Carbocations, benzylic charge distribution

This modified charge distribution in the transition state leads to a mismatch between substituent effects on the rate of reaction and on the equilibrium constant. With respect to the fluorine substituents in Scheme 31, these decrease both the stability of the carbocation and the stability of the transition state. However, while there must be less carbocation character in the transition state than in the carbocation itself the positive charge is located to a greater degree on the benzylic carbon atom and therefore will be more sensitive to stabilization by substituents. If substituent effects at the a-carbon atom in the carbocation and in the transition state are then of comparable magnitude, there will be no net effect on the rate of reaction, as is observed. 

Since cations B, F and G do not undergo electron delocalization, they are less stable than the other cations Cations A, C, D and E have the positive charge distributed amongst four carbons, three of which are the same (ring carbons) Since carbocation stability follows the order 3 > 2° > 1°, cation C is the most stable one. Cation C is a tertiary benzylic cation, cations A, D, and E are either secondary or primary benzylic cations. Hence,... 

Repeat the computational experiment described in Part A, using density-electrostatic potential maps for the allyl and benzyl carbocations. These two experiments can be performed without displaying them both on the same screen. What do you observe about the charge distribution in these two carbocations ... 

Like carbocations, carbanions are also stabilized by resonance. Thus, benzyl carbanion and allyl carbanion are more stable than ethyl carbanion. The stabilization by resonance is due to the delocalization of the negative charge, which is distributed over other carbon atoms. The canonical (resonating) forms of benzyl carbanion and allyl carbanion are given below ... 

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