(E)-Alkene

Definition

(E)-alkenes, also known as trans-alkenes, are a type of alkene isomer where the two largest substituents are on opposite sides of the carbon-carbon double bond. This structural arrangement has important implications in the context of the E2 reaction and the deuterium isotope effect.

5 Must Know Facts For Your Next Test

1. The (E) prefix indicates that the two largest substituents on the carbon-carbon double bond are on opposite (trans) sides.
2. The geometry of (E)-alkenes results in a more stable configuration compared to (Z)-alkenes, where the substituents are on the same side.
3. In the E2 reaction, the formation of (E)-alkenes is favored due to the anti-periplanar arrangement of the leaving group and the β-hydrogen.
4. The deuterium isotope effect in the E2 reaction is more pronounced for the formation of (E)-alkenes, as the cleavage of the C-H bond is the rate-determining step.
5. The stability and stereochemistry of (E)-alkenes play a crucial role in various organic reactions, including addition, substitution, and elimination reactions.

Review Questions

• Explain the structural features and stability of (E)-alkenes compared to (Z)-alkenes.
  • The (E)-alkenes, also known as trans-alkenes, have the two largest substituents positioned on opposite sides of the carbon-carbon double bond. This arrangement results in a more stable configuration compared to (Z)-alkenes, where the substituents are on the same side (cis). The greater stability of (E)-alkenes is due to the minimization of steric interactions between the bulky substituents, leading to a more favorable spatial orientation of the molecule.
• Describe the role of (E)-alkene formation in the E2 reaction mechanism.
  • In the E2 reaction, the formation of (E)-alkenes is favored due to the anti-periplanar arrangement of the leaving group and the β-hydrogen. This geometry allows for the efficient overlap of the C-H bond and the σ* orbital of the C-X bond, facilitating the concerted removal of the leaving group and the β-hydrogen. The resulting (E)-alkene product is more stable than the potential (Z)-alkene, making the E2 elimination a stereoselective process.
• Analyze the impact of the deuterium isotope effect on the formation of (E)-alkenes in the E2 reaction.
  • The deuterium isotope effect is more pronounced in the formation of (E)-alkenes during the E2 reaction. This is because the cleavage of the C-H bond is the rate-determining step, and the substitution of hydrogen with the heavier deuterium isotope leads to a slower reaction rate. The kinetic isotope effect is greater for the formation of (E)-alkenes compared to (Z)-alkenes, as the former requires the cleavage of the C-H bond in the rate-determining step. This observation provides insights into the mechanism and stereochemistry of the E2 reaction.

Related terms

Alkene: An unsaturated hydrocarbon compound containing a carbon-carbon double bond.

Stereoisomerism: The phenomenon where molecules have the same molecular formula and connectivity but differ in the spatial arrangement of their atoms.

E2 Reaction: A bimolecular elimination reaction that results in the formation of an alkene product.