E2 Reaction, Mechanism and Examples

E2 Reaction

An E2 reaction is a type of elimination reaction, where atoms are removed from a compound to form a new pi bond (in the form of a double or triple bond). There are two elimination reactions that differ based on how the reaction mechanism proceeds. Both end up with a new pi bond. The E2 elimination reaction occurs in a single concerted step while the E1 elimination reaction occurs in two separate steps. The 'E' refers to it being an elimination reaction, while the '2' is explaining how many reactants are involved in the rate-limiting step. As there are a total of 2 reactants, and only one step occurring, both reactants are involved in the rate-limiting step. This makes it a bimolecular reaction, where there are two reactants in the rate equation: {eq}rate = k[alkane][^-OH] {/eq}.

E2 reactions start with an alkane (carbon-carbon single bond) and produce an alkene (carbon-carbon double bond), or they start with an alkene and produce an alkyne (carbon-carbon triple bond). The starting alkane or alkene needs to have a good leaving group attached for the reaction to occur. This is often a halide (fluorine, chlorine, bromine, and iodine), but it can also be an alcohol that has had a hydrogen added to it. The better the leaving the group, the more reactive the alkane will be. The rate of the reaction also increases the more substituted the carbon with the leaving group is. In other words, the more R groups that are attached to the carbon with the leaving group, the faster the reaction will occur.

The other reactant is a base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). The hydroxide in the base ends up as water, and the cation forms an ionic bond with the leaving group. The reaction is often written only showing the hydroxide with it implied that any cation could be included.

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E2 Mechanism

The E2 mechanism occurs in a single, concerted step. Concerted means that more than one thing is happening at the same time. While the reaction is often talked about as though one thing happens first, in reality, any part of the reaction could happen first and the rest of the reaction occurs at the same time.

In the E2 reaction mechanism there are three things that are occurring at the same time:

  • The hydrogen attached to the carbon adjacent to the carbon with the leaving group is removed by the base
  • The leaving group leaves
  • The carbon-hydrogen atoms form a new pi bond (such as a new double bond)

Any one of these can spontaneously occur, causing the rest of the reaction to simultaneously occur as well.

In the E2 reaction all three things occur at the same time in a single, concerted, step.

Mechanism of the E2 reaction

Stereochemistry of E2 Reactions

The E2 reaction stereochemistry is both regioselective and stereoselective. Regioselective refers to a reaction that can produce more than one constitutional isomer (the chemical formula is the same, but the structure is different), but one is preferred over the others. For the E2 reaction, the more substituted alkene is preferred. Stereoselective means that there are two or more possible stereoisomers (the structure is the same, but the 3D orientation is different) as a product, but one is formed predominantly. For the E2 reaction, the trans product is preferred.

In this halide, there are 3 possible hydrogen atoms that could be removed on carbon atoms adjacent to the carbon-chlorine bond,, creating the possibility of 3 different compounds

Secondary halide

In the above secondary halide, there are three possible products. If the hydrogen circled in red is removed then 2-methyl-2-pentene is formed:

The more substituted alkene is 2 methyl 2 pentene

Structure of 2 methyl 2 pentene

If either of the hydrogen atoms circled in blue is removed then 4-methyl-2-pentene is formed:

The minor product is 4 methyl 2 pentene, with the alkene being less substituted.

Structure of 4 methyl 2 pentene

2-methyl-2-pentene and 4-methyl-2-pentene both have the chemical formula {eq}C_6H_12 {/eq}, but have different structures, making them constitutional isomers. The major product in an E2 reaction is the more substituted alkene. In other words, the alkene that has a carbon atom attached to more R groups. The alkene in 2-methyl-2-pentene is attached to a total of 3 R groups, while the alkene in 4-methyl-2-pentene is only attached to a total of 2 R groups. This makes 2-methyl-2-pentene the major product.

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Frequently Asked Questions

How does E2 reaction work?

An E2 reaction occurs in a single concerted step. In this step a base removes the hydrogen, the carbon-hydrogen electrons form a new pi bond, and the leaving groups leaves with its electrons.

What is meant by E2 reaction?

An E2 reaction is an elimination reaction where the concentration of 2 different reactants is involved in the rate limiting step and equation. It forms a new pi bond.

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