Which of the following best describes the major product formed when $3-\mathrm{methylbutanal}$ undergoes aldol condensation under basic conditions?

• A. A $\beta$-hydroxy aldehyde formed by the addition of two molecules of $3-\mathrm{methylbutanal}$ without subsequent dehydration
• B. A carboxylic acid formed by oxidation of $3-\mathrm{methylbutanal}$
• C. A cyclic hemiacetal formed by intramolecular reaction of $3-\mathrm{methylbutanal}$
• D. A $\beta ,\alpha$-unsaturated aldehyde formed by the condensation of two molecules of $3-\mathrm{methylbutanal}$, specifically $2,4-\mathrm{dimethyl}-2-\mathrm{pentenal}$

Answer: (D)

Verified step by step guidance

1. Identify the structure of 3-methylbutanal and recognize that it contains an aldehyde functional group with alpha hydrogens adjacent to the carbonyl carbon, which are acidic under basic conditions.
2. Under basic conditions, the alpha hydrogen is abstracted to form an enolate ion, which acts as a nucleophile and attacks the carbonyl carbon of another molecule of 3-methylbutanal, leading to the formation of a β-hydroxy aldehyde (aldol addition product).
3. Recognize that under typical aldol condensation conditions (heat and base), the β-hydroxy aldehyde undergoes dehydration (loss of water) to form an α,β-unsaturated aldehyde, which is more stable due to conjugation.
4. Understand that the major product is not the simple β-hydroxy aldehyde (no dehydration), nor a carboxylic acid (which would require oxidation), nor a cyclic hemiacetal (which involves intramolecular nucleophilic attack on the aldehyde by an alcohol group, not present here).
5. Conclude that the major product is the α,β-unsaturated aldehyde formed by condensation of two molecules of 3-methylbutanal, specifically 2,4-dimethyl-2-pentenal, which results from aldol condensation followed by dehydration.