20.6.4 Explain the difference in the physical and chemical properties of geometrical isomers.

Geometric Isomers

• Cis-1,4-Dichloroethene

  • Same groups on the same side of the double bond.

  • Polar molecule.

  • Boiling Point: 60°C

  • Melting Point: 80°C

  • Higher boiling point due to dipole-dipole attractions.

• Trans-1,4-Dichloroethene

  • Same groups on opposite sides of the double bond.

  • Non-polar molecule.

  • Boiling Point: 48°C

  • Melting Point: 50°C

  • Lower boiling point due to Van der Waals forces.

Physical Properties

• Restricted Rotation:

  • Caused by the pi bond in the double bond, preventing rotation within the molecule.

• Polarity Effects on Boiling Points:

  • Cis isomer has a higher boiling point than trans isomer due to stronger intermolecular forces.

• Symmetry and Melting Points:

  • Trans isomer has a higher melting point due to greater symmetry allowing closer packing of molecules.

Example: Butenedioic Acids

• Cis-Butenedioic Acid

  • Melting Point: 139°C

  • Forms intramolecular hydrogen bonds.

• Trans-Butenedioic Acid

  • Melting Point: 287°C

  • Forms intermolecular hydrogen bonds, leading to a higher melting point.

Chemical Properties

• Cis-Butenedioic Acid:

  • Upon heating to 160°C, it forms butenedioic anhydride plus water.

• Trans-Butenedioic Acid:

  • Upon heating to 160°C, it sublimes without a chemical change.