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🥼organic chemistry review

Restricted Rotation

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

Definition

Restricted rotation refers to the limited ability of certain molecules, particularly alkenes, to freely rotate around a carbon-carbon bond due to steric hindrance or electronic effects. This concept is crucial in understanding cis-trans isomerism in alkenes.

5 Must Know Facts For Your Next Test

  1. Restricted rotation in alkenes is the primary reason for the existence of cis-trans isomers, as it prevents the free rotation around the carbon-carbon double bond.
  2. The presence of bulky substituents on either side of the carbon-carbon double bond creates steric hindrance, which limits the ability of the molecule to rotate freely.
  3. Torsional strain is another factor that contributes to restricted rotation in alkenes, as it arises when atoms or groups are forced into unfavorable positions.
  4. Restricted rotation in alkenes leads to the formation of two distinct geometric isomers: cis (substituents on the same side) and trans (substituents on opposite sides).
  5. The stability of cis and trans isomers can be influenced by the nature and size of the substituents, with the more stable isomer typically being the one with less steric hindrance.

Review Questions

  • Explain how restricted rotation in alkenes leads to the formation of cis-trans isomers.
    • Restricted rotation in alkenes is the primary reason for the existence of cis-trans isomers. The presence of bulky substituents on either side of the carbon-carbon double bond creates steric hindrance, which limits the ability of the molecule to rotate freely. This restricted rotation prevents the substituents from freely moving to the opposite side of the double bond, leading to the formation of two distinct geometric isomers: cis (substituents on the same side) and trans (substituents on opposite sides). The stability of these isomers can be influenced by the nature and size of the substituents, with the more stable isomer typically being the one with less steric hindrance.
  • Describe the role of torsional strain in contributing to restricted rotation in alkenes.
    • Torsional strain is another factor that contributes to restricted rotation in alkenes. When atoms or groups are forced into unfavorable positions due to the limited ability to rotate freely around the carbon-carbon double bond, torsional strain arises. This strain creates an energetic barrier that prevents the molecule from easily rotating and adopting different conformations. The presence of torsional strain, in addition to steric hindrance, further reinforces the restricted rotation in alkenes, leading to the stabilization of cis-trans isomers and the inability of the molecule to freely interconvert between these geometric forms.
  • Analyze how the nature and size of substituents can influence the stability of cis and trans isomers in alkenes with restricted rotation.
    • The stability of cis and trans isomers in alkenes with restricted rotation can be influenced by the nature and size of the substituents. Generally, the more stable isomer is the one with less steric hindrance, as this minimizes the repulsive interactions between bulky groups. Larger substituents, such as phenyl or tert-butyl groups, can create more significant steric hindrance when placed on the same side of the double bond, favoring the trans isomer. Conversely, smaller substituents, like hydrogen or methyl groups, tend to have less steric impact, allowing the cis isomer to be more stable in some cases. The electronic effects of the substituents can also play a role, with electron-withdrawing groups potentially stabilizing the cis isomer through dipole-dipole interactions. By carefully considering the size and electronic properties of the substituents, one can predict and understand the relative stability of cis and trans isomers in alkenes with restricted rotation.