aldehyde
Summary
An aldehyde is an organic compound whose carbonyl group (C=O) carbon is bonded to one hydrogen and one R group (RCHO), at the end of a chain. Its electrophilic carbonyl carbon is usually more reactive toward nucleophiles than a ketone's, and it is easily oxidized to a carboxylic acid.
Detail
Meaning: An aldehyde has a carbonyl group (C=O) whose carbon is bonded to one hydrogen and one R group (RCHO), so it sits at the end of a chain. Oxygen's electronegativity makes the carbonyl carbon electrophilic, so nucleophiles attack it. Aldehydes are usually more reactive than ketones because they have less steric hindrance and only one electron-donating alkyl group. Names end in -al (ethanal).
On the MCAT: Expect product prediction. PCC oxidizes a primary alcohol and stops at the aldehyde; stronger oxidants (Jones reagent, KMnO₄) go on to the carboxylic acid. NaBH₄ or LiAlH₄ reduces an aldehyde to a primary alcohol. Alcohols add to give hemiacetals and acetals, amines give imines, and Grignard reagents give secondary alcohols (a primary alcohol with formaldehyde). Tollens' reagent (silver mirror) detects aldehydes. In IR, C=O absorbs near 1700 cm⁻¹; in ¹H NMR the aldehyde proton appears near 9–10 ppm.
Common confusion: Aldehydes and ketones both contain C=O. Look at the carbonyl carbon: an attached H means aldehyde, while two carbon groups means ketone. Because of that H, aldehydes oxidize easily to acids, while ketones resist mild oxidation.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.