Energetics of Intra- and Inter-molecular Bonds in the Three Nitrophenols

Abstract

A thermodynamic study of the three nitrophenol isomers (general formula C₆H₅NO₃) was realized by combustion calorimetry of small amounts of substance (a few milligrams), sublimation calorimetry, differential thermal analysis and heat capacity measurements. The experimental enthalpies of combustion, sublimation and fusion of these compounds are as follows:

ortho para -ΔcH°m(s,298.15K)/kJ mol^-1 2871.0±1.3 2875.1±0.9 2868.5±1.0 Δ_subH°m(298.15K)/kJ mol^-1 72.30±0.28 91.23±0.49 92.39±0.43 Δ_fusHm/kJ mol^-1 18.32±0.35 20.54±0.34 17.33±0.10 T_{triple point}/K 318.40±0.01 370.51±0.01 387.26±0.05  

The strength of the intramolecular hydrogen bond in the ortho isomer was estimated equal to 20.09 kJ mol-¹. The relative stability of the three isomers is discussed, and the intermolecular bond enthalpies have been determined. The experimental resonance energies E_exp,conj are 168.7, 142.8 and 148.2 kJ mol-¹ for ortho -, meta- and para-nitrophenol respectively, and are in good agreement with theoretical values. The experimental atomization enthalpies Δ_a,expH°m(298.15K) are 6742.5±1.9, 6719.5±1.7 and 6724.9±1.8 kJ mol-¹ for ortho -, meta- and para-nitrophenol respectively.