An Electronic Mechanism of Addition Reactions

Abstract

IT has now become abundantly clear that in ethylene (CH₂=CH₂) one of the two bonds is of the normal covalent type (σ-bond), while the other belongs to a distinct and separate class (π-bond). Thermochemical studies have further revealed that the bond-strength of a C=C double bond is appreciably less than twice the value for a C—C single bond, showing that the π-bond is not as strong as the σ-bond. Thus it was quite natural to assume that addition reactions occur by the direct rupture of the weaker π-bond. However, it has been found that addition proceeds generally in the trans sense, which is exactly contrary to expectation. The reactivity towards addition reactions also decreases unexpectedly with increase in the number of σ-bonds (except in conjugation). Acetylene (CH≡CH) with two π-bonds is less reactive than ethylene¹. The first stage addition to one of the cumulated double bonds in allene (CH₂=C=CH₂), which is expected to be very much strained, is also much slower than the second stage addition to its remaining simple double bond². It is remarkable that higher cumulenes (>C=C=C=C=C=C<) are quite stable³.