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Elimination Reactions (E1 & E2)

Mechanisms, Kinetics, Stereochemistry, and Regioselectivity.

By chemca Team • Updated Jan 2026

Elimination reactions involve the removal of two atoms or groups from a molecule, typically forming a double bond. In alkyl halides, this is usually Dehydrohalogenation ($-\text{HX}$). The two main pathways are E1 (Unimolecular) and E2 (Bimolecular).

1. E1 Mechanism (Elimination Unimolecular)

Two-Step Process

Similar to $S_N1$, E1 proceeds via a carbocation intermediate. It typically occurs with weak bases and tertiary substrates.

Step 1: Ionization (Slow/RDS)
Formation of Carbocation.

$$ R-X \rightleftharpoons R^+ + X^- $$

Step 2: Deprotonation (Fast)
A weak base (solvent) abstracts a $\beta$-proton to form the alkene.

$$ B: + H-\overset{\beta}{C}-\overset{+}{C} \rightarrow B-H^+ + >C=C< $$

Kinetics: Rate $= k[RX]$ (First Order).
Reactivity: $3^\circ > 2^\circ > 1^\circ$ (Based on Carbocation stability).

Rearrangement: Since a carbocation is formed, rearrangements (Hydride/Methyl shift) to a more stable carbocation are possible before elimination.

2. E2 Mechanism (Elimination Bimolecular)

One-Step Concerted Process

Occurs with strong bases. The proton abstraction and leaving group departure happen simultaneously.

$$ Base^- + H-C_{\beta}-C_{\alpha}-X \rightarrow [Base\cdot\cdot H\cdot\cdot C=C \cdot\cdot X]^\ddagger \rightarrow Base-H + >C=C< + X^- $$

Kinetics: Rate $= k[RX][Base]$ (Second Order).
Reactivity: $3^\circ > 2^\circ > 1^\circ$ (Based on stability of alkene formed in Transition State).

Stereochemistry (Anti-Elimination):

The $\beta$-Hydrogen and the Leaving Group ($X$) must be Anti-periplanar (180° dihedral angle) to allow orbital overlap for $\pi$-bond formation.

3. Regioselectivity: Zaitsev vs Hofmann

Which Alkene is Major?

Zaitsev (Saytzeff) Rule

The more substituted alkene is the major product.

Favored when:

• Small, unhindered bases (e.g., $EtO^-, OH^-$).
• Good leaving groups ($I^-, Br^-, Cl^-$).

Hofmann Rule

The less substituted alkene is the major product.

Favored when:

• Bulky, sterically hindered bases (e.g., $t\text{-BuO}^-$).
• Poor leaving groups (e.g., $F^-$).

Example (2-Bromobutane + $EtO^-$):
$CH_3-CH=CH-CH_3$ (81% Zaitsev) + $CH_3-CH_2-CH=CH_2$ (19% Hofmann)

4. E1 vs E2 Comparison

Feature	E1 Mechanism	E2 Mechanism
Steps	Two Steps	One Step (Concerted)
Base	Weak Base ($H_2O, ROH$)	Strong Base ($RO^-, OH^-$)
Intermediate	Carbocation	Transition State
Rearrangement	Possible	Not Possible
Stereochem	No specific requirement	Anti-periplanar required
Order	Unimolecular (1)	Bimolecular (2)

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