How large is the conjugative stabilization of diynes?

Abstract

The conjugation stabilization energies of dienes and diynes are considerably larger than estimates based on heat of hydrogenation differences between 1,3-butadiyne and 1-butyne as well as between 1,3-butadiene and 1-butene. Such comparisons do not take into account the counterbalancing hyperconjugative stabilization of the partially hydrogenated products by their ethyl groups. When alkyl hyperconjugation is considered, the conjugation stabilization of diynes (≈9.3 kcal/mol) is found by two methods (involving isomerization of nonconjugated into conjugated isomers and heats of hydrogenation) to be larger than that of dienes (≈8.2 kcal/mol). Copyright © 2004 American Chemical Society.