SN1 and SN2 Mechanisms?

Question

Explain why, even though both are 1 degree substrates, 1-chloro-2,2-dimethylpentane prefers the SN1 mechanism whereas 1-chloropentane prefers the SN2 mechanism.

Answer 1

The fact that 1-chloro-2,2-dimethylpentane prefers an SN1 mechanism suggests that it can form a more stable carbocation. Hopefully you have been taught that a primary carbocation should never be drawn. In addition, the site of substitution is heavily branched (it is analogus to neopentyl chloride, which is notoriously inert under SN2 reaction conditions). So what happens?
It is possible to get a concerted migration / dissociation step. At the same time as the C-Cl is breaking (which would form a carbocation), a methyl group on the next carbon over does a 1,2-migration.
This gives a 3-methyl-3-hexyl cation, which is tertiary.
In chloropentane, no tertiary carbocation can be formed, so SN1 is slower here.

Answer 2

1-chloro-2,2-dimethylpentane prefers SN1 reaction mechanism because the carbon adjacent to it is branched - tertiary carbon, a steric hindrance. It prefers to form carbocation intermediate; and in this case, may shift the carbocation to the tertiary carbon.

1-chloropentane, on the other hand, prefers SN2 reaction mechanism because there are no steric hindrance and can easily undergo inversion of configuration during the back-side attack of the nucleophile.

Answer 3

Mechanism will depend on the halide. HCl alone will not react usually use zinc chloride catalyst so completely different mechanism. HBr reaction very slow usually use sulphuric acid catalyst. HI reacts at reflux in water. In most real cases nucleophilic substitutions are a mixture of Sn1 and Sn2 and by careful choice of conditions and solvent the reaction can be made to operate one mechanism. It is necessary to do kinetics etc to demonstrate mechanism.

Answer 4

Maybe it has something to do with the reagents that are being used. Take a look at those and see if you can figure it out.