[ramboacid]

On a recent test there was a question asking to order the five isomers of C4H9Cl (including enantiomers) in order of reactivity by an SN1 mechanism from least to greatest. The two primary alkyl halides (1-chloro-2-methylpropane and 1-chlorobutane) would be the least reactive by an SN1 mechanism, but I was wondering which one would form the more stable carbocation. Both would form primary carbocations. I thought it would be 1-chloro-2-methylpropane because there is a tertiary carbon to delocalize some of the positive charge, but I've also figured that a methyl group cation would be really unstable as well, pointing to 1-chlorobutane as forming a more stable carbocation by default. If you were to consider the methyl cation as part of the chain however, I would still choose 1-chloro-2-methylpropane for the reason that there is a tertiary carbon to delocalize the charge. Is my reasoning correct, or am I making a fundamental error with respect to the stability of carbocations?

[orgopete]

Your reasoning is sound. Not only will the tertiary carbon donate electrons, but that donation will enable a hydride shift to a more stable tertiary carbocation to also occur.