Physical properties monohydric alcohols

Alcohols and Phenols consist of two parts, an alkyl/aryl group and a hydroxyl group. The properties of alcohols and phenols are due to the -OH group.

The alkyl and aryl groups modify these properties.

1. The lower members of alcohols are colourless, volatile liquids with a characteristic alcoholic smell and burning taste whereas higher alcohols are odourless and tasteless.

Higher alcohols having 12 or more carbon atoms are colourless waxy solids. Phenols are colourless, crystalline solids or liquids. (They may become coloured due to slow oxidation with air).

2. Solubility of alcohols

The first three members are completely miscible with water. The solubility rapidly decreases with increase in molecular mass. The higher members are almost insoluble in water but are soluble in organic solvents like benzene, ether etc.

The solubility of lower alcohols is due to the existence of hydrogen bonds between water and polar -OH group of alcohol molecules. Phenols too are sparingly soluble in water.

The -OH group in alcohols and phenols contain a hydrogen bonded to an electronegative oxygen atom. Thus they form hydrogen bonds with water molecules.

The solubility of alcohols in water decreases with increase in molecular mass because the increase in molecular mass, the non polar alkyl group becomes predominant and masks the effect of polar -OH group.

In addition, among the isomeric alcohols, the solubility increases with branching of chain. As the surface area of the non-polar part in the molecule decreases, the solubility increases.

Phenols are sparingly soluble in water but readily soluble in organic solvents such as alcohol and ether.