1,2-dihaloalkane 1-halo-1-alkene

Bis-magnesium-halide reagents are difficult to prepare. 1,2-Dihaloalkanes react readily with Mg metal this is the reason for the usefulness of 1,2-dibromoethane as an entrainment reagent [see 5.4.2.2.1(iii)]. However, the intermediate step of an a-halomagnesio-/ -halo-substituted species occurs as is discussed below, such species tend to eliminate Mg salts and form alkenes. Alkenes are the products actually observed ... 

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