The Acid Strength of Mono and Diesters of Phosphoric Acid. The n-Alkyl Esters from Methyl to Butyl, the Esters of Biological Importance, and the Natural Guanidine Phosphoric Acids

Abstract

1. The dissociation constants of monomethyl, monoethyl, mono-n-propyl, mono-n-butyl, dimethyl, diethyl, di-n-propyl, and di-n-butyl phosphoric acids have been measured. The monoalkyl acids are stronger than phosphoric and the dialkyl acids are stronger than the monoalkyl acids. 2. The factors affecting the acid strength of these compounds are the solvation, inductive, statistical and steric effects. Of these the solvation effect is chiefly responsible for the substituted acids being stronger than phosphoric acid. 3. A steric effect similar to that found in the fatty acids is present in the alkyl phosphoric acids. Ethyl phosphoric acid in this series corresponds to butyric acid in the other series, each being abnormally strong. 4. Information obtained from the study of the alkyl compounds has been applied to the glycerol and sugar esters of phosphoric acid. The greater strength of these compounds is due in small part to the positive inductive effect of the negative oxygen atoms, and in larger part to the solvation of the alcoholic hydroxyl groups. Chelation due to hydrogen bonding occurs in these compounds and either increases or decreases the acid strength depending on whether or not the hydrogen in the bond comes from an alcoholic or phosphoric acid hydroxyl group. 5. The groups responsible for the dissociation constants of the natural guanidine phosphoric acids have been assigned on a basis of analogies and theoretical considerations. 6. The factors of chelation through hydrogen bonding and the inductive effect of the phosphoric acid group offer a possible mechanism whereby the presence of the phosphoric acid moiety facilitates some of the reactions of biological importance. © 1943, American Chemical Society. All rights reserved.