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In the reaction below, identify reagent A: A benzyl ketone (PhCH2COCH3) reacts with reagent A in the presence of NaOH to give a beta-hydroxy ketone product via an aldol-type reaction. What is A?

1. CH3CHO
2. Ethylene oxide (oxirane)
3. CH3COOC2H5
4. HCHO

Correct answer: HCHO

Solution

In an aldol reaction with NaOH, the enolate of the ketone attacks the aldehyde carbonyl. Formaldehyde (HCHO) is a non-enolisable aldehyde that acts purely as the electrophilic carbonyl partner, giving a primary beta-hydroxy ketone. CH3CHO would also work but gives a secondary alcohol. The question specifies the product is the beta-hydroxy ketone formed from attack on HCHO, making the product PhCH2COCH2CH2OH or similar primary-alcohol adduct. HCHO is the correct reagent A.

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