formaldehyde

Meaning: Formaldehyde, H₂C=O (IUPAC name methanal), is the simplest aldehyde. Its carbonyl carbon is sp² and trigonal planar, with bond angles near 120°. Because that carbon carries two hydrogens and no alkyl group, nothing donates electron density to it or blocks a nucleophile, so it is a more reactive electrophile than larger aldehydes. Aldehydes in general are more reactive than ketones. In water it is mostly hydrated to the gem-diol H₂C(OH)₂. It has no α-carbon, so it cannot form an enolate.

On the MCAT: Expect nucleophilic addition. A Grignard reagent RMgX adds to formaldehyde and, after acid workup, gives a primary alcohol with one more carbon than R; other aldehydes give secondary alcohols and ketones give tertiary alcohols. Reduction gives methanol, and oxidation gives formic acid, HCOOH. In a crossed aldol reaction, formaldehyde can act as the electrophile but not as the enolate donor. A passage may also mention formalin, its aqueous solution, as a preservative that cross-links proteins.

Common confusion: Students mix up formaldehyde, formic acid and formalin, or assume that having no alkyl groups makes it less reactive. Formic acid is its oxidation product, formalin is its solution in water, and missing alkyl groups raise electrophilicity because alkyl groups are electron donating.