What is E1 Reaction?
E1 reaction is a type of elimination reaction in organic compounds. A beta-hydrogen and a leaving group attached to two adjacent carbon atoms are removed from a compound (substrate) to form a double bond. The removal usually occurs in two steps in the presence of a Lewis base or acid, resulting in an alkene. The reaction rate is proportional to the concentration of the substrate and not of the base. Since the base is not involved in the rate-determining step, its nature is unimportant. The E1 reaction is common in alkyl halide during dehydrohalogenation and alcohol during dehydration [1-10].
Characteristics of E1 Reaction
- Also known as unimolecular elimination
- Two-step removal mechanism process – ionization and deprotonation
- Non-concerted with a carbocation intermediate
- Carbon goes from sp3 to sp2 hybridization state
- A first-order reaction, i.e., the rate is proportional to the concentration of the substrate (R = k [substrate])
- The rate-determining step is the loss of the leaving group.
- The reaction rate increases as the number of alkyl groups on the carbocation increases. The rate is highest for tertiary carbocation, followed by secondary and primary.
- Regioselective and stereospecific – the reaction follows Zaitsev’s rule, i.e., favors the formation of Zaitsev product.
Examples of E1 Reaction [1-11]
1. Dehydration of Alcohols
In the presence of sulfuric acid (H2SO4), 2-propanol (C3H7OH) loses a molecule of water (H2O) to form propene (C3H6). This process is known as acid-catalyzed dehydration.
C3H7OH → C3H6 + H2O
2. Dehydrohalogenation of Alkyl Halides
In the presence of methanol (CH3OH), 2-Bromo-3-methyl butane (C5H11Br) converts into a combination of 2-methyl-2-butene (C5H10) and 2-methyl-3-butene (C5H10).
C5H11Br → C5H10 + HBr
Mechanism of E1 Reaction
The mechanism of the E1 reaction takes place in two steps [1-10].
Step 1: Formation of carbocation – The leaving group leaves the alpha-carbon in the presence of a polar protic solvent.
Step 2: Deprotonation – The alpha-carbon reacts with the base, resulting in the deprotonation at beta-carbon. The final product is an alkene.
Zaitsev’s rule is applied where the most substituted alkene is formed.
SN1 and E1 Reaction
SN1 and E1 reactions have a similar first step in their reaction mechanisms, where a carbocation forms the intermediate. Due to carbocation’s stability, the two reactions favor tertiary (3rd degree) carbocation over primary (1st degree). Both are first-order reactions in which the rate depends on the concentration of the substrate. The following table lists the differences between the two reactions.
|E1 Reaction||SN1 Reaction|
|Type of reaction||Elimination of a functional group||Substitution of a nucleophile|
|Mechanism||Base pulls off a beta-hydrogen||Nucleophile attacks the carbocation|
|Heat||Important||Not so important|
|Carbon atom involvement||Two adjacent||One central|