Dehydrohalogenation of Alkyl Halides – 1,2 Elimination, E2 and E1
In organic chemistry, alkyl halides can undergo elimination reactions. These reactions remove two groups from a molecule and form a double bond. This is important for preparing alkenes.
The most common elimination reaction in alkyl halides is dehydrohalogenation, where a hydrogen atom (H) and a halogen (Cl, Br, I) are removed. These two atoms are removed from adjacent carbon atoms, also called the α-carbon and β-carbon.
What is 1,2-Elimination (β-Elimination)?
When the hydrogen is removed from the carbon next to the halogen-bearing carbon, it is called β-elimination or 1,2-elimination. This forms a double bond between the two carbons.
Examples
• Alkyl chlorides → eliminate HCl
* Alkyl bromides → eliminate HBr
* Alkyl iodides → eliminate HI
All these reactions form alkenes.
Important Terms
- α-carbon: Carbon attached to the halogen (leaving group).
- β-carbon: The carbon next to α-carbon.
- β-hydrogen: Hydrogen on β-carbon, removed during elimination.
Mechanisms of Dehydrohalogenation
Elimination reactions mainly follow two mechanisms:
- E2 Mechanism – bimolecular, single-step reaction
- E1 Mechanism – unimolecular, two-step reaction
E2 Mechanism (Bimolecular Elimination)
E2 means “elimination, second order.” The rate depends on:
- Concentration of alkyl halide
- Concentration of base
Rate = k [alkyl halide] [base]
Key Features of E2 Reaction
- Happens in one single step.
- Strong base removes β-hydrogen.
- Leaving group (halide) leaves at the same time.
- Forms a double bond immediately.
- Base must “pull” the H from the opposite side of the leaving group (anti-periplanar geometry).
Why This Geometry?
The electron pair from the C–H bond pushes out the halide group. This is similar to the backside attack in SN2 reactions.
E1 Mechanism (Unimolecular Elimination)
E1 means “elimination, first order.” The rate depends only on the concentration of the alkyl halide.
- Occurs in two steps.
- Step 1: Leaving group leaves → forms a carbocation (slow step).
- Step 2: Base removes a β-hydrogen → double bond forms (fast step).
E1 vs SN1
E1 and SN1 share the same first step: carbocation formation. After that:
- SN1: A nucleophile attacks the carbocation → substitution product.
- E1: A base removes β-hydrogen → elimination product (alkene).
So, E1 usually gives a mixture of substitution and elimination products.
When Does a Reaction Go by E1?
- With tertiary alkyl halides
- Polar protic solvents (like ethanol)
- Weak bases
Summary of Differences: E2 vs E1
| E2 | E1 |
|---|---|
| One-step reaction | Two-step reaction |
| Strong base required | Weak base sufficient |
| No carbocation | Carbocation formed |
| Occurs with 1°, 2°, 3° halides | Mainly with 3° halides |
| Rate depends on substrate + base | Rate depends only on substrate |
PDF with diagrams and mechanisms will be embedded below for students.
SHORT VERSION:
PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/Dehydrohalogenation of alkyl halides – 1,2 elimination, kinetics, E2 and E1.