E1 elimination reaction is the type of β-elimination reaction. In these reactions, two groups are removed from the adjacent carbon atoms one by one. It results in the formation of a double bond. E1 elimination reaction is also called a unimolecular elimination reaction.
The E1 elimination reaction consists of two steps. The first step is the slow step also the rate-determining step whereas, the second step is the fast step.
Further, E1 reactions are analogous to the SN1 reaction that why compete with nucleophilic substitution (SN1) reactions. Tertiary alkyl halides give E1 elimination reactions. Additionally, the E1 reaction is the regioselective reaction and favors the formation of more substituted alkenes (Zaitsev Product).
Moreover, E1 elimination reaction proceeds through a carbonation intermediate in the presence of a weak base. The first step of the reaction ionization of the substrate. This led to the formation of a carbocation. This step is similar to the SN1 reaction. There is no role of the base in the first step whereas, In the second step base removes β- hydrogen and produces the corresponding alkene.
Furthermore, This reaction is also known as the dehydrohalogenation reaction. Because in this reaction hydrogen and halogen atom are removed from adjacent carbons. For instance, the leaving group is at α carbon whereas hydrogen is removed from β-carbon atoms. In the β-elimination reaction leaving group leaves with electrons while β-hydrogen leaves without electrons.
Competition of SN1 and E1 elimination reaction
E1 reactions compete with SN1 reactions these reactions are taking place side by side. E1 and SN1 reactions are taking place simultaneously. Both of the reactions are first-order reactions. Their rate of reaction depends upon the concentration of substrate. The first step of the SN1 and E1 reactions is the same. It results in the formation of a stable carbocation intermediate.
The second step is an important step of the reaction. E1 reaction or Sn1 depends upon this step. If a solvent act as a nucleophile then the reaction would proceed through the SN1 pathway whereas, if the solvent behaves as a base then the reaction would proceed through the E1 elimination pathway.
Temperature also plays a role to determine whether a reaction will go through elimination or substitution. At low temperatures, the SN1 reaction is favored whereas at elevated temperature E1 reaction is favored.
Moreover, polar solvents also favor E1 elimination reactions. This is because polar solvents stabilize the carbocation intermediate.
E1 elimination reaction Mechanism
Elimination reactions can take place by several mechanisms. Here we are discussing the mechanism following the solvolysis.
The first step involves the ionization of the substrate. In this step, the C-X bond cleave and leaving groups leave the substrate. This results in the formation of a carbocation intermediate, which may arrange toward stability. Note that, there is no role of the base in this step, and it is similar to the first step of the SN1 reaction.
In the second step of the E1 reaction base attack the substrate to abstract a proton. This led to the formation of the double bond.
This is a first-order reaction. In E1 reaction rate of reaction depends on the concentration of substrate. Reactivity relies on carbocation. It is directly proportional to the stability of carbocation. The greater the stability of carbocation greater would be the reactivity. Because the first step of the E1 reaction is the rate-determining step and carbocation is formed in this step.
Rate ∝ [ substrate concentration]
Rate = k [substrate concentration]
- k is the constant of proportionality
Stability of carbocation
Tertiary carbocations are the most stable whereas methyl carbocation is the least stable. Their stability can be explained on the basis of hyperconjugation. Tertiary Carbocation has eight hyper conjugative structures whereas methyl carbocation has no hyper conjugative structure. Hyperconjugation involves the delocalization of electrons from the filled bonding orbital to adjacent unfiled orbitals.
Evidence of E1 mechanism
E1 elimination reaction exhibits first-order kinetics. It means it only depends upon the concentration of substrate. There is no effect of the base in the rate-determining step.
Secondly, the nature of leaving the group does not affect the second step of the E1 elimination reaction. Although it has an effect on the first step of the chemical reaction after the formation of carbocation it has no effect on the second step. If carbocation is the reaction intermediate then there must be rearrangements taking place during the reaction.
E1 reaction does not show deuterium isotopes. This means that there is a cleavage of the C-H bond. There are no geometrical requirements because the halide and hydrogen atom leaves in two steps.
Stereochemistry of E1 Elimination reaction
E1 reactions are stereoselective reactions. Carbocation forms have trigonal planer geometry. The base can remove any β-hydrogen from the second carbon atom. We get a mixture of isomers in different ratios. One of the isomers is produced in excess whereas the other is produced in smaller quantities. In these reactions, trans alkenes are favored over cis alkenes to avoid steric hindrance.
However, the new pi(π) bond can only be formed if the vacant p-orbitals of the carbocation and the breaking of the C-H bond are aligned parallel. But still, one of the isomers is produced in excess because it has less steric crowding.
E1 elimination reaction can also be regioselective. They may result in a mixture of regioisomers. These isomers produce in different ratios. One of the regioisomers will produce in excess based on the stability of the product.
Examples of E1 elimination reactions
Dehydration of alcohols (acid-catalyzed)
Secondary and tertiary alcohols are dehydrated through the E1 mechanism. This is acid-catalyzed dehydration. Secondary alcohol requires a higher temperature than tertiary alcohol. Tertiary alcohols are easily dehydrated under mild conditions. This type of elimination is also known as hydro-hydroxy elimination.
Dehydration of ethers
Ethers can be converted into alkenes when they are treated with strong bases like alkyl sodium, alkyl lithium, sodium amide, etc. This elimination is also known as hydroxy-alkoxy elimination. Note that, tertiary butyl ethers are more easily converted than others.
- E1 elimination is a unimolecular elimination reaction.
- It is a two-step chemical reaction.
- The first step is the slow step hence the rate-determining step.
- There is no effect of the base in the first step.
- The second step is a fast step.
- In the second step, the base removes β-hydrogen.
- Tertiary substrate gives an E1 eliminations reaction.
- E1 is the first-order reaction.
- The rate of reaction depends upon the substrate concentration.
- Mostly syn elimination takes place during the E1 elimination reaction.
- Weak bases are required for E1 elimination reactions.
- E1 reactions are stereoselective reactions.
- Trans alkenes are favored over cis alkene.
E1 is the stereoselctive or stereospecfic reaction?
E1 elimination reactions are stereoselective reactions. We get a mixture of isomers(cis/trans) in different ratios.
What are the similarities between the Sn1 and E1 reactions?
Similarities between SN1 and E1 reactions
- Both are unimolecular reactions.
- These are two-step reactions.
- In both reactions, carbocation is formed in the first step.
- E1 and Sn1 have first-order kinetics.
- Similarly, the rate of reaction depends on substrate concentration.
- The tertiary substrate is required for these reactions.
What are the differences between the E1 and Sn1 reactions?
Differences between SN1 and E1 reactions
- SN1 is the substitution reaction whereas E1 is the elimination reaction.
- The solvent acts as a base in the E1 reaction and as a nucleophile in the SN1 reaction.
- E1 reaction led to the formation of a double bond whereas there is no double bond formation in SN1 reactions.
- β-hydrogens are involved in the e1 reaction whereas there is no involvement of β-hydrogens in the SN1 reaction.
- Low temperature favors SN1 reaction whereas high temperate leads to E1 reaction.
What product do we obtain as a result of E1 elimination reaction?
We get Zaitsev product (more substituted alkene) as a result of the E1 reaction.
Why weak base required for E1 reactions?
Because a strong base leads to the E2 reaction.
Why the trans product is mostly produced in excess in E1 reactions?
Trans products have less steric hindrance that’s why it is produced in excess.
What is meant by hyperconjugation?
Hyperconjugation is a type of delocalization shown by sigma (σ) electrons. It is the system of conjugation in which σ electrons are in conjugation with π electrons.
What is the major product of the E1 elimination reaction?
More substituted alkene is the major product of the E1 elimination reaction.
Solovlysis is a process in which a solvent molecule is present in excess and it dissolves the solute particles through solvation.
What factors favor E1 reactions over SN1 reactions?
There are different factors that prefer E1 reactions over SN1 reactions:
- High temperature
- Weak base
- Polar solvent
- Stable carbocation
- Bulky base
- 12th edition of Organic Chemistry by T.W Graham Solomons, Craig B. Fryhle, and Scott A. Synder.
- 7th edition of March’s Advanced Organic Chemistry by Michael B. Smith.
- Chap 5 (University of Calgary)