E1 and E2 Elimination Reactions: Mechanism, Energy Profile and Differences
Elimination reactions are an important class of organic reactions in which atoms or groups are removed from a molecule to form a carbon-carbon double bond. Two major mechanisms of elimination are E1 and E2.
Both E1 and E2 reactions commonly occur in the reactions of alkyl halides and other suitable organic compounds. Although both mechanisms generally produce alkenes, their reaction pathways are quite different.
What Does E1 and E2 Mean?
The letter E stands for elimination. The numbers 1 and 2 indicate the molecularity of the rate-determining step.
- E1: Unimolecular elimination.
- E2: Bimolecular elimination.
The most important difference is that E1 occurs in two steps through a carbocation intermediate, whereas E2 occurs in one concerted step without forming a carbocation.
Basic Idea of an Elimination Reaction
In a typical elimination reaction, a leaving group is attached to the alpha carbon, while a hydrogen is removed from an adjacent beta carbon.
The removal of these groups results in the formation of a carbon-carbon double bond.
General representation:
Cα–Cβ–X → Cα=Cβ + HX
Here, X represents the leaving group.
E1 Elimination Reaction
E1 stands for unimolecular elimination. It is a stepwise elimination mechanism that involves the formation of a carbocation intermediate.
Rate Law of E1 Reaction
The rate of an E1 reaction depends only on the concentration of the substrate:
Rate = k[Substrate]
The base does not participate in the slow, rate-determining step.
Mechanism of E1 Reaction
Step 1: Formation of Carbocation
The first step is the departure of the leaving group from the substrate. The carbon-leaving group bond breaks and a carbocation is formed.
R–CH2–X → R–CH2+ + X−
This step is usually the slow and rate-determining step of the E1 reaction.
Step 2: Removal of Beta Hydrogen
In the second step, a base removes a hydrogen atom from the beta carbon. The electrons from the C–H bond form a carbon-carbon double bond.
Carbocation → Alkene
Therefore, E1 is a two-step reaction:
- Formation of carbocation.
- Removal of beta hydrogen and formation of the alkene.
E1 Energy Profile
Because E1 occurs in two steps, its energy profile contains two transition states and one intermediate.
| Stage | Energy Profile Feature |
|---|---|
| Reactants | Starting energy level |
| Transition State 1 | First energy maximum |
| Carbocation | Intermediate between the two peaks |
| Transition State 2 | Second energy maximum |
| Products | Final energy level |
The E1 energy profile can therefore be represented as:
Reactants → TS1 → Carbocation → TS2 → Products
The important feature is that the energy diagram contains two peaks. The valley between the two peaks represents the carbocation intermediate.
Carbocation Rearrangement in E1
Since a carbocation is formed during E1, rearrangement may occur if it produces a more stable carbocation.
Common rearrangements include:
- Hydride shift
- Alkyl shift
Therefore, the final alkene may sometimes be formed through a rearranged carbocation.
E2 Elimination Reaction
E2 stands for bimolecular elimination. Unlike E1, E2 occurs in a single concerted step.
Rate Law of E2 Reaction
The rate depends on both the substrate and the base:
Rate = k[Substrate][Base]
Therefore, E2 is a second-order reaction overall.
Mechanism of E2 Reaction
In an E2 reaction, the base removes the beta hydrogen at the same time that the leaving group leaves and the double bond forms.
- The base removes the beta hydrogen.
- The C–H electrons form the C=C bond.
- The leaving group leaves.
All three processes occur in a single step.
Base + substrate → alkene + leaving group products
No carbocation intermediate is formed.
E2 Energy Profile
Because E2 is a one-step reaction, it has only one transition state.
| Stage | Energy Profile Feature |
|---|---|
| Reactants | Starting energy level |
| Transition State | Single energy maximum |
| Products | Final energy level |
The E2 energy profile can therefore be represented as:
Reactants → Transition State → Products
There is only one peak and there is no intermediate.
Why Does E2 Have No Carbocation?
In E2, the removal of beta hydrogen, formation of the double bond and departure of the leaving group occur simultaneously.
Because the carbon-carbon double bond begins forming while the leaving group is leaving, there is no stage at which a free carbocation exists.
Consequently, carbocation rearrangement does not occur in E2 reactions.
Important Difference in Energy Profiles
| E1 | E2 |
|---|---|
| Two-step mechanism | One-step mechanism |
| Two transition states | One transition state |
| One carbocation intermediate | No intermediate |
| Two energy peaks | One energy peak |
Example of E1 Reaction
Consider the elimination of 2-bromo-2-methylpropane.
(CH3)3C–Br → (CH3)2C=CH2
The reaction first produces a tertiary carbocation. A base then removes a beta hydrogen and the alkene is formed.
Example of E2 Reaction
Consider the reaction of bromoethane with a strong base.
CH3CH2Br + Base → CH2=CH2 + Products
The base removes a beta hydrogen while bromide leaves in the same step. No carbocation is formed.
E1 vs E2: Effect of Substrate
Tertiary Substrates
Tertiary substrates can form relatively stable tertiary carbocations, so E1 is often possible under suitable conditions. With a strong base, the same type of substrate can also undergo E2.
Secondary Substrates
Secondary substrates can undergo either E1 or E2 depending on the reaction conditions.
Primary Substrates
Primary substrates generally do not favour E1 because formation of a primary carbocation is highly unstable. E2 is more commonly associated with primary substrates when a suitable strong base is present.
Role of the Base
The nature of the base is one of the most important factors in determining whether E1 or E2 is favoured.
- A strong base generally favours E2.
- A weaker base can be involved in E1 reactions.
- Bulky strong bases can favour elimination and may influence which alkene is formed.
Saytzeff's Rule
In many elimination reactions, more than one alkene can be formed. According to Saytzeff's rule, the major product is generally the more substituted alkene.
For example, elimination from 2-bromobutane can produce both but-1-ene and but-2-ene.
CH3–CHBr–CH2–CH3
Possible products include:
- But-1-ene
- But-2-ene
According to Saytzeff's rule, but-2-ene is generally the major product because it is the more substituted alkene.
Anti-Periplanar Requirement in E2
E2 reactions have an important stereochemical requirement. The beta hydrogen and the leaving group generally need to have an appropriate anti-periplanar orientation for effective elimination.
This arrangement allows the orbital overlap necessary for the formation of the new pi bond.
This stereochemical requirement is particularly important when studying cyclic compounds and stereoisomeric substrates.
Complete Comparison of E1 and E2
| Property | E1 | E2 |
|---|---|---|
| Meaning | Unimolecular elimination | Bimolecular elimination |
| Number of steps | Two | One |
| Intermediate | Carbocation | None |
| Number of transition states | Two | One |
| Rate law | k[Substrate] | k[Substrate][Base] |
| Order of reaction | First order | Second order |
| Base | Usually weak base may be sufficient | Usually strong base |
| Carbocation rearrangement | Possible | Not possible |
| Stereochemical requirement | Less restrictive | Important |
| Energy profile | Two peaks | One peak |
| Typical favourable substrate | 3° > 2° | 3°, 2° and suitable 1° substrates |
Quick Revision
- E1 = two-step elimination.
- E2 = one-step elimination.
- E1 forms a carbocation.
- E2 does not form a carbocation.
- E1 has two transition states.
- E2 has one transition state.
- E1 has two energy peaks.
- E2 has one energy peak.
- E1 follows first-order kinetics.
- E2 follows second-order kinetics.
- Carbocation rearrangement is possible in E1.
- Carbocation rearrangement is not possible in E2.
- Strong bases generally favour E2.
- Saytzeff's rule often predicts the more substituted alkene as the major product.
Easy Memory Trick
E1: Two steps → Carbocation → Two transition states → Two peaks.
E2: One concerted step → No carbocation → One transition state → One peak.
Conclusion
E1 and E2 are two fundamental mechanisms of elimination reactions. Both can produce alkenes, but their pathways are different.
The key distinction is that E1 is a stepwise mechanism involving a carbocation intermediate, while E2 is a concerted mechanism in which the base removes the beta hydrogen while the leaving group departs simultaneously.
Understanding the number of steps, rate law, carbocation formation, strength of the base and energy profile makes it much easier to distinguish E1 from E2 reactions in organic chemistry.