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.
Two products are formed from this reaction, the desired alkene via E1 and also the undesired ethyl ether from SN1. Here, EtOH serves as the base abstracting the proton alpha to the carbocation.
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-.
In the presence of fairly good nucleophiles, it’s important to remember the E1 reaction will proceed with SN1 as a competing side reaction, and vice versa.
© 2011 Joseph Lennox, Ph.D.
This blog article was brought to you FREE by Lennox Organic Chemistry & MCAT Tutoring of San Diego, where “Total Comprehension = Key to Success”™.
Copyright © 2012-2022 Lennox Organic Chemistry & MCAT Tutoring, the Physical and Life Sciences PH.D. Tutors™, PH.D. San Diego Organic Chemistry Tutor. All rights reserved.
