Enols, Enolates and Alpha-Carbon Chemistry: every key term you need (+ practice quiz)
25 flashcard terms for Organic Chemistry II Topic 5, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
The carbon next to a carbonyl. Its hydrogens are unusually acidic because the resulting anion delocalizes onto the carbonyl oxygen.
Enol
The alkene alcohol tautomer of a carbonyl compound. It is usually a minor species at equilibrium but is the reactive form in acid-catalysed alpha substitution.
Keto-enol tautomerism
A rapid equilibrium in which a proton moves between the alpha carbon and the carbonyl oxygen. It is catalysed by either acid or base and is not a resonance relationship.
Enolate
The delocalized anion formed by removing an alpha proton. Charge sits on both the alpha carbon and the oxygen, so it can react at either site.
Kinetic enolate
Formed by deprotonating the less hindered side quickly with a bulky strong base at low temperature, with no chance to equilibrate.
Thermodynamic enolate
Formed under reversible conditions with a weaker base, favouring the more substituted and therefore more stable alkene.
Lithium diisopropylamide
A hindered, very strong base that deprotonates alpha positions completely without adding to the carbonyl, making it the standard kinetic enolate reagent.
Alpha halogenation under acid
Proceeds through the enol and stops cleanly after one substitution because the added halogen slows further enolization.
Alpha halogenation under base
Runs through the enolate and tends to over-react, because each halogen added makes the remaining alpha protons more acidic.
Haloform reaction
Exhaustive halogenation of a methyl ketone followed by cleavage, producing a carboxylate and a trihalomethane.
Alkylation of an enolate
The enolate carbon attacks an alkyl halide. It works well only for unhindered primary halides, since hindered ones eliminate instead.
Acetoacetic ester synthesis
Uses a beta-keto ester to build substituted ketones, ending in hydrolysis and loss of carbon dioxide from the beta-keto acid.
Malonic ester synthesis
The same strategy applied to a diester, giving substituted carboxylic acids after hydrolysis and loss of carbon dioxide.
Aldol addition
One enolate attacks the carbonyl of another molecule, giving a beta-hydroxy carbonyl compound and forming a new carbon-carbon bond.
Aldol condensation
The dehydration that follows aldol addition on warming, giving a conjugated unsaturated carbonyl that is more stable than the starting material.
Crossed aldol problem
Mixing two enolizable carbonyls gives four products. The fix is to use one partner with no alpha hydrogen or to form one enolate completely first.
Intramolecular aldol
Closes a ring when a molecule holds two carbonyls at the right spacing, again favouring five- and six-membered products.
Claisen condensation
Two esters combine through an enolate to give a beta-keto ester, with an alkoxide expelled rather than a hydroxide.
Dieckmann cyclization
The intramolecular version of the ester condensation, forming a cyclic beta-keto ester from a diester.
Michael addition
A stabilized enolate adds to the carbon beta to a conjugated carbonyl, forming a carbon-carbon bond away from the carbonyl itself.
Robinson annulation
Combines conjugate addition with an intramolecular condensation to build a new six-membered ring fused to the original system.
Beta-dicarbonyl acidity
A proton flanked by two carbonyls is far more acidic than an ordinary alpha proton because the anion is delocalized over both.
Enamine as an enolate equivalent
A neutral nucleophile that alkylates or acylates at the alpha carbon and is hydrolysed back to the carbonyl afterward, avoiding strongly basic conditions.
Mannich reaction
Joins an enol, formaldehyde and an amine into an amino-substituted carbonyl compound, a common route into alkaloid skeletons.
Alpha racemization
A stereocentre at the alpha carbon loses its configuration whenever an enol or enolate forms, because that carbon becomes planar.