formyl group
Summary
A formyl group is –CHO, a carbonyl carbon bonded to one hydrogen; it is the functional group of an aldehyde. Testable point: it is easily oxidized to a carboxylic acid and reduced to a primary alcohol, and it is named with the prefix formyl when a higher-priority group is present.
Detail
Meaning: A formyl group is –CHO: a carbonyl carbon double-bonded to oxygen and single-bonded to one hydrogen and one other atom (a carbon, or a second H in formaldehyde). It is the functional group of an aldehyde. With only one carbon substituent, it is less hindered and receives less electron donation than a ketone carbonyl, so it is generally more electrophilic toward nucleophilic addition. Its C–H bond also lets it oxidize readily to a carboxylic acid.
On the MCAT: Naming: an aldehyde takes the suffix -al and its carbon is C1. When a higher-priority group such as a carboxylic acid is present, the –CHO is named with the prefix formyl.
Reactions: NaBH₄ or LiAlH₄ reduces it to a primary alcohol. PCC stops the oxidation of a primary alcohol at the aldehyde, while stronger oxidants go on to the acid. Tollens' reagent oxidizes it, leaving a silver mirror.
Spectra: IR shows a C=O stretch near 1720 cm⁻¹ plus weak C–H stretches near 2720 and 2820 cm⁻¹. ¹H NMR shows a signal near 9–10 ppm.
Common confusion: Students mix up –CHO with –COOH or with a ketone. Formyl has an H on the carbonyl carbon, a carboxyl group has an OH there, and a ketone carbonyl has two carbon substituents. Formyl is also not hydroxymethyl (–CH₂OH), which is its reduced form.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.