Caibocation allylic

Any structural effect which reduces the electron deficiency at the tricoordinate carbon will have flie effect of stabilizing the caibocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. 

In contiaat, die isomer, in which tire double bond is not in a position to participate in die iooizsdon step, reacts 10 times slower than die anti isomer. The reaction product is derived fiom a rearranged caibocation ion that is stabilized by virtue of being allylic. ... 

Benzyl and allyl alcohols which can generate stabilized caibocations give Friedel-Crafts alkylation products with mild Lewis acid catalysts such as scandium triflate. ... 

Both the 1,2-addition product and the 1,4-addition product aie derived from the same allylic caibocation. 

The caibocations fonned as intennediates when allylic halides undergo SnI reactions have their positive chaige shaied by the two end caibons of the allylic system and may be attacked by nucleophiles at either site. Pioducts may be fonned with the same pattern of bonds as the starting allylic halide or with allylic rearrangement. 

Section 10.10 Piotonation at the temninal caibon of a conjugated diene system gives an allylic caibocation that can be captured by the halide nucleophile at either of the two sites that share the positive charge. Nucleophilic attack at the caibon adjacent to the one that is protonated gives the product of direct addition (1,2 addition). Capture at the other site gives the product of conjugate addition (1,4 addition). 

Benzylic halides resemble allylic halides in the readiness with which they fonn caibocations. On comparing the rate of SnI hydrolysis in aqueous acetone of the following two tertiary chlorides, we find that the benzylic chloride reacts over 600 times faster than does rerr-butyl chloride. 

Section 26.9 Carbon-caibon bond fonnation between isoprene units can be understood on the basis of nucleophilic attack of the tt electrons of a double bond on a caibocation or an allylic caibon that bears a pyrophosphate leaving group. 

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