CHEM20018 · Chemistry: Reactions And Synthesis
The Carbonyl & Enolate Foundation
This is the bedrock of the entire organic section. A carbonyl group C=O is polarised so the carbon is electrophilic (δ+) and the oxygen nucleophilic (δ−); equally important, the hydrogen on the adjacent α-carbon is weakly acidic (pKa ≈ 20 for ketones/aldehydes, ≈ 25 for esters) because removing it gives a resonance-stabilised enolate. Master keto–enol tautomerism, enolate formation, and the difference between the kinetic and thermodynamic enolate, and every aldol/Claisen/Michael/Mannich reaction downstream becomes a variation on one idea: make a carbon nucleophile, then react it with a carbon electrophile.
What this chapter covers
- 01Polar C=O: nucleophiles attack the δ+ carbon, electrophiles meet the δ− oxygen
- 02α-hydrogen acidity and why the enolate is resonance-stabilised (pKa ≈ 20 vs ≈ 25 for esters)
- 03Keto–enol tautomerism: keto form usually dominates, enol/enolate is the reactive species
- 04Kinetic enolate (LDA, low T, less-substituted) vs thermodynamic enolate (weak base, equilibrium, more-substituted)
- 05Drawing all enol tautomers of a polycarbonyl (e.g. 1,3-diketones favour the conjugated, H-bonded central enol)
- 06Hell–Volhard–Zelinsky (HVZ): RCOOH + Br₂ / cat. PBr₃ → α-bromo acid
- 07Enolate α-alkylation: mono- and double alkylation, intramolecular ring closure, nitrile α-anions
Enol tautomers of a 1,3-diketone + a kinetic α-alkylation
- 2 marks — identify the central C3 position as special(a) Hexane-2,4-dione has two non-equivalent α-CH positions next to a single carbonyl (the terminal CH₃ of the C2 ketone and the CH₃CH₂ end), plus the central CH₂ that sits between BOTH carbonyls (C3). Enolisation at C3 gives a conjugated enol whose O–H can hydrogen-bond to the second carbonyl oxygen.
- 1 mark — correct ranking with justificationThe central 1,3-enol is the major tautomer: it is conjugated (C=C–C=O) and stabilised by an intramolecular six-membered hydrogen bond, which raises its equilibrium population far above the terminal mono-enols.
- 1 mark — recognise LDA/−78 °C = kinetic control(b)(i) LDA is a bulky, very strong, non-nucleophilic base used at low temperature, so deprotonation is irreversible and occurs at the LESS hindered α-carbon → the kinetic enolate of cyclohexanone.
- 1 mark — enolate alkylation step(b)(ii) The kinetic enolate is a carbon nucleophile; it performs an SN2 displacement on iodoethane (a good 1° electrophile), forming a new C–C bond at the α-carbon.
Key terms
- α-carbon
- The carbon directly bonded to a carbonyl carbon; its hydrogens are the acidic α-hydrogens that can be removed to form an enolate.
- Enol
- The tautomer with a C=C–OH unit; in equilibrium with the keto form, it is the neutral nucleophilic species that reacts at carbon.
- Kinetic enolate
- The enolate formed fastest under irreversible conditions (bulky strong base such as LDA, low temperature); it is the less-substituted enolate.
- Thermodynamic enolate
- The more stable, more-substituted enolate that predominates at equilibrium under weaker-base, reversible conditions.
- Hell–Volhard–Zelinsky reaction
- Bromination of the α-carbon of a carboxylic acid using Br₂ with catalytic PBr₃, proceeding through the acyl bromide enol to give an α-bromo acid.
The Carbonyl & Enolate Foundation FAQ
Why is an α-hydrogen acidic but an ordinary C–H is not?
Removing the α-H gives an enolate whose negative charge is delocalised onto the electronegative carbonyl oxygen by resonance. That stabilisation lowers the pKa to about 20 (ketones/aldehydes) or 25 (esters), millions of times more acidic than a typical alkane C–H at pKa ≈ 50.
How do I decide kinetic vs thermodynamic enolate in an exam?
Read the base and temperature. Bulky, strong, non-nucleophilic base (LDA) at low temperature → kinetic, less-substituted enolate. Weaker base (NaOEt, NaOH) at equilibrium / higher temperature → thermodynamic, more-substituted enolate. Then alkylate or condense at that position.
Is the keto or the enol form usually present in larger amount?
For simple ketones and aldehydes the keto form dominates massively at equilibrium. The exception worth remembering is 1,3-dicarbonyls, where conjugation plus an intramolecular hydrogen bond can make the enol a major or even dominant species.
Exam move
Lock this chapter down first because everything in Section A depends on it. Make a one-page mental picture: C=O polarisation, α-H acidity, the enolate resonance pair. Then drill two reflexes until they are automatic — (1) given a base and temperature, state kinetic vs thermodynamic enolate and which carbon deprotonates; (2) given a carbonyl, draw every distinct enol and rank them (watch for 1,3-dicarbonyls). Practise HVZ and simple α-alkylations forward and backward. If you can confidently form the right enolate and recognise it as a carbon nucleophile, the aldol, Claisen, malonate and Michael chapters are just different electrophiles bolted onto the same nucleophile.
