Question

The boiling point of para nitrophenol is greater than ortho nitrophenol, because:

• A. there is intermolecular hydrogen bonding in para nitrophenol and intramolecular hydrogen bonding in ortho nitrophenol
• B. there is intramolecular hydrogen bonding in para nitrophenol and intermolecular hydrogen bonding in ortho nitrophenol
• C. both have the same kind of hydrogen bonding
• D. para nitrophenol is polar, while ortho nitrophenol is non-polar
• E. van der Waals forces are stronger in ortho nitrophenol

Answer 1

Answer: (A)

Phenols containing group like $N{O}_2$ in the ortho position to the $-OH$ group can form intramolecular hydrogen bonds (e. g., o-nitrophenol) which is responsible for their lower boiling points and less solubility in water than the corresponding meta or para isomer. Due to possibility of intramolecular H-bonding (also known as chelation) in the ortho isomer, intermolecular H-bonding is not possible and hence, the ortho isomer can neither associate nor can form H-bonding with $H_{2}O$ with the result it has a low boiling point and less solubility in $H_{2}O$ than the meta and para isomers which can associate (union of 2 or more molecules of the same species) as well as can form H-bonding with $H_{2}O$ .