Elimination Reactions in Organic Chemistry

Elimination reaction in organic chemistry has main types as E1-reaction, E2-reaction and E1cB-elimination reaction. 

Key words: E1-reaction, E2-reaction and E1cB-elimination reaction, Dehydrohalogenation, Dehalogenation, Dehydration

Hi Friends, in this article we will learn elimination reactions in organic chemistry.

Table of Contents

1. Definition of Elimination Reaction
2. E1-reaction 
3. E1cB Reaction
4. E2 Reaction
5. Conclusion

1. Definition of Elimination Reaction

An elimination reaction involves removal of fragment of molecule to produce alkene product. The group which is being removed is known as Leaving group. The reaction takes place in basic or acidic medium.

Fig 1 : The Elimination Reaction

In organic chemistry, most common elimination reactions are;

Dehydrohalogenation (1)

Dehalogenation (2)

Dehydration (3)

We have discussed these reactions in detail. Please check out the post for further readings.

Here in this article, we will learn about types of mechanism of elimination reactions.

2.1 E1-reaction

The abbreviation E1 stands for “unimolecular elimination”; that is, an E1 reaction is an elimination reaction in which only one species is involved in the rate-limiting step.

The mechanism of E1-reactions consists of two steps,

Step 1: Loss of leaving group to form carbocation.

Step 2:  Deprotonation on adjacent carbon to for alkene product.

In these types of reaction mechanism mild base required. Here the step 1 in rate determining step.

Requirements:

Acidic Media
Good leaving groups

For example; the reaction of 2-bromo-2methyl butane with ethanol in presence of heat gives alkene product 2-methylbut-2-ene. Here in the first step formation of carbocation takes place followed by deprotection on adjacent carbon by mild base gives the major product. 

Fig 2 : The E1-Reaction

We have discussed IUPAC nomenclature of alkene compounds in previous article. Please see the article for details. (4)

Since the reaction intermediate is carbocation, there are chances of rearrangements to form more stable carbocation followed by alkene product. For example;

Fig 3: Rearrangement of Carbocation 

3. E1cB Reaction

The abbreviation E1-cB stands for Elimination Unimolecular conjugate Base.

This is stepwise reaction.

Step1: A base abstract the acidic proton to form a new base which is more stable due to conjugation of double bond.

Step 2: Attack of conjugate base on adjacent carbon which bears Leaving group to form alkene product.

Here in this type reactions, intermediate is Carbanion and Step 1 is the rate determining step.

Requirements:

Strong base
Poor leaving group

For example; check out the reaction of hydroxy ketone with sodium ethoxide. It provides elimination product under basic conditions. The hydroxy ketone which is shown below is can be synthesized by Aldol Reaction. For the details of Aldol reactions please check the previous article.(5)

Coming to the reaction mechanism,

Step 1: Ethoxide ion deprotonates the carbon which is next to carbonyl group to generate carbanion intermediate

Step 2: Attack of lone pair of electrons to remove hydroxyl group and form elimination product.

Fig 4: The E1cB Reaction

4. E2 Reaction

E2 elimination reaction is Concerted reaction. Here in this reaction strong base is required. Since this is one step reaction, there is no chance for rearrangement product.  In this type of reactions highly substituted alkene product is favored.

Requirements:

Strong & bulky base
Good Leaving group

For example; reaction of 2-chloro-3methylpentane with sodium ethoxide to provide elimination product.

Here ethoxide ion deprotonates the carbon and removal of leaving group (Cl) talks place in concerted manner to provide elimination product.

We have discussed geometric isomerism in alkene compounds in previous article. Please see the article for details. (6)

Fig 5: E2 Reaction

5. Conclusion

Let’s summarize the all three reactions.

Fig 6 : Comparison of E1, E2 & E1cB Reaction

That's all for this topic. If you feel this information helpful, please feel free to ask me in the comment box. Thank you...! See you in the next blog ..!!