= Solution
\b[The reactive occupied orbital of <ethoxide> is an <oxygen> <lone pair>; the acyl <electrophile> accepts into its <carbonyl> $\pi^*$ orbital.] Draw the <ethoxide> <lone pair> as a directional oxygen-centred lobe directed toward an <electrophile>. In a lithium-ion-pair picture the low unoccupied acceptor on <lithium> is an empty, diffuse Li-centred orbital, with 2s/2p character depending on coordination. It should not be confused with an occupied Li–O covalent bonding pair.
For <methyl acetate>, the occupied frontier donor is principally an <oxygen> nonbonding orbital, mixed with the <ester> orbital system. Its low-lying reactive acceptor is $\pi^*(\mathrm{C=O})$, with opposite phases on <carbon> and <oxygen> and a relatively large accepting lobe on the <carbonyl> <carbon>. The corresponding sketch must show a <node> between the <carbonyl> atoms. These are chemically useful frontier-orbital descriptions; <lithium> <alkoxide> aggregation and <ethanol> solvation make a unique isolated-molecule orbital energy diagram an oversimplification.
\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-frontiers.png]
{title=Original frontier-orbital sketches and the two complementary donor–acceptor contacts in the ester–lithium ethoxide encounter}
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