Which of the following compounds does not have an $\alpha$ hydrogen?

• A. ${\mathrm{CH}}_3\mathrm{CHO}$ (acetaldehyde)
• B. ${\mathrm{CH}}_3\mathrm{CO}{\mathrm{CH}}_3$ (acetone)
• C. ${\mathrm{CH}}_3{\mathrm{CO}}_{2}H$ (acetic acid)
• D. $H_2\mathrm{CO}$ (formaldehyde)

Answer: (D)

Verified step by step guidance

1. Understand that an \( \alpha \) hydrogen is a hydrogen atom attached to the carbon atom directly adjacent (\( \alpha \)-position) to a carbonyl group (C=O).
2. Identify the carbonyl group in each compound and locate the carbon atom next to it (the \( \alpha \)-carbon).
3. Check if the \( \alpha \)-carbon has any hydrogen atoms attached. If it does, the compound has \( \alpha \) hydrogens; if not, it does not.
4. For acetaldehyde (\( \mathrm{CH_3CHO} \)), the carbon next to the carbonyl is the methyl group (\( \mathrm{CH_3} \)), which has hydrogens, so it has \( \alpha \) hydrogens.
5. For formaldehyde (\( \mathrm{H_2CO} \)), the carbonyl carbon is bonded only to hydrogens and has no adjacent carbon, so there is no \( \alpha \)-carbon and thus no \( \alpha \) hydrogens.