Chromogenic reactions of tertiary amines with polycarboxylic acids and acetic anhydride: carbon suboxide as the reactive species in the malonic acid reagent.

Analytical methods based on the title reactions are reviewed, and the malonic acid-acetic anhydride system was selected for detailed study. It is postulated that carbon suboxide, O = C = C = C = O, formed by the action of acetic anhydride on malonic acid, is the effective reactive species in this system. Carbon suboxide was prepared and identified, and spectrophotometric observations of its reactions with tertiary amines are described. Aliphatic and aromatic tertiary amines generate colored products upon reaction with carbon suboxide in the presence of acetic anhydride. It was found that aliphatic tertiary amines form colors upon reaction with carbon suboxide in the absence of acetic anhydride, whereas aromatic tertiary amines require the presence of acetic anhydride.