Which of the following reagents gives yellow, orange or red solid derivatives with aldehydes and ketones that are useful for their characterisation?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 2,4-Dinitrophenylhydrazine
Ammonia derivatives of the type H2N–Z add to the carbonyl group of aldehydes and ketones, and the addition product loses water to give a >C=N–Z derivative. With 2,4-dinitrophenylhydrazine (2,4-DNP) the product is a 2,4-dinitrophenylhydrazone:
\( \displaystyle \ce{R2C=O + H2N-NH-C6H3(NO2)2 -> R2C=N-NH-C6H3(NO2)2 + H2O} \)
These 2,4-DNP derivatives are yellow, orange or red solids, so the reagent is used to detect and characterise aldehydes and ketones.
NCERT (2026-27 edition):
8.4 Chemical Reactions
Table 8.2: Some N-Substituted Derivatives of Aldehydes and Ketones (>C=N-Z)
