Question

Which of the following explanations accounts for o-nitrophenol to be more volatile than p-nitrophenol.
(a) Intermolecular hydrogen bonding
(b) Resonance
(c) Intramolecular hydrogen bonding
(d) Inductive effect

Answer 1

Hint: In o-nitrophenol, there is hydrogen bonding within the molecule and thus, doesn’t exist as associated molecules whereas on the other hand, in p-nitrophenol, there is hydrogen bonding between the two molecules and thus, exist as associated molecules and is not easily volatile. Now you can easily answer the statement.

Complete Solution :
- First of let’s discuss what are phenols. Phenols are the hydroxyl derivatives of the hydrocarbons in which the hydroxyl group -OH is directly attached to the carbon atom of the aromatic ring.
- Phenols are colourless, crystalline solids or liquids and have characteristic phenolic odour and are sparingly soluble in water but completely soluble in alcohols, ethers etc. and their boiling point is also very high.
- In o-nitrophenol, there is weak intramolecular H-bonding (i.e. within the molecule) and the release of hydrogen ion is not as difficult as in p-nitrophenol and is thus, easily volatile than the p-nitrophenol .
- On the other hand, in case of p-nitrophenol, there is strong intermolecular H-bonding (i.e. between the two molecules) and thus, exist as associated molecule and the release of hydrogen ion is not as difficult as in p-nitrophenol and is thus, is not easily volatile as the o-nitrophenol .
So, thus o-nitrophenol is more volatile than p-nitrophenol because of the intramolecular hydrogen bonding.
So, the correct answer is “Option C”.

Note: Phenols are more acidic then the alcohols due to the dispersal of the negative charge and thus, turn blue litmus red and react with alkali metals and alkalis to form their salts. But phenols are weaker acid than the carboxylic acids and therefore, they do not react with the sodium carbonate and sodium bicarbonate.