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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.

The Meaning of E2

E2 is one of four major reaction mechanisms you'll encounter early in your study of organic chemistry. The E stands for elimination. Two atoms on adjacent carbons are removed, or eliminated, in order to obtain the product. The end result is an alkene with a new C-C pi bond. The 2 in the mechanism name stands for bimolecular. This means that the rate of the reaction depends on the concentration of the substrate, the organic molecule undergoing elimination, as well as the base that is required for the reaction.

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  • 0:36 Examples of E2 Reactions
  • 1:06 The E2 Mechanism
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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.

Examples of E2 Reactions

All E2 reactions have two things in common: a good leaving group and a hydrogen atom on a carbon adjacent to the one with the leaving group. Alkyl halides and alcohols are the most common reactants in an E2 reaction. Here are some examples of E2 reactions.

Alkyl halides undergo elimination to produce alkenes.
E2 elimination from RX

Alcohols undergo elimination to produce alkenes.
E2 ROH

Notice that E2 elimination of an alcohol uses acid, not base. As we'll see when we look at the mechanism, the acid is used to make the -OH group into a better leaving group, which is water.

The E2 Mechanism

Here we see the mechanism of E2 elimination from an alkyl halide, RX.

Bimolecular elimination occurs in a single, concerted step.
E2 mechanism

This reaction works best when the halide is primary or secondary. As noted earlier, a base takes a proton (H+) from a carbon adjacent to the one with the X. The electrons that had formed the C-H bond are now used to make a new C-C pi bond. When the new bond forms, the leaving group leaves with its electron pair.

Elimination of water from a primary alcohol is possible, but difficult. However, if it occurs it will use the E2 mechanism. Unimolecular elimination reactions, also called E1, use a different mechanism that involves a carbocation. Since primary carbocations rarely form, elimination of a primary alcohol will occur via E2 as you can see.

Bimolecular elimination of a primary alcohol requires acid to convert the -OH group to a good leaving group.
E2 alcohol

Stereochemistry & Regiochemistry of E2 Reactions

Let's take another look at the E2 mechanism involving an alkyl halide. Notice that the hydrogen atom removed by the base is anti to the leaving group, or the halide ion, X. This is an important stereochemical feature of the reaction mechanism, and it determines the stereochemistry of the product alkene when the alkyl halide is secondary.

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Video Transcript

The Meaning of E2

E2 is one of four major reaction mechanisms you'll encounter early in your study of organic chemistry. The E stands for elimination. Two atoms on adjacent carbons are removed, or eliminated, in order to obtain the product. The end result is an alkene with a new C-C pi bond. The 2 in the mechanism name stands for bimolecular. This means that the rate of the reaction depends on the concentration of the substrate, the organic molecule undergoing elimination, as well as the base that is required for the reaction.

Examples of E2 Reactions

All E2 reactions have two things in common: a good leaving group and a hydrogen atom on a carbon adjacent to the one with the leaving group. Alkyl halides and alcohols are the most common reactants in an E2 reaction. Here are some examples of E2 reactions.

Alkyl halides undergo elimination to produce alkenes.
E2 elimination from RX

Alcohols undergo elimination to produce alkenes.
E2 ROH

Notice that E2 elimination of an alcohol uses acid, not base. As we'll see when we look at the mechanism, the acid is used to make the -OH group into a better leaving group, which is water.

The E2 Mechanism

Here we see the mechanism of E2 elimination from an alkyl halide, RX.

Bimolecular elimination occurs in a single, concerted step.
E2 mechanism

This reaction works best when the halide is primary or secondary. As noted earlier, a base takes a proton (H+) from a carbon adjacent to the one with the X. The electrons that had formed the C-H bond are now used to make a new C-C pi bond. When the new bond forms, the leaving group leaves with its electron pair.

Elimination of water from a primary alcohol is possible, but difficult. However, if it occurs it will use the E2 mechanism. Unimolecular elimination reactions, also called E1, use a different mechanism that involves a carbocation. Since primary carbocations rarely form, elimination of a primary alcohol will occur via E2 as you can see.

Bimolecular elimination of a primary alcohol requires acid to convert the -OH group to a good leaving group.
E2 alcohol

Stereochemistry & Regiochemistry of E2 Reactions

Let's take another look at the E2 mechanism involving an alkyl halide. Notice that the hydrogen atom removed by the base is anti to the leaving group, or the halide ion, X. This is an important stereochemical feature of the reaction mechanism, and it determines the stereochemistry of the product alkene when the alkyl halide is secondary.

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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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