pKa values of protonated α,β-unsaturated cyclic carboxylic acids. Effect of ring size on acidities

Abstract

The pKa values of protonated α,β-unsaturated acids, namely tiglic, cyclopentene carboxylic, cyclohexene carboxylic, and cycloheptene carboxylic acids have been determined spectrophotometrically and the values are −4.08, −4.05, −3.88, and −3.84, respectively. The conjugate acid of tiglic acid is a stronger acid than that of crotonic acid (pKa − 3.94) and the difference in acid strength is explained on the basis of steric inhibition of resonance by the α-methyl group. All the protonated cyclic acids are shown to be somewhat weaker than the corresponding acyclic ones. Several factors have been considered to explain the acid strength of these cyclic α,β-unsaturated acids. It has been shown that the ease of placing the double bond exo to the ring system is responsible for the changes in acidity of the above cyclic systems.