= Solution
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 $\mathrm{R^1COR}$. 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:
$$
\mathrm{R^1CO_2Et\ \longrightarrow\ R^1C(O^-)(OEt)R\ \longrightarrow\ R^1COR\ \longrightarrow\ R^1C(O^-)(R)_2\ \xrightarrow{H^+}\ R^1C(OH)(R)_2}.
$$
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 \b[n-butylmagnesium <bromide>]. The <carbon> chain has four carbons, so its acetylation produces \b[hexan-2-one, $\mathrm{CH_3CO(CH_2)_3CH_3}$], not pentan-2-one. Attack on <acetic anhydride> forms $\mathrm{CH_3C(O^-)(nBu)OCOCH_3}$. 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.<The Preparation of Ketones from Grignard Reagents by Melvin S. Newman and William T. Booth Jr.>[1945, printed p. 154: proposed intermediate I and low-temperature dependence]{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 $\mathrm{PhC(nBu)=N^-MgBr^+}$. This anionic nitrogen-containing intermediate resists a second <nucleophilic addition> much more than a neutral <carbonyl> does. The \b[<ketone> $\mathrm{PhC(=O)nBu}$ 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 is
$$
\begin{aligned}
\mathrm{PhC(nBu)=N^-}&\xrightarrow{2H^+}\mathrm{PhC(nBu)=NH_2^+}\\
&\xrightarrow{H_2O}\mathrm{PhC(nBu)(OH_2^+)NH_2}\\
&\xrightarrow{\text{proton transfers}}\mathrm{PhC(nBu)(OH)NH_3^+}\\
&\longrightarrow\mathrm{PhC(nBu)=OH^++NH_3}\longrightarrow\mathrm{PhC(nBu)=O}.
\end{aligned}
$$
Water'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.
\Image[/past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-1-grignard-ketones.png]
{title=Original electron-flow summary contrasting ester overaddition with anhydride trapping and nitrile hydrolysis}
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