E1 and E2 reactions

E1 and E2 reactions – kinetics, order of reactivity of alkyl halides, rearrangement of carbocations, Saytzeffs orientation and evidence.

• E1 and E2 reactions are two types of elimination reactions that occur in organic chemistry, where elements of a molecule are removed, leading to the formation of a double bond.

• The key difference between them lies in the mechanism by which this elimination occurs.
  

Let's delve into each reaction type in detail:

E1 Reaction (Unimolecular Elimination)

1. The E1 reaction is a two-step process involving the formation of a carbocation intermediate.

Mechanism of E1 Reactions

Formation of Carbocation (Step 1):

1. The reaction begins with the departure of the leaving group from the substrate, leading to the creation of a carbocation. This step is crucial as it sets the stage for the formation of the double bond in the subsequent step.

2. Equation: R-LG → R⁺ + LG⁻

Proton Abstraction and Double Bond Formation (Step 2):

1. Following the formation of the carbocation, a base abstracts a proton from a carbon atom adjacent to the carbocation. This action results in the formation of a double bond, generating the alkene product.

2. Equation: R⁺ + B → R=CR₂ + BH⁺

3. Example: Dehydration of 2-Methyl-2-Butanol

4. To illustrate the E1 mechanism, consider the dehydration of 2-methyl-2-butanol, a tertiary alcohol, in the presence of a strong acid like sulfuric acid.

Proton Abstraction and Double Bond Formation

Step 1: Protonation of the Alcohol:

• The hydroxyl group's oxygen in 2-methyl-2-butanol bonds with a proton (H⁺) from sulfuric acid, transforming the -OH into a better leaving group (water).

Step 2: Formation of Carbocation:

• The bond between the carbon and oxygen breaks as the water molecule departs, resulting in the formation of a tertiary carbocation. This step is the rate-determining step of the E1 reaction.

Step 3: Proton Abstraction by a Base:

• A base present in the solution (could be a water molecule or a bisulfate ion, HSO₄⁻) then abstracts a proton from a carbon adjacent to the carbocation.

• The electrons from the C-H bond move to form a double bond, leading to the formation of 2-methyl-2-butene.

Factors Influencing E1 Reactions

1. Stability of Carbocation:

1. The rate and feasibility of E1 reactions depend significantly on the ability of the substrate to form a stable carbocation.

2. Tertiary and allylic or benzylic substrates, which can form more stable carbocations, are more prone to undergo E1 reactions.

2. Nature of the Leaving Group:

1. A good leaving group is essential for the first step of the E1 mechanism.

2. Better leaving groups facilitate the formation of the carbocation.

3. Solvent Effects:

1. Polar solvents can stabilize the carbocation and the leaving group, aiding the reaction process.

4. Temperature:

Higher temperatures favor the E1 reaction by providing the necessary energy for the formation of the carbocation.

E2 Reaction (Bimolecular Elimination)

• The E2 reaction is a one-step process where the elimination of the leaving group and the deprotonation of a β-hydrogen (hydrogen on a carbon adjacent to the carbon bearing the leaving group) occur simultaneously.

Mechanism:

1. In an E2 reaction, a strong base attacks a β-hydrogen, and the electrons from the C-H bond are used to form a double bond between the α and β carbons, while simultaneously ejecting the leaving group.

2. This process is bimolecular, with the rate of reaction depending on the concentration of both the substrate and the base.

3. The stereochemistry of the E2 reaction is often important, with the most common outcome being anti-periplanar geometry, where the hydrogen and the leaving group are on opposite sides of the molecule.

Example:

1. CH3​CH2​CH2​Br + OH− → CH3​CH=CH2 ​+ Br− + H2​O

2. In this reaction, 1-bromopropane reacts with a hydroxide ion (a strong base) to produce propene, bromide ion, and water.

3. The hydroxide ion attacks a β-hydrogen, leading to the formation of the double bond and the simultaneous ejection of the bromide leaving group.