ChemInform Abstract: MOLECULAR ORBITAL THEORY OF THE ELECTRONIC STRUCTURE OF MOLECULES. 33. THE EFFECT OF α‐ELECTROPOSITIVE SUBSTITUENTS ON THE STABILITIES OF CARBENIUM IONS

Abstract

The effect of substituents, X, on the stabilities of a-substituted methyl, ethyl, and vinyl cations is studied by standard ab initio procedures. X is varied systematically along the whole series of the first short period substituents, Li, BeH, BH2, CH3, NH2, OH, and F. SiH3 was included because of the availability of experimental evidence. It is found that electropositive sub- stituents can be extremely effective in stabilizing carbenium ions. Lithium stabilizes the ethyl and vinyl cations even better than the best ?r donor, the amino group; BeH, BH2, and SiH3 are roughly as effective as CH3. The methyl cation is the most sensitive to ?r donors, while the vinyl cation is the most sensitive to u donors. Consequently, the a-lithiovinyl cation is more sta- ble than the a-lithioethyl cation, and BeH is more effective than OH for the stabilization of thevinyl cation. The binding ener- gies of the simple Lewis acids H+, Li+, BeH+, BH2+, and CH3+ to the carbenes :CH2, :CHCH3, and :C=CH2 follow the order: H+ > CH3+ > BH2+ > BeH+ > Li+. The basicity sequence of the carbenes toward all these Lewis acids is :CHCH3 > :CH2 > :C=CH*.