Identification of Organic Compound S

To determine the structure of Compound S, we analyze the reaction sequence step-by-step:

Step 1: Formation of Compound R

The passage describes the reaction of isobutyraldehyde (2-methylpropanal) with formaldehyde ($\text{HCHO}$) in the presence of a base ($\text{OH}^-$).

Isobutyraldehyde has only one $\alpha$-hydrogen:
$\text{(CH}_3)_2\text{CH-CHO}$

When treated with aqueous base and formaldehyde, an aldol condensation (specifically, a cross-aldol addition) takes place at the single $\alpha$-carbon:
$\text{(CH}_3)_2\text{CH-CHO} + \text{HCHO} \xrightarrow{\text{OH}^-} \text{(CH}_3)_2\text{C(CH}_2\text{OH)CHO}$

Thus, Compound R is:
$\text{R} = \text{(CH}_3)_2\text{C(CH}_2\text{OH)CHO}$

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Step 2: Formation of Compound S

Compound R contains a reactive aldehyde group ($\text{-CHO}$). When treated with hydrogen cyanide ($\text{HCN}$), nucleophilic addition of the cyanide ion ($\text{CN}^-$) occurs at the carbonyl carbon to form a cyanohydrin:

$\text{(CH}_3)_2\text{C(CH}_2\text{OH)CHO} + \text{HCN} \longrightarrow \text{(CH}_3)_2\text{C(CH}_2\text{OH)CH(OH)CN}$

Thus, Compound S is the cyanohydrin derivative:
$\text{S} = \text{(CH}_3)_2\text{C(CH}_2\text{OH)CH(OH)CN}$

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Step 3: Identification of the Structure in Options

• Option D represents a central quaternary carbon bonded to:
  1. Two methyl groups: $\text{-CH}_3$ and $\text{-CH}_3$
  2. A hydroxymethyl group: $\text{-CH}_2\text{OH}$
  3. A cyanohydrin group: $\text{-CH(OH)CN}$

This structure matches Compound S.

Correct Answer: D