Order of increasing boiling point is p-nitrophenol > m-nitrophenol > o-nitrophenol. The difference between bp of p- and o-nitrophenol is that one form intermolecular hydrogen bonding while other form intramolecular hydrogen bonding. Then what is the reason for high bp of p- than m-nitrophenol as both form intermolecular hydrogen bonding?
In case of p-nitrophenol the lone pair on oxygen of OH takes part in an extended resonance upto NO2 , more delocalisation of lone pair results into more delta minus on H atom ,so stronger H bonding, whereas in case of m-nitrophenol it cant have resonace structures upto NO2. Since less delocalisation results in comparitively less delta minus, H bonding is comparitively weak.