Allylic carbocation, electrostatic

Allylic bromination, 339-340 mechanism of, 339-340 Allylic carbocation, electrostatic potential map of, 377, 489 resonance in, 488-489 SN1 reaction and, 376-377 stability of, 488-489 Allylic halide, S l reaction and. 377 S j2 reaction and, 377-378 Allylic protons, ]H NMR spectroscopy and, 457-458... 

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.

The electrostatic potential maps in Figure 16.2 compare the resonance-stabilized allyl carbocation with CH3CH2CH2, a localized 1° carbocation. The electron-deficient region—the site of the positive charge—is concentrated on a single carbon atom in the 1° carbocation CH3CH2CH2 . In the allyl carbocation, however, the electron-poor region is spread out on both terminal carbons. 

Allylic carbocations, like allylic radicals (Section 8.6), have a double bond next to the electron-deficient carbon. The allyl cation is the simplest allylic carbocation. Because the allyl cation has only one substituent on the carbon bearing the positive charge, it is a primary allylic carbocation. Allylic carbocations are considerably more stable than comparably substituted alkyl carbocations because delocalization is associated with the resonance interaction between the positively charged carbon and the adjacent tt bond. The allyl cation, for example, can be represented as a hybrid of two equivalent contributing structures. The result is that the positive charge appears only on carbons 1 and 3, as shown in the accompanying electrostatic potential map. 

We know from other experimental evidence that the location of the positive charge in the allylic carbocation is more important than the location of the double bond. Therefore, in the hybrid, the greater fraction of positive charge is on the secondary carbon. Reaction with bromide ion occurs more rapidly at this carbon, giving 1,2-addition, simply because it has a greater density of positive charge. The electrostatic potential map shows that the positive charge (blue) is more intense on the secondary carbon. 

Electrostatic potential map of the allylic carbocation formed by protonating 1,3-butadiene. 

Figure 14.4 An electrostatic potential map of the allylic carbocation produced by protonation of 1,3-butadiene shows that the positive charge is shared by carbons 1 and 3. Reaction of Br with the more positive carbon (C3) gives predominantly the 1,2-addition product.

The carbocation intermediate in these reactions is a single species, a resonance hybrid. This type of carbocation, with a carbon-carbon double bond adjacent to the positive carbon, is called an allylic cation. The parent allyl cation, shown below as a resonance hybrid, is a primary carbocation, but it is more stable than simple primary ions (such as propyl) because its positive charge is delocalized over the two end carbon atoms as shown in the electrostatic potential map accompanying eq. 3.34. 

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 ... 

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