Common features of E1 and E2 mechanisms

BothE1 andE2 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 andE1 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.

TheE1 reactions are favored by weak bases. The most common weak bases are water and alcohols:

Because theE1 goes by forming a carbocation,rearrangements are possible just like in S_N1 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 astrong base, the mechanism isE2

If aweak base is used, then the mechanism isE1

Remember also thatE1 reactions cannot occur on primary substrates sinceprimary carbocations are very unstable. Other than this, the reactivity pattern is the same for bothE1 and E2 – they go faster with more substituted alkyl halides:

AlthoughS_N2is a substitution and not an elimination reaction, it is themain competitor of the E2 elimination. The reason for this is thatE2 and S_N2 are both bimolecular reactionsfavored by strong bases or good nucleophiles. Once again,E1 is a unimolecular, stepwise mechanism that proceeds via formation of acarbocation. In that sense, it iscloser to theS_N1 reaction, and both are favored whenweak bases and nucleophiles are used.

Check these two articles for comparison, competition, and deciding betweenS_N1/E1 andS_N2/E2 reactions.

For a broader coverage of deciding betweenS_N1, S_N2, E1, and E2, check the linked article.

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2 thoughts on “E1 vs. E2: How to Tell if the Mechanism is E1 or E2 with Practice Problems”

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