JEE Main 2014 · 9 Apr · Shift 1 · Chemistry Q56

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ChemistrySingle correct+4 / -1Question 56 of 90

In a nucleophilic substitution reaction :
$\ce{R-Br + Cl- ->[DMF] R-Cl + Br-},$
which one of the following undergoes complete inversion of configuration ?

\ce{C6H5CHC6H5Br}

\ce{C6H5CH2Br}

\ce{C6H5CHCH3Br}

\ce{C6H5CCH3C6H5Br}

Answer and worked solution

Official answer: B

Complete inversion is the mark of a clean $S_N2$ reaction, favoured by the polar aprotic DMF. Benzyl bromide (primary, unhindered, benzylic) reacts this way.

The secondary and tertiary diaryl or aryl-alkyl bromides form stabilised benzylic cations, so they react partly by $S_N1$ (racemisation).

Topics:SN1 and SN2

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JEE Main 2014 · 9 Apr · Shift 1, Chemistry question 56