JEE Main 2021 · 24 February · evening · Single correct · +4 / −1
What is the correct sequence of reagents used for converting nitrobenzene into m-dibromobenzene?
- (a) →(NaNO₂)/→(HCl)/→(KBr)/→(H⁺)
- (b) →(Sn/HCl)/→(KBr)/→(Br₂)/→(H⁺)
- (c) →(BB₂/Fe)/→(Sn/HCl)/→(NaNO₂/HCl)/→(CuBr/HBr)
- (d) →(Sn/HCl)/→(Br₂)/→(NaNO₂)/→(NaBr)
Answer and solution
Answer (c) →(BB₂/Fe)/→(Sn/HCl)/→(NaNO₂/HCl)/→(CuBr/HBr)
Solution
Chemistry ma’am · 3 steps
Brominate while NO₂ is still on the ring, because only it sends bromine to the meta position.
- Plan backwards. The two Br atoms are meta to each other. One Br must come from the nitrogen of NO₂, through a diazonium salt. The other must come from bromination of the ring, and it must go meta to that nitrogen.
- Order matters. NO₂ is a meta director, while NH₂ is a strong ortho/para director. So the ring must be brominated before the nitro group is reduced.
Step Reagent Product 1 Br₂/Fe (printed as BB2/Fe) m-bromonitrobenzene (NO₂ directs meta) 2 Sn/HCl m-bromoaniline (NO₂ → NH₂) 3 NaNO₂/HCl, 0 to 5 °C m-bromobenzenediazonium chloride 4 CuBr/HBr (Sandmeyer) m-dibromobenzene (N₂⁺ replaced by Br) - So. Only (c) brominates first, then reduces, diazotises and finishes with a Sandmeyer reaction. That places the second Br exactly where the nitrogen was, meta to the first.
The idea to keep. In ring synthesis, use a group's directing effect before you change it. NO₂ steers meta. Once it becomes NH₂, everything goes ortho/para. Then the diazonium salt swaps N for Br in place.
Answer from the NTA final answer key. Solution by PrepWiser.