11.2 Aldehydes & Ketones: Nucleophilic Addition Reactions

Nomenclature and Physical Properties

Aldehydes carry a carbonyl (C=O) bonded to at least one hydrogen, named with the suffix -al (propanal) or, when the carbonyl carbon cannot be the parent chain's terminus, the suffix -carbaldehyde. Ketones carry a carbonyl bonded to two carbon substituents, named with the suffix -one and a locant for the carbonyl's position (butan-2-one).

Both groups have a strongly polarized C=O bond — carbon partial positive, oxygen partial negative — giving dipole-dipole intermolecular forces stronger than those in alkanes or ethers of similar mass. Aldehydes and ketones therefore boil higher than comparably sized alkanes but lower than alcohols of similar mass: a carbonyl oxygen can accept a hydrogen bond, but lacking an O–H, the molecule cannot donate one to another carbonyl compound. Small aldehydes and ketones (acetaldehyde, acetone) are water-soluble because they hydrogen-bond as acceptors with water's O–H.

Nucleophilic Addition at the C=O Bond

The carbonyl carbon is electrophilic and sp2-hybridized. A nucleophile attacks it from a trajectory roughly perpendicular to the C=O plane, pushing the pi electrons onto oxygen and forming a tetrahedral alkoxide or alcohol intermediate. This single mechanistic pattern — nucleophilic addition — underlies every reaction below.

Hemiacetals and Acetals

One equivalent of alcohol adds to an aldehyde or ketone, acid- or base-catalyzed, to give a hemiacetal (one OR group and one OH group on the former carbonyl carbon). Under acidic conditions with excess alcohol, the hemiacetal loses water through an oxocarbenium ion and picks up a second alcohol to form an acetal (two OR groups). Acetal formation is reversible and acid-catalyzed in both directions. Acetals are stable to base, which is exactly why chemists use them to protect a ketone or aldehyde from base-sensitive reagents elsewhere in a molecule, removing the protecting group later with aqueous acid.

Imines and Enamines

A primary amine adds to a carbonyl to give a hemiaminal, which loses water to form a carbon-nitrogen double bond — an imine (Schiff base). A secondary amine cannot form a stable neutral imine, because it has no second N–H left to lose alongside the departing water. Instead, the intermediate loses a proton from the alpha carbon to form an enamine (a C=C conjugated with nitrogen's lone pair). Both reactions run fastest around mildly acidic pH (roughly 4–5): acid is needed to protonate the hydroxyl leaving group during dehydration, but too much acid protonates the amine nucleophile itself and shuts the reaction down — a classic bell-shaped pH-rate profile.

Hydride Reagents

Sodium borohydride (NaBH4) is a mild hydride source that reduces aldehydes to primary alcohols and ketones to secondary alcohols but is generally too weak to touch esters or carboxylic acids. Lithium aluminum hydride (LiAlH4) is a stronger, less selective hydride source that reduces aldehydes, ketones, esters, and carboxylic acids all the way to alcohols. If a passage describes reducing an ester or acid to an alcohol, the reagent must be LiAlH4 or something comparably strong — NaBH4 alone will not accomplish it.

Cyanohydrin Formation

Cyanide ion, from HCN or NaCN/KCN, adds to a carbonyl to give a cyanohydrin: a new stereocenter bearing both –OH and –C≡N on the same carbon. If the starting carbonyl is planar and achiral, the nucleophile can attack either face with equal probability, giving a racemic mixture of the cyanohydrin product.

Oxidation of Aldehydes

Aldehydes have a hydrogen directly on the carbonyl carbon, making them easy to oxidize one step further to a carboxylic acid; ketones have no such hydrogen and strongly resist oxidation under the same mild conditions. This reactivity gap underlies two classic diagnostic tests: Tollens' test, in which Ag(NH3)2+ is reduced to metallic silver, depositing a silver mirror, and Fehling's/Benedict's test, in which Cu2+ is reduced to brick-red Cu2O. A positive result on either test identifies an aldehyde, or a sugar capable of presenting one; simple ketones are negative unless they can tautomerize into an aldehyde-bearing form.

Enolate Chemistry: Reactions at the Adjacent (Alpha) Position

Keto-Enol Tautomerism and Alpha-Racemization

A hydrogen on the carbon adjacent to a carbonyl — the alpha carbon — is acidic enough to be removed by acid or base catalysis, generating an enol tautomer, or its conjugate base enolate, that can reprotonate on either face of the now-planar alpha carbon. If the alpha carbon was originally a stereocenter, this tautomerization racemizes it, a mechanistic fact tested directly on the MCAT.

Aldol Condensation and Retro-Aldol

An enolate generated from one carbonyl compound can act as a nucleophile and attack the electrophilic carbonyl carbon of a second molecule, forming a new C–C bond and a beta-hydroxy carbonyl — the aldol addition product. Under thermodynamic conditions, typically heat, that beta-hydroxy carbonyl undergoes E1cb-type dehydration, in which loss of the acidic alpha-hydrogen facilitates loss of the beta-hydroxide, giving a conjugated alpha,beta-unsaturated carbonyl (an enone) — the full aldol condensation. The retro-aldol is this sequence run in reverse, cleaving a C–C bond beta to a carbonyl to regenerate two smaller carbonyl fragments; this is exactly the reaction fructose-1,6-bisphosphate undergoes in glycolysis, catalyzed by the enzyme aldolase.

Kinetic vs. Thermodynamic Enolate

An unsymmetrical ketone has two different sets of alpha-hydrogens and can form two different enolates. A bulky, strong, non-nucleophilic base such as LDA, used at low temperature in an aprotic solvent, removes the more accessible, less hindered alpha-hydrogen fastest, favoring the less-substituted kinetic enolate. A weaker base under equilibrating conditions — for example, sodium ethoxide in ethanol, with heat and time — favors the more stable, more substituted thermodynamic enolate, whose C=C carries more alkyl substitution.

Effect of Substituents on Carbonyl Reactivity

Electron-withdrawing substituents near the carbonyl carbon make it more electrophilic and speed up nucleophilic addition; electron-donating alkyl groups do the opposite and also add steric bulk that physically blocks the nucleophile's approach. Both effects point the same direction down the series formaldehyde > other aldehydes > ketones in nucleophilic-addition reactivity: formaldehyde has two small, non-donating hydrogens on its carbonyl carbon; other aldehydes have one small hydrogen and one alkyl group; ketones have two alkyl groups, both donating electron density into the carbonyl and shielding it sterically. Bulkier ketones react even more slowly than small ones like acetone.

Acidity of the Alpha-Hydrogen

An alpha-hydrogen on a simple aldehyde or ketone has a pKa around 19–20 — far more acidic than a typical alkane C–H (pKa about 50) but much less acidic than water or an alcohol. The reason is resonance: removing that proton generates a carbanion that delocalizes onto the electronegative carbonyl oxygen, forming the enolate and spreading out the negative charge. Flanking the alpha carbon with a second electron-withdrawing carbonyl increases acidity dramatically further — a beta-ketoester alpha-hydrogen (pKa around 11) or a malonic ester alpha-hydrogen (pKa around 13) is acidic enough to be fully deprotonated by common alkoxide bases, which is why these doubly activated positions are the workhorses of classic carbanion alkylation chemistry.

Aldehyde vs ketone comparison: