Question

What is the effect of steric hindrance on reaction rate? Specifically compare the rate of reaction of the less hindered 1º alkyl chloride (1-chlorobutane) with the more hindered 1º alkyl chloride (1-chloro-2-methylpropane) in both SN1 and SN2 reactions.

Answer 1

Steric hindrance has no effect on SN1 reaction because in SN1 reaction a carbocation is formed which is planar and nucleophile has to attack on this carbocation and not on the sterically hindered reactant. Thus rate of SN1 reaction is unaffected with steric hindrance.

In SN2 reaction, nucleophile attack on reactant from the backside and it is difficult for nucleophile to approach at sterically hindered carbon bearing leaving group. Thus rate SN2 reaction decreases with increase in steric hindrance.

In 1-chlorobutane is sterically less hindered thus have high rate of SN2 reaction whereas 1-chloro-2-methylpropane is sterically hindered thus rate of reaction towards SN2 is less.

Since 1-chloro-2-methylpropane give stable carbocation as compared to 1-chlorobutane, thus 1-chloro-2-methylpropane has high rate of reaction towards SN1 as compared to 1-chlorobutane.

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