Question

Which of the following halides is most likely to undergo nucleophilic substitution with the cyanide ion via SN2 mechanism? 2-chlorobutane 2-chloro-2-methylpropane 2-bromobutane 2-bromo-2-methylpropane 2-iodobutane

Answer 1

S_N2 mechanism: Substitution reaction in which the nucleophile replaces the halogen in the alkyl halide. The mechanism is a one step process. S_N2 is a type of nucleophillic substitution reaction in which one bond is formed and one is broken simultaneously.

The rate of the reaction depends on both of the nucleophile and the alkyl halide.

Suitable alkyl halide to undergo this type of substitution reaction is the alkyl halide with less substituent.

The order of the alkyl halide for S_N2 is:

1⁰ > 2⁰ > 3⁰

The halogen attached to the alkyl halide (leaving group) must be almost always expelled with full negative charge. As a result, the best leaving groups are those that stabilize the transition state.

The order halide atom present on the alkyl halide is:

I^- > Br^- > Cl^-

2-chloro-2-methylpropane, 2-bromo-2-methylpropane are 3⁰ alkyl halides and thus is least reactive to follow S_N2 kinetics.

2-chlorobutane, 2-bromobutane and 2-iodobutane are 2⁰ alkyl halides, but between them 2-iodobutane (Best leaving group) is most likely to undergo nucleophilic substitution with the cyanide ion via S_N2 mechanism.