Elimination Reaction
Habish Ribin Haneef
An elimination reaction is a reaction in which saturated compounds are transformed into unsaturated compounds. Saturated compounds are those organic compounds which contain single carbon-carbon bonds and unsaturated compounds are those organic compounds which contain double or triple carbon-carbon bonds. In an elimination reaction, atoms are removed as molecules or compounds. The removal process usually occurs due to the action of acids or bases or metals. Elimination reactions are endothermic reactions that occur at high temperatures. For the preparation of alkenes and alkyl halides, elimination reactions are used.
Before going to the detailed explanation of elimination reaction, you should have a clear picture about α and β Carbon and Hydrogen atoms. α-carbon is the first carbon atom attached to a functional group in an organic molecule. The carbon atom that is directly attached to a halogen group, aromatic group, or double or triple bond is also known as α carbon. The hydrogen atoms attached to α carbon are called α hydrogens.
β Carbon is the carbon atom present next to the α carbon. The hydrogen atoms attached to the β Carbons are known as β hydrogens.
The two important methods of elimination reaction are Dehydration and Dehydrohalogenation. In the dehydration method, water molecules are eliminated mainly from compounds like alcohol. The dehydration method is sometimes known as beta elimination method, where the living group and hydrogen atoms are placed at neighbouring carbon atoms. On the other hand, in dehydrohalogenation, hydrogen atoms and halogen atoms are eliminated
Dehydration method
Dehydrogenation method
Mechanism of Elimination Reaction:
Based on reaction kinetics, there can be mainly two mechanisms of elimination reaction- E1 reaction and E2 reaction. Here E denotes elimination and the corresponding number denotes molecularity.
E1 Reaction:
E1 reaction is also called unimolecular reaction and it takes place in two steps. The two steps in E1 reaction are ionization and deprotonation. E1 reaction is endothermic and occurs at high temperatures. This reaction takes place in the presence of weak bases only. Now, let’s come to the two steps of the E1 reaction. The first one is ionization. In ionization, the formation of carbocation as an intermediate occurs. Here, in the presence of a polar protic solvent, the leaving group leaves the α-carbon as anion and carbocation is formed. This is a slow and rate-determining step. The second one is deprotonation where a proton is lost by the carbocation. Here, the carbocation reacts with a weak base and deprotonation occurs at β Carbon. Alkene is formed as a result of this. The E1 reaction displays first-order kinetics. In this process, the reaction rate is proportional to the concentration of the substance formed.
Generally, E1 reaction can be expressed as shown below:
Examples
- C3H7OH ———–> C3H6 + H2O
This is the dehydration of alcohol. Here, 2-propanol ( C3H7OH ) loses a molecule of water ( H2O ) and forms propene ( C3H6 ) in the presence of sulfuric acid ( H2SO4). This process is called acid-catalyzed dehydration.
- C5H11Br ———-> C5H10 + HBr
This is the dehydrohalogenation of alkyl halides. Here, 2-Bromo-3-methyl butane ( C5H11Br) converts into a combination of 2-methyl-2-butene ( C5H10 ) and 2-methyl-3-butene ( C5H10 ) in the presence of methanol ( CH3OH ).
E2 Reaction:
E2 reaction is also called bimolecular elimination and it takes place in a single step. This reaction is endothermic and takes place at high temperatures similar to the E1 reaction. E2 reaction occurs in the presence of a strong base only. Here the reaction rate is proportional to the concentrations of the eliminating agent and the substrate. The E2 reaction displays second-order kinetics. Polar aprotic solvent is used in the E2 reaction. This is the rate-determining step as the process involves only one step. In the E2 reaction, the carbon-hydrogen and carbon-halogen bonds get broken to form a new double bond.
The rate of E2 reaction is expressed as,
Rate= k[RX][Base]
Hence, it is evident that the reaction rate depends on both the substrate and the base. The major product formed in the elimination reaction is the most stable alkene.
Generally, the E2 reaction can be expressed as shown below:
Example:
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An elimination reaction is a reaction in which saturated compounds are transformed into unsaturated compounds.
Dehydration and Dehydrohalogenation
E1 reaction and E2 reaction