How the Arrangement of Alkyl Substituents Affects the Stability of Delocalized Carbocations

G-4 calculations are used to explore which carbon atoms of methylated butadienes, methylated cyclopentadienes, and methylated benzenes are most readily protonated to yield delocalized allyl and pentadienyl cations. While it is not surprising that alkylation of the positions bearing formal positive charge stabilizes these cations, several other effects are less obvious. First, alkylation of positions in the delocalized cation that do not bear formal charge is beneficial, to an extent about a quarter to a third as great as at charged positions. Second, alkylation of the position receiving the proton disfavors protonation. Finally, at least in the acyclic systems, the more symmetrical substitution pattern that is 2° at both ends is moderately preferred to the less symmetrical pattern that is 3° at one end and 1° at the other. Taking all three of these factors into account, as well as substitution at the formally charged centers, models the stability of all 94 delocalized cations quite well.