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Physical Properties of Carboxylic Acids

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Odour and colour of Carboxylic Acids

Carboxylic acids up to nine carbon atoms are colourless liquids with unpleasant odour. Carboxylic acids containing more than nine carbon atoms are colourless waxy solids and are odourless due to their low volatility.

Boiling point of Carboxylic Acids

The boiling point of carboxylic acid is higher than those of aldehydes, ketones and alcohols of comparable molecular masses because carboxylic acids form very strong hydrogen bonds.

Why is the boiling point of carboxylic acid higher than that of alcohols even though both of them form H-bonds?

  1. The O-H bond in carboxylic acid is attached to the electron withdrawing carbonyl group; therefore, the O-H bond in carboxylic acid is more polarised as compared to the O-H group of alcohols.

    -O-H in carboxylic acid

    Presence of carbonyl group in carboxylic acids

    -O-H in alcohol

    alcohol molecule
  2. Since the oxygen atom of the carbonyl group is polar, it can form H-bonds with the hydrogen atom of the O-H of the other molecule. These hydrogen bonds are so strong that they do not break completely even in the vapour phase. As a result, most carboxylic acids exist as dimer in the vapour phase or in the aprotic solvents.
    Carboxylic acids exist as dimer in vapour phase

Aprotic solvents are solvents that do not have O-H or N-H bonds such as hexane, chloroform, acetone etc. Therefore, they cannot form H-bonds with themselves.

Solubility of Carboxylic Acids

Solubility in water : Carboxylic acids containing up to four carbon atoms are miscible in water because they form H-bond with water. The solubility decreases with increase in the number of carbon atoms as the larger hydrocarbon part resists the formation of H-bonds. Benzoic acid is nearly insoluble in cold water because of the larger hydrocarbon part.

Solubility in organic solvents : Carboxylic acids are soluble in less polar organic solvents such as benzene, ether, alcohols etc.

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