Substituent effects on SN1 reactivity

We observed quite similar substituent effects in Section 1.2.1 in connection with the stability of radicals of type R1 R2R3C . These effects were interpreted both in VB and in MO terms. For the carbenium ions, completely analogous explanations apply, which are shown in Tables 2.2 (effect of conjugating groups) and 2.3 (effect of alkyl groups). These tables use res- 

The very large inherent differences in the stability of carbenium ions are reduced in solution—because of a solvent effect—but they are not eliminated. This solvent effect arises because of the dependence of the free energy of solvation AGhyd(R ) on the structure of the carbenium ions (Table 2.1, row 2). This energy becomes less negative going from Me to Ph— CH2 , as well as in the series Me — Et — r Pr — tert-Bu . The reason for this is hindrance of solvation. It increases with increasing size or number of the substituents at the carbenium ion center. 

Allyl halides heterolyze just as easily as benzyl halides because they also produce a resonance-stabilized carbenium ion. Even faster heterolyses are possible when the charge of the resulting carbenium ion can be delocalized by more than one unsaturated substituent and can thereby be stabilized especially well. This explains the remarkably high SN1 reactivities of the benzhydryl halides (via the benzhydryl cation) and especially of the triphenylmethyl halides (via the trityl cation)  

Side Note 2.3. The Structures of Alkyl-and Aryl-substituted Carbenium Ions 

The four resonance forms of the benzylic cations (bottom of Table 2.2) allow for the prediction of structural details. They are confirmed, for example, by crystal structure analysis of the 

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