Protonation of alkylpyridines: polarizability and steric effects in the base and in the cation

The energies of alkyl‐substituted pyridines in the 2‐, 3‐, 4‐ and 2,6‐positions, of their protonated forms and of reference derivatives of benzene were calculated at the RHF/6–31G(d,p) and B3LYP/6–31G(d,p) levels. The results were processed by isodesmic reactions, expressing the substituent effect separately for the base molecules, their protonated forms and for the gas‐phase basicity. The substituent effect is greatest in the protonated forms, and in the uncharged bases it is approximately three times smaller. In both cases, it is stabilizing: basicity is given by the difference. No actual steric effect was revealed, merely a stabilizing effect in the 2‐position, which can be explained in terms of polarization. This effect is almost equal in the protonated and unprotonated forms, so that it is not operative in the basicity. In water solution, all effects are attenuated and steric inhibition of solvation appears, particularly with 2,6‐di‐ tert ‐butylpyridine and 2,6‐diisopropylpyridine.