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The correct sequential addition of reagents in the preparation of 3-nitrobenzoic acid from benzene is:

1. Br2/AlBr3, HNO3/H2SO4, Mg/ether, CO2, H3O+
2. Br2/AlBr3, NaCN, H3O+, HNO3/H2SO4
3. Br2/AlBr3, HNO3/H2SO4, NaCN, H3O+
4. HNO3/H2SO4, Br2/AlBr3, Mg/ether, CO2, H3O+

Correct answer: HNO3/H2SO4, Br2/AlBr3, Mg/ether, CO2, H3O+

Solution

For 3-nitrobenzoic acid the NO2 and COOH must be meta. Nitrate benzene first (NO2 is a meta-director), then brominate to place Br meta to NO2, then convert C-Br to COOH via Mg/ether, CO2, H3O+. Sequence: HNO3/H2SO4, Br2/AlBr3, Mg/ether, CO2, H3O+. (Brominating first makes Br an o/p-director, giving the para nitro product instead.)

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