Question

7
(
10.7 Which alkyl halide from the following pairs would you expect to react more
rapidly by an S 2 mechanism? Explain your answer.
CH, ,
1
(1) CH,CH,CH,CH,Br or CH,CH,CHCH, (ii) CH,CH,CHCH, or H,C-C-Br.
CH
Br
(iii) CH,CHCH,CH,Br or CH,CH,CHCH,Br
1
CH,
CH,
1
1
1
Br
1​

Answer 1

Answer:

The rate of an S N 2 reaction is dependent on a number of factors, including the size of the alkyl group attached to the carbon bearing the leaving group, the stability of the leaving group, and the polarity of the solvent.

In general, a tertiary alkyl halide will react more rapidly in an S N 2 reaction than a primary or secondary alkyl halide due to the larger size of the tertiary alkyl group, which results in greater solvolysis and a more favorable transition state for the reaction.

When comparing CH3CH2CH2CH2Br and CH3CH2CHCH2, the former would be expected to react more rapidly in an S N 2 reaction due to its larger size.

When comparing CH3CH2CHCH2 and HC-C-Br, the former would be expected to react more rapidly in an S N 2 reaction due to the stability of the leaving group. Bromine is a better leaving group than a hydrogen, so CH3CH2CHCH2 would react more rapidly.

When comparing CH3CHCH2CH2Br and CH3CH2CH2CH2Br, the former would be expected to react more rapidly in an S N 2 reaction due to the polarity of the solvent. A more polar solvent would increase the rate of the reaction, so CH3CHCH2CH2Br would be expected to react more rapidly.