NEET 2019 Odisha · ChemistryPrevious Year Question

The reaction that does not give benzoic acid as the major product is:

Answer: (C) (C) $C X_{6} H X_{5} CH X_{2} OH$ with PCC (pyridinium chlorochromate). \textbf{Answer:} (C) $C X_{6} H X_{5} CH X_{2} OH$ with PCC (pyridinium chlorochromate) \textbf{Solution:} PCC is a mild oxidising agent that oxidises a primary (here benzylic) alcohol only up to the aldehyde stage, stopping at benzaldehyde $C X_{6} H X_{5} CHO$ ; it does not oxidise further to the acid.

A.(A) $C X_{6} H X_{5} CH X_{2} OH$ with $K X_{2} Cr X_{2} O X_{7}$
B.(B) $C X_{6} H X_{5} COCH X_{3}$ with (i) $NaOCl$ ; (ii) $H X_{3} O X^{+}$
C.(C) $C X_{6} H X_{5} CH X_{2} OH$ with PCC (pyridinium chlorochromate)✓
D.(D) $C X_{6} H X_{5} CH X_{2} OH$ with $KMnO X_{4}$ / $H X^{+}$

Correct Answer

Solution & Explanation

\textbf{Answer:} (C) $C X_{6} H X_{5} CH X_{2} OH$ with PCC (pyridinium chlorochromate) \textbf{Solution:} PCC is a mild oxidising agent that oxidises a primary (here benzylic) alcohol only up to the aldehyde stage, stopping at benzaldehyde $C X_{6} H X_{5} CHO$ ; it does not oxidise further to the acid. Hence this reaction does not give benzoic acid. In contrast, strong oxidants $K X_{2} Cr X_{2} O X_{7}$ (A) and $KMnO X_{4}$ / $H X^{+}$ (D) oxidise benzyl alcohol all the way to benzoic acid $C X_{6} H X_{5} COOH$ . Acetophenone $C X_{6} H X_{5} COCH X_{3}$ (B) undergoes the haloform reaction with $NaOCl$ and, after acidification, gives benzoic acid (plus $CHCl X_{3}$ ). Therefore option (C) is the exception.

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