E2 Eliminations

FIGURE 5 10 Potential en ergy diagram for concerted E2 elimination of an alkyl halide... 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

How many alkenes would you expect to be formed from each of the following alkyl bro mides under conditions of E2 elimination Identify the alkenes in each case... 

Choose the compound of molecular formula C7Hi3Br that gives each alkene shown as the exclusive product of E2 elimination... 

Compound A (CgHi4) gives three different monochlondes on photochemical chlonnation One of these monochlondes is inert to E2 elimination The other two monochlondes yield the same alkene B (CgHi2) on being heated with potassium tert butoxide in tert butyl alcohol Men tify compound A the three monochlondes and alkene B... 

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

Secondary and tertiary alkyl halides are not suitable because they react with alkox ide bases by E2 elimination rather than by 8 2 substitution Whether the alkoxide base IS primary secondary or tertiary is much less important than the nature of the alkyl halide Thus benzyl isopropyl ether is prepared m high yield from benzyl chloride a pri mary chloride that is incapable of undergoing elimination and sodium isopropoxide... 

Fig. 6.5. Representation of changes in transition-state character in the variable transition state E2 elimination reaetion, showing displacement of transition-state location as a result of substituent effects (a) substituent Z stabilizes catfaanion character of Elcb-like transition state (b) substituent R stabilizes carbocation character of El-like transitions state.

Table 6.S. Orientation in E2 Elimination as a Function of Base Strength...

In most cases, E2 elimination proceeds via a transition state involving the anti arrangement. Nevertheless, syn elimination is possible, and, when special structural features retard anti elimination, syn elimination becomes the dominant mode. 

For example, c/s-4-t-butyleyelohexyl bromide undergoes E2 elimination at a rate about 500 times greater than the tram isomer because only the cis isomer permits anti elimination from the favored chair conformation. ... 

Table 6.8. Stereochemistry of E2 Eliminations for Some Acyclic Substrates...

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

Predict the effect on the 1-butene Z-2-butene E-2-butene product ratio when the E2 elimination of erythro- i-deuteno-lAxomobutaae is compared with that of 2-bromo-butane. Which alkene(s) will increase in relative amount and which will decrease in relative amount Explain the basis of your prediction. 

Is the electrophilic addition of hydrogen chloride to 2-methyl-propene the reverse of the El or the E2 elimination reaction of fe/t-butyl chloride ... 

Hydrolysis of alkyl halides (Section 8.1) A reaction useful only with substrates that do not undergo E2 elimination readily. It is rarely used for the synthesis of alcohols, since alkyl halides are normally prepared from alcohols. 

---