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Elimination Reactions and Formation of Alkenes: Synthesis via E2 Mechanism, Exercises of Organic Chemistry

University of Detroit MercyOrganic Chemistry

An in-depth exploration of elimination reactions and their role in the formation of alkenes through the E2 mechanism. Topics covered include the difference between unimolecular (E1) and bimolecular (E2) mechanisms, the advantages of E2 reactions in dehydrohalogenation, the reaction mechanism and energy diagram, effects of leaving group and solvent, and the role of alkyl halide structure. The document also discusses the classification and relative stabilities of alkenes, stereoisomers, and the Zaitsev rule.

What you will learn

  • What is the Zaitsev rule and how does it apply to E2 reactions?
  • What are the advantages of using the E2 mechanism in dehydrohalogenation reactions?
  • How does the structure of the alkyl halide affect the rate of E2 reactions?

Typology: Exercises

2021/2022

Uploaded on 09/27/2022

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Elimination Reactions
&
Formation of Alkenes
Chapter 8
Synthesis of Alkenes via Elimination Reactions
Dehydrohalogenation
u Reactions are possible by unimolecular (E1) or bimolecular (E2)
mechanisms.
u The E2 mechanism is more synthetically useful in
dehydrohalogenation reactions because regioslectivity and side
reactions can be more easily limited by controlling the reaction
conditions
u General Reaction & Mechanism:
The E2 Reaction
E2 reaction involves concerted removal of the proton, formation of
the double bond, and departure of the leaving group.
The reaction is 2nd order.
E2 means bimolecular elimination.
E2 reactions are β-eleminations
E2 reactions are favored by:
u Secondary or tertiary alkyl halides.
u Alkoxide bases such as sodium ethoxide or other strong bases
u Bulky bases such as potassium tert-butoxide should be used for
E2 reactions of primary alkyl halides
Energy Diagram for an E2 Reaction
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Elimination Reactions

Formation of Alkenes

Chapter 8

Synthesis of Alkenes via Elimination Reactions

Dehydrohalogenation u Reactions are possible by unimolecular (E1) or bimolecular (E2) mechanisms. u The E2 mechanism is more synthetically useful in dehydrohalogenation reactions because regioslectivity and side reactions can be more easily limited by controlling the reaction conditions u General Reaction & Mechanism:

The E2 Reaction

E2 reaction involves concerted removal of the proton, formation of the double bond, and departure of the leaving group. The reaction is 2nd order. E2 means bimolecular elimination. E2 reactions are β-eleminations E2 reactions are favored by: u Secondary or tertiary alkyl halides. u Alkoxide bases such as sodium ethoxide or other strong bases u Bulky bases such as potassium tert-butoxide should be used for E2 reactions of primary alkyl halides

Energy Diagram for an E2 Reaction

  • The most common bases used in elimination reactions are negatively charged oxygen compounds, such as HO¯ and its alkyl derivatives, RO¯, called alkoxides.
  • Alkoxide synthesis:

Common Bases for Dehydrohalogenation Effects of Leaving Group and Solvent on E2 Reactions

  • Because the bond to the leaving group is partially broken in the transition state, the better the leaving group the faster the E2 reaction.
  • Polar aprotic solvents increase the rate of E2 reactions.
  • The SN2 and E2 mechanisms differ in how the alkyl halide structure affects the reaction rate.
  • As the number of R groups on the carbon with the leaving group increases, the rate of the E2 reaction increases.

Effect of Alkyl Halide Structure on E2 Reactions

  • The double bond of an alkene consists of a σ bond and a π bond.

Alkene Structure

Overall Relative Stabilities of Alkenes

The greater the number of attached alkyl groups

( i.e. the more highly substituted the carbon

atoms of the double bond), the greater the

alkenes stability.

The ( E )-( Z ) System for Designating Alkene Isomers

The Cahn-Ingold-Prelog convention is used to assign

the groups of highest priority on each carbon.

If the group of highest priority on one carbon is on the

same side as the group of highest priority on the other

carbon the double bond is Z (zusammen).

If the highest priority groups are on opposite sides the

alkene is E (entgegen).

Z / E is useful when cis - trans is ambiguous.

Z / E vs. cis- / trans - The Zaitsev (Saytzeff) Rule

Some hydrogen halides can eliminate to give two or

more different alkene products.

Zaitsev  s Rule: Formation of the most substituted

alkene is favored with a small base.

Additionally, for the same level of substitution of the

double bond, the less sterically hindered isomer is

favored ( if both E/Z isomers are possible).

  • A reaction is regioselective when it yields predominantly or exclusively one constitutional isomer when more than one is possible.
  • Thus, the E2 reaction is regioselective.

Regioselectivity of E2 Reactions

The transition state in this E2 reaction has double bond character. The trisubstituted alkene-like transition state will be most stable and have the lowest Δ G ‡. This reaction is under kinetic control. Kinetic control of product formation: When one of two products is formed because its free energy of activation is lower and therefore the rate of its formation is higher.

Zaitsevs

Rule

E

Mechanism

  • A reaction is stereoselective when it forms predominantly or exclusively one stereoisomer when two or more are possible.
  • The E2 reaction is stereoselective because the more stable stereoisomer is formed preferentially.

Stereoselectivity of E2 Reactions Hoffman Rule

Bulky bases such as potassium tert -butoxide have

difficulty removing sterically hindered hydrogens and

generally only react with more accessible hydrogens

(e.g. primary hydrogens)

Hoffman Rule: Formation of the least substituted alkene

will be favored when using a bulky base.

This will result in the anti-Zaitsev product being the

major product.