Ionization Constants and Reactivity of Isomers of Eugenol

To determine the scope of the reaction of zinc oxide with isomers of eugenol, the effect of changes in the position of the substituents in the benzene ring on the ionization constants and reactivity of these isomers has been studied. The ionization constants of eugenol isomers as well as those of newly synthesized allyl-and propenylbenzoic acids were determined by spectrophotometric and potentiometric techniques. The influence of inductive, resonance and steric effects of the substituents on the ionization constants has been discussed and the substituent constant for the Hammett equation pK ₀ - pK = σρ has been calculated. For the 4- and 5- substituted allyl and propenyl derivatives, the Hammett equation is valid. Vicinal trisubstituted isomers do not harden readily with zinc oxide due to the steric hindrance of the side chain. The unsymmetrically trisubstituted derivatives react rapidly in the presence of zinc acetate. Besides the steric effects of the substituent groups the rate of the chelation reaction is to a lesser degree dependent on the ionization constants as indicated by the shorter setting time of chavibetol-zinc oxide slurries compared to those containing eugenol.