Q3 / 180 · COMEDK 2024 (Evening Shift) · Chemistry

Which one of the following is the correct order of reagents to be used to convent [A] to [X] ? Figure $ \text { Reagents: } \mathrm{PCC}, \mathrm{PCl}_5, \mathrm{LiAlH}_4, \dfrac{\mathrm{KCN}}{\mathrm{H}_3 \mathrm{O}^{+}}$

  • A
    $\mathrm{PCC} \quad \mathrm{PCl}_5 \quad \dfrac{K C N}{\mathrm{H}_3 \mathrm{O}^{+}} \quad \mathrm{LiAlH}_4$
  • B
    $\dfrac{K C N}{H_3 \mathrm{O}^{+}} \quad \mathrm{PCl}_5 \quad \mathrm{PCC} \quad \mathrm{LiAlH}_4$
  • C
    $\mathrm{LiAlH}_4, \dfrac{K C N}{H_3 \mathrm{O}^{+}} \quad \mathrm{PCl}_5 \quad \mathrm{PCC}$
  • Correct answer
    D
    $\mathrm{PCl}_5, \quad \dfrac{K C N}{H_3 \mathrm{O}^{+}} \quad \mathrm{LiAlH}_4 \quad \mathrm{PCC}$

Solution

The goal is to convert $R-CH_2-OH$ to $R-CH_2-CHO$ (adding one carbon via chain extension).

$PCl_5$: Converts alcohol to alkyl chloride (good leaving group).
$R-CH_2-OH \xrightarrow{PCl_5} R-CH_2-Cl$

$KCN$: Nucleophilic substitution adds cyanide, extending the chain by one carbon.
$R-CH_2-Cl \xrightarrow{KCN} R-CH_2-CN$

$H_3O^+$: Acidic hydrolysis converts nitrile to carboxylic acid.
$R-CH_2-CN \xrightarrow{H_3O^+} R-CH_2-COOH$

$LiAlH_4$: Reduces carboxylic acid to primary alcohol.
$R-CH_2-COOH \xrightarrow{LiAlH_4} R-CH_2-CH_2-OH$

$PCC$: Selectively oxidises primary alcohol to aldehyde (stops at aldehyde stage).
$R-CH_2-CH_2-OH \xrightarrow{PCC} R-CH_2-CHO$

Correct order: $PCl_5 \to \dfrac{KCN}{H_3O^+} \to LiAlH_4 \to PCC$

Hence, the correct answer is option: 'D'.

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