Question

Which of the following is steam volatile?
(A) Phenol
(B) o-Nitrophenol
(C) m-Nitrophenol
(D) p-Nitrophenol

Answer 1

Hint: Some of the given compounds include intermolecular hydrogen bonding and intramolecular hydrogen bonding. They affect the boiling point of a compound significantly.

Complete answer:
Hydrogen bonding significantly affects the boiling point of a compound. Hydrogen bonding is a special type of dipole-dipole attraction between molecules. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as a N, O, or F atom and another very electronegative atom. Hydrogen bond strengths range from 4 kJ to 50 kJ per mole of hydrogen bonds. Because of the difference in electronegativity, the H atom bears a large partial positive charge and the other electronegative atom bears a partial negative charge. So, the H atom is electrostatically attracted to the other electronegative atom. Hydrogen bonds have about a tenth of the strength of an average covalent bond, and are being constantly broken and reformed.
If we observe the given compounds, o – Nitrophenol has a nitro group very close to the -OH group. The hydrogen of the hydroxyl group (-OH) is electrostatically attracted to the oxygen of the nitro group. This type of hydrogen bonding is called intramolecular hydrogen bonding. Intramolecular hydrogen bonding decreases the boiling point of a molecule significantly as the molecule binds strongly to itself and decreases the tendency of intermolecular hydrogen bonding.
If we observe p – Nitrophenol, the hydrogen of the hydroxyl group is very far from the oxygen of the nitro group, so they can’t perform intramolecular hydrogen bonding. So, they are involved in intermolecular hydrogen bonding with the adjacent molecules. Intermolecular hydrogen bonding increases the boiling point significantly as the molecules bind to each other strongly thus increasing boiling point.

Hence, o -Nitrophenol is most volatile

Thus, the correct answer is (B).

Note:
A student must know the effects of intermolecular Hydrogen bonding and intramolecular Hydrogen bonding on the boiling point of a compound. The reasons for the same have been explained in the solution.