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
Double bonds	Yes	No
Carbon atom involvement	Two adjacent	One central