Question

explain intermolecular hydrogen bonding takes place in ortho-nitrophenol but not in para-nitrophenol

Answer 1

Intramolecular hydrogen bonding takes place in ortho-nitrophenol because of the NO2 and OH molecules close proximity to each other and this disrupts possibility of a molecular association.
In para-nitrophenol intramolecular hydrogen bonding takes place due to extensive hydrogen present in it.