Cyclobutylmethyl carbocations

The cyclopropylmethyl ion 11 is unusually stable and has a 14kcal/mol barrier to rotation about the cyclopropyl-carbocation bond.69 In contrast, the corresponding cyclobutylmcthyl ion 12 quickly rearranges to a cyclopentyl cation. Here, some strain relief occurs in the rearrangement, but this is opposed by the conversion of the stable tertiary carbocation to the less stable secondary ion. Although rearrangement is the normal process for cyclobutylmethyl cations, there is one case 13 in which rearrangement does not occur, and a small rate acceleration is observed.70... 

Another method by which a carbocation can be generated is by protonation of either an alkene or a carbonyl group. Thus, cyclobutylmethyl cations are formed by treatment of a ketone or an alkene with acid the cyclobutylmethyl cations subsequently rearrange to give cyclopentyl cations. 

One very interesting structure that might at first glance appear to be a 1° carbocation is the cyclopropylmethyl cation. Calculations indicated, however, that this species is best described by a pair of rapidly equilibrating nonclassical structures (62 and 63, Figure 5.46) that are approximately equal in energyAttempts to prepare the 1° cyclobutylmethyl cation were unsuccessful, however, with cyclopentyl cations being formed instead. ... 

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