Common features of E1 and E2 mechanisms
Both E1 and E2 are elimination reactions with some common features.
First, in both reactions, there is a loss of hydrogen and the leaving group, and the result is the formation of a π bond. This hydrogen must be at the β position:
Difference between E1 and E2 mechanisms
The key differences between the E2 and E1 mechanisms are:
1) E2 is a concerted mechanism where all the bonds are broken and formed in a single step. The E1, on the other hand, is a stepwise mechanism.
2) E2 reactions are favored by strong bases such as the methoxide (MeO–), ethoxide (EtO–), potassium tert-butoxide (tBuOK), DBN, DBU, LDA and etc.
The E1 reactions are favored by weak bases. The most common weak bases are water and alcohols:
Because the E1 goes by forming a carbocation, rearrangements are possible just like in SN1 reactions:
3) E2 is a second-order reaction, and the rate depends on the concentration of both the substrate and the base.
Both reactions are regio- and stereoselective, and this will be covered in separate posts.
Choosing Between E1 and E2 mechanisms
The key factor in determining if the mechanism is E1 or E2 is to look at the base:
If it is a strong base, the mechanism is E2
If a weak base is used, then the mechanism is E1
Remember also that E1 reactions cannot occur on primary substrates since primary carbocations are very unstable. Other than this, the reactivity pattern is the same for both E1 and E2 – they go faster with more substituted alkyl halides:
Although SN2 is a substitution and not an elimination reaction, it is the main competitor of the E2 elimination. The reason for this is that E2 and SN2 are both bimolecular reactions favored by strong bases or good nucleophiles. Once again, E1 is a unimolecular, stepwise mechanism that proceeds via formation of a carbocation. In that sense, it is closer to the SN1 reaction, and both are favored when weak bases and nucleophiles are used.
Check these two articles for comparison, competition, and deciding between SN1/E1 and SN2/E2 reactions.
For a broader coverage of deciding between SN1, SN2, E1, and E2, check the linked article.
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