= Solution
\Image[/past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2021/paper-1-q2-amine-methylation.png]
{title=Nitrogen lone-pair attack and iodide departure form choline iodide}
Draw a curly arrow from the <nitrogen> <lone pair> to methyl <carbon>, and a second from the C-I bond to I:
$$
\mathrm{HOCH_2CH_2NMe_2+MeI
\longrightarrow[HOCH_2CH_2NMe_3]^+I^-}.
$$
This is concerted \b[<SN2> quaternization]. The product is <choline> iodide; <nitrogen> has four <carbon> substituents and positive charge, while the <alcohol> remains OH.
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