Question

The major product formed in the following reaction is

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is A

$\text{1. The Reaction Type: Intramolecular Cannizzaro}$

Reactant: Phthalaldehyde ($\text{Benzene}-1,2-\text{dicarbaldehyde}$) contains two aldehyde ($\text{CHO}$) groups. Since there are no $\text{H}$ atoms on the carbon adjacent to the carbonyl groups ($\text{C}1$ and $\text{C}2$ of the benzene ring), it has no $\alpha$-hydrogens.

Reagent: Concentrated $\text{NaOH}$ (a strong base).

Result: A Cannizzaro reaction occurs. Since both reacting aldehyde groups belong to the same molecule, it is an intramolecular Cannizzaro reaction.

$\text{2. Cannizzaro Mechanism}$

In the Cannizzaro reaction, two molecules of the aldehyde (or two aldehyde groups in an intramolecular reaction) disproportionate: one aldehyde group is oxidized to a carboxylic acid, and the other is reduced to an alcohol.

The mechanism proceeds as follows:

Nucleophilic Attack: The hydroxide ion ($\text{OH}^-$) attacks one of the carbonyl carbons ($\text{C}1$), forming a tetrahedral intermediate.

Hydride Transfer: A hydride ion ($\text{H}^-$) is transferred from the anionic form of the first aldehyde group ($\text{C}1$) to the carbonyl carbon of the second aldehyde group ($\text{C}2$).

Product Formation:

The group that loses the hydride ($\text{C}1$) is oxidized to the carboxylate ion ($\text{COO}^-$).

The group that accepts the hydride ($\text{C}2$) is reduced to an alkoxide ion, which upon protonation (from the aqueous solvent) becomes a primary alcohol ($\text{CH}_2\text{OH}$).

$\text{3. Final Product}$

The final product before workup is the salt: Sodium 2-hydroxymethylbenzoate.

$$\text{Phthalaldehyde} \xrightarrow[\text{reflux}]{\text{conc. } \text{NaOH}(\text{aq})} \text{Sodium 2-hydroxymethylbenzoate}$$

This structure matches Option 1: $\text{Sodium}$ salt of 2-hydroxymethylbenzoic acid (where the $\text{OH}$ group is the alcohol and the $\text{COO}^-\text{Na}^+$ group is the carboxylate).

$$\text{The major product formed is } \mathbf{\text{Option 1}}$$