Secondary β-Hydrogen Isotope Effects in Ketal Formation Equilibria

An n.m.r. technique was used to measure K H /K D of ketal formation in methylene chloride solution for acetone and phenylacetone. For acetone, K H /K D = 0.95 per D for methyl ketal formation. Phenylacetone-1,1-d 2 , -3,3,3-d 3 , and -1,1,1,3,3-d 5 were examined for both dimethyl and ethylene ketal formation. For both ketals, K H /K D = 0.91 per D, for each ketone. Liquid scintillation counting was used to examine K H /K T for ketal formation in methylene chloride. An extraordinarily large isotope effect was found for ethylene ketal formation in 1,3-diphenyl-acetone. Here K H /K T = 0.67 for one tritium atom. This isotope effect is too large to arise from hyperconjugative effects alone. It must have a substantial steric component.