Cis and Trans Isomers

Definitions

• Cis Isomer: Two groups on the same side.
• Trans Isomer: Two groups on opposite sides.

Physical Properties

• Dipole Moments:
  • Cis Isomer: ~3 Debye (polar)
  • Trans Isomer: 0 Debye (nonpolar)
• Boiling Points:
  • Cis Isomer: ~60°C
  • Trans Isomer: ~47.5°C

Explanation of Properties

• The carbon-chlorine bond is relatively polar, causing different dipole moments.
  • Cis: Dipoles do not cancel, resulting in a net dipole moment.
  • Trans: Dipoles cancel each other out, hence nonpolar.
• Boiling Points:
  • Polar molecules (cis) have higher boiling points than nonpolar molecules (trans).

Structural Considerations

• Double Bonds:
  • Cis and trans isomers lack free rotation due to the presence of a double bond.
  • High energy barrier to convert between cis and trans:
    • Requires breaking the pi bond.
• Single Bonds:
  • In cases like ethane, free rotation is possible (energy barrier ~12 kJ/mol).

Representation of Isomers

• Isomers can also be represented using rings:
  • Cis: Both hydroxyl groups on the same side (e.g., both on wedges).
  • Trans: One hydroxyl group going out of the page (wedge) and the other going into the page (dash).

Identifying Cis and Trans Isomers in Alkenes

• Criteria for Isomerism:
  • Each carbon in the double bond must be attached to two different groups.
• Examples:
  1. Example 1: Cannot form isomers (two identical groups on one carbon).
  2. Example 2: Can form isomers (attached groups are different).
     • Trans Isomer: One carbon has H and CH3, the other has H and C2H5.
     • Cis Isomer: Both hydrogens facing the same direction.
  3. Example 3: Cannot form isomers (same group on one carbon).
  4. Example 4: Cannot form isomers (two identical groups).

Conclusion

• To determine if an alkene can show cis and trans isomerism, analyze the substituents on each carbon of the double bond to check if they are different.