Chemistry Aldehydes & Ketones MDCAT 2015
PMDC Verified Question 79 of 94
When acetaldehyde reacts with 2,4-dinitrophenylhydrazine (2,4-DNPH), which one of the following products is formed?
A
\( (CH_3)_2C=N-NH-C_6H_3(NO_2)_2 \)
B
\( H_2C=N-NH-C_6H_3(NO_2)_2 \)
C
\( C_2H_5CH=N-NH-C_6H_3(NO_2)_2 \)
D
\( CH_3CH=N-NH-C_6H_3(NO_2)_2 \)
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Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: \( CH_3CH=N-NH-C_6H_3(NO_2)_2 \)
Concept:

Reactions of carbonyl compounds with 2,4-DNPH are condensation reactions where the \( =O \) of the carbonyl is replaced by \( =N-NH-Ar \).

Formula:

$$ CH_3CHO + H_2N-NH-Ar \longrightarrow CH_3CH=N-NH-Ar + H_2O $$

Solution:

  • Acetaldehyde has the formula \( CH_3CHO \). The carbonyl carbon is bonded to one methyl group and one hydrogen atom.


  • During the condensation with 2,4-DNPH, water is eliminated.


  • The resulting product structure retains the \( CH_3 \) and \( H \) attached to the \( C=N \) double bond, giving \( CH_3CH=N-NH-C_6H_3(NO_2)_2 \).


Why other options are incorrect:

Option A is the derivative of acetone. Option B is the derivative of formaldehyde. Option C is the derivative of propanal.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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