Nucleophilic Addition to Aldehydes and Ketones

A representative example — structures drawn live.

The mechanism, step by step

Why the carbonyl carbon is electrophilic

The carbon–oxygen double bond is the defining feature of aldehydes and ketones, and it is strongly polarized. Oxygen is far more electronegative than carbon, so it pulls the shared electrons toward itself: the oxygen carries a partial negative charge (δ−) and the carbon carries a partial positive charge (δ+). A useful resonance picture puts a full negative charge on oxygen and a full positive charge on carbon, emphasizing that the carbonyl carbon is an electrophilic site hungry for electrons.

Just as important, the carbonyl carbon is sp² hybridized and trigonal planar, leaving both faces open. A nucleophile can approach from above or below the plane along the so-called Bürgi–Dunitz angle, with little to block it. This combination — a partial-positive carbon that is also sterically accessible — is what makes nucleophilic addition the signature reaction of aldehydes and ketones.

The general addition mechanism

Nucleophilic addition converts the flat, sp² carbonyl into a tetrahedral, sp³ center. The core sequence is short:

1. Nucleophile attacks the carbonyl carbon. The nucleophile donates a lone pair to the δ+ carbon. To make room, the C=O π electrons are pushed up onto oxygen.
2. An alkoxide forms. Oxygen now carries a negative charge (an alkoxide, R₂C–O⁻) and the carbon is sp³ with four groups.
3. Protonation. The alkoxide picks up H⁺ — from water, from acidic workup, or from solvent — to give a neutral alcohol or related product.

Under acidic conditions the order flips slightly: the oxygen is protonated first, which makes the carbon even more electrophilic, and then a weaker neutral nucleophile attacks. Either way, the mechanism runs through a tetrahedral intermediate. Because aldehydes and ketones have no leaving group on the carbonyl carbon, that tetrahedral species keeps the added nucleophile — the reaction is addition, not substitution.

Aldehydes react faster than ketones

Given the same nucleophile, aldehydes are more reactive than ketones, for two reinforcing reasons:

• Sterics. An aldehyde carbonyl carries one H and one R group; a ketone carries two R groups. The bulkier ketone crowds the incoming nucleophile and crowds the developing tetrahedral center, raising the barrier.
• Electronics. Alkyl groups are weakly electron-donating. A ketone's two alkyl groups spread more electron density onto the carbonyl carbon, partly quenching its δ+ character. An aldehyde, with only one alkyl group, leaves the carbon more electrophilic.

The same logic explains why formaldehyde (two hydrogens) is the most reactive carbonyl of all, and why very hindered ketones can be sluggish or unreactive toward bulky nucleophiles.

The key nucleophiles and their products

The reactions differ only in which nucleophile attacks; the mechanism above stays the same:

• Hydride (NaBH₄ or LiAlH₄). Delivers H⁻ to the carbon; after protonation you get an alcohol. NaBH₄ is mild and selective for aldehydes/ketones; LiAlH₄ is far stronger and also reduces esters, acids, and amides.
• Grignard (RMgX) or organolithium (RLi). The carbanion-like carbon attacks, forming a new C–C bond and, after workup, an alcohol. These reagents build the carbon skeleton and are the workhorses of carbonyl chemistry.
• Water. Adds to give a hydrate (a gem-diol). The equilibrium favors the hydrate only for reactive carbonyls such as formaldehyde or chloral.
• Alcohols. One equivalent gives a hemiacetal; with acid catalysis and a second equivalent you reach the acetal, a common protecting group for carbonyls.
• Cyanide (HCN / ⁻CN). Adds to form a cyanohydrin, installing a nitrile next to a new hydroxyl and adding one carbon to the chain.

Reversibility and driving the reaction

Some of these additions are effectively irreversible (hydride and organometallic additions form strong, stable products), while others — hydrate, hemiacetal, and cyanohydrin formation — are reversible equilibria. For the reversible cases you can shift the position of equilibrium using Le Châtelier's principle: an excess of the nucleophile or removal of a product (for example, removing water during acetal formation) pushes the reaction toward the addition product. Recognizing whether a given addition is reversible tells you whether you can protect a group and later unmask it, which is central to planning carbonyl syntheses.