Substituent Effects of the Alkyl Groups: Polarity vs. Polarizability

Substituents effects of the alkyl groups, both straight‐chain and branched, were evaluated by means of model reaction series, each comparing the acidities and basicities of the same or very similar compounds. Four such models wereexamined, one based on known gas‐phase experimental data (acidity and basicity of alcohols), the three others on density‐functional calculations at the levels B3LYP/6‐311++G(2df,2pd)//B3LYP/6‐311++G(2df,2pd) or B3LYP/6‐311+G(d,p)//B3LYP/6‐311+G(d,p): acidity and basicity of 4‐substituted bicyclo[2.2.2]octan‐1‐ols, stability of borate anions compared to the stability of ammonium cations, and acidity of imines compared to the basicity of ketones. In all cases the alkyl groups stabilize both the anions and cations; with straight‐chain alkyls the effect is proportional and can be called polarizability effect. However, simple mathematical expression of the polarizability is not correct because the effect on the cations is always stronger than on the anions. Effects of secondary and tertiary alkyls, in some cases also of branched primary alkyls, are different and deviate from the proportionality. They can be interpreted in some cases as steric effects or hyperconjugation. In any case they are different from the effects of straight‐chain alkyls and they are also fundamentally different from the effects of dipolar substituents; both groups should not be correlated together in one reaction series.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)