Halogenoalkanes, elimination reactions

Halogenoalkanes react with nucleophiles in substitution reactions and with bases in elimination reactions. 

The two main mechanisms for elimination reactions of halogenoalkanes are El and E2. In both cases, the mechanism involves the loss of HX from RX (e.g. R2CH-CXR2) to form an alkene (e.g. R2C=CR2). 

The mechanism of an elimination reaction is influenced by the nature of the halogenoalkane, the base and the solvent. 

Halogenoalkanes undergo competitive substitution and elimination reactions. The ratio of products derived from substitution and elimination depends on the structure of the halogenoalkane, the choice of base or nucleophile, the reaction solvent and the temperature. Sn2 reactions are normally in competition with E2 reactions, while S jl reactions are normally in competition with El reactions. 

A close look at the nature of a nucleophile will emphasize that it shares common features with a Lewis hase (see Chapter 18). Indeed, a nucleophilic species can act as such a base if the reaction conditions are appropriate - it can remove a proton (H ion) from a halc enoalkane and thereby initiate an elimination reaction. In this type of reaction HX is eliminated from the halogenoalkane and an alkene is produced. It is essential to realize that, given the similarity of the reagents involved, the two processes of nucleophilic substitution and elimination are generally in competition with each other. If a primary halogenoalkane is reacted with aqueous alkali (OH (aq)) then the substitution reaction we have discussed earlier is favoured. However, if ethanolic alkali (OH (ethanol)) is used, then the elimination reaction is favoured. 

Halogenoalkanes also undergo elimination reactions. An elimination reaction involves the loss of a small molecule from the original organic molecule. In the case of halogenoalkanes, this small molecule is a hydrogen halide, such as HCl or HBr. 

Halogenoalkanes will also undergo elimination reactions when heated with ethanolic sodium hydroxide, forming alkenes. 

Analysis of the butene fraction obtained in reactions between 2-halogeno-butanes and the alkoxides CHa-CHj ONa and CFs-CHg-ONa at 25 C in dipolar aprotic solvents (DMF and DMSO, to minimize solvation effects) has revealed that in each case change from ethoxide to 2,2,2-trifluoroethoxide results in a decrease in the tendency for but-l-ene to be formed. This demonstrates that base strength and not size is of prime importance in determining orientation in elimination reactions between 2-halogenoalkanes and alkoxides of modest proportions. ... 

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