2-Fluorobutane is heated with $\mathrm{NaOEt}$ in ethanol. Unlike 2-bromobutane, the major alkene is

Fluoride is a poor leaving group and the strong C-F bond is broken late, so the E2 transition state has considerable carbanion character. The most acidic, least hindered primary hydrogen is removed, giving the Hofmann product but-1-ene. 2-Bromobutane, in contrast, gives mainly but-2-ene.