Dehalogenation

Dehalogenation

Dehalogenation or removal of halogen of 1, 1-dihalogen derivatives of the alkanes by means of zinc dust and methanol produces alkenes, e.g., propane from propylidene dibromide :

If sodium is used instead of zinc, and the reaction is carried out preferably in either solution, comparatively little propene is formed, the main product being hex-3-ene.

2CH₃CH₂CHBr₂ + 4Na  →  CH₃CH₂CH=CHCH₂CH₃ + 4NaBr

This reaction is really an extension of the Wurtz Synthesis, and the important point to note is that the use of sodium tends to produce lengthening of the carbon chain.

Zinc dust and methanol also dehalogenate 1, 2-dihalogen derivatives of the alkanes, e.g., propene from propene dibromide:

CH₃CHBrCH₂Br + Zn → CH₃CH=CH₂ + ZnBr₂

A possible mechanism is:

Neither method is useful for preparing alkenes, since the necessary dihalogen compounds are not readily accessible. The second method, however, is useful for purifying alkenes or for ‘protecting’ a double bond. Sodium iodide may be used instead of zinc dust, and according to Mulders et al (1963), the mechanism is:

Other means of removing the two bromine atoms to regenerate the alkene are chromous chloride (Allen et al., 1960), chromous sulphate (Castro et al., 1963), or sodium trimethoxyborohydride (King et al., 1968).

Both addition (to the alkene) and elimination (from the dibromide) of the two bromine atoms are predominantly trans, i.e., stereoselective, and these observations may be explained in terms of the formation of a bridged intermediate. It also follows from this that the configuration of the ‘protected’ alkene remains unchanged.