Nucleophilic Substitution Reactions (II) Problem 7

Description:

Problem

Arrange the following in their reactivity towards S_N2 reaction.

1. 2 − Bromo − 2 − methylbutane, 1 − bromopentane, 2 − bromopentane.

2. 1 − Bromobutane, 1 − Bromo − 2,2 − dimethylpropane, 1 − Bromo − 2 − methylbutane, 1 − Bromo − 3 − methylbutane.

Solution

• S_N2 reaction is a one-step process which involves the formation of a transition state.

• Since transition state is highly unstable due to carbon bearing 5 bonds, steric hindrance needs to be minimized.

• Thus, the order of reactivity of alkyl halides towards S_N2 reaction is 1₀ > 2⁰ > 3⁰

Set 1 − 2 − Bromo − 2 − methylbutane, 1 − Bromopentane, 2 − Bromopentane.

The reactivity of the alkyl halides towards 𝑆𝑁2 reaction will follow the order: 1₀ > 2⁰ > 3⁰

1 − Bromopentane > 2 − Bromopentane > 2 − Bromo − 2 − methylbutane

Set 2 − 1 − Bromobutane, 1 − Bromo − 2,2 − dimethylpropane, 1 − Bromo − 2 − methylbutane, 1 − Bromo − 3 − methylbutane

Substrate	Structure	Type
1 − Bromobutane (I)		10 Primary
1 − Bromo − 2,2 − dimethylpropane (II)		10 Primary
1 − Bromo − 2 − methylbutane (III)		10 Primary
1 − Bromo − 3 − methylbutane (IV)		10 Primary

Since all the alkyl halides given are primary, reactivity towards S_N2 reaction will be determined based on steric inhibition.

Steric hindrance is determined by crowding around carbon bearing halogen atom.

Thus, the order of reactivity towards S_N2 reaction is −

(I) > (IV) > (III) > (II)