IIT-JEE 2008 · Paper 1 · Chemistry Q48 · Dronacharya Exams

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ChemistrySingle correct+3 / -1Question 48 of 69

The major product of the following reaction is

1-(1-Bromoethyl)-2-fluoro-4-nitrobenzene with the Br on a wedge at the benzylic stereocentre, treated with PhS⁻ Na⁺ in dimethylformamide.

A

The SPh group replaces Br at the benzylic carbon with inversion (SPh hashed); F remains on the ring.

B

The SPh group replaces Br at the benzylic carbon with retention (SPh on a wedge); F remains on the ring.

C

Br retained at the benzylic carbon; the ring F replaced by SPh.

D

Both the benzylic Br and the ring F replaced by SPh (benzylic SPh on a wedge).

Answer and worked solution

Official answer: A

Which site reacts?

The benzylic C-Br is a good $\mathrm S_\mathrm N2$ site, and thiophenoxide in DMF (a polar aprotic solvent) is a strong, soft nucleophile.
The ring F is meta to the nitro group (F at C2, $\ce{NO2}$ at C4), so it is not activated for aromatic substitution.

Stereochemistry. $\mathrm S_\mathrm N2$ goes with inversion: the SPh group takes the opposite (hashed) position to the departing Br.

(A)

Topics:SN1 and SN2 Haloarenes

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