keto
Summary
Keto names the carbonyl (C=O) form of a compound, most often the keto tautomer that interconverts with an enol (C=C–OH) by moving a proton and shifting a double bond. For simple aldehydes and ketones the keto form is usually strongly favored at equilibrium, though some structures are exceptions.
Detail
Meaning: In keto-enol tautomerism, the keto form has a C=O and an α-carbon bearing at least one hydrogen. The enol form has a C=C with an OH on one carbon. Tautomers are constitutional isomers that differ in the position of a proton and a double bond. They interconvert quickly, and acid or base catalyzes the change. The keto form is usually favored because a C=O bond plus a C–H bond is more stable than a C=C bond plus an O–H bond. Enol content rises with conjugation and intramolecular hydrogen bonding (as in 1,3-dicarbonyls) and with aromaticity (phenol is mostly enol). "Keto" is also a naming prefix, as in β-keto acid, for a ketone group on a molecule whose principal group is higher priority.
Equation: keto ⇌ enol, with Keq = [enol]/[keto] (dimensionless). Keq is very small for simple ketones.
On the MCAT: Expect questions that ask you to draw the enol from a ketone, or to predict which tautomer is favored. The same enol or enolate intermediate explains α-hydrogen acidity, aldol reactions, racemization of a chiral α-carbon, and heating a β-keto acid to lose CO₂.
Common confusion: Tautomers are not resonance structures. Tautomers are different compounds because a hydrogen has moved, while resonance forms differ only in electron placement. A carbonyl with no α-hydrogen, such as benzophenone, cannot form an enol.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.