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Which among the following react fastest with NaOMe ?

  • A.Bromobenzene with NO2 at the ortho position and NO2 at the para position
  • B.Bromobenzene with NO2 at the ortho position and CN at the para position
  • C.Bromobenzene with NO2 at the ortho position and CH3 at the para position
  • D.Bromobenzene with CN at the ortho position and CH3 at the para position

Answer: A

(A), 1-bromo-2,4-dinitrobenzene, reacts fastest.

Methoxide replaces bromide by nucleophilic aromatic substitution: OMe− adds to the carbon bearing Br, forming a negatively charged Meisenheimer complex, and Br− is then lost. The addition is rate-determining, so anything that stabilizes the negative charge speeds the reaction. Electron-withdrawing groups ortho and para to the leaving group do this by taking the charge onto themselves.

(A) has two nitro groups, ortho and para, the strongest such combination. (B) has a nitro group ortho but a cyano group para; cyano withdraws less strongly than nitro, so it is slower. (C) has only one nitro group, and the para methyl donates electrons and destabilizes the anion. (D), with a single cyano group and a para methyl, is the least activated.

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