Hydrocyanic acid Constitution

In many of its reactions hydrocyanic acid behaves as the nitrile of formic add H.CN. Many facts, in particular its great chemical and pharmacological similarity to the isonitriles >C = NR suggest another constitution, namely, that of carbimide >C =NH with bivalent carbon. The addition reactions of the nitriles (see above), which reactions are also characteristic of hydrocyanic acid, can equally well be explained on the basis of this second structural formula. In the nitrile form it is at the triple bond between carbon and nitrogen that addition takes place. In the methylene form this occurs at the two free valencies of the bivalent carbon atom, e.g. ... 

The question of the constitution of hydrocyanic acid has already been considered (p. 139). Here it need only be remarked that the isonitriles are converted by hydrolysis into primary amines and formic acid no carbon monoxide is produced, although from the formula this might be expected. The reason for this is to be sought in the fact that the first stage in the reaction consists in the addition of water to the two free valencies of the carbon atom. The reaction must therefore be formulated thus ... 

Cryoscopic measurements on the acid in benzene, etc., indicate a mixture of single and double molecules. Single molecules of thiocyanie acid may be represented by the formula NsC — SH or S C=NH. Probably the inorganic salts and the esters derived from them have the constitution represented by the first formula. The esters are converted into sulphonic acids and hydrocyanic acid by oxidation,... 

Voto6ek and Wichterle " investigated the reaction of JV-substituted gly-cosylamines with hydrocyanic acid, but they did not start with the isolated glycosylamine. The sugar and amine were heated in alcohol, the hydrocyanic acid was added, and the mixture was allowed to cool. The structure of the crystalline nitrile isolated was not determined. Because the hydrogen cyanide could have added to the carbonyl group of the ketose after an Amadori rearrangement had occurred, the constitution of their products is open to question. 

Hydrocyanic acid is most easily prepared from its potassium salt, K(CN), which is obtained principally by the decomposition of the complex double cyanides of iron as we shall soon consider. The acid is also obtained by the hydrolysis of certain glucosides, e.g., amygdalin, in bitter almonds. It is prepared synthetically by reactions to be discussed presently in connection with the constitution of it and its salts. It is a colorless liquid with a characteristic odor and burns with a violet flame. It boils at 26.1 and solidifies to crystals which melt at —14°. It is an extremely strong poison the best antidotes being chlorine and hydrogen dioxide. It is readily absorbed by metallic nickel which is thus used in gas masks for this purpose. It is stable in dry air but in presence of water is readily hydrolyzed yielding ammonia and formic acid as the chief products. 

S3mthesis from Cyanogen.—Two methods of synthesis of hydrocyanic acid support the view that this compound has the cyanide structure and not the iso-cyanide. Cyanogen gas because it yields oxalic acid on hydrolysis must have the constitution in which the two cyanogen groups are linked by the carbon atoms rather than nitrogen. 

From Acetylene.—The second synthesis of hydrocyanic acid supporting this same constitution is from acetylene by reaction with nitrogen under the influence of an electrical discharge. The nitrogen would split the acetylene molecule at the triple linkage of the two carbons leaving each hydrogen linked to carbon in the hydrocyanic acid. 

From Ammonia.—A third method for the synthesis of hydrocyanic acid supports the constitution of an iso-cyanide with the linkage of hydrogen to nitrogen. The Hofmann iso-nitrile reaction (p. 71) consists in the formation of iso-cyanides (iso-nitriles) by the reaction between chloroform and primary amines in the presence of an alkali. 

While these reactions support one constitution for potassium cyanide and the other for silver cyanide yet the two compounds are each prepared from hydrocyanic acid by the action of the respective metallic hydroxides. 

The above formula for leuoio acid is founded upon a reaction for the synthetical produotion of this acid from valeric aldehyde and hydrocyanic acid. Valerio acid contains bntyl consequently valerio aldehyde has the constitution expressed by the... 

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