Question

Question: Which compound in each pair undergoes a faster S_N2 reaction?

a.

and

b.

and

Short answer

a. The compound mentioned below undergoes a faster S_N2 reaction.

b.The structure provided below undergoes a faster S_N2 reaction.

Step-by-step solution

  SN2 reaction

The S_N2 reaction involves two steps, and a carbocation is not created in this reaction. Their reactions occur smoothly with unhindered halides as a hindered substrate makes the nucleophilic attack more difficult.

Compound that undergoes faster SN2  reaction in question a

The reaction rate of S_N2 reaction depends on the nature of the alkyl halide. The S_N2 reaction occurs faster in less bulky and unhindered alkyl halides. In the compounds given in the question, one is a primary alkyl halide, and the other is a secondary alkyl halide. The rate of S_N2 reaction is faster for a primary halide. Hence, the compound showing a more rapid S_N2 reaction is illustrated below:

Compound exhibiting faster S_N2 reaction in question a

Compound that undergoes faster SN2  reaction in question b

The reaction is more rapid for an unhindered alkyl halide than a hindered alkyl halide. One of the compounds in question b is a secondary halide, and the other is a tertiary halide.

Tertiary halides do not undergo reaction because of the bulky groups present in them. The compound that exhibits a faster reaction is provided below:

Compound exhibiting faster reaction in question b