Reactions

The haloalkane dehydrohalogenation mechanism depends on the haloalkane type. Primary haloalkanes favor an E2 mechanism, whereas tertiary haloalkanes favor an E1 mechanism. Secondary haloalkanes can react via either pathway, depending on the stability of the carbocation and the orientation of the C-H bond relative to the departing group. Greater carbocation stability favors the E1 mechanism. The E2 mechanism is only possible if the C-H bond is in antiperiplanar (180°) orientation to the leaving group (here, the halide). For secondary and tertiary haloalkanes, the formation of the most substituted alkene is favored (Zaitsev’s rule).

Regarding stereochemistry, the E2 mechanism is stereospecific: because elimination requires strict antiperiplanar alignment of the C-H and C-X bonds, the configuration of the starting haloalkane dictates which alkene stereoisomer (E or Z) is formed. In contrast, the E1 mechanism proceeds through a planar carbocation intermediate, allowing free rotation before elimination of the proton and leading to a mixture of E and Z alkenes.