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🥼organic chemistry review

Trans-Alkene

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

A trans-alkene is a type of alkene, or carbon-carbon double bond, where the two largest substituents are positioned on opposite sides of the double bond. This geometric arrangement contrasts with cis-alkenes, where the largest substituents are on the same side of the double bond.

5 Must Know Facts For Your Next Test

  1. Trans-alkenes are more thermodynamically stable than cis-alkenes due to reduced steric strain.
  2. The trans configuration is favored in alkene products formed by the reduction of alkynes.
  3. Catalytic hydrogenation of alkynes using H2 and a metal catalyst such as Pt or Pd typically yields trans-alkenes.
  4. The Lindlar catalyst, which contains Pd on CaCO3 with Pb(OAc)2 and quinoline, is used to selectively reduce alkynes to cis-alkenes.
  5. Trans-alkenes have a more linear shape compared to the bent structure of cis-alkenes, impacting their physical and chemical properties.

Review Questions

  • Explain the relationship between the reduction of alkynes and the formation of trans-alkenes.
    • The reduction of alkynes, which contain carbon-carbon triple bonds, typically yields trans-alkenes as the major product. This is because the trans configuration is more thermodynamically stable than the cis configuration, as it minimizes steric strain between the largest substituents on the double bond. Catalytic hydrogenation of alkynes using hydrogen gas (H2) and a metal catalyst, such as platinum or palladium, selectively produces the trans-alkene product.
  • Describe how the Lindlar catalyst is used to selectively reduce alkynes to cis-alkenes.
    • The Lindlar catalyst, which contains palladium on calcium carbonate with lead(II) acetate and quinoline, is used to selectively reduce alkynes to cis-alkenes. Unlike the typical catalytic hydrogenation that produces trans-alkenes, the Lindlar catalyst is able to partially hydrogenate the alkyne while maintaining the cis geometry of the resulting alkene. This is achieved by the catalyst's ability to control the degree of hydrogenation and the stereochemical outcome, allowing for the selective formation of the less stable but more useful cis-alkene product.
  • Analyze how the geometric arrangement of substituents in trans-alkenes influences their physical and chemical properties compared to cis-alkenes.
    • The trans configuration of substituents in trans-alkenes results in a more linear molecular shape, which differs from the bent structure of cis-alkenes. This geometric difference impacts the physical and chemical properties of these alkene isomers. Trans-alkenes generally have higher melting and boiling points, as well as greater stability, due to the reduced steric strain compared to cis-alkenes. Additionally, the linear shape of trans-alkenes can affect their reactivity and interactions with other molecules, leading to distinct chemical behaviors compared to the bent cis-alkene counterparts.