Structure & Physical Properties of Alcohols

Electronegativity of Oxygen

The structure of alcohols is similar to that of water. The electronegativity of oxygen is more than that of hydrogen. Therefore, in alcohols, the O–H bond is polar in nature. In other words, oxygen has a slight negative charge on it whereas hydrogen has a slight positive charge.

Hydrogen Bonding & Boiling Point

Normally, hydrogen bonding is responsible for higher boiling points of alcohols. The negatively polarised oxygen atom of one alcohol molecule attracts the positively polarised hydrogen atom of the other molecule. Thus, alcohol molecules are associated or are held together. This force of attraction is to be overcome before a molecule is set free from the liquid state and vaporises.

Thus, more heat energy is required to break the hydrogen bonds and hence, the boiling points of alcohols are higher than alkanes and haloalkanes of comparable molecular mass.

Solubility

Alcohols have high solubilities in water. The lower alcohols are completely miscible and their solubilities decrease as the hydrocarbon portion of the molecule becomes larger. The higher solubility of alcohols can be attributed to the hydrogen bonding. In this case, hydrogen bonding takes place between the alcohol and water molecules.