The Enol of Mandelic Acid, Detection, Acidity in Aqueous Solution, and Estimation of the Keto‐Enol Equilibrium Constant and Carbon Acidity of Mandelic Acid

An acidity constant pK of 6.62 and a constant pKE of 15.4 were determined for the keto–enol equilibrium of 2, the enol of mandelic acid, in water. Compound 2 is accessible from 1 and 3 by photoelimination. The values confirm the assumption that simple carboxylic acids contain a much lower amount of the enol form than simple aldehydes and ketones, since their keto form is stabilized by interaction of carbonyl and hydroxyl groups