The Regiochemical Outcome of an El Reaction

For now, let s consider the effect of the substrate on the rate of an El process. The rate is fonnd to be very sensitive to the nature of the starting aUcyl halide, with tertiary halides reacting more readily than secondary halides and primary halides generally do not nndergo El reactions. This trend is identical to the trend we saw for SnI reactions, and the reason for the trend is the same as well. Specihcally, the rate-determining step of the mechanism involves formation of a carbocation intermediate, so the rate of the reaction will be dependent on the stability of the carbocation (recall that tertiary carbocations are more stable than secondary carbocations). 

In the previons chapter, we saw that an OH gronp is a terrible leaving group, and that an SnI reaction can only occur if the OH group is hrst protonated to give a better leaving gronp  

The same is trne with an El process. If the substrate is an alcohol, a strong acid will be required in order to protonate the OH group  

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

The more-substituted alkene (Zaitsev product) is the major product. However, there is one critical difference between the regiochemical outcomes of El and E2 reactions. Specifically, we have seen that the regiochemical outcome of an E2 reaction can often be controlled by carefully choosing the base (sterically hindered or not sterically hindered), hi contrast, the regiochemical outcome of an El process cannot be controlled. The Zaitsev product will generally be obtained. 

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