Carbenium ions effects, positive charge interaction

The precise reactivities of various types of carbenium ions (free, paired, and aggregates) are not known, but it seems that the differences in reactivities are not very large. This may be due to solvation effects, which are similar for all types of carbocations, and also due to the large size of the counterions which interact weakly with the cations. A positive charge in carbocations is located partially on the sp2-hybridized C atom and is widely delocalized over the /3-protons and substituents, especially in the case of aromatic and alkoxy a-substituents. 

Poor Hammett a correlations are often obtained for reactions in which a positively charged centre is formed that can resonate with an electron donating substituent, because of the additional resonance transmission for these substituents This phenomenon is illustrated by the solvolysis of substituted 2-phenyl-2-chloropropane (Figure 5). Scheme 6 shows how a transition structure may obtain extra stabilisation by resonance interaction between carbenium ion and 4-methoxy and 4-dimethylamino groups. In these cases there is no effect on the energy of the reactant molecules. 

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