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Q8.15(ii):
How will you bring about the following conversion in not more than two steps? (ii) Benzoic acid to Benzaldehyde

Solution :

Given Reactants & Targeted Product

Reactant: Benzoic acid ($C_6H_5COOH$)

Product: Benzaldehyde ($C_6H_5CHO$)

Step 1: Conversion of Benzoic acid to Benzoyl chloride

Treat Benzoic acid with thionyl chloride ($SOCl_2$) or phosphorus pentachloride ($PCl_5$). Thionyl chloride is preferred as the by-products ($SO_2$ and $HCl$) are gaseous and escape, leaving pure Benzoyl chloride.

Chemical Equation:

$C_6H_5COOH + SOCl_2 \xrightarrow{\Delta} C_6H_5COCl + SO_2(g) + HCl(g)$

[Justification: This is a nucleophilic acyl substitution reaction where the $-OH$ group is replaced by a $-Cl$ atom.]

Step 2: Rosenmund Reduction of Benzoyl chloride

The resulting Benzoyl chloride is subjected to partial reduction using hydrogen gas in the presence of palladium catalyst supported on barium sulfate ($Pd/BaSO_4$). This is known as the Rosenmund reduction.

Chemical Equation:

$C_6H_5COCl + H_2 \xrightarrow{Pd/BaSO_4} C_6H_5CHO + HCl$

[Justification: $BaSO_4$ acts as a poison to the $Pd$ catalyst, preventing the further reduction of the aldehyde into a primary alcohol.]

Visual Representation of the Transformation

C6H5COOH SOCl2 C6H5COCl H2, Pd/BaSO4 C6H5CHO

Summary of Reaction Conditions

Step Reagent Name of Reaction
1 $SOCl_2$ Acyl Chloride Formation
2 $H_2 / Pd / BaSO_4$ Rosenmund Reduction

Final Solution: Benzoic acid is converted to Benzaldehyde in two steps: first by reaction with $SOCl_2$ to form Benzoyl chloride, followed by a Rosenmund reduction using $H_2$ over $Pd/BaSO_4$.


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