There are two principle sets of conditions favoring the typical E1 reaction (elimination, unimolecular). First, compounds bearing a 3° leaving group (LG) will, in the presence of a weakly basic nucleophile, result in E1 especially when the reaction is conducted in a polar protic solvent. The E1 reaction often occurs simultaneously and competitively with the SN1 reaction. The slow step for each pathway is the formation of a stable carbocation intermediate, and the rate of the reaction is first order in R3C-LG.
Rate = k [R3C-LG]
Polar protic solvents have the capacity to solvate the departing LG and also stabilize the intermediate carbocation. An example is the solvolysis of tert-butyl bromide in the presence of ethanol.
The second principle set of conditions, involving compounds containing a 2° LG in the presence of a weak base and a polar protic solvent, will also lead to E1 along with competing SN1. The rate for this reaction is indicated by the expression,
Rate = k [R2CH-LG]
An example is the dehydration of cyclohexanol in the presence of H2SO4.
In this example, HSO4- serves as the base abstracting the alpha proton. Here, there is very little competition via the SN1 pathway due to the low nucleophilicty of the resonance statilized HSO4-.
© 2011 Joseph Lennox, Ph.D.
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