nucleus," referred to above, which constitutes the distinctive feature of the aro matic compounds as a class. It will be observed that in Kekule's structural formula the carbon atoms are all tetravalent, just as the carbon is in carbon dioxide (CO.), and that three of the four valencies of each carbon atom are satisfied by the valencies of other carbon atoms, while the fourth is satisfied, in each case, by a hydro gen atom. Von Baeyer has proposed a slightly different structural formula for benzene, even more symmetrical in appearance than Kekules, but which raises certain questions that are not yet answered. His formula is as shown here with. The closed carbon chain is present here also, but only three of the valencies of each carbon atom are definitely provided for, and it is assumed that the six remaining valencies (one to each carbon atom) are satisfied by a sort of "central linkage,' whose precise nature is not determined or defined. It is customary, at the present time, to express the structural formula of benzene in the simple form without attempting any explanation of the fact that the carbon atoms are here apparently trivalent. Much thought has been expended upon this matter, and the constitution of benzene is one of the most interesting problems in the realm of organic chemistry. The structural formulas presented above are not to be taken in any sense as pictorial representations of the actual geometrical configuration of the benzene mole cule. We know nothing at all about the shape of a molecule, nor about the way in which its parts are associated with one another, in space. The structural formulas employed in chemistry are mere empirical diagrams, for representing, to the eye, the chemical properties and relations that have been observed in the laboratory.
Compounds have been prepared which con tain closed rings of three, four and five atoms of carbon, respectively, but these are not classed as aromatic compounds. They are intermediate, in general properties, between the aromatic series and the fatty series, but resemble the latter more closely. Compounds are also known in which the chain is closed by an atom of oxygen, or of sulphur, or of nitrogen. Thus the structural formula of pyridine is as shown herewith. Such substances could be classed as 'aromatic compounds' by an extension of the definition of the aromatic group, but are usually regarded as outside of the limits of that group. The aromatic compounds are so numerous, and include so many substances of technical im portance, that only the merest outline of their general character can be given in this place. In general it may be said that they are derived from benzene by replacing one or more of its typical hydrogen atoms by an equal number of monovalent radicals (either simple or com pound). The essential features of these substi
tutions may be illustrated by considering the chlorobenzenes. By the action of chlorine upon cold benzene, several substitution products are formed, having the formulas GIlsCl, G1-1,.C1,, C.41-12C13, etc. according to the number of atoms of hydrogen that are replaced by the chlorine. The first of these substitution products, is called simply echlorobenzene," and it is to be noted that since the hydrogen atoms in the orig inal benzene are all °of equal value° (that is, all involved symmetrically), it makes no differ ence which hydrogen atom is replaced by the chlorine; hence only one chlorobenzene having the formula G1-I,.C1 is possible: But when a second atom of hydrogen is replaced by chlorine, the resulting compound, C.Hs.Cl, (known as di chlorobenzene), can exist in no less than three distinct isomeric forms, according to the rela tive positions of the chlorine atoms in the benzene ring. Let the structural formula of chlorobenzene (GHs.CI) be represented by the skeletonized scheme, the numbers represent ing the several groups of CH, in one of which the hydrogen is to be replaced by a further substitution of chlorine. It is evident that the next chlorine atom may replace a hydrogen atom at any one of the five vertices to which numbers have been attached; but it is also evi dent from symmetry that the two positions numbered al) must be considered as essentially identical, as far as the product resulting from a substitution is concerned, and the same is also true of the two positions marked °2.') Only three essentially different ways of substituting the second chlorine atom need therefore be considered. When the second chlorine atom is situated at an angle adjacent to the first, the product is known as ortho-dichlorobenzene; when the second chlorine atom is separated from the first by one vertex which still retains its hydrogen, the product is known as meta-dichlo robenzene ; and, finally, when the two substituted atoms of chlorine are opposite one another, the product is known as para-dichlorobenzene. The three different dichlorobenzencs thus shown by the structural formula to be possible are ac tually known. All aromatic compounds having the general formula GHsYs (where Y is a monovalent element or radical) occur in three isomeric series, just as the chlorobenzenes do, and the separate compounds are distinguished, as already explained in the case of dichloroben zene, by the prefixes ortho-, meta- and para-. These prefixes are frequently abbreviated to single letters, in works on chemistry. Thus p dihydroxybenzene is often written in the place of the full name, This particular substance (used in photography and commonly known as ahydroquinone) is formed, as its name implies, by the substitution of two molecules of hydroxyl (HO) for two atoms of hydrogen in the benzene ring, the hydroxyl molecules being opposite each other (as indicated by the prefix para-). Its struc tural formula, therefore, is as below. The ortho- compound having the same composition (except that its two hydroxyl molecules are in the °orthop positions), is a different substance, known more familiarly as catechol, or pyro catechin, and the meta- compound (where the two molecules of hydroxyl are in the °meta* positions) is quite different from either of the others, and is known as resorcinol. The sub stitutions of monovalent radicals for the hydro gen atoms in benzene are by no means limited to two, nor need the radicals that are substi tuted be alike. Thus pyrogallic add, so exten sively used as a developer in photography, is obtained from benzene by the substitution of three molecules of hydroxyl for three mole cules of the benzene hydrogen; and it there fore has the formula Celi,.(OH)8. Vanillin, now largely used in the place of extract of vanilla for flavoring confectionery and ices, is benzene in which three atoms of the original hydrogen have been replaced, respectively, by the groups (CHO), (OCHs) and (OH). On account of the typographical difficulties in volved in printing the structural formulas of the aromatic compounds, chemists often specify the constitution of these compounds by num bering the original hydrogen atoms of the ben zene from 1 to 6, and then specifying by num ber, which hydrogen atom has been replaced by each of the substituted radicals. Thus vanillin (referred to above) consists of benzene in which CHO has been substituted for the first hydrogen atom, OCR, for the third, and OH for the fourth: and with this convention the constitution of vanillin may be expressed thus: CHO: OCHs: OH-1 :3 :4.