Question

The melting point of 2-nitrophenol is lower than that of 4-nitrophenol and this is because of:
A.intramolecular hydrogen bonding in 2- nitrophenol
B.intramolecular hydrogen bonding in 4- nitrophenol
C.intermolecular hydrogen bonding in 2- nitrophenol
D.hydrogen bonding has no relation to the melting point

Answer 1

Hint: When the hydrogen bond acts within a molecule, then it’s called as intramolecular hydrogen bonding and when it acts between different molecules of the same or different type it is called as intermolecular hydrogen bonding.

Complete step by step answer:
In the structure of 2-nitrophenol (ortho-nitrophenol) there is a possibility of formation of intramolecular hydrogen bonding. But for 4-nitrophenol (para nitrophenol) there is possibility of formation of intermolecular hydrogen bonds. As explained above intramolecular bonding takes place within a molecule and intermolecular bonding takes place between different molecules of the same or different type
Hence in case of 2-nitrophenol, because to intramolecular Hydrogen-bonding it acts as a monomer whereas in the case of 4-nitrophenol because of intermolecular Hydrogen-bonding it form polymer-like structure by interacting with the neighbouring molecules which increases the molecular weight of the system in case of 4-nitrophenol thereby making it less volatile which in return increases the boiling point of 4-nitrophenol.
Hence, Boiling point of 2-nitrophenol is lower than that of 4-nitrophenol because of formation of intramolecular hydrogen bonds.
Therefore, the correct option is B.


Note:
It is helpful to know the relationship between vapour pressure, volatility and boiling point to understand this better. Higher the vapour pressure of a liquid at a given temperature, higher the volatility and lower the boiling point of that liquid. Thus in case of 4-nitrophenol, its molecular weight increases making it less volatile which increases its boiling point. And this doesn’t happen in 2-nitrophenol that is why the boiling point of 2-nitrophenol is less and is more volatile.