= Solution
\b[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>:
$$
\mathrm{MeC(=O)OMe+EtO^-\rightleftharpoons MeC(O^-)(OMe)(OEt)}.
$$
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>:
$$
\mathrm{MeC(O^-)(OMe)(OEt)\rightleftharpoons MeC(=O)OEt+MeO^-}.
$$
<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.
\Image[/past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2023/paper-1-acyl-exchange.png]
{title=Original curved-arrow addition–elimination mechanism for ethoxide-catalysed ester exchange}
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