Thermochemistry of substituted benzenes: quantification of ortho-, para-, meta-, and buttress interactions in alkyl-substituted nitrobenzenes

The standard (p(O) = 0.1 MPa) molar enthalpies of formation Delta H-f(m)0(1 , or cr) at T = 298.15 K were determined by means of combustion calorimetry for 2-methylnitrobenzene, 2-ethylnitrobenzene, 2-isopropylnitrobenzene, 2- tert-butylnitrobenzene, 2,6-dimethylnitrobenzene, 2,6-di-isopropyl-nitroben zene, and 2,4,6-tri-tert-butylnitrobenzene. The molar enthalpies of sublima tion (or vaporization) of these compounds were obtained from the temperatur e dependence of the vapor pressure measured in a flow system. The resulting values of Delta H-f(m)0(g) were obtained at T = 298.15 K and used to deriv e strain enthalpies of alkylnitrobenzenes. The intramolecular interactions of the substituents were discussed in terms of deviations of Delta H-f(m)0( g) from group additivity rules. These values provide a further improvement on the group-contribution methodology for the estimation of the thermodynam ic properties of organic compounds. (C) 2000 Academic Press.