JEE Main 2020 · 7 January · morning · Single correct · +4 / −1
Consider the following reaction :
The product 'X' is used :
- (a) as food grade colourant
- (b) in acid base titration as an indicatior.
- (c) in laboratory test for phenols
- (d) in protein estimation as an alternative to ninhydrin.
Answer and solution
Answer (b) in acid base titration as an indicatior.
Solution
Chemistry ma’am · 4 steps
Diazotised sulphanilic acid plus N,N-dimethylaniline is the classic recipe for methyl orange.
- Spot the partners. The diazonium ion comes from sulphanilic acid. It is a weak electrophile, so it attacks only a strongly activated ring. The -N(CH₃)₂ group makes the ring of N,N-dimethylaniline very electron rich.
- Where it couples. Coupling happens at the para position of N,N-dimethylaniline. The two methyl groups crowd the ortho positions, so para is preferred. The two rings become joined through an azo link, -N=N⁻.
(CH₃)₂N-C₆H₅ + ⁺N₂-C₆H₄-SO₃⁻ →(OH⁻) (CH₃)₂N-C₆H₄-N=N-C₆H₄-SO₃⁻Na⁺
- Name the product. This azo compound is methyl orange. It is red below about pH 3.1 and yellow above about pH 4.4, because protonation changes its conjugated structure.
- So. X is methyl orange, used as an indicator in acid–base titrations, especially strong acid against weak base. The answer is (b).
The idea to keep. Sulphanilic acid diazonium coupled with N,N-dimethylaniline gives methyl orange; coupled with phenol or 2-naphthol it gives orange azo dyes. Learn which partner gives which named dye.
Answer from the NTA final answer key. Solution by PrepWiser.