362

EFFECTS OF HEAT AND OF CHLORINE UPON THE ACIDS.

Again, malic acid, by the loss of water, yields fumaric (paramalæic) acid :

and fumaric acid by sublimation is converted into its isomeride, maleic acid.

In like manner, gallic acid by the loss of 1 atom of carbonic anhydride yields pyrogallin; and pyrogallin in its turn, by the loss of 1 atom of water, furnishes metagallin (1334):

Results closely analogous are afforded by the decomposition of meconic acid: a solution of which, at a temperature not exceeding that of boiling water, is converted into comenic acid by the loss of I atom of carbonic anhydride; and comenic acid itself if subjected to distillation loses an atom of carbonic anhydride, and furnishes another distinct volatile acid, the pyromeconic :

(1264) Effects of Chlorine and other Halogens upon the Acids.Chlorine generally acts upon the monobasic acids simply by displacing hydrogen, thus producing a body of the same chemical type as the original acid, and containing the same number of

COMBINED ACTION OF HEAT AND BASES ON THE ACIDS. 363

2

2

elements similarly arranged. In the radicle of acetic acid, for instance, the 3 atoms of hydrogen may be displaced by 3 of chlorine; and trichloracetic acid, which is analogous to the original acetic acid, is produced; H¤‚H ̧Ð1⁄2 + 3 Cl2=HE2Cl ̧Ð ̧ + 3 HCl. +3 By a substitution of a similar kind, benzoic acid (H¤‚H ̧Ð1⁄2) yields chlorobenzoic acid, (HЄ,H,ClO). In other instances, especially in the more complex acids, the original group of elements is broken up into two or more groups of simple constitution. For example, citric acid (H ̧¤¿н ̧‡,), when treated with chlorine, yields a neutral chlorinated body (E,C,,,), besides other products. The action of bromine and of iodine is analogous to that of chlorine, but with differences depending chiefly upon the feebler character of their attractions. Peroxide of nitrogen may also be substituted for a part of the hydrogen, atom for atom; but in this case the acid character of the nitric acid whence the peroxide was derived entirely disappears.

3 6 5

8 12

It is remarkable that several of the organic acids have a composition identical with that of anhydrous glucose, if the ultimate analysis only be considered; thus furnishing additional evidence that chemical character depends quite as much upon the mode of arrangement among the elements as upon the nature of those elements themselves :

3 Ats. Acetic acid. 2 Ats. Lactic acid. 1 At. Glucose.

Many of the acids contain oxygen and hydrogen in exactly the proportions required to form water: such, for example, as the lactic, the acetic, the quinic, the pyrotartaric, and some others.

(1265) Combined Action of Heat and Bases on the Acids.— Many of the organic acids when fused with caustic potash are decomposed by it into salts of acetic and oxalic acid, with separation of water, and sometimes with evolution of hydrogen; for example:

Malic acid.

Potassic oxalate. Potassic acetate.

HỌC,H,O, + 3 KHO = K,C,O + K€,H,O + 2H,O + Hạ;

4

4

When the salts of any of the monobasic acids, which in their state of normal hydrates contain 2 atoms of oxygen, are distilled with hydrate of baryta, they undergo a peculiar decomposition;

364

OXYCHLORIDES OF THE ACIDS.

baric carbonate is formed, and a hydro-carbon, the hydride of the alcohol radicle of the next inferior term of the series, is given off; for example:

In this manner the salts of the following acids may be decom

When the salts of acids, which in the state of normal hydrates contain 3 atoms of oxygen, are similarly treated, oxidized compounds are obtained, and a carbonate of the metal remains in the retort, as in the former case :

Reference has already been made to the ketones, which are the result of submitting the calcium or barium salt of any of the volatile acids (the normal hydrates of which contain 2 atoms of oxygen) to dry distillation (1258).

a. Oxychlorides of the Acids or Chlorides of the Acid Radicles.

In

(1266) Chlorides of the Radicles of the Monobasic Acids.The acids of the lower members of the series HЄH2-102, can readily be made to furnish volatile compounds in which one atom of hydroxyl (HO), as contained in the normal acid, has its place supplied by chlorine. With acetic acid [(Є2H ̧O)H→] acetyl chloride may be obtained, consisting of ‚H ̧ÐСl. preparing these compounds Gerhardt (Ann. de Chimie, III. xxxvii. 285) heats together a mixture of 1 atom of the oxychloride of phosphorus (phosphoryl chloride, POCI,; 460), with 3 atoms of the anhydrous potassium or sodium salt of the acid. For instance:

In some cases it is more advantageous to mix phosphorous trichloride with the normal acid, in the proportion of 2 atoms of the acid to 1 atom of the trichloride. In the case in which acetic acid is employed, the reaction is as follows:

Acetic acid.

:

Acetyl chloride.

and on distilling the mixture, the new compound passes over, whilst phosphorous acid remains in the retort. By these methods the following oxychlorides have been prepared :

:

One molecule of each of these bodies yields two volumes of vapour. The vapour of these compounds is extremely irritating to the lungs, and their manipulation consequently demands great care, otherwise the health of the operator would be seriously endangered. Water immediately decomposes these oxychlorides into the normal acid and free hydrochloric acid. The action of a few drops of water upon acetyl chloride is so violent as to amount to an explosion; €,H,OC1+H,→ becoming H¤‚H ̧Ð2+ HCl. When they are mixed with an alcohol, they furnish hydrochloric acid, and a compound ether of the acid to which they correspond; valeryl-chloride, Є.H,→CI+(¤2H.)H‡, becoming valeric ether (Є2H2)¤ ̧н,→2+нCI. With ammonia they react violently, producing an amide, and ammonium chloride :

5 9 2

Benzoyl chloride.

5

Benzamide.

2 H2N + €,H2OCI = H1NCI + ¤‚Í‚Ðн2N.

Oxychlorides corresponding to the foregoing compounds may also be obtained with the aromatic acids, such as the benzoic, toluic, cinnamic, and cuminic acids:

(Cahours, Ann. de Chimie, III. xxiii. 337.)

These oxychlorides, though unimportant in themselves, have in the hands of Gerhardt led to the discovery of a general method of procuring the anhydrides of the acids from which they are derived (1268).

If oxybromide of phosphorus (phosphoryl bromide, POBr、) be used instead of oxychloride for decomposing the salt of the organic acid, oxybromides of corresponding composition may be procured.

4 2

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(1267) Oxychlorides of the dibasic acids, such as succinyl chloride (HCl) and citraconyl chloride (EH12Cl2), may also be procured; but it is best in these cases to act upon the anhydrides of these acids with phosphoric pentachloride : they are obtained in a state of purity with difficulty, since they are generally decomposed at a temperature below their point of volatilization.

b. The Anhydrides, or Anhydrous Acids.

be em

(1268) The oxychlorides of the monobasic acids may ployed to furnish the anhydrides of their respective acids, by causing an atom of the oxychloride to decompose an atom of the dry potassium or sodium salt of the acid to which it corresponds. For instance, in the preparation of acetic anhydride, an equivalent of acetyl chloride is treated with an equivalent of anhydrous potassic acetate, when the following reaction

It is not necessary to isolate the oxychloride in order to procure the anhydride of the acid, for if one atom of phosphoryl chloride be made to act upon six atoms of the anhydrous salt, instead of upon three atoms, as would be required in the preparation of the oxychloride of the organic acid, a compound reac