Physical Properties of Carboxylic Acid Preparation and Properties
→ Physical State : Aliphatic carboxylic acids upto 9 carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax like solids and less volatile generally odourless. Benzoic acid is aromatic carboxylic acid which is colourless crystalline solid.
→ Odour : First three members (C1 – C3) have unpleasant odour, C4 to C9 have odour of like rancid butter and higher member are odourless.
→ Solubility : Carboxylic acid are soluble in water mostly simple aliphatic carboxylic acid having upto four carbon atoms are miscible with water due to the formation of hydrogen bond with water. The solubility decreases with increasing number of carbon atoms.
→ Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part. Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water. Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol etc.
→ Lower carboxylic acid forms hydrogen bonds with water in the following way :
→ Boiling Point: Alkanoic acid have higher boiling point. As the molecular weight increases boiling point also increases. Carboxylic acids have higher boiling point than aldehydes, ketones and even alcohols of comparable molecular masses.
Ether ≅ Alkane < alcohols < carboxylic acid
|CH3COOH Acetic acid
|CH3—CH2— CH2—CH3 Butane ‘
|Boiling Point (K)
→ It is due to intermolecular hydrogen bonding in between the molecules of carboxylic acid which is more stronger compared to hydrogen bonding in between the molecules of alcohol. As carbonyl group is present nearer to O-H bond in carboxylic acid therefore the O-H bond of acid is more polar than O-H bond of alcohols.
→ Due to polarity of carboxylic acid molecules, the oxygen of carbonyl group gets negative charge and this negatively charged oxygen atom makes hydrogen bond with positively charged hydrogen atom of other molecule.
→ That’s why not only in solid state but also in vapour state the molecules of carboxylic acid remains associated. Example : Two molecules of acetic acid forme dimer by hydrogen bonding.
→ The existence of dimer is supported by the fact that the observed molecular mass of acetic acid as determined by measurement of colligative properties is 120 whereas the actual molecular mass of acetic acid is 60.
→ Melting Point : Melting points of aliphatic carboxylic acid (upto C10) shows alternation effect i.e. the melting point of an acid with even number of carbon atom is higher than the next homologous containing odd number of carbon atoms.
→ The X-ray diffraction studies have shown that the acids with odd number of carbon atom have carboxyl group and terminal methyl group. On the same sides of the zig-zag carbon chain.
→ Therefore these fit in the crystal lattice poorly and hence have weak intermolecular forces. On the other hand the acids with even number of carbon atom have carboxyl group and terminal methyl group on the opposite sides of the zig-zag carbon chain.
→ As result, they fit into the crystal lattice better and results in stronger intermolecular forces so their melting points are higher.
This trend is not noticeable in carboxylic acid having more than 10 carbon atom.