The ortho and para isomers can be separated by steam distillation . This is because o-nitrophenol is steam volatile due to intramolecular hydrogen bonding while p nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules .
Reimer-Tiemann reaction
This reaction is an electrophilic substitution reaction and electrophile is dichlorocarbene . Similarly with carbon tetrachloride and alkali , c- and p- hydroxybenzoic acid are obtained