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Acetone is the ketone $\mathrm{C}{\mathrm{H}}_3\mathrm{COC}{\mathrm{H}}_3$, with a carbonyl group between two methyl groups. Its polar carbonyl makes it a useful solvent and an electrophilic center for nucleophilic addition.

Hydrogens on its adjacent methyl groups can be removed to form an enolate. Thus one molecule illustrates both carbonyl electrophilicity and the carbon acidity used in carbon-carbon bond-forming reactions.

The Bürgi-Dunitz trajectory describes the preferred approach of a nucleophile toward a carbonyl carbon, usually at an obtuse angle to the carbon-oxygen bond, often near $107^{\circ }$. It is a geometrical trend, not a fixed angle obeyed by every reacting system.

An oblique approach balances favorable interaction with the carbonyl antibonding orbital against repulsion from occupied orbitals. Crystallographic observations of close contacts motivated this picture, linking static molecular structures with a possible path of nucleophilic addition. Steric constraints and the other substituents can modify the accessible trajectory.

Hans-Beat Bürgi coauthored the structural-correlation work associated with the Bürgi-Dunitz trajectory. Comparing close nucleophile-carbonyl contacts in crystal structures helps identify a preferred oblique approach to a carbonyl carbon.

The characteristic Bürgi-Dunitz angle connects three-dimensional geometry with orbital overlap and repulsion. It is a useful trend for understanding nucleophilic addition, while steric constraints and molecular environment can change the accessible path.^[ref]

Jack D. Dunitz used X-ray diffraction to investigate molecular structure, motion, and reaction paths. His work with Hans-Beat Bürgi relates structural correlations among molecules to possible trajectories of nucleophilic addition at a carbonyl group.

A set of static structures can sample geometries resembling different stages of bond formation. This provides experimental constraints on a reaction path, although each crystal is not itself a time-resolved frame of one reacting molecule.^[ref]

Nguyễn Trọng Anh is associated with the Felkin-Anh model of stereoselective nucleophilic addition to a carbonyl beside a stereogenic centre. The model connects substrate conformation and electronic interactions with the preferred face of attack.

Its usefulness is conditional: an open, non-chelating pathway can favour a different product from a metal-chelated pathway. Comparing these possibilities makes the model a mechanistic prediction under specified conditions rather than a universal stereochemical rule.^[ref]

A nucleophilic addition forms a bond by attack of a nucleophile on an electrophilic multiple bond, reducing its bond order. A common case is attack on the carbon of a carbonyl group, with its pi electrons displaced toward oxygen.

Protonation of the resulting oxygen-centered intermediate can give an alcohol. Unlike nucleophilic substitution, the basic addition step does not expel a leaving group from the attacked carbon. Carbonyl polarization and orbital geometry explain both why carbon is attacked and why the incoming bond follows a preferred approach direction.

For the ordinary ester, the carbon nucleophile of a grignard reagent attacks the carbonyl carbon and moves the C=O pi pair onto oxygen. Collapse of the tetrahedral intermediate expels ethoxide, transiently giving ${\mathrm{R}}^1\mathrm{COR}$. This ketone is more electrophilic than the starting ester because it lacks an alkoxy group's stabilizing resonance donation. A second equivalent of the grignard reagent therefore adds to it before isolation:The negatively charged intermediates are associated with magnesium counterions. Merely reducing the amount of reagent to one equivalent does not generally trap the ketone selectively; it can leave unreacted ester together with the double-addition product.

In the first successful preparation, magnesium insertion into the primary C–Br bond gives n-butylmagnesium bromide. The carbon chain has four carbons, so its acetylation produces hexan-2-one, $\mathrm{C}{\mathrm{H}}_3\mathrm{CO}(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{C}{\mathrm{H}}_3$, not pentan-2-one. Attack on acetic anhydride forms $\mathrm{C}{\mathrm{H}}_3\mathrm{C}({\mathrm{O}}^{-})(\mathrm{nBu})\mathrm{OCOC}{\mathrm{H}}_3$. Coordination of the anhydride-derived oxygens to magnesium can stabilize this tetrahedral addition complex. Under suitably cold, controlled-addition conditions it survives sufficiently to delay release of the free ketone until aqueous work-up has destroyed the remaining grignard reagent. Protonation followed by collapse releases acetate/acetic acid and the ketone.

This interpretation has direct experimental precedent: Melvin S. Newman and William T. Booth Jr. obtained methyl ketones by cold addition of grignard reagents to acetic anhydride, attributing selectivity to a persistent addition complex. The temperature and reagent-addition regime are genuine conditions on the explanation, rather than proof that an anhydride can never undergo overaddition; the exam scheme omits them.^[ref]

For the benzonitrile route, n-butyl attacks the nitrile carbon and one C≡N pi pair moves to nitrogen, producing the magnesium ketimine salt $\mathrm{PhC}(\mathrm{nBu})={\mathrm{N}}^{-}\mathrm{MgB}{\mathrm{r}}^{+}$. This anionic nitrogen-containing intermediate resists a second nucleophilic addition much more than a neutral carbonyl does. The ketone $\mathrm{PhC}(=\mathrm{O})\mathrm{nBu}$ appears only during acidic aqueous work-up, after organomagnesium has been quenched. Protonation gives the iminium ion; water attacks its carbon, proton transfers give a carbinolamine, and C–N cleavage from its nitrogen-protonated form produces the protonated ketone and Ammonia, followed by deprotonation. In the acidic mixture Ammonia is present largely as ammonium. This sequence explains why nitrile addition stops at a ketone without requiring the ketone itself to be resistant to grignard reagents.

The atom-labelled hydrolysis sequence isWater's lone pair attacks the iminium carbon while its C=N pi pair moves to nitrogen; the carbinolamine oxygen then reforms C=O while the C–N pair moves to the departing protonated amino group. Final deprotonation gives the neutral ketone. The arrows involving protonation and hydrolysis also exchange protons with the acidic aqueous reservoir; the condensed sequence tracks the carbon–nitrogen and carbon–oxygen transformations rather than displaying every solvent molecule.

The mechanism is nucleophilic addition followed by elimination of methoxide. An ethoxide oxygen lone pair attacks the carbonyl carbon, while the C=O pi pair moves to oxygen:The tetrahedral intermediate collapses when its anionic oxygen reforms the carbonyl and the C–OMe bond breaks with its electron pair going to the methoxy oxygen:lithium is a coordinating counterion; it does not supply an electron pair to the acyl carbon. proton exchange with ethanol connects the methoxide produced to the ethoxide catalyst pool and gives methanol, so the net solvent-mediated exchange is methyl acetate plus ethanol to ethyl acetate plus methanol. The reversibility of the elementary acyl steps is important: changing an alkoxy group is not automatically an irreversible reaction.

A tetrahedral intermediate has a formerly trigonal carbonyl group carbon bonded to four groups after nucleophilic addition. Its subsequent elimination can restore a carbonyl group.