Relative carbocation stabilities

The major influence of the methyl group is electronic The most important factor IS relative carbocation stability To a small extent the methyl group sterically hinders the ortho positions making attack slightly more likely at the para carbon than at a single ortho carbon However para substitution is at a statistical disadvantage because there are two equivalent ortho positions but only one para position... 

Model computational studies aimed at understanding structure-reactivity relationships and substituent effects on carbocation stability for aza-PAHs derivatives were performed by density functional theory (DFT). Comparisons were made with the biological activity data when available. Protonation of the epoxides and diol epoxides, and subsequent epoxide ring opening reactions were analyzed for several families of compounds. Bay-region carbocations were formed via the O-protonated epoxides in barrierless processes. Relative carbocation stabilities were determined in the gas phase and in water as solvent (by the PCM method). 

Recent advances in the study of ion-molecule reactions have opened another approach to the determination of relative carbocation stabilities. The halide transfer... 

Using the rule that the electrophile adds to the sp carbon bonded to the greater number of hydrogens is simply a quick way to determine the relative stabilities of the intermediates that could be formed in the rate-determining step. You will get the same answer, whether you identify the major product of an electrophilic addition reaction by using the rule or whether you identify it by determining relative carbocation stabilities. In the following reaction for example, H is the electrophile ... 

Draw all of the contributing resonance structures and the resonance hybrid for the carbocation that would result from ionization of bromine from 5-bromo-1,3-pentadiene. Open the computer molecular model at the book s website depicting a map of electrostatic potential for the pentadienyl carbocation. Based on the model, which is the most important contributing resonance structure for this cation Is this consistent with what you would have predicted based on your knowledge of relative carbocation stabilities ... 

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