The aldehydes and ketones undergo many nucleophilic reactions. Aldehydes are generally more reactive than Ketones in nucleophilic addition reactions due to steric and electronic reasons. The aldehydes and ketones undergo a number of reactions due to acidic nature of α-Hydrogen also. Aldol and cross aldol reactions occur in carbonyl compounds which contain α-Hydrogen. Aldehydes and ketones having at least one α-Hydrogen undergo reaction in dil. alkali to form β-hydroxy aldehyde or β-hydroxy ketone. This is aldol reaction. Aldol or ketol lose water to form α,β unsaturated carbonyl compounds which is called aldol condensation. Carbonyl compounds react with ammonia derivatives such as NH2OH, NH2NHCONH2, 2,4 DNP to give products useful in their identification.
Identify the reagent which is used to check the presence of an aldehyde or a keto group in an organic compound
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2,4-dinitrophenylhydrazine
Aldehydes and ketones react with ammonia derivatives (Z–NH2) by addition followed by loss of water, which converts C=O into C=N–Z. With 2,4-dinitrophenylhydrazine (2,4-DNP) the product, a 2,4-dinitrophenylhydrazone, separates as a yellow, orange or red solid, so the reagent is used as a test for the C=O group of aldehydes and ketones:
\( \displaystyle \ce{R2C=O + H2N-NH-C6H3(NO2)2 -> R2C=N-NH-C6H3(NO2)2 + H2O} \)
Why the other options are incorrect
Option (1) H2NNHOH — Incorrect. There is no such reagent in use for testing aldehydes and ketones.
Option (3) semicarbazide — Incorrect. It also condenses with aldehydes and ketones (giving semicarbazones), but these are not the coloured precipitates used as the standard test.
Option (4) phenylhydrazine — Incorrect. It gives phenylhydrazones, but the test reagent for aldehydes and ketones is the 2,4-dinitro derivative, whose products are brightly coloured and crystallise well.
NCERT (2026-27 edition):
Addition of Ammonia and Its Derivatives – “Nucleophiles, such as ammonia and its derivatives H2N-Z add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid. The equilibrium favours the product formation due to rapid dehydration of the intermediate to form >C=N-Z. Z = Alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.
* 2,4-DNP-derivatives are yellow, orange or red solids, useful for characterisation of aldehydes and ketones.”