Question

Separation of o-nitrophenol from the mixture of o - and p - nitrophenol can be achieved by:
A. Sublimation
B. Steam distillation
C. Crystallisation
D. Fractional distillation

Answer 1

The presence of an intramolecular hydrogen bond is observed in ortho-nitrophenol and intermolecular hydrogen bond para-nitrophenol. This pattern of hydrogen bonding changes the physical properties of the organic molecules.

Complete step by step answer:
Specific mixtures of organic molecules include o–nitrophenol and p-nitrophenol which are to be separated from each other by a physical separation process. The structure of p-nitrophenol is a phenol molecule with a $N{O_2}$ group as the functional group present at the para position. The structure of o-nitrophenol is formed with a phenol molecule where the $N{O_2}$ group is present at the ortho position.
The ortho position is considered as the carbon residue just beside the carbon to which the alcoholic functional group is attached. The para position is present at the opposite carbon to that of the carbon to which the alcoholic functional group is attached.
Due to the close association of the $N{O_2}$ group and the $OH$ group in the ortho-nitrophenol, there can be formation of intramolecular $H$-bonding. The intramolecular hydrogen bond means that each of the molecules have the presence of hydrogen bond in it. But in the case of para-nitrophenol the $N{O_2}$ group is not present in close vicinity to that of the $OH$ group. This is why there is formation of $H$-bonding between two molecules of para-nitrophenol. This is known as intermolecular $H$ bonding.
The separation of these two organic molecules from the mixture is possible using two processes. Steam distillation is used to separate the organic molecules based on their evaporation under the effects of steam. The evaporation of the molecule after the introduction of steam takes place due to an increase in temperature. Hence the molecule with higher tendency to evaporate gets removed faster from the mixture. Hence the ortho-nitrophenol is removed first as the $H$-bond is internal. The para-nitrophenol has intermolecular $H$-linkage and hence the interaction between multiple molecules increases the temperature for evaporation.
The mixture of these two molecules can also be separated by fractional distillation. This is because the solubility differs in the two specific molecules. The intramolecular $H$-bonding makes the organic molecule more soluble while the other molecules are less soluble compared to them. This is why through distillation the separation can be done based on solubility.
Therefore, the correct answer is both B. steam distillation and D. fractional distillation.

Note: Distillation is a process of separation of components of a mixture based on specific physical or chemical properties. Mode of separation depends on the differences on conformation or chemical structure, volatile nature, solubility and many more. Specific separation techniques need to be used for specific purposes.