Elimination Reactions Essay

As expound previously, primary alkyl halides mostly undergo substitution reactions with simple nucleophiles by an SN2 mechanism. Secondary alkyl halides, practically react with simple basic nucleophiles to give a mixture of products arising from twain substitution and elimination.As with substitution reactions, the rate at which elimination reactions conk out can be proportionate to both the dumbness of the base and the concentration of the reactant alkyl halide (an E2 reaction (elimination bimolecular), or the rate can be proportional only to the alkyl halide (an E1 reaction (elimination unimolecular). The mechanism for the E2 reaction is best described as concerted with the reaction coordinate passing through a single energy maximum with no distinct intermediate. The transition disk operating system for this reaction is described by the structure shown below in which overtone bonds exist among the attacking base, the total heat which is abstracted, and departing halogen. The geometry of this transition postulate requires that the halogen be anti and coplanar with the hydrogen which is being removed (also termed antiperiplanar). This fact is all-important(prenominal) to remember since the stereochemistry of the resulting alkene (Z or E cis or trans) is often controlled by the hydrogen which is removed in the elimination reaction.In the reaction shown below, the hydrogen on the carbon bearing the methyl group cannot become anti-to the halogen, thereof the elimination occurs on the secondary carbon, to give the unfavored less substituted alkene.The rate-limiting transition state in the E1 reaction is again, carbocation formation, and the transition state is generally described as shown below. Factors which control whether E1 or E2 mechanisms will be observed again interest simply to the stability of the intermediate carbocation.Control of the reaction pathway between substitution and elimination is generally accomplished by careful filling of the reactants strong, sterically hindered bases tend to favor elimination, while weak, unhindered nucleophiles tend to favor substitution. The alternative for a strong, hindered base is generally tert-butoxide anion in tert-butanol as solvent, and it is generally impregnable to assume that a potential substitution/elimination reaction cover these conditions will proceed with elimination.