For E2 reaction

Anti periplanar geometry for E2 reactions is particularly important in cyclohexane rings, where chair geometry forces a rigid relationship between the substituents on neighboring carbon atoms (Section 4.8). As pointed out by Derek Barton in a landmark 1950 paper, much of the chemical reactivity of substituted cyclohexanes is controlled by their conformation. Let s look at the E2 dehydro-halogenation of chlorocyclohexanes to see an example. 

The anti periplanar requirement for E2 reactions overrides Zaitsev s rule and can be met in cyclohexanes only if the hydrogen and the leaving group are trans diaxial (Figure 11.19). If either the leaving group or the hydrogen is equatorial, E2 elimination can t occur. 

Figure 11.19 The geometric requirement for E2 reaction in a substituted cyclohexane. The leaving group and the hydrogen must both be axial for anti peri-planar elimination to occur.

Although anti periplanar geometry is preferred for E2 reactions, it isn t absolutely necessary. The deuterated bromo compound shown here reacts with strong base to yield an undeuterated alkene. Clearly, a svn elimination has occurred. Make a molecular model of the reactant, and explain the result. 

El processes show a regiochemical preference for the Zaitsev product, just as we saw for E2 reactions. For example ... 

For secondary halides in aqueous solvents, unimolecular and bimolecular processes compete, and the result is usually a mixture of products. With strong bases and protic solvents other than water, bimolecular elimination is usually faster than substitution, although this is only an assumption and accurate predictions can be difficult with secondary substrates. In polar aprotic solvents, bimolecular processes are usually faster. If a strong base is present and a protic solvent is used, bimolecular elimination is usually preferred to bimolecular substitution, but this is another assumption. If ethanol is used as a solvent and sodium ethoxide is a nucleophilic base. Table 2.9 shows the competition between bimolecular substitution (Sn2) and bimolecular elimination (E2) for a series of alkyl bromides. The preference for E2 reactions of secondary and tertiary halides in this protic solvent is clearly shown. 

Fig. 9. The relationship between the Bronsted coefficient, j8 for E2 reactions (catalysed by substituted thiophenoxides) and the ratio of the rate coefficients for elimination by thiophenoxide to...

From the earlier discussion on the nature of the transition state for E2 reactions, two salient factors affecting reactivity can be recognised, these being polar and steric effects. The polar effect can be divided into inductive and conjugative or electromeric components . The influence of a substituent will depend principally on the nature of the transition state, which to a large extent is determined by the leaving group and the base and solvent. A reaction... 

TABLE 5.8 Observed and Calculated Free Energies of Activation for E2 Reactions of Alkyl Bromides with Ethanolic Ethoxide Ion at 25°C, and Transition State Positions ... 

Cross section through potential energy surface for E2 reaction. 

Experimental data indicate that the anti pathway for E2 reactions is favored over the syn pathway. In one of the earliest studies of the stereochemistry of the E2 reaction, Cristol found the rate constant for the dehydrochlorination of the )3 isomer of benzene hexachloride (1,2,3,4,5,6-hexachlorocyclohexane, 6), in which each chlorine is cis to the hydrogen atoms on either side of it, to be only 10" times those of the other benzene hexachloride isomers. Since each of the other isomers has at least one hydrogen atom trans to a chlorine atom on an adjacent carbon atom, the low reactivity of 6 suggested that the E2 reaction occurs preferentially when there is a trans relationship for the hydrogen atom and chlorine atom on cyclohexane. ... 

Table 10.7 lists changes to the transition state structure for E2 reactions that occur when structural changes are made to the reactants and solvent. Confirm each of these by looking at More O Ferrall-Jencks plots. 

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