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Q8.7(iii):
Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction. (iii) Benzaldehyde

Solution :

Step 1: Structural Analysis of Benzaldehyde

The molecular structure of benzaldehyde is $C_6H_5CHO$. Structurally, it consists of a benzene ring attached to a formyl group ($-CHO$).

CHO Benzene Ring

A critical observation regarding the alpha-carbon ($C_\alpha$): In the formyl group of benzaldehyde, the carbonyl carbon is attached directly to the aromatic ring. There is no hydrogen atom on the carbon adjacent to the carbonyl group (alpha-hydrogen). [Per the definition of Aldol Condensation requirements].

Step 2: Determining Reaction Classification

  • Aldol Condensation: Requires the presence of at least one $\alpha$-hydrogen atom in an aldehyde or ketone to facilitate the formation of an enolate ion. Since benzaldehyde lacks $\alpha$-hydrogen atoms, it cannot undergo aldol condensation.
  • Cannizzaro Reaction: Applies to aldehydes that lack $\alpha$-hydrogen atoms. In the presence of a concentrated base, these undergo self-oxidation and reduction (disproportionation). Since benzaldehyde satisfies this condition, it undergoes the Cannizzaro reaction.

Step 3: Mechanism and Products of the Cannizzaro Reaction

When benzaldehyde ($2C_6H_5CHO$) reacts with concentrated alkali (e.g., $NaOH$ or $KOH$), one molecule is reduced to an alcohol (benzyl alcohol) and the other is oxidized to a carboxylic acid salt (sodium benzoate).

Chemical Equation:

$2C_6H_5CHO + NaOH_{(conc.)} \xrightarrow{\Delta} C_6H_5CH_2OH + C_6H_5COONa$

Reactant Product 1 (Reduction) Product 2 (Oxidation)
Benzaldehyde Benzyl Alcohol ($C_6H_5CH_2OH$) Sodium Benzoate ($C_6H_5COONa$)

Step 4: Structural Representation of Products

  • Benzyl Alcohol: The formyl group is reduced to a primary alcohol group ($-CH_2OH$).
  • Sodium Benzoate: The formyl group is oxidized to a carboxylate salt group ($-COO^-Na^+$).

Summary Classification:

  • Aldol Condensation: Neither (due to absence of $\alpha$-hydrogens).
  • Cannizzaro Reaction: Undergoes reaction.

Final Solution: Benzaldehyde undergoes the Cannizzaro reaction because it lacks $\alpha$-hydrogen atoms. The products formed are Benzyl Alcohol ($C_6H_5CH_2OH$) and Sodium Benzoate ($C_6H_5COONa$).


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