In a nucleophilic substitution reaction : \(\mathrm{R-Br + Cl^-} \xrightarrow{\mathrm{DMF}} \mathrm{R-Cl + Br^-}\), which one of the following undergoes complete inversion of configuration ?
Show Hint
- \(\mathrm{C_6H_5CHC_6H_5Br}\)
- \(\mathrm{C_6H_5CCH_3C_6H_5Br}\)
- \(\mathrm{C_6H_5CH_2Br}\)
- \(\mathrm{C_6H_5CH_2CH_2Br}\)
The Correct Option is C
Solution and Explanation
Option (2) is a tertiary, doubly benzylic bromide and option (1) a secondary, doubly benzylic bromide; both ionise easily to very stable carbocations (partial/complete \(\mathrm{S_N1}\), racemisation) and are sterically hindered for backside attack.
Option (3) is a primary benzylic bromide: unhindered, and the adjacent phenyl ring stabilises the \(\mathrm{S_N2}\) transition state, so with \(\mathrm{Cl^-}\) in DMF it reacts by a clean \(\mathrm{S_N2}\) with inversion.
Option (4) is a primary but non-benzylic bromide, less reactive than (3); the intended answer is the benzylic primary bromide, option (3).
Answer: (3) \(\mathrm{C_6H_5CH_2Br}\)
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