E1: Elimination reaction-Mechanism-Examples and Stereochemistry

E1 elimination is the type of 𝜷-elimination reaction. In these reactions, two groups are removed from the adjacent atoms. It results in the formation of the double bond. E1 elimination is two steps reaction. The first step is the slow step also known as the rate determining step whereas, the second step is the fast step. E1 elimination is also called a unimolecular elimination reaction.

 

general example of e1 elimination reaction

The IUPAC designation of E1 reaction is DN+DE. E stands for elimination whereas 1 stands for the unimolecular. E1 reactions are analogous to the SN1 reactions. They also compete with SN1. Tertiary alkyl halides give E1 elimination reactions. E1 is the regioselective reaction and favors the formation of more substituted alkenes. (Zaitsev Product)

E1 reaction proceeds in the presence of a weak base. This is two step reaction. 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 hydrogens and produces the corresponding alkene.

These reactions are also known as dehydrohalogenation reactions. Because in this reaction hydrogen and halogen atom are removed from adjacent carbons. In these reactions, the leaving group is at 𝛂 carbon whereas hydrogen is removed from 𝜷 carbon atoms. In the 𝜷-elimination reaction leaving group leave with electrons while 𝜷 hydrogen leaves without electrons.

exAMPLE

example of e1 elimination reaction

Mechanism

Elimination reactions can take place by several mechanisms. Here we are discussing the mechanism following solvolysis.

E1 is the unimolecular elimination reaction. it consists of two steps.

The first step is the slow step. It is also known as the rate determining step. This 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 stable carbocation intermediate. There is no role of the base in this step. This is similar to the first step of the SN1 reaction.

first step of e1 elimination reaction

In the second step of the E1 reaction base attack the 𝜷 hydrogen. It removes one of the 𝜷 hydrogens as a proton. This led to the formation of the double bond. E1 reactions usually favor the more substituted products. (Zaitsev Product)

second step of e1 reaction

Reaction kinetics

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 8 hyper conjugative structures whereas methyl carbocation has no hyper conjugative structure. Hyper conjugation involves the delocalization of electrons from filled bonding orbital to adjacent u filed orbitals.

Stability of carbocations

Evidence of E1 mechanism

  1. 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.
  2. 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 but after the formation of carbocation it has no effect on the second step.
  3. If carbocation is reaction intermediate then there must be rearrangements taking place during the reaction.
  4. E1 reaction does not show deuterium isotopes. This means that there is a cleavage of the C-H bond.
  5. There are no geometrical requirements because halide and hydrogen atom leaves in two steps.

Competition of E1 and SN1 reactions

E1 reactions compete with SN1 reactions. these reactions are taking place side by side. E1 and S1 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. This is a slow step and is also called the rate determining step.

The second step is an important step of the reaction. E1 elimination reaction or SN1 substitution reaction 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 its role to determine whether a reaction will go through elimination or substitution. At low temperature, SN1 reaction is favored whereas at elevated temperature E1 reaction is favored.

effect of temeprature on e1 elimination reaction

Polar solvents also favor e1 elimination reactions. This is because polar solvents stabilize the carbocation intermediate.

E1 vs SN1 reaction

ComparisonE1 reactionSN1 reaction
DefinitionIt is a unimolecular elimination reactionIt is a unimolecular substitution reaction.
Order of reactionFirst order reaction.First order reaction.
SolventThe solvent acts as a baseThe solvent acts as a nucleophile
Type of reactionEliminationSubstitution
TemperatureHigh temperature favors e1 elimination.Low temperature favors the SN1 reaction.
Double bondDouble bond formation takes placeNo double bond formation.
ProductAlkenes are produced at the end of the reaction.Alkane is produced at the end of the chemical reaction.

Stereochemistry of E1 Elimination reactions

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. This is to avoid steric hindrance.

stereochemistry of e1 elimination reaction

The geometry of the product depends is determined at the moment that the 𝛃-hydrogen is lost from the carbocation. The new 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.

Regioselectivity

E1 reaction can also be regioselective. They may result in a mixture of regioisomers. These isomers produce in a different ratios. One of the regioisomers will produce in excess. It depends on the stability of the product. More substituted alkenes are a major product because these are more stable.

regioselectivity of e1 elimination

Examples of E1 reaction

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 alcohols. Tertiary alcohols are easily dehydrated under mild conditions. This type of elimination is also known as hydro hydroxy elimination.

dehydration of alcohols

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. tertiary butyl ethers are more easily converted than others.

dehydration of ethers

Differences between E1 and E2 elimination reactions

ComparisonE1 elimination reactionE2 elimination reaction
DefinitionIt is a unimolecular elimination reaction.It is a bimolecular elimination reaction
MechanismIt is two step reaction.It is a single-step reaction
SubstrateTertiary alkyl halides are required.Primary or secondary alkyl halides are required
Order of reactionFirst order reactionSecond order reaction
BaseA weak base is required for e1 elimination reactionsA strong base is required for e2 elimination reaction.
SolventA polar solvent is requiredPolar aprotic solvent is required.
Leaving groupGood leaving group favor E1 elimination reaction.The weak leaving group favors E2 elimination.
eliminationMostly syn elimination takes place.Mostly anti elimination takes place.
IsomersTrans isomer predominately formedCis isomer predominately formed
ProductZaitsev’s product is favored.Hoffman’s product is favored.

Summary

  • E1 elimination is a unimolecular elimination reaction.
  • It is two step chemical reaction.
  • The first step is the slow step also known as 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 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.
  • Similarly, these are regioselective reactions.
  • Usually, Zaitsev product is favored.

Concepts berg

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 reactions?

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?

Hyper conjugation is a type of delocalization shown by 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.

Define solvolysis.

Solovlysis is a process in which a solvent molecule is present in excess and it also acts as a nucleophile or a base.

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

References

  • 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.

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