Ques. 1AIPMT 2008
In an SN2 substitution reaction of the type R-Br + Cl- DMF R-Cl + Br- which one of the following has the highest relative rate?
- CH3- CH3 | CH3 |C-CH2Br
- CH3CH2Br
- CH3CH2CH2Br
- CH3- CH3 |CH-CH2Br
Answer
(2) CH3CH2Br
Solution
As per NCERT, in an SN2 reaction the nucleophile attacks the carbon from the side opposite to the leaving group; bulky groups on or near this carbon hinder the attack and slow the reaction.
All four are primary bromides, so the rate depends on the crowding near the -CH2Br carbon.
CH3CH2Br has the smallest alkyl group; CH3CH2CH2Br is slightly more hindered; branching at the β-carbon in (CH3)2CHCH2Br and especially in (CH3)3CCH2Br blocks the back-side attack.
Relative rate: CH3CH2Br > CH3CH2CH2Br > (CH3)2CHCH2Br > (CH3)3CCH2Br
Answer: (2)