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Why does a Grignard reagent make a primary alcohol with formaldehyde?

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oxidation of alcohols — aldehyde Alcohols may be oxidized to give aldehydes, ketones, and carboxylic acids. The oxidation of organic compounds generally increases the number of bonds from carbon to oxygen, and it may decrease the number of bonds to hydrogen.

A Grignard reagent reacts with formaldehyde to produce a primary alcohol because of the specific structure of the formaldehyde molecule.

In all aldehydes other than formaldehyde, the carbonyl group is bonded to one hydrogen atom and one carbon-containing group. However, in formaldehyde, the carbonyl carbon is bonded to two hydrogen atoms. When a Grignard reagent (RMgX) acts as a nucleophile and adds to the carbonyl carbon of formaldehyde, the resulting intermediate—after acidification—retains those two original hydrogen atoms attached to the carbon bearing the hydroxyl group.123

Because the carbon atom bonded to the hydroxyl group is attached to only one alkyl group (the one provided by the Grignard reagent) and two hydrogens, the product is classified as a primary alcohol. In contrast, adding a Grignard reagent to other aldehydes results in a secondary alcohol, as those aldehydes already possess an additional carbon group attached to the carbonyl carbon.432