ALIPHATIC DIVISION In this section are outlined the classification, general methods of preparation, and characteristics of the very large number of sub stances concerned. There are special articles on most of the technically valuable members of this subdivision and on important theoretical considerations such as valency, isomerism and stereo chemistry.
(i.) Paraffins. Analysis and molecular-weight determination of the simplest possible hydrocarbon, marsh gas or methane, lead to the formula Organic chemistry has been built up on the conception that carbon is quadrivalent ; i.e., that this element is capable of combining with four atoms of hydrogen to form a stable compound. Further, it has been shown experimentally that the four hydrogen atoms are of equal value, and consequently the only method of picturing the arrangements of the atoms in this molecule is to regard the carbon as situated in the centre of a sphere, with its four valencies directed symmetrically in space. This view of the molecular architecture of marsh gas will be referred to later; in the meanwhile, the graphic formula will be written as When chlorine acts on marsh gas, the first substance formed is methyl chloride, a sweet-smelling gas boiling at —24° C ; one hydrogen atom in the original substance has been replaced by chlorine with the simultaneous formation of hydrogen chloride. The further action of chlorine replaces a second and so on, until finally tetrachloromethane or carbon tetrachloride, CC1, (b.p. 76° C), results; this is a valuable commercial solvent (see CARBON) . This behaviour of marsh gas with chlorine is typical of the whole group of the paraffins, and it is for this reason that they are termed saturated, and are alluded to as the limit hydrocarbons.
Methyl iodide, corresponding closely to the chloride, is a liquid boiling at 43° C. If dissolved in an inert solvent and treated with metallic sodium, it forms sodium iodide, and the two methyl groups join together forming ethane, CH„---CH,, the next member of the series. It is possible similarly to synthesize propane, CH„—CH,—CH,, boiling at —44° C, butane, CH_ CH,— CH,, boiling at I° C, and so on, with a complete knowledge of the molecular structure of these substances. Hydrocarbons of this type, the molecules of which consist of carbon linked to carbon in unbranched chains, are known as normal (written as n-), and the term homologous is applied to such a series in which each member differs by —CH, from the next. The general physical properties of homologous series vary in much th'6 same way; the lower members are gases and as they become more complex, they pass to liquids of low, then high, boiling points, and finally to solids of low, then high, melting points.
Paraffins of this type occur in various petroleums. Ethane, a gas, is found dissolved in the oils from certain wells, such as those of Pittsburgh. Pentane, boiling at 36° C, hexane at 69° C, and heptane at 93° C, etc., have been isolated from Pennsylvanian oils. Similar oils also contain in varying amounts the solid members of this series known as paraffin wax, a mixture of n-hydrocarbons, which is also manufactured by the distillation of shale or brown coal. Waxes of Scottish origin, and probably all such materials, are composed of about Zo to 1 a normal hydrocarbons of which the most important are : docosane tetracosane penta cosane hexacosane octacosane CJI„, nonacosane C,BH,, and hentriacontane Many of these solid hydro carbons have also been isolated from the animal or plant world, the most complex being hexacontane C,,H,.:_, melting at 102° C.
There are two hydrocarbons known of the formula : one, n-butane, has been mentioned above; the other boils at —17° C, and has exactly the same percentage composition and vapour density and hence molecular weight. This phenomenon, termed isomerism (q.v.), receives a most satisfactory explanation on, the conception of valency previously mentioned. With four carbon and ten hydrogen atoms, it is possible to form two and only two different structures, viz., Hydrocarbons containing such a branched chain are designated by the prefix iso; hence, in this case, as but two isomerides exist, the names n-butane and isobutane are sufficient to identify these substances.
Another example of isomerism is seen in the existence of three different hydrocarbons for the formula The first, n-pentane, has been mentioned; the second boils at 3o° C, and the third at C. As it is only possible to arrange five quadrivalent carbon atoms and 12 hydrogen atoms in three ways, these hydrocarbons must be As the hydrocarbons become more complex, the number of theoretically possible isomerides increases very rapidly indeed; in fact, it can be calculated that the hydrocarbon tridecane, could exist in 802 different forms. The normal hydrocarbon, how ever, is the only one known. After the seventh member of the series, the number of known isomerides is very considerably less than those theoretically possible, and after the eleventh, with but few exceptions, the normal isomerides only have been examined.
The whole of this group of hydrocarbons is characterized by great inertness towards chemical reagents, thus differing very markedly from the benzene series. With chlorine (or bromine), however, they give substitution products, some of which are of industrial value. Partial oxidation (when this operation is effected without breaking down the hydrocarbon to carbon dioxide and water—the ultimate oxidation products of all organic substances) leads to valuable derivatives. Both formaldehyde (q.v.) and methyl alcohol (q.v.) can be prepared by the partial oxidation of marsh gas, and since this hydrocarbon can be obtained by the reduction of either carbon monoxide or dioxide, the possibility of preparing organic substances such as acetic acid and acetone from coal or even from limestone becomes realizable (see METHYL ALCOHOL).
(ii.) Olefines. When ethyl iodide, is heated with alco holic soda, a gas called ethylene (or olefine) is evolved which, on analysis and density determination, is found to have the molec ular formula The same substance can be formed by the de hydration of ethyl alcohol. These reactions may be depicted thus: CH,I —HI = CH, CH, and — H_0 = In ethylene and all its homologues two adjoining carbon atoms appear each to be trivalent. Ethylene and its homologues show a set of reactions which differentiate them sharply from the par affins; they readily absorb hydrogen, becoming paraffins, and combine with chlorine or bromine to yield oily di-substitution products of saturated hydrocarbons. It is to the formation of its oily dichloride that ethylene owes its historic name of olefine or olefiant (oil-forming) gas . Owing to these and similar "additive" reactions, the olefines are termed unsaturated, and the conventional method of indicating such characteristics is to regard the carbon atoms as linked together by two valencies : e.g., CH, = CH,. This arrangement retains the conception of the quadrivalency of carbon, but it is not intended to indicate that the carbon atoms are more tightly held together than they are in the case of ethane CH,-CH, with a single linkage between each other.
Some of the reactions of ethylene may be summarized : (i.) CH, = CH,—CH,; (ii.) CH,=CH,+ Br, (or = CH,Br—CH,Br, ethylene dibromide (or CH.C1—CH.C1, ethylene dichloride) ; (iii.) CH,=CH, + HI = CH,-CH,I, ethyl iodide ; (iv.) CH,= CH,+H,S0.,= C•,H,HSO,, ethyl hydrogen sulphate; (v.) mild oxidation converts ethylene into glycol, Generally these reactions are manifested by all organic substances containing the double linkage. In the homol ogous series of olefines, the phenomenon of isomerism is first noticed in the case of butylene, which occurs in the three theoretically possible forms:— (iii.) Acetylenes. When ethylene dibromide is treated with alcoholic potash or soda, acetylene is formed : CH,Br—CH,Br — 2HBr =CH—CH. In this hydrocarbon, and its homologues, carbon appears to be bivalent, and for reasons similar to those just ascribed in the case of ethylene, a triple linkage is written between the carbon atoms; e.g., CH—CH. This gas, which is much more easily and inexpensively prepared by the action of water on calcium carbide (see CARBIDES), readily takes up hydro gen, giving ethylene. The homologous hydrocarbons of this series readily undergo a change known as polymerization (q.v.); thus acetylene passed through a red-hot tube gives benzene : 3C,H, = C,H,. The hydrogen atoms in acetylene can be replaced by metals, and the explosive red copper and white silver com pounds are used for the characterization of this substance. Under the catalytic influence of mercuric salts, acetylene combines with water giving acetaldehyde (see ALDEHYDES), a substance which may be readily oxidized to acetic acid (q.v.), or reduced to ethyl alcohol.
(iv.) Di-Olefines. These hydrocarbons are isomeric with the homologues of acetylene, butadiene, being isomeric with ethylacetylene, Unlike the lenes, they do not form copper or silver salts. The most interesting members of this series are those which undergo polymerization to a substance resembling caoutchouc. This is the case with butadiene and with isoprene, The latter hydro carbon, warmed for 5o hours at 6o° C with sodium in a sealed tube, gives a nearly quantitative yield of a substance with proper ties very similar to those of rubber.
The Alcohols. Methyl alcohol, when acted upon by metallic sodium, evolves hydrogen and gives sodium methoxide, CH3ONa. This and other reactions show that one hydrogen atom is different from the other three, indicated graphically by linking that hydrogen to oxygen, forming the hydroxyl group (OH), thus methyl alcohol is ethyl alcohol and so on. It has been observed that when once oxygen has entered into such stable substances as the paraffins, further oxidation is easy. Ethyl alcohol, for example, is readily oxidized to acetaldehyde : Substances containing two hydroxyl groups attached to one carbon atom are very unstable and at once lose water giving the aldehyde. It will be noticed that oxygen has attacked the carbon atom which is already partly oxidized. This is invariably the case provided that such a carbon atom has hydrogen attached to it.
In the hydrocarbon propane, CH„—CH,—CH,, the terminal hydrogen atoms are symmetrically situated in the molecule, hence theoretically but one n-propyl alcohol, CH,.OH, is capable of existence, and one only is actually known. This alcohol behaves on oxidation in a similar manner to ethyl alcohol, giving first propyl aldehyde, CH,.CH,•CHO, and then propionic acid, When the two central hydrogen atoms in propane are considered, it will be seen that a second isomeric alcohol, is possible, and actually, two isomerides are invariably found when a hydrogen atom in propane is replaced by univalent groups or such elements as chlorine, bromine, etc. Isopropyl alcohol, is oxidized to acetone, a typical member of the group of ketones (q.v.):— The Aldehydes. When acted upon by phosphorus pentachlo ride, the oxygen of the aldehydes is replaced by two atoms of chlorine; thus, acetaldehyde, CH,•CHO, gives ethylidene chloride, CH,•CHC12, isomeric with ethylene dichloride, When aldehydes are further oxidized, they pass to acids as follows :— the third substance, for reasons already mentioned, being unstable and losing a molecule of water.
The Organic Acids, (see ACIDS), all contain the car boxyl group -COOH. If acetic acid, is acted upon by phosphorus pentachloride, acetyl chloride, results, a proof that the acid contained the OH group. The chlorine atom in this substance can be replaced by by acting upon it with ammonia, and the resulting substance, can be dehy drated, giving acetonitrile, which can also be synthesized from methyl iodide and potassium cyanide, CH,I+KCN = KI+CH,CN. These observations, which show that acetic acid contains a methyl group (CH,) linked to carbon, and in addition, a hydroxyl group, support the foregoing formula for acetic acid.
The alcohols, ketones, aldehydes and acids constitute the most important oxygen-containing materials of organic chemistry. Derivatives are known containing two or more of the same charac teristic groups; e.g., glycerine, CH,(OH).CH(OH).CH2(OH), or malonic acid, and, on the other hand, organic substances have been synthesized, or occur naturally, con taining two or more different groups such as lactic acid, (q.v.) or one of the tartaric acids, COOH•CH(OH) .CH (OH).COOH (q.v.).
Alcohols can be synthesized from the corresponding halogen derivative by the action of hydrated silver oxide or lead oxide and water; thus ethyl iodide, C,H,I, gives ethyl alcohol. Such a reaction as this may be used for introducing several OH groups. Ethylene dibromide, treated in a similar manner, gives glycol, Alcohols also result from the reduction of ketones and aldehydes. Ethylene dissolves in strong sulphuric acid to give ethyl hydrogen sulphate, ; when this is treated with warm water, it generates sulphuric acid and gives ethyl alcohol, which may be separated by distillation. Propylene, CH-CH„, dissolves in the concentrated acid to give iso propyl hydrogen sulphate, (CH,),CH•HSO,, which passes to isopropyl alcohol, (CH,),CH.OH, on treatment with water. Since propylene is formed by dehydrating n-propyl alcohol, this process effects the conversion of a primary into a secondary alcohol. The properties of the more important alcohols are described under ALCOHOLS.
The lower members of the paraffin alcohols are miscible with water in all proportions, but as the series is ascended, the solu bility diminishes rapidly. Both methyl and ethyl alcohols are extremely valuable solvents for organic substances.
With sulphuric acid, alcohols give hydrogen sulphates: HSO4+H2O. If this acid sulphate be dis tilled, dimethyl sulphate is formed a substance largely used in industry for the introduction of the methyl (CH,) group. The corresponding organic analogues are prepared by heating to gether alcohol and an organic acid, and are called esters. From acetic acid and ethyl alcohol, for instance, ethyl acetate CH,•COOC,H,, is formed.
The simplest ketone, acetone, CH,.CO•CH, (q.v.), is one of the products of the bacterial fermentation of maize, and is made tech nically by the distillation of calcium acetate (CH,C00),Ca Ketones neither oxidize as readily as aldehydes, nor polymerize; otherwise their reactions are strikingly similar. Like the aldehydes, the ketones combine with sodium bisulphite and hydrocyanic acid; both aldehydes and ketones con dense with hydroxylamine to form "aldoximes," R•CH : N•OH, and "ketoximes," RR'C : N•OH, respectively.
A valuable synthetic method for the preparation of acids con sists in the formation of nitrites by the interaction of an alkyl iodide such as ethyl iodide with potassium cyanide, C_>H.;I+KCN = and the subsequent saponification or hydrolysis of nitrites with aqueous alkali, the alkali salt of the acid being formed : The reactions of the acids are described under that heading, and the acid-amides derived therefrom by replacement of a hy droxyl group by an amino-group are also described separately. The action of phosphorus pentachloride upon acids gives the acid chloride, e.g., = CH,•COC1+POCI,-j-HC1, and these react with the alkali salts of the acids to give 'anhydrides: The anhydrides react with water, alcohol or ammonia to give the acid, ester or amide respectively :— Palmitic, stearic and oleic acids are of great commercial im portance and are dealt with in the article FATTY ACIDS. The struc tures of the first two are known from application of methods al ready indicated to be respectively and all the carbon atoms being in a straight chain. Since oleic acid is it is of importance to determine the actual position of the double linkage, and this has been shown by the investigation of the ozone addition product. Such compounds result from the interaction of the two doubly linked carbon atoms with this reagent, and when decomposed break down at this point. The oleic acid derivative yields an aldehyde CH,• and an aldehyde-acid CHO•(CH,),•COOH. Consequently the double link age lies in the centre of the CH,• : CH• molecule.
Dicarboxylic Acids.—These acids contain two carboxyl groups. Glycol, CH.2(OH)•CHLOH, on oxidation gives oxalic acid, COOH.COOH. Ethylene dibromide gives ethylene dicyanide, or succinonitrile, CH,CN•CH,CN, when acted upon by aqueous potassium cyanide; this nitrile when saponified gives succinic acid, CH, CO2H.
The properties of these acids depend on the relative position in the molecule of the two carboxyl groups. 'Malonic acid, when heated evolves carbon dioxide and forms acetic acid, whereas succinic acid gives an anhydride If succinic acid is chlorinated, the first product is chlorosuccinic acid, When this is acted upon by alcoholic alkalis, fumaric acid is formed, COOH•CH=CH•COOH. This unsaturated acid shows the properties both of ethylene and of succinic acid. It sublimes at 200° C, and at a higher tempera ture gives an anhydride which on warming with water, does not revert to fumaric acid, but gives an isomeride, maleic acid, which melts at 13o° C, and reverts to fumaric acid when its aqueous solution is heated to about 200° C. Both of these acids have the same structural formula, viz., C0011—CH = CH—COOH, and con sequently this is the first case considered where the uniplanar conception of valency fails to account for the existence of a pair of isomerides. An extension of the conception used hitherto is therefore necessary, and this is made along the lines of STEREO CHEMISTRY (q.v.). Briefly, the explanation of this type of iso merism (or stereoisomerism) is to be found in the different rela tive positions in space of the groups attached to the carbon atoms, thus :— The decision as to which formula should be ascribed to the individual acids may be determined by their chemical reactivity. Thus, maleic acid very readily loses water and passes into maleic anhydride, and for this reason the two carboxyl groups are sup posed to be near together in space. By this and similar arguments, it is possible to determine the configuration of such pairs of isomerides.
Substances Containing Two or More of the Functional Groups.—Alcohols containing one or more hydroxyl groups have been mentioned. Similarly aldehydes, ketones and acids have been discussed, and in addition a few dicarboxylic acids and their derivatives have been alluded to in order to bring out points of theoretical importance. A knowledge of the characteristics of these functional groups in these four classes, and of the alteration in the properties of the hydrocarbons when such groups are sub stituted for hydrogen atoms, affords very valuable evidence for the determination of the structure of new organic substances dis covered in the animal or plant world. Large and important groups of aliphatic derivatives have been synthesized containing two or more different functional groups, and of these a few selections will now be discussed.
Hydroxy-Aldehydes and -Ketones.—The hexahydric alcohols such as mannitol or mannite, are soluble in water and have a sweet taste, but unlike the sugars they do not reduce alkaline copper solutions, are not fermented by yeast, and cannot be utilized as a food by the body. When a group is oxidized to the CHO group, or an adjacent (CHOH) group is converted into a CO group, the sugars result. These important compounds are also termed carbohydrates and are described more fully under that heading. Grape-sugar (d glucose), occurs in many fruits and in honey, and in most sweet fruits laevulose (fruit-sugar or d-fruc tose), is found.
Owing to the different arrangements of the valencies of the carbon atoms in space, numerous other compounds are capable of existence having the same plane formulae ; thus there are 16 possible stereoisomerides all having the same structure as glucose, and 14 of these are known. The carbohydrates are divided for convenience into four main groups according to the number of carbon atoms in space, numerous other compounds are capable hydrogen atoms to oxygen atoms is 2:1, as in water—hence the name carbohydrate.
The monosaccharides are grouped about the hexoses, of which glucose and fructose (see above) are the most important; pentoses, CJ111005, are included in this group. The disaccharides, may be regarded as derived by the loss of water from two similar or dissimilar monosaccharide molecules Cane-sugar (sucrose or saccharose) is the most important member of this group, which also includes lactose (or milk-sugar) and maltose (or malt-sugar) ; on hydrolysis it yields equal parts of glucose and fructose, whereas maltose gives only glucose. The trisaccharides, (= are rare, but the polysaccharides where n is generally a large number, are very well known in the form of starch and cellulose.
Hydroxy-Acids.—When acetic acid is chlorinated, the first prod uct is monochloroacetic acid, which, on boiling with water, yields hydroxy-acetic or glycollic acid, also found in unripe grapes. One of the most characteristic reactions of aldehydes is the formation of cyanohydrins with hydrocyanic acid. Thus, acetaldehyde readily gives acetaldehyde cyanohydrin, When this is saponified, inactive lactic acid, CH,•CH(OH)•COOH, is formed. Lactic acid, which contains an asymmetric carbon atom (see STEREOCHEMISTRY), occurs in three forms ; the optically inert isomeride is formed during the lactic acid fermentation of carbohydrates such as lactose and glucose, and is consequently present in sour milk. Sarcolactic acid, the dextrorotary modification, occurs in meat juice, and the laevoform may be prepared by resolution of the ordinary inert acid. Isomeric with lactic acid is /3-hydroxy-propionic acid CH2(OH)-CH2•COOH, which may be taken as an illustration of the nomenclature employed in this group of acids. The next higher is termed 7-hydroxybutyric acid On heating these hydroxy-carboxylic acids, it is found that their behaviour depends on the relative position of the two functional groups. Lactic acid, where both are linked to the same carbon atom, loses two molecules of water from two of the acid and gives lactide. j3-hydroxy-propionic acid loses water and gives the unsaturated acrylic acid, But the most inter esting anhydride formation takes place with the next homologues —the •y- and 6-hydroxy-acids—which give rise to stable y- and 6-lactones, containing 5- and 6-membered rings :— Citric acid, widely disseminated in many varieties of fruit—lemons, currants, goose berries, etc.—is manufactured from lemon juice which contains 6-7%, and also by a fermentation process (Citrosnycetes) from carbohydrates, glycerine and similar substances. It has been syn thesized from dichloroacetone, which absorbs hydrocyanic acid, giving CH2C1•C(OH) a cyano hydrin, saponifiable to the dichlorohydroxy-acid. The chlorine atoms are then replaced by cyanogen (CN) groups through the agency of potassium cyanide, and saponification of the resulting dinitrile leads to citric acid.
Ketonic Acids.—The most interesting member of this group is acetoacetic acid, best known as ethyl acetoacetate, which, owing to its peculiar properties, and great chemical reactivity, has been more fully investigated than any other aliphatic derivative, and is therefore described separately. Its behaviour is often in accord not only with the above formula, but also with the constitution A sub stance such as this, the constitution of which appears to vary according to conditions, giving rise to two types of derivatives, is an example of the phenomenon of tautomerism (q.v.).
Acetoacetic ester, when treated with sodium, or with alcoholic sodium ethoxide, gives a highly reactive sodium derivative which, when acted upon by ethyl iodide, yields ethyl ethylacetoacetate, A repetition of these processes gives ethyl diethylacetoacetate, Acet oacetic ester itself or its mono- or di-ethyl derivative can be decomposed by alkalis or acids in suitable concentrations to give a preponderating yield of either ketones or acids. The mono-ethyl derivative can give either a ketone, or an acid, and when it is remembered that a variety of groups can be introduced into the original ester, it will be realized that a large number of derivatives can be synthesized, of acetone on the one hand, and acetic acid on the other.
Moreover, the sodium derivatives of acetoacetic ester and of its mono-alkyl homologues, when acted upon by alcoholic iodine. furnish a condensation product, with elimination of sodium as sodium iodide. The new derivative can then be decomposed to give either di-ketones, or dicarboxylic acids, where R is a hydro gen atom or an alkyl group.
Malonic ester, is another very reactive ester of considerable utility in syntheses. Thus, with either sodium or sodium ethoxide, it gives a reactive sodium derivative which readily decomposes with ethyl iodide to give ethylmalonic ester, The acids derived from any of these malonoid esters decompose when heated, giving off carbon dioxide, and yielding derivatives of acetic acid, Moreover, the sodium derivative of the ester may be acted upon by iodine to give a tetracarboxylic ester, CH (COOC2H:,) 2, and by repeating these processes, acids containing a large number of carboxyl groups have been synthesized.
Ethyl iodide dissolved in dry ether is rapidly acted upon by metallic magnesium to form a solution of magnesium ethyl iodide, C,H,..MgI [or C2H6•MgI+(C2H5)20] (see GRIGNARD RE AGENT), and when this acts on lead chloride, lead tetraethyl, or lead ethide, is formed. This substance is of impor tance as an "anti-knock" in petrol or gasolene, a suitable mixture being sold as the proprietary brand "Ethyl." Chloroform, prepared by the action of bleaching powder on alcohol, and iodo form, by the interaction of iodine, alcohol and alkali, are used in medicine, the former as an anaesthetic, the latter an anti septic. When acetic acid is chlorinated, it gives mono-, then di-, and finally tri-chloroacetic acid, and all of these are very much stronger acids than acetic itself.
The Acid-Amides (q.v.), of the general type are hydrolyzed by boiling alkalis to ammonia and the salt of the corresponding acid, whereas amines are completely unaffected by this treatment. Urea (q.v.), the acid-amide of carbonic acid, obtained artificially in a memorable synthesis from ammo nium cyanate by Wohler in 1828, may also be synthesized by the action of ammonia on phosgene, It is one of the most im portant of the final products of protein metabolism, and occurs in the urine of mammals; a human adult, for instance, excretes about 3o grams per day. The nitriles, R.CN, and the hydroxy-nitriles, or cyanohydrins, e.g., acetaldehyde-cyanohydrin, have been previously discussed. The former on reduction give primary amines such as propylamine, When the cyanohydrins are treated with ammonia, they are converted into amino-nitriles, e.g., and on saponification, these yield the amino-acids.
The Amino-Acids are of great physiological importance since it has been shown that the proteins (q.v.) on hydrolysis yield a variety of acids belonging to this group. Glycine, glycocoll or amino-acetic acid, the simplest representative of the class, is a solid melting at C, soluble in water, and can be obtained by boiling glue with dilute sulphuric acid, or syn thetically by the action of ammonia on mono-chloroacetic acid. By the hydrolysis of different proteins, a large number of amino acids have been obtained and their constitution determined by synthesis. These acids have been very closely studied as a preliminary step in the attempts that have been made to synthe size the proteins themselves.
The Proteins are of profound biological importance, for they appear to be the essential constituents of the living cell, and are intimately bound up with the processes of life. Their molecular weight is very large, but no exact estimations have yet been made. The simplest proteins include such substances as the albumins, the plant globulins, and the albuminoids such as gelatin, or silk fibroin, caseinogen from cheese, etc. Conjugated proteins are such individuals as haemoglobin, etc. When boiled with concentrated hydrochloric acid, or with 25% sulphuric acid, they break down, giving various amino-acids. From all the proteins investigated by this method up to the present, 20 of these amino-acids have been isolated, that found most frequently being leucine, (CH:.).CH•CH..CH(NH_)•COOH. The conclusion that different proteins are built up of varying numbers of amino-acids is obvious, and attempts have been made to reconstruct them, commencing with these simpler acids of known molecular structure. The result has been the synthesis of a group of substances called the polypep tides, of which the molecular architecture is therefore known, and some of which are of considerable molecular magnitude. They show many of the characteristic reactions of the proteins them selves but are not identical with them, so the constitution of this important group of substances is still unknown.
Laboratory Synthesis and Photosynthesis.—Throughout this summary of the aliphatic derivatives, many have been men tioned as occurring in the plant kingdom. It is well to point out that although a considerable number of these have been synthe sized in the laboratory, and are identical in every respect with the natural products, the methods employed in their preparation are entirely different from those taking place in the living cell. The two sugars, dextrose and laevulose, and palmitic, stearic and malic acids, for instance, have been obtained synthetically, although starch, cellulose and the proteins have not. All these complex substances, however, are manufactured by the living cell from the carbonic acid present in the atmosphere, from nitrogenous material and various inorganic constituents in the soil, under the influence of moisture and with but slight variations of temperature, the energy required being obtained directly or indirectly from the sun ; so that, without the last factor, the whole process would cease. Attempts have been made in the laboratory to imitate the reactions which are believed to take place in the living cell; these are meeting with an increasing measure of success, which should give rise to a new organic chemistry of great importance to man.
See advanced works on organic chemistry, and numerous articles in the Journal and Annual Report of the Chemical Society, British Chemical Abstracts and see also list of journals, in which original papers appear, in F. A. Mason, Introduction to the Literature of Chemistry (i924). (F. FR.)