Q.How will you obtain monobromobenzene from aniline?

Concept: Sandmeyer Reaction — a method to replace the diazonium group ($-\mathrm{N}_2^+$) with a halogen using a copper(I) halide.

Steps:

1. Diazotisation: Treat aniline ($\mathrm{C_6H_5NH_2}$) with $\mathrm{NaNO_2}$ and dilute $\mathrm{HCl}$ at $0-5^\circ\mathrm{C}$ to form benzene diazonium chloride ($\mathrm{C_6H_5N_2^+Cl^-}$).

2. Substitution: Add the diazonium salt solution to a cold solution of cuprous bromide ($\mathrm{CuBr}$) in $\mathrm{HBr}$ (Sandmeyer reagent). The diazonium group is replaced by bromine. …

The target, monobromobenzene, has no nitrogen at all — so the cleanest strategy is not to brominate the ring while the amino group is still on it, but to use the –NH₂ group itself as the handle: convert it into a diazonium salt and then swap that group for bromine.

Why not just brominate aniline directly? The –NH₂ group is a powerfully activating, ortho/para-directing group. Treating aniline with bromine water doesn't stop at one bromine — it gives 2,4,6-tribromoaniline as a white precipitate. That over-bromination problem is exactly what the diazonium route sidesteps: it never brominates the ring at all.

The standard (NCERT) sequence is:

1. Diazotisation: Treat aniline with $\text{NaNO}_2$ and dilute $\text{HCl}$ at 0–5 °C to form benzenediazonium chloride:

The low temperature matters — diazonium salts decompose readily above ~5 °C.

2. Sandmeyer reaction: Add the cold diazonium salt solution to cuprous bromide dissolved in HBr. The diazonium group is replaced by bromine, with nitrogen gas escaping:

(The Gattermann variation — copper powder with HBr — achieves the same substitution.)

The loss of N₂, a supremely stable gas, is what makes this replacement so clean: the reaction is driven forward and the product is a single monosubstituted arene, exactly what we want.

Concept: Diazotization & Sandmeyer Reaction (or Replacement by Halogen)

This method converts a primary aromatic amine (like aniline) into a diazonium salt, which is then replaced by a halogen (bromine) to give the aryl halide.

Method: Sandmeyer Reaction (using CuBr)

Why this works:

Aniline has an –NH₂ group that is strongly activating and ortho/para-directing. Direct bromination would give a mixture of polybrominated products (e.g., 2,4,6-tribromoaniline). To get monobromobenzene, we must first remove the amino group after using it to position the bromine.

Steps:

1. Diazotization Treat aniline ($\ce{C6H5NH2}$) with sodium nitrite ($\ce{NaNO2}$) and dilute HCl at 0–5°C to form the benzenediazonium chloride salt:

2. Sandmeyer substitution Add the diazonium salt solution to a cold solution of cuprous bromide ($\ce{CuBr}$) in HBr. The diazonium group is replaced by bromine: …

Here is a breakdown of the common mistakes students make when converting aniline to monobromobenzene, along with the correct reasoning to avoid them.

The Core Concept: Replace, Don't Protect-and-Brominate

Since the target is monobromobenzene (a plain benzene ring with a single Br, no nitrogen anywhere), the most direct route is to replace the -NH2 group with -Br entirely, via a diazonium salt -- NOT to protect the ring, brominate it while -NH2 is still there, and then separately remove the nitrogen. Both approaches can technically reach the same product, but the diazotisation + Sandmeyer route is far simpler and is the one this question's short_answer/method use.

Common Mistake #1: Assuming You Must Protect the -NH2 Group First

The Mistake: Students assume that since -NH2 is a very strong activating, ortho/para-directing group, you must first "protect" it (e.g. by acetylation) before doing anything else -- as if the goal were to brominate the ring while keeping nitrogen attached.

Why it's wrong: That assumption only applies if you're trying to install Br on the ring while ALSO keeping the -NH2 group in the final product (e.g. making a bromoaniline). Here, the target has NO nitrogen at all -- so instead of protecting -NH2 to survive a ring bromination, it's far simpler to convert -NH2 into a diazonium salt and then directly REPLACE it with Br via the Sandmeyer reaction.

How to Avoid: Before reaching for a protection strategy, check whether the final product still needs the amino group. If it doesn't (as here), diazotisation + direct replacement is usually the shorter route.

Common Mistake #2: Confusing This with Direct Ring Bromination

The Mistake: Students write Aniline + Br2/FeBr3 -> Monobromobenzene directly, or worry about getting 2,4,6-tribromoaniline.

Why it's wrong: Direct bromination of free aniline's ring genuinely WOULD overreact to 2,4,6-tribromoaniline (since -NH2 is such a strong activator) -- but that concern only applies if you're trying to brominate the ring while nitrogen is still attached. The diazotisation/Sandmeyer route never brominates the ring at all -- it replaces the nitrogen-bearing carbon's substituent directly, sidestepping the over-bromination problem entirely. …