Grignard reaction with formaldehyde followed by workup yields what product?

• A. Secondary alcohol
• B. Primary alcohol
• C. Aldehyde
• D. Carboxylic acid

Answer: (B)

Grignard reagents are strong nucleophiles that add to carbonyl compounds, turning the C=O into an O–MgX alkoxide after the attack. When the aldehyde is formaldehyde, its carbonyl carbon has only hydrogens besides the carbonyl oxygen. After the Grignard adds, that carbon becomes attached to the R group from the Grignard and to one hydrogen, giving an alkoxide of the form R–CH2–O–MgX. A subsequent acidic workup protonates the oxygen to give the primary alcohol R–CH2OH. The key point is that formaldehyde has no carbon substituent of its own, so the product ends up being a primary alcohol after hydrolysis.