carbonyl
Summary
A carbonyl is a carbon double-bonded to oxygen (C=O). Oxygen's electronegativity makes the carbon partially positive and electrophilic, so nucleophiles attack it. Most testable: aldehydes, ketones, acids and their derivatives contain it, and an IR peak near 1700 cm⁻¹ signals it.
Detail
Meaning: A carbonyl is a carbon double-bonded to oxygen (C=O). The carbon is sp² hybridized and trigonal planar, with bond angles near 120°. Oxygen is more electronegative, so the bond is polar: carbon is partially positive (electrophilic) and oxygen partially negative. Carbonyls occur in aldehydes, ketones, carboxylic acids, esters, amides, anhydrides and acyl chlorides. Alpha hydrogens are weakly acidic because the enolate is resonance stabilized.
On the MCAT: Nucleophiles attack the carbonyl carbon, the π bond breaks, and a tetrahedral intermediate forms. Aldehydes and ketones then typically undergo addition; acid derivatives, which carry a leaving group, undergo nucleophilic acyl substitution. Aldehydes are generally more reactive than ketones (less steric hindrance, less electron donation to carbon). In IR spectra, look for a strong peak near 1700 cm⁻¹; in ¹³C NMR, the carbonyl carbon appears far downfield, about 160–220 ppm.
Common confusion: Carbonyl is not carboxyl. A carbonyl is just C=O; a carboxyl group is a carbonyl with an OH on the same carbon (COOH). Aldehydes and ketones are specific carbonyl compounds: the carbonyl carbon bears only carbon or hydrogen substituents, with no heteroatom attached.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (Clayden, Greeves, Warren)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.