Question about Sn1 and Sn2 reactions?

Question

Hi,

I'm so confused about how to identify the order of reactivity between halides in Sn1 and Sn2 conditions. Is it the bulkiness that is considered for both, or just Sn2 and Sn1 is just the stability of the carbocation.

For the following, what would be the order for Sn1 and Sn2 reactions.

bromocyclopentane, 2-bromo-2-methylpropane, bromocyclohexane, 2-bromobutane (sec-butyl bromide), 1-bromobutane

Thanks

Answer 1

It is both but mainly the stability of the carbocation (I<II<III) for SN1. If the compound is to bulky the nucleophile won't be able to come in for SN2.

SN1:2-bromo-2-methylpropane (III)

SN2: 1-bromobutane (I), 2-bromobutane (II), bromocyclohexane (II), bromocyclopentane (II),

The IIairy cation can also give SN1 reactions especially if there is a good leaving group (Br>Cl) or if the carbon atome is hindred (cyclobupentane>cyclohexane)

Answer 2

Sn1 reactions are governed by stabilities of carbocations. So 2-bromo-2-methylpropane would be fastest, 1-bromobutane would be slowest, and the others would fall in between.

Sn2 reactions are governed by bulkiness. 1-Bromobutane would be fastest. Bromocyclopentaneand bromocyclohexane would be slower, but still fairly fast, because the ring "ties back" the substituents, letting the nucleophile have good access. 2-Bromo-2-methylpropane would be slowest.