JEE Main 2023 · 30 January · evening · Numerical answer · +4 / −0
Number of compounds from the following which will not dissolve in cold NaHCO₃ and NaOH solutions but will dissolve in hot NaOH solution is ________.
Answer and solution
Answer 3
Solution
Chemistry ma’am · 4 steps
Sort by acid strength first: acids go in bicarbonate, phenols in cold alkali, and only esters wait for heat.
- Three tiers. A carboxylic acid is strong enough to react with NaHCO₃. A phenol is too weak for NaHCO₃ but dissolves in cold NaOH. An ester has no acidic H, so it dissolves only in hot NaOH, where it is hydrolysed to soluble salts.
- Ether stays out. An aryl alkyl ether has no acidic H and is not hydrolysed by NaOH, even when hot, so it does not dissolve at all.
Compound Group Cold NaHCO₃ / cold NaOH Hot NaOH 4-methylbenzoic acid carboxylic acid dissolves in both dissolves methyl benzoate ester no / no dissolves (hydrolysis) 4-methylphenyl formate ester no / no dissolves (hydrolysis) 2-hydroxyacetophenone phenol no / yes dissolves 3-hydroxy-5-methylbenzaldehyde phenol no / yes dissolves 3-isopropylanisole ether no / no no phenyl acetate ester no / no dissolves (hydrolysis) - Hot NaOH reaction. For example, CH₃COOC₆H₅ + 2NaOH →(Δ) CH₃COONa + C₆H₅ONa + H₂O. Both products are salts, so the ester goes into solution.
- So. Only the three esters, methyl benzoate, 4-methylphenyl formate and phenyl acetate, fit all three conditions. The answer is 3.
The common slip: 5. Treating the two phenols as insoluble in cold NaOH gives 5. Phenols are too weak for NaHCO₃, but cold NaOH does dissolve them.
The idea to keep. Solubility in bases ranks acidity: RCOOH dissolves in NaHCO₃, phenols in cold NaOH, esters only in hot NaOH by hydrolysis, and ethers in none.
Answer from the NTA final answer key. Solution by PrepWiser.