A substrate $\mathrm{Z}-\mathrm{CH}{}_2-\mathrm{CH}{}_2-\mathrm{OR}$ (Z is a strong electron-withdrawing group) is treated with $\mathrm{EtO}{}^{-}$ in $\mathrm{EtOD}$. Observations: (i) the rate is first order in substrate and first order in $\mathrm{EtO}{}^{-}$; (ii) at partial conversion the recovered substrate contains deuterium on the carbon bearing Z; (iii) no rearranged skeleton is found. The mechanism is

Deuterium incorporation at the carbon bearing Z shows that a carbanion is formed there and is reprotonated (deuterated) in the medium; absence of rearrangement excludes a carbocation. First order in base and substrate with a poor OR leaving group fits the E1cB mechanism with reversible carbanion formation. A concerted E2 would not exchange hydrogen for deuterium in the recovered substrate.