Question

Which of the following has the lowest boiling point?
A. p-nitrophenol
B. m-nitrophenol
C. o-nitrophenol
D. Phenol

Answer 1

Hint: The temperature at which the vapour pressure of the substance becomes equal to the vapour pressure of liquid present at the surface is known as its boiling point. The boiling point of a compound depends upon the force of attraction between different atoms of the complex.

Complete step by step answer:
Intermolecular Hydrogen Bonding: This hydrogen bond forms between the Hydrogen atom of one molecule and almost electronegative element of another molecule.
Intramolecular hydrogen bonding – This hydrogen bond forms within the same molecule, then this intramolecular hydrogen bond results in the cyclization of the molecule and prevents the association of the molecule.
Intramolecular hydrogen bonding causes intramolecular forces and these forces are stronger because it contains two groups such that one group contains hydrogen atom associate with the highly electronegative atom and the other group should contain a highly electronegative atom linked to a less electronegative atom.
The first option is p-nitrophenol, it is having intermolecular nitrogen bonding because the nitro group is far from the alcohol group and it has the highest boiling point.
Coming to the second option- nitrophenol, it is having intermolecular hydrogen bonding and associates with another molecule so m- nitrophenol shows the highest melting point.
Third option is option O-nitrophenol is having intramolecular hydrogen bond and undergoes cyclization of molecule .This makes the ortho nitrophenol highly volatile and shows lowest boiling point.
Fourth option is phenol. It also forms strong intermolecular hydrogen bonding and it is also having a high boiling point compared to ortho nitrophenol.
So from the above options concluded that o-nitrophenol forms intramolecular Hydrogen bond and no association occurs whereas molecules of p-nitrophenol and meta nitrophenol get associated through intermolecular Hydrogen bonding and boils relatively at higher temperature so only ortho-nitrophenol is steam volatile.

So, the correct answer is Option C.

Note:
Due to intramolecular hydrogen bonding the boiling point of compounds decreases because this bond prevents the association of one compound with another. Hydrogen bonding is comparatively weak.