4-Methoxybutan-2-one, $\mathrm{CH}{}_3\mathrm{COCH}{}_2\mathrm{CH}{}_2\mathrm{OCH}{}_3$, is treated with $\mathrm{NaOD}$ in $\mathrm{D}{}_2\mathrm{O}$. At low conversion the recovered starting material is found to have exchanged hydrogen for deuterium at C-3 faster than the ketone is converted to but-3-en-2-one. This is best explained by

1. Aan E1 reaction through a primary carbocation
2. Ba concerted E2 reaction without any intermediate
3. CSN2 displacement of methoxide by $\mathrm{OD}{}^{-}$
4. Dreversible formation of an enolate carbanion followed by slower loss of methoxide (E1cB)