DERIVATIVES OF RADICALS POORER IN HYDROGEN.

942. Aconitic acid, C ̧¤ ̧0; = C ̧H1(CO.OH),, occurs in the form of calcic salt in various species of aconitum, also in Delphinium Consolida and Equisetum fluviatile. It can be obtained from citric acid, with elimination of water, by heating at 175 or by long heating with concentrated hydrochloric or hydrobromie acids at 140°. It is best prepared by heating citric acid in a retort until oily streaks are observed on the neck, when the residue is cooled and extracted with ether. It crystallises in white plates or granules, soluble in water, alcohol, and ether, and melts at 140°. It is a tribasic acid and unites with nascent hydrogen, forming tricarballylic acid (§ 936) :

and on dry distillation yields itaconic acid (§ 897, 1):

The normal calcic salt, (C¿H ̧06)¿Ca1,6H2O, crystallises in prisms and is difficultly soluble.

Aceconitic acid, whose ethylic salt is formed in the decomposition of ethylic brom-acetate with sodium, is probably an isomer of aconitic acid.

943. Meconic acid, C,H,O,, occurs in the milky juice of poppies and in the opium prepared therefrom, and passes into the alcoholic extract of the latter. From this baric chloride precipitates baric meconate nearly in a state of purity, the free acid being obtained by addition of sulphuric acid; it is then converted into the ammonic salt, and this recrystallised several times from hot water and finally decomposed by hydrochloric acid.

Meconic acid crystallises in colourless, mica-like plates or prisms containing three molecules of water, which is given off at 100°. It is difficultly soluble in cold water or ether, more readily in hot water and alcohol. The alkali and ammonium salts are readily soluble, all

others difficultly soluble or insoluble. The above-mentioned baric salt has the formula C-H2BaO,,H2O. Calcic meconate :

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dissolves in warm nitric acid, the solution on cooling yielding brilliant plates of the monobasic salt (C,H307),Ca,2H2O. Two silver salts are known, a white, C,H,Ag2O7, and a yellow, C-HAg307, only obtainable by aid of an ammoniacal solution of argentic nitrate. Meconic acid yields a most intense blood-red coloration with ferric chloride.

On saturating an alcoholic solution of meconic acid with hydrochloric acid gas, and cooling, feathery crystals of hydric ethylic meconate, C,H3(C2H5)O7, separate, and on evaporation of the mother liquor diethylic meconate, C,H2(C2H5)2O7, is obtained as an oily liquid which solidifies in crystals.

Sodium amalgam and water convert meconic acid, with union of six hydrogen atoms, into hydromeconic acid, C4H1007.

944. By long boiling with water, or better dilute hydrochloric acid, or by heating alone to 200°, meconic acid gives off carbonic anhydride:

and is converted into comenic acid, which crystallises in hard granules or plates, is difficultly soluble, and readily yields salts containing one equivalent of metal, CH,MOs. Compounds of the formula CH2M205 are only obtained in the presence of much free ammonia. The only ethylic salt, CH3(C2H5)О5, crystallises in needles melting at 135°. On dry distillation comenic acid splits up into carbonic anhydride and pyrocomenic acid:

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which, however, can scarcely be termed an acid, as it crystallises unchanged from a hot solution of potassic hydrate. Metallic derivatives, such as (CH3O3)2Cа,H2O and СH2AgО, can only be obtained in the presence of much free ammonia by addition of the acetates of calcium, barium, and lead, or of argentic nitrate. Ethylic derivatives have not been obtained.

From these properties meconic acid would appear to be a dibasic hydroxy-acid, CHÔ2(OH)(CO.OH)2, comenic acid :

C5H2O2(OH).CO.OH,

and pyrocomenic acid, CH3O.OH. Nothing is known of the inner constitution of these bodies.

945. Chelidonic acid, CH,O,, is obtained from the sap of Chelidonium majus, by slightly acidulating with nitric acid, precipitating with plumbic acetate, and decomposing the resulting.lead salt with hydric sulphide. It crystallises in silky needles containing one molecule of water of crystallisation, difficultly soluble in cold water or alcohol. It decomposes at 220° with evolution of carbonic anhydride. It yields salts containing three equivalents of metals; they are of a yellow colour, and, with exception of the alkali salts, insoluble in water. The hydric salts are colourless, like the acid, and are mostly crystalline.

AROMATIC COMPOUNDS.

946. The term 'aromatic compounds' has been applied to a large class of bodies derived more or less directly from benzene, C6H6, by the substitution of one or more of its hydrogen atoms by other elements or compound radicals, similarly to the bodies of the fatty or alkyl series which are derived from marsh gas or methane, CH (§ 141, &c.)

The term aromatic was applied at a time when fewer benzene derivatives were known than at present, and on the supposition that the presence of the benzene nucleus in any body accounted for the more or less strongly pronounced aromatic odour possessed by these bodies; and in this work the term is used generally to indicate that the body contains the benzene nucleus, whether it be derived from benzene by substitution of an alkyl group or from the introduction of a benzene group into a body belonging to the fatty series.

947. Various theories have been proposed as to the constitution of the benzene nucleus. The one which has perhaps been most fruitful in results, and best supported by facts, is that proposed in 1865 by Kekulé, according to which the benzene nucleus must be considered as a ring or closed chain of carbon atoms, combined to each other with alternating single and double union, and so that each carbon atom is combined with one hydrogen atom only.

This may be expressed in one plane thus :

Several important facts seem to support this theory of its constitution.

1. The synthesis of benzene from acetylene, by passing that gas through a red-hot tube, when it may be supposed to combine :

2. The power which benzene possesses of combining with two, four, or six atoms of bromine or chlorine to form so-called 'additive' compounds, behaving towards the halogens in this respect like three molecules of a compound each of which possesses a double carbon combination.

The fact that not more than six atoms of chlorine or bromine can be combined with benzene is a decided indication of a closed chain form of combination of the carbon atoms; for in the case of an open chain a compound of six carbon with six hydrogen atoms would be able to combine with eight atoms of chlorine or bromine, as in the case of the only known isomer of benzene, dipropargyl (§ 932), which combines with eight atoms of bromine.

In these additive compounds of benzene with monad elements the divalent unions are supposed to be changed into monovalent, the closed formation of the group still being maintained:

The comparative instability of these compounds also supports the above view of their constitution.

3. Many benzene derivatives, especially the benzene carboxylic acids, CH,(CO.OH)2, C6(CO.OH)6, &c., are acted upon by nascent hydrogen, of which up to six atoms may be united, and frequently can be again removed by chlorine or oxidising agents, with regeneration of the original acid.

On the other hand, if eight hydrogen atoms are added to a saturated aromatic compound, which can be effected in some cases by heating to about 200° with concentrated hydriodic acid, derivatives of the paraffin series are obtained, which no longer possess the character of aromatic compounds and cannot be converted into benzene or benzene derivatives by simple means.

4. The six hydrogen atoms in benzene all appear to possess the same value; in cases where only one hydrogen atom in benzene is replaced by an element or compound radical it is a matter of perfect indifference which atom is replaced, the mono-substituted compound obtained having exactly the same properties, although prepared by methods differing so as to ensure that different hydrogen atoms should be replaced in each case. No isomeric monosubstituted benzenes have been obtained.

948. Chemical Character of the Benzene Nucleus. Substituted products are somewhat easily obtained from benzene and such of its derivatives as still contain some hydrogen atoms in connection with the benzene nucleus, by the action of the halogens, nitric acid, sulphuric acid, &c., in the case of the two last-mentioned bodies much more readily than with compounds of the alkyl' series.

The peculiarities of the benzene group itself are exhibited in all

its derivatives, especially in some of its hydroxyl, nitro, and amido compounds.

The hydroxyl substitution products or hydrates of this series correspond in many respects to the alcohols, only that the hydroxylic hydrogen is much more readily replaced by basic metals. Barium, calcium, or alkali metal compounds may be obtained from monhydroxy-benzene, or phenol, CH,.OH, by mixing it with aqueous solutions of the respective bases and evaporating. And it is from this readiness which phenol exhibits to form compounds with strong bases that it obtained the name carbolic acid, although it has no real acid nature, for its basic compounds are easily decomposed by carbonic acid with reproduction of phenol :

2CH.ONa+ OH + CO2 = Na2CO3 + 2C6H5.OH.

The hydrates of the benzene nucleus occupy electro chemically a position between the alcohols or carbinols and the acids proper, and are termed ' phenols.' When, however, several hydrogen atoms in benzene are substituted by hydroxyl, or by hydroxyt and nitryl or halogens, the products are of decidedly acid character.

The peculiar chemical properties of the benzene group are also exhibited in its amido derivatives. In the case of alkylamines (§ 264), produced by the substitution of the ammonic hydrogen atoms in ammonia by alcohol radicals, the basic character is intensified by each such substitution, C2H5.NH, being more positive than NH3, and so on. But in the benzene amides the reverse is the case. Aniline, phenylamine, CH5.NH2, for instance, will form comparatively stable salts; the salts of diphenylamine, (C6H5)2NH, are decomposed by water, and triphenylamine, (C6H5)3N, does not combine with acids.

949. Isomerism in the Polysubstituted Products of Benzene.Most important support is given to the above view of the constitution of benzene by the study of the isomeric bodies produced by the substitution of several hydrogen atoms in the benzene nucleus, the differences in physical and chemical properties being referable, as also in the case of ethane derivatives, to the relative position of the substituting groups to each other.

The simplest products result when the substituting elements or radicals are the same.

950. If two hydrogen atoms in benzene be substituted by the same element or compound radical three cases of isomerism arise, depending on the relative position of these substituting elements or radicals.

1. If the substitution take place at neighbouring carbon atoms, ortho derivatives are produced,

thus: