Beginning Organic Chemistry (BOC)
7. Physical Properties.

Return to BOC Index.

1. Effect of structure on boiling point.

Boiling points of molecular compounds depend on weak intermolecular forces (van der Waals forces) of three major types. For further details, click on one of the types.

London forces (induced dipole - induced dipole interactions).
dipole - dipole interactions.
hydrogen bonds.

2. Solubility.

Solubility depends on the similarity of the intermolecular forces between the two compounds. Similar intermolecular forces lead to mixing of the two compounds as one molecule type will attract the other.

Dissimilar intermolecular forces will lead to one molecular type attracting only its own type, squeezing the other out so that mixing does not occur.

This leads to 'Like dissolves like' as the general rule.

The non-polar alkanes will dissolve other non-polar compounds. Acetone, a fairly polar organic compound, will dissolve many organic compounds. Water, a polar compound, will dissolve only polar organic compounds (with few carbons).

The table below compares the solubility of some alcohols (the -OH makes the alcohol similar to water (H-OH), whereas the carbon chain makes it dissimilar) in the very polar solvent, water.

Compound Solubility in Water Model
methanol completely
ethanol completely
propan-1-ol completely
butan-1-ol 7.4 g per 100 g water
pentan-1-ol 2.7 g per 100 g water
hexan-1-ol 0.8 g per 100 g water
heptan-1-ol 0.1 g per 100 g water

Date created: 2005 06 11.