Polarity
1- Polar (water) acts as a solvent according to the following mechanisms:
a) Dielectric constant:
due to their high dielectric constant, polar solvents reduce the force of attraction between oppositely charged ions in crystals.
Example: water possessing a high dielectric constant (> = 80) can dissolve NaCl, while chloroform (> = 5) & benzene (> = 2) cannot. Ionic compounds are practically insoluble in these 2 solvents.
b) Hydrogen bond formation:
Water dissolves phenols, alcohols and other oxygen & nitrogen containing compounds that can form hydrogen bonds with water.
c) Solvation through dipole interaction:
Polar solvents are capable of solvating molecules & ions through dipole interaction forces.
The solute must be polar to compete for the bonds of the already associated solvent molecules.
Example: Ion-dipole interaction between sodium salt of oleic acid & water
2. Semi-Polar
Semipolar solvents, such as ketones can induce a certain degree of polarity in non polar solvent molecules.
• They can act as intermediate solvents to bring about miscibility of polar & non polar liquids.
Example: acetone increases solubility of ether in water.
--Induces polarity in nonpolar solvents
--Co-Solvents
