Question

Find the Product A and B in the below reaction

• A. A = 2,4,6-tribromoaniline, B = 1,3,5-tribromobenzene
• B. A = 2,4,6-tribromobenzene, B = 1,3,5-tribromoaniline
• C. A = 2-bromoaniline, B = 3-bromobenzene
• D. A = 4-bromoaniline, B = 1-bromobenzene

Answer 1

Answer: (A)

A = 2,4,6-tribromoaniline, B = 1,3,5-tribromobenzene

Answer 2

The reaction sequence involves two main steps: bromination of aniline and then the removal of the amino group.

Step 1: Formation of Product A

• Reactant: Aniline (aminobenzene)
• Reagent: $\text{Br}_2/\text{H}_2\text{O}$ (Bromine water)
• Mechanism: Electrophilic Aromatic Substitution (Bromination)

The amino group ($\text{-NH}_2$) is a powerful activating group and an ortho-para director towards electrophilic aromatic substitution. In the presence of aqueous bromine, which is a highly reactive brominating agent, all available ortho (positions 2 and 6) and para (position 4) positions relative to the amino group are simultaneously brominated. This results in the formation of 2,4,6-tribromoaniline as a white precipitate.

Aniline (C1=CC=C(C=C1)N) reacts with $\text{Br}_2/\text{H}_2\text{O}$ to give 2,4,6-tribromoaniline.

Product A (2,4,6-tribromoaniline): N(C1=CC(Br)=CC(Br)=C1Br)

Step 2: Formation of Product B

• Reactant: Product A (2,4,6-tribromoaniline)

• Reagents: (a) $\text{NaNO}_2/\text{HCl}$ (0-5 °C), (b) $\text{H}_3\text{PO}_2$

• Step 2(a): Diazotization

The primary aromatic amine (2,4,6-tribromoaniline) reacts with nitrous acid (generated in situ from $\text{NaNO}_2$ and $\text{HCl}$) at low temperatures (0-5 °C). This reaction, known as diazotization, converts the amino group into a diazonium group ($\text{-N}_2^+$). The intermediate formed is 2,4,6-tribromobenzenediazonium chloride.

2,4,6-tribromoaniline (N(C1=CC(Br)=CC(Br)=C1Br)) reacts with $\text{NaNO}_2/\text{HCl}$ to form 2,4,6-tribromobenzenediazonium chloride.

• Step 2(b): Reduction of Diazonium Salt

The arenediazonium salt (2,4,6-tribromobenzenediazonium chloride) is then treated with hypophosphorous acid ($\text{H}_3\text{PO}_2$). Hypophosphorous acid is a reducing agent that replaces the diazonium group ($\text{-N}_2^+$) with a hydrogen atom. This is a useful method to remove an amino group from an aromatic ring after it has been used to direct other substitutions.

The diazonium group ($\text{-N}_2^+$) is replaced by a hydrogen atom. The bromines remain on the ring. The numbering of the product is adjusted to give the lowest possible locants to the substituents. Therefore, 2,4,6-tribromobenzene becomes 1,3,5-tribromobenzene.

2,4,6-tribromobenzenediazonium chloride reacts with $\text{H}_3\text{PO}_2$ to give 1,3,5-tribromobenzene.

Product B (1,3,5-tribromobenzene): C1=CC(Br)=CC(Br)=C1Br

Summary of Products:

• Product A: 2,4,6-tribromoaniline Smiles: N(C1=CC(Br)=CC(Br)=C1Br)
• Product B: 1,3,5-tribromobenzene Smiles: C1=CC(Br)=CC(Br)=C1Br