Question

Ortho-nitrophenol has lower boiling point than para-nitro phenol, why?

Answer 1

Hint: We know intermolecular forces are electrostatic and include van der Waals forces and hydrogen bonds. The higher the intermolecular forces are higher the melting and boiling point.

Complete step by step answer:
Let us discuss the hydrogen bond in detail.
In a molecule, when an atom is linked to a highly electronegative atom, it attracts the shared pair of electrons more so it becomes a slightly negative end while the opposite end becomes slightly positive. The negative end of the molecule attracts the positive end of the opposite and as a result, a weak bond is made between them. This bond is named a hydrogen bond.
There are two types of the hydrogen bond. They are,
1.Intermolecular hydrogen bond.
2.Intramolecular Hydrogen bond.
1.Intermolecular Hydrogen Bonding:
If the hydrogen bond takes place between different molecules of the same or different compounds, it is called intermolecular hydrogen bonding.
Example – hydrogen bonding in water.
2.Intramolecular Hydrogen Bonding:
If the hydrogen bonding takes place within a molecule itself is called intramolecular hydrogen bonding. It takes place in compounds containing two groups such that one group contains a hydrogen atom linked to an electronegative atom and the other group contains a highly electronegative atom linked to a lesser electronegative atom of the other group.
Now, we see the structure of ortho- and para-nitro phenol,

From the above structures, we found that ortho isomer has intramolecular hydrogen bonding while para isomer has intermolecular hydrogen bonding. Ortho isomers cannot form effective hydrogen bonding because their hydrogen atoms are involved in intramolecular hydrogen bonding. Thus Ortho-nitro phenol has a lower boiling point than para-nitro phenol.

Note:
As we know molecules in liquids are held together by intermolecular interactions, which are weaker than the intramolecular interactions that hold the atoms together within molecules and polyatomic ions.