Phenols- Physical and Chemical Properties

Phenols are the organic compounds containing benzene ring bonded to a hydroxyl group. They are also known as carbolic acids. They exhibit unique physical and chemical properties in comparison to alcohols. These physical and chemical properties of phenols are mainly due to the presence of hydroxyl group. Some prominent physical and chemical properties of phenols are given below:

Boiling point of phenols

Phenols generally have higher boiling points in comparison to other hydrocarbons having equal molecular masses. This is due to the presence of intermolecular hydrogen bonding between hydroxyl groups of phenol molecules. In general, the boiling point of phenols increases with increase in the number of carbon atoms.


Solubility of phenols

Solubility of phenol in water is governed by the hydroxyl group present. The hydroxyl group in phenol is involved in the formation of intermolecular hydrogen bonding. Thus, hydrogen bonds are formed between water and phenol molecules which make phenol soluble in water. However the aryl group attached to the hydroxyl group is hydrophobic in nature. Thus, the solubility of phenol decreases with the increase in size of aryl group.

Acidity of phenols

Phenols react with active metals such as sodium, potassium etc. to form corresponding phenoxide. These reactions of phenols indicate its acidic nature. In phenol, the sp2 hybridized carbon of benzene ring attached directly to the hydroxyl group acts as an electron withdrawing group. Thus, it decreases the electron density on oxygen. Due to the delocalization of negative charge in the benzene ring, phenoxide ions are more stable than alkoxide ions. As a result phenols are more acidic than alcohols. In case of substituted phenols the acidity decreases if an electron donating group is attached to the ring while the acidity increases in case of electron withdrawing group.

 Chirality of phenols

Phenols exhibit chirality within their molecules, for example: catechin. This chirality is due to the absence of planar and axial symmetry in the phenol molecule.

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Practise This Question

An organic liquid A containing C,H and O has a pleasant odour with a boiling point of 78o On boiling A with 

conc. H2SO4 a colourless gas is produced which decolourises bromine water and alkaline KMnO4 One mole of 

this gas also takes one mole of H2 The organic liquid A is   [KCET 1993]