Organic Chemistry 2 · Reaction Mechanism

Nucleophilic Addition Reactions of Aldehydes and Ketones

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Study tools

In 30 seconds

is the signature reaction of aldehydes and ketones: a nucleophile attacks the electrophilic carbonyl carbon, the π bond breaks onto oxygen, and a forms, which is protonated to give an alcohol-type product. The product depends on the nucleophile — water gives hydrates, alcohols give hemiacetals and acetals, amines give imines and enamines, hydride gives alcohols, Grignard reagents build carbon–carbon bonds, and cyanide gives cyanohydrins. Acid catalysis activates the carbonyl toward weak nucleophiles; strong nucleophiles add directly.

Why this matters

and formation is central to biology: enzyme cofactors such as pyridoxal phosphate form imines ("Schiff bases") with amine substrates, a key step in amino-acid metabolism. Acetal protecting groups keep a carbonyl inert during multi-step drug synthesis. Cyanohydrins and their hydrolysis products (α-hydroxy acids) appear in metabolism and in natural-product chemistry. Reductive amination (imine then reduction) is a standard medicinal-chemistry route to amines.

The college version

1. Carbonyl Electrophilicity and the Tetrahedral Intermediate

The polarized C=O group (resonance: C=O ↔ C⁺-O⁻) makes the carbonyl carbon a soft target for nucleophiles. Attack converts the planar, sp² carbon into a tetrahedral, sp³ carbon bearing an alkoxide oxygen. This tetrahedral intermediate is the central species of every reaction in this topic; whether it is protonated and isolated, or collapses onward, determines the product.

2. Acidic vs. Basic Conditions

Strong nucleophiles (hydride, Grignard reagents, cyanide) add directly under basic or neutral conditions — the carbonyl is already electrophilic enough. Weak nucleophiles (water, alcohols, amines) need acid catalysis: a proton first attaches to the carbonyl oxygen, pulling even more electron density away from carbon and making it more electrophilic. Acid is catalytic (regenerated), whereas strong-nucleophile reactions consume the nucleophile stoichiometrically.

3. Product Families

Oxygen nucleophiles: water → ; one alcohol → hemiacetal (aldehyde) or hemiketal (ketone); a second alcohol with acid → .
Nitrogen nucleophiles: primary amine → imine; secondary amine → enamine; hydroxylamine → .
Sulfur nucleophiles: thiol → , which can be desulfurized to a CH₂ group.
Hydride and carbon nucleophiles: NaBH₄/LiAlH₄ → alcohols; Grignard reagents → alcohols with new C–C bonds; cyanide → cyanohydrins.

How it works

A nucleophile attacks the electrophilic carbonyl carbon.
The π electrons move onto oxygen, forming a tetrahedral alkoxide.
Protonation (or a collapse step) converts the intermediate to product.
Acid catalysis protonates the carbonyl oxygen first when the nucleophile is weak.
Water gives hydrates; alcohols give hemiacetals and then acetals/ketals.
Primary amines give imines; secondary amines give enamines; hydroxylamine gives oximes.
Thiols give thioacetals, which desulfurize to CH₂ groups.
NaBH₄/LiAlH₄ reduce aldehydes and ketones to alcohols.
Grignard reagents add carbon nucleophiles to make 1°, 2°, or 3° alcohols.
Cyanide gives cyanohydrins, adding one carbon to the chain.

Common confusions

Do not confuse	With	Difference
Hemiacetal	Acetal	Hemiacetal has one -OR and one -OH; acetal has two -OR
Hemiketal	Ketal	Hemiketal/ketal are the ketone versions of hemiacetal/acetal
Imine	Enamine	Imine has C=N; enamine has C=C-N (secondary amine)
Nucleophilic addition	Nucleophilic acyl substitution	Addition stops at the tetrahedral intermediate; acyl substitution expels a leaving group
Hydride reduction	Grignard addition	Hydride adds H; Grignard adds an R group (a new C–C bond)
Oxime	Imine	Oxime has =N-OH; imine has =N-R

Memory aids

"Water, Alcohol, Amine, Thiol, Hydride, Grignard, Cyanide — WAATHGC ('what goes in tells the product')." Products in order: hydrate, (hemi)acetal, imine/enamine, thioacetal, alcohol, alcohol (C–C), . Add: "weak nucleophiles want acid; strong ones just add."

Quick review

Topic Recap

Nucleophilic addition is governed by one idea: the electrophilic carbonyl carbon accepts a nucleophile while the π electrons retreat onto oxygen, forming a tetrahedral intermediate. Acid catalysis activates weak nucleophiles; strong ones add directly. The nucleophile's identity — water, alcohol, amine, thiol, hydride, Grignard, or cyanide — sets the product, from hydrates and acetals to imines, enamines, alcohols, and cyanohydrins.

Knowledge Check

Give the product of NaBH₄ reduction of butanal.
What reagent converts acetone into a tertiary alcohol with a new C–C bond?
Which forms an enamine — a primary or a secondary amine — and why?
Why are acetals used as protecting groups, and under what conditions are they removed?
What product results from adding HCN to propanal, and what does this reveal about stereochemistry?

Answers and Rationales

1-Butanol (a primary alcohol). Hydride adds to the aldehyde carbon and the alkoxide is protonated; an aldehyde always reduces to a primary alcohol.
A Grignard reagent (e.g., CH₃MgBr) adds a methyl group to acetone to give tert-butyl alcohol. Ketone + carbon nucleophile → tertiary alcohol.
A secondary amine forms an enamine. The primary amine has an N–H that can be lost to give an imine; a secondary amine has no N–H left after addition, so it loses an α-proton instead, giving the C=C-N enamine.
Acetals protect the carbonyl because they are stable to bases and nucleophiles but hydrolyze back to the carbonyl in aqueous acid. This lets other reactions proceed without touching the carbonyl.
2-Hydroxybutanenitrile (a cyanohydrin). The planar carbonyl is attacked from either face, so the new stereocenter forms as a racemic mixture.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Picture the carbonyl carbon as a magnet for anything carrying extra electrons. Oxygen is greedy and pulls the shared electrons toward itself, leaving carbon hungry for electrons. When an electron-rich molecule (a nucleophile) approaches, it "donates" a pair of electrons to carbon. To make room, carbon pushes its double-bond electrons up onto oxygen, so oxygen gains a negative charge and carbon now holds four groups — a temporary, crowded, pyramid-shaped intermediate.

A good comparison: nucleophilic addition is like adding one more person to a full three-person bench. The bench (the flat, three-group carbonyl carbon) has room for one more if one occupant (the double-bond electron pair) slides to the side (onto oxygen). Once everyone settles, oxygen grabs a proton from the surroundings.

This comparison stops being exact because the "shifting" is a flow of electron pairs (the double bond breaking and re-forming as a lone pair on oxygen), not a physical move of people. The intermediate also is not a stable endpoint — it usually grabs a proton quickly, and whether the reaction proceeds at all depends on the nucleophile's strength and on acid or base assistance.

Simple Example

Sodium borohydride (NaBH₄) reduces acetone to 2-propanol. The reagent delivers a hydride (H⁻) that attacks the carbonyl carbon; the C=O π electrons move onto oxygen to form an alkoxide; a proton (from water or alcohol during workup) then converts the alkoxide to 2-propanol.

Worked example

Generic nucleophilic addition (basic/neutral conditions, strong nucleophile):

The nucleophile Nu⁻ uses its lone pair to attack the partially positive carbonyl carbon. A double-headed curved arrow runs from the nucleophile's lone pair to carbon.
Simultaneously, the C=O π bond breaks: a curved arrow from the middle of the double bond moves that electron pair onto oxygen, giving a tetrahedral alkoxide R₂C(O⁻)Nu. Electron accounting: two electrons from the nucleophile form the new C–Nu bond, and two from the π bond become a lone pair on oxygen — the negative charge simply transfers from the nucleophile to oxygen.
Protonation of the alkoxide (from solvent or added acid) gives the neutral tetrahedral product R₂C(OH)Nu.

Acid-catalyzed variant (weak nucleophile, e.g., acetal formation):

The carbonyl oxygen is protonated, forming an oxonium ion that increases the positive character of carbon.
The weak nucleophile attacks carbon while the π electrons move onto oxygen.
Deprotonation gives the addition product (e.g., a hemiacetal). For an acetal, the hemiacetal's OH is protonated, water leaves to form an oxocarbenium ion, a second alcohol attacks, and deprotonation yields the acetal.

In every step, the curved arrows showing where electron pairs move are drawn before the bonds and charges of the products are written.

Key takeaways

High yield: Every nucleophilic addition goes through a tetrahedral intermediate; the π electrons move onto oxygen, not away.
High yield: Aldehydes reduce to primary alcohols; ketones reduce to secondary alcohols.
High yield: Grignard + formaldehyde → 1° alcohol; + aldehyde → 2° alcohol; + ketone → 3° alcohol.
High yield: Acetals are stable to base and nucleophiles but hydrolyze in aqueous acid — ideal carbonyl protecting groups.
High yield: Imines come from primary amines; enamines come from secondary amines.
Hemiacetal = aldehyde + one alcohol; acetal = aldehyde + two alcohols (with acid, loss of water).
Reaction selection: choose the nucleophile by the group you need — hydride/Grignard for alcohols, amines for imines/enamines, alcohols for acetals, thiols for thioacetals, cyanide for cyanohydrins.
Addition to a planar carbonyl gives a racemic mixture whenever a new stereocenter is created.
NaBH₄ is milder (aldehydes/ketones only); LiAlH₄ is stronger (also esters and acids).

Keep learning

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Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

Explain why the carbonyl carbon is electrophilic and write the general mechanism of nucleophilic addition.
Distinguish acidic (catalytic) conditions from basic conditions and predict which nucleophiles need which.
Predict products for reactions with water, alcohols, amines, thiols, hydride reagents, Grignard reagents, and cyanide.
Use acetal protecting groups and thioacetal desulfurization strategically in synthesis and state the regiochemistry/stereochemistry of addition.

Key vocabulary

Nucleophilic addition: A nucleophile adds to the carbonyl carbon
Carbonyl electrophilicity: Partial positive charge on the carbonyl carbon
Acidic vs. basic conditions: Acid activates weak nucleophiles; strong ones add directly
Tetrahedral intermediate: The sp³ intermediate with an alkoxide oxygen
Hydrate (gem-diol): Product of water addition
Hemiacetal/hemiketal: One alcohol added to an aldehyde/ketone
Acetal/ketal: Two alkoxy groups replacing C=O
Acetal protecting group: Masks a carbonyl from basic/nucleophilic reagents
Imine: C=N formed from a primary amine
Enamine: C=C-N formed from a secondary amine
Oxime: C=N-OH from hydroxylamine
Thioacetal: Two -SR groups replacing C=O
Desulfurization: Removal of sulfur (Raney Ni/H₂) to give CH₂
Hydride reduction: NaBH₄ or LiAlH₄ delivers H⁻
Grignard addition: RMgX adds a carbon nucleophile
Cyanohydrin: R₂C(OH)CN from cyanide addition
Reaction selection: Choosing the nucleophile to match the desired product

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