Question

Consider the two substances below.  

Which substance would you expect to have the greatest solubility in a polar solvent? Explain your reasoning based on the intermolecular forces present between molecules of the substance and molecules of the solvent.

Answer 1

Part I : Cis isomer

1. In cis-1,2-dichloroethene, each C—Cl bond is polar, so the overall molecule is polar.

2. Because of this, there will be dipole-dipole interactions as well as dispersion forces.

3. And intermolecular forces between the cis isomer and molecules of the solvent are greater due to greater polarity.

3. Hence Cis isomer has greater solubility in a polar solvent.

Note:

[As like dissolves in like] or [Polar substance soluble in Polar solvent]

Part II : Trans isomer

1.However, in trans-1,2-dichloroethene, the individual dipoles in the bonds cancel out, and so the molecule is non-polar.

2. In the solid state, the trans isomers can pack more closely than the cis isomers, making the intermolecular forces more effective.

3. Hence the intermolecular force between the molecules of the substance are greater in solid state.

4. And the intermolecular forces between trans isomer and molecules of the solvent are lesser due to lesser polarity.

5. Only intermolecular attractions these molecules experience are van der Waals dispersion forces.

6. Hence trans isomer has lesser solubility in polar solvent.

Conclusion:

There must be stronger intermolecular forces between the molecules of the cis isomers than between trans isomers to exhibit greater solubility in polar solvents.