Benzylic carbocation, electrostatic

Figure 11.12 Resonance forms of the allyl and benzyl carbocations. Electrostatic potential maps show that the positive charge (blue) is delocalized over the ir system in both. Electron-poor atoms are indicated by blue arrows.

Benzylic acid rearrangement, 836 Benzylic carbocation, electrostatic potential map of, 377 resonance in, 377 SN1 reaction and, 376-377... 

Electrostatic potential map—cont d ammonia, 145 aniline, 925 anilinium ion, 925 anisole, 111 18]annulene, 535 arene, 74 azulene, 541 benzaldehyde, 565, 704 benzene, 44, 521, 565 benzenediazonium ion, 945 benzoquinone, 631 benzyl carbocation, 377 benzyne, 576 borane, 223... 

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 ... 

Part Three. The benzyl (and allyl) halides are a special case they have resonance. To see how the charge is delocalized in the benzyl carbocation, request two plots the electrostatic potential mapped onto a density surface and the LUMO mapped onto a density surface. Submit these for calculation at the AMI semiempirical level. On a piece of paper, draw the resonance-contributing structures for the benzyl cation. Do the computational results agree with the conclusions you draw from your resonance hybrid ... 

To avoid oversimplification, we note that changes in m values can occur independently of changes in l values when charge delocalization occurs either within the carbocation [e.g., solvolyses of benzyl substrates (3, 65, 66)] or within the anionic leaving group [e.g., solvolyses of picrates (58)]. (It has recently been shown (74) that solvolyses of benzyl tosylate do fit equation 12 satisfactorily.) Such effects appear to be general electrostatic as opposed to specific electrophilic or nucleophilic. These effects can be observed in a... 

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