Final answer:
The reaction of formaldehyde with one equivalent of a Grignard reagent yields a secondary alcohol, as the Grignard reagent adds an alkyl group to the aldehyde's carbonyl carbon.
Step-by-step explanation:
When formaldehyde reacts with one equivalent of a Grignard reagent, the product formed is a secondary alcohol. The Grignard reagent adds to the carbonyl carbon of formaldehyde, followed by protonation to yield the alcohol when the reaction is quenched with acid. It's important to note that formaldehyde, being the simplest aldehyde, will form a secondary alcohol even though it has no alkyl groups on its own because the Grignard reagent provides the alkyl part.
By contrast, if an aldehyde other than formaldehyde (which has additional alkyl groups) reacts with a Grignard reagent, the typical product would be a tertiary alcohol. However, in the case of formaldehyde, which lacks alkyl groups attached to its carbonyl carbon, a secondary alcohol is formed instead.