Elimination reaction : E1 and E2 reaction – Examples, Mechanism, Orientation and Reactivity

Elimination reaction

An elimination reaction is a type of organic reaction in which a pair of atoms or group of atoms are removed from a organic molecule. Elimination reaction is the principal process by which saturated organic compounds (i.e compounds containing carbon – carbon single bonds) are converted to unsaturated organic compounds (i.e. compounds containing carbon – carbon double or triple bonds). For example- Dehydrohalogenation reaction of alkyl halides.

When an alkyl halide is heated with an alcoholic solution of KOH or NaOH, an alkene is formed by the elimination of a molecule of hydrogen halide. This reaction is known as dehydrohalogenation of alkyl halide. This reaction involves the removal of hydrogen atom together with the halogen atom from the adjacent carbon atom, so this reaction is also called the α,β- elimination reaction.

Types of elimination reactions

Elimination reaction is of two types:

1. E₂ reaction
2. E₁ reaction

E₂ reaction

E₂ reaction is also known as elimination bimolecular reaction. This reaction occurs when an alkyl halide is treated with a strong base such as hydroxide ion (OH-) and forms a carbon-carbon double bond. Example:

Kinetics of E₂ reaction :

In E₂ reaction, the rate of dehydrohalogenation (i.e. alkene formation) depends upon the concentration of both alkyl halide and base. It follows second order kinetics.

Rate α [Alkyl halide] [Base]

R = k[RX] [:B]

Mechanism of E₂ reaction :

E₂ mechanism is a one step process. Base attacks the hydrogen atom of β-carbon and begins to remove the H atom and at the same time as the carbon-carbon double bond starts to form, the –X group starts to leave as shown below in transition state. After the transition state, C-H bond and C-X bond are completely broken and carbon-carbon double bond is formed.

Energy profile diagram of E₂ reaction:

Orientation and reactivity of E₂ reaction:

In some cases, this reaction yields a single alkene but in other cases a mixture of alkenes are formed. For example, 2-bromopropane forms only propene whereas 2-bromobutane forms a mixture of 1-butene and 2-butene.

If there is a possibility of formation of mixture of alkenes, the E₂ reaction follows Saytzeff rule. This rule states that the major product is the alkene that has greater number of alkyl groups attached to the double bonded carbon atoms. Therefore, the stability order of various alkenes is:

R₂CH=CR₂ > R₂C=CHR > R₂C=CH₂ > RCH=CHR > RCH=CH₂ > CH₂=CH₂

Hence, in dehydrohalogenation reaction, the more stable the alkene more easily or faster it is formed.

E₁ reaction

E₁ reaction is also known as elimination unimolecular reaction. This reaction is particularly common in secondary and tertiary alkyl halides in absence of a strong base. For example, when 2-bromo-2-methylpropane is treated with aqueous ethanol, 2-methyl propene is formed.

Kinetics of E₁ reaction:

In E1 reaction, the rate of alkene formation depends upon the concentration of alkyl halide only. It follows first order kinetics.

Rate α [Alkyl halide]

R = k [RX]

Mechanism of E₁ reaction:

E₁ mechanism is a two step process.

Step I : In this step the molecule of alkyl halide undergoes ionization to give a carbocation (carbonium ion) and halide ion.

Step II : In this step, the carbocation loses a proton from the adjacent carbon to yield the alkene.

In a chemical reaction the slow step is rate determining step, so first step is rate determining step of E₁ reaction.

In certain alkyl halides, the carbocation initially formed undergoes rearrangement to form more stable carbocation and thus highly branched alkene is formed as the major product.

For example, dehydrohalogenation of 3-chloro-2,2-dimethylbutane yields 2,3-dimethylbut-2-ene as major product.

Energy profile diagram of E₁ reaction:

Orientation and reactivity of E₁ reaction:

Elimination by E₁ reaction shows Saytzeff’s orientation. For example;

Reactivity of alkyl halide to E₁ reaction is determined by the rate of formation of carbocation which in turn depends on stability of carbocation. Hence, order of reactivity of alkyl halides in E₁ reaction is:

3⁰ haloalkane > 2⁰ haloalkane > 1⁰ haloalkane

References

• Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
• Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
• Morrison, R.T. , Boyd, R.N., Organic Chemistry, Sixth edition, Prentice-Hall of India Pvt. Ltd., 2008.
• March, j., Advanced Organic Chemistry, Fourth edition, Wiley Eastern Ltd. India, 2005.
• https://www.britannica.com/science/elimination-reaction