An Electronic Mechanism of Addition Reactions

Abstract

A RECENT communication from Mr. Lakhbir Singh¹ seems to require a reply based rather more firmly on conventional theoretical chemistry. He refers to the decrease in reactivity of acetylene compared to ethylene, and implies that this shows that an increase in the number of π-bonds leads consequently to a decrease in reactivity. Duchesne² has, however, pointed out that the greater reactivity of ethylene compared to acetylene is due to the fact that the torsional oscillations in ethylene uncouple the π-electrons, whereas this cannot happen in acetylene. Singh also describes allene as “very much strained”. It is difficult to see why this should be so, unless reliance is placed on the Baeyer³ strain theory. In modern terms the central atom may be described as having two σ-bonds in an sp-hybrid, with two p-orbitals available to form two π-bonds, the planes of which are at right-angles to each other. The low reactivity in cumulenes compared to polyenes can be attributed to the small effect of torsional oscillations on the environment of the central atoms.