Aldehydes and ketones are two important classes of organic compounds which are widely used in the synthesis of a variety of other organic compounds. Due to the polarity of the carbonyl group, they undergo nucleophilic addition reactions. Broadly speaking, these addition reactions are of two types- one in which simply the addition of the nucleophilic reagent occurs across \(>C=O\) bond and the other in which addition is followed by the elimination of a molecule of water. A number of ammonia derivatives such as hydroxylamine, hydrazine, phenylhydrazine, 2,4-dinitrophenylhydrazine, and semicarbazide belong to the second category of reactions. These derivatives are used for the identification and characterization of aldehydes/ketones. Both types of nucleophilic addition reactions are influenced by steric and electronic factors. In general, aliphatic aldehydes are more reactive than aromatic aldehydes. The aliphatic aldehydes reduce both Fehling's solution and Tollen's reagent but aromatic aldehydes reduce only Tollen's reagent. Like aldehydes, aromatic ketones are less reactive than aliphatic ketones towards nucleophilic addition reactions.
Phenols and carboxylic acids are acidic in nature. Both dissolve in \(NaOH\) solution and turn blue litmus red. However carboxylic acids are much stronger acids than phenols and decompose \(NaHCO_3\) with the evolution of \(CO_2\) but phenols do not. Both electron-donating and electron-withdrawing substituents influence the acid strength of aliphatic as well as aromatic acids. Unlike all other aliphatic acids, formic acid has reducing properties and also does not show reactions of the alkyl group.
Which of the following reagents can be used to distinguish between a carboxylic acid and a phenol?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(NaHCO_3\) solution
Both carboxylic acids and phenols are acidic, so they share many reactions. A test that tells them apart must use a reagent that reacts with only one of them. Carboxylic acids are much stronger acids than phenols, so only they can decompose a weak base such as sodium hydrogencarbonate.
A carboxylic acid gives brisk effervescence of \( \ce{CO2} \) with \( \ce{NaHCO3} \) solution:
\( \ce{RCOOH + NaHCO3 -> RCOONa + H2O + CO2 ^} \)
Phenol is too weak an acid to decompose \( \ce{NaHCO3} \), so there is no effervescence. This test tells the two apart.
Why the other options are incorrect
Option (1) NaOH solution — Incorrect. Both are acidic enough to react with a strong base. Both dissolve in NaOH, giving sodium carboxylate and sodium phenoxide, so it does not tell them apart.
\( \ce{RCOOH + NaOH -> RCOONa + H2O} \)
\( \ce{C6H5OH + NaOH -> C6H5ONa + H2O} \)Option (2) Blue litmus — Incorrect. Both are acids and both turn blue litmus red.
Option (4) Sodium metal — Incorrect. Both have an acidic O–H group, and both evolve hydrogen with sodium.
\( \ce{2RCOOH + 2Na -> 2RCOONa + H2 ^} \)
\( \ce{2C6H5OH + 2Na -> 2C6H5ONa + H2 ^} \)
NCERT (2026-27 edition):
Reactions with Metals and Alkalies
