Evidence for the E2 Mechanism
E2 is a one-step elimination reaction where the base removes a β-hydrogen at the same time the leaving group leaves. Because everything happens in a single step, this mechanism shows unique and very predictable behavior.
The major evidence supporting E2 includes:
- Second-order kinetics
- Absence of rearrangement
- Isotope effect
- Absence of hydrogen exchange
- The element effect
- Product orientation and reactivity
1. Absence of Rearrangement
Carbocation rearrangement happens only in the E1 mechanism because E1 forms a carbocation intermediate. But E2 happens in one step and never forms a carbocation. Therefore, rearrangements are not possible in the E2 pathway.
2. Isotope Effect
This is very strong evidence for the E2 mechanism. In E2, breaking the C–H bond is part of the rate-determining step. If the β-hydrogen is replaced by deuterium (D), the C–D bond is stronger and harder to break.
As a result:
- The reaction becomes slower when deuterium is present.
- Experiments show the hydrogen-containing compound reacts about seven times faster than the deuterated compound.
This proves that C–H bond breaking is directly involved in the slow step, which is consistent with E2 but not with E1.
3. Absence of Hydrogen Exchange
Hydrogen exchange would only occur if a reversible carbanion intermediate is formed. That happens in the E1cB mechanism, not in E2.
Experiments using deuterium tracers show:
- No hydrogen exchange occurs during the reaction.
- This rules out the presence of any reversible carbanion.
Therefore, the reaction must follow the E2 pathway.
4. The Element Effect
The nature of the leaving group affects reaction speed. Leaving groups lower in the periodic table (I⁻ > Br⁻ > Cl⁻) generally give faster eliminations because their bonds break more easily.
This observation supports the E2 mechanism because the breaking of the C–X bond happens in the rate-determining step.
5. Orientation and Reactivity
E2 reactions often produce more than one alkene. However, one alkene usually forms as the major product. This depends on alkene stability.
The general rule is:
More substituted, more stable alkenes form faster.
This is known as Zaitsev’s rule.
For example, in the elimination of sec-butyl bromide:
- Elimination from carbon-1 gives 1-butene.
- Elimination from carbon-3 gives 2-butene.
Although 1-butene is easier to form statistically, 2-butene becomes the major product because it is more stable.
Why does the more substituted alkene form faster?
Because the E2 transition state already has partial double-bond character. Anything that stabilizes the double bond also stabilizes the transition state, lowering activation energy and increasing reaction speed.
So, alkyl-substituted alkenes form more rapidly in E2 reactions.
Summary
- E2 shows second-order kinetics.
- No rearrangement occurs.
- Isotope effect clearly supports C–H breaking in the slow step.
- No hydrogen exchange means no carbanion intermediate.
- Leaving group ability strongly affects rate.
- Product formation follows Zaitsev’s rule based on alkene stability.
All these points strongly prove the single-step E2 elimination mechanism.
Detailed Notes:
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PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/ EVIDENCE FOR E2 MECHANISM, ABSENCE OF REARRANGEMENT ISOTOPE EFFECT, ABSENCE HYDROGEN EXCHANGE, THE ELEMENT EFFECT, ORIENTATION AND REACTIVITY.