Typical Reactions of Carbocations Rearrangements

Pi-bond and o-bond nucleophiles can also add to carbocations. In the former case, a new carbocation is formed, as discussed above. 

The major mechanisms for carbocationic rearrangements are 1,2-hydride shifts and 1,2-alkyl shifts. Both of these occur in the rearrangement of the cyclo-hexylium ion (a 2° carbocation) to the 1-methylcyclopentylium ion (a 3° carbocation). A 1,2-alkyl shift of the C3—C2 bond of cyclohexylium from C2 to Cl gives a cyclopentylmethylium ion (a 1° carbocation). This reaction is uphill in energy. However, a 1,2-hydride shift of the Cl—H bond from Cl to C2 gives a much more stable product. 

The cation is generated at Cl. Then C7—C2 migrates to C7—Cl, and C4—C3 then migrates to C4—C2. Addition of AcOH to the C3 carbocation and H+ loss gives the product. 

Note how the atoms are kept in the same relative positions in the drawings as the bonds are moved around. In rearrangement reactions especially, draw all intermediates exactly as the starting material was drawn. Only when you have made and broken all bonds in your list should you try to redraw the structure. 

Transannular 1,5-hydride shifts are often seen across medium-sized (8-, 9-, and 

Transannular 1,5-hydride shifts are often seen across medium-sized (8-, 9-, and 10-membered) rings. The atoms directly across from each other in these rings are quite close together, so migration is facile. In general, though, most cationic rearrangements can be accounted for by a series of 1,2-shifts. 

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