Mechanism of dehydrohalogenation

Mechanism and stereochemistry of reductions of ketones, 66 Mechanism of dehydrohalogenation, 292 Mechanism of hydrogenation, 111 Mechanism of reduction of aromatic compounds, 12... 

The mechanism of dehydrohalogenation of haloalkanes varies from a concerted E2 mechanism to a carbanionic ElcB mechanism, where the /j-hydrogen has been made sufficiently acidic to be removed, leaving a carbanion.4,5 The E2 mechanism of dehydrohalogenation takes place in one step. The ElcB or carbanionic mechanism of dehydrohalogenation has two steps the first step being deprotonation, and the second one elimination of the halide ion. 

The mechanism of the above elimination reaction is similar to the E2 mechanism of dehydrohalogenation. The reaction proceeds by nucleophilic attack of a base on silicon, simultaneous elimination of halide ion and formation of double bond (Scheme 4.4). 

The general mechanism of dehydrohalogenation may be illustrated by considering the reactions of 4-halo-3-methoxymethyl-6-methylpyridazine with amide ion in ammonia, Scheme 2. Amide ion removes a proton at a position adjacent to a halogen ... 

The mechanism of dehydrohalogenation under basic conditions of trons-fused bicyclo[4,n,0]alkane halohydrins (563)—(565) has been studied. Three reaction types are noted (i) epoxide formation, (ii) ketone formation, and (iii) ring contraction. trans-Diaxial chlorohydrins corresponding to (563)—(565) gave epoxides (566)—(568) with relative rates (derived from bimolecular rate constants) of 1 3 17. This rate sequence was rationalized in terms of deformation of the cyclohexane ring brought about by the nature of the fused ring. In particular, deformation is probably towards the half-chair conformation favoured by the cyclohexane epoxide which is formed in the slow step. trans-Diequatorial chlorohydrins represented by (563)—(565)... 

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