Carboxylic Acids and Derivatives
3.0 Physical Properties of Carboxylic Acid
3.0 Physical Properties of Carboxylic Acid
1. These are polar substances and can form $H-$bonds with each other to form dimmer structures.
(a) Boiling Point: Due to dimeric structure, the effective molecular mass of the acid becomes double the actual mass. Hence carboxylic acids have higher boiling points than alcohols of comparable molecular masses. Due to hydrogen bonding carboxylic acid show appreciable solubility in water. Its solubility in water is greater than alcohol because $H-$bonding strength is greater in carboxylic acid than alcohol.
(b) Melting Point: The carboxylic acid with even number of carbon atoms is higher than acid with odd number of carbon atoms. Such effect is observed in first ten members of the homologous series.This feature is based on the fact that in the carboxylic acids with even number of carbon atoms, the terminal methyl group and carboxylic group are on the opposite sides of zig-zag carbon chain.Hence they fit better in the crystal lattice resulting in stronger inter molecular forces on the other hands acids with odd number of carbon atom have carboxyl and terminal methyl group on the same side of zig-zag carbon chain which result in poor fitting in the crystal lattice. This causes a weak forces among molecular and for the relatively lower melting point.
The melting point and boiling points are usually higher than those of aliphatic acid of comparable molecular masses. This is due to planar structure of benzene ring in the acid which can pack closely in the crystal than aliphatic acids.
