The medium (pressure, temperature, and solvent) effects on the conformational equilibrium (trans-gauche <-> gauche-gauche) of 1,3-dichloropropane have been investigated using Raman spectroscopy. We have determined the volume (Delta V) and enthalpy (Delta H) differences from the pressure and temperature dependences of the intensities of the C-Cl stretching mode. The differences in the trans-gauche to the gauche-gauche are Delta V(15 degrees C) = -0.51 +/- 0.08 cm(3)/mol, Delta V(65 degrees C) = -0.44 +/- 0.10 cm(3)/mol, Delta H(1 atm) = -3.25 +/- 0.14 kJ/mol, Delta H(3 kbar) = -4.07 +/- 0.18 kJ/mol in the neat liquid and Delta H(30% in n-hexane) = -4.03 +/- 0.29 kJ/mol. The volume differences show that the globular conformer is more compact in volume. This agrees with the van der Waals picture of liquids. However, the solvent and pressure effects on Delta H cannot be explained only by the packing effects. The local attractive interactions in the liquid, which are detected from the solvent-induced frequency shifts, would contribute to the latter effect. The limits of the van der Waals picture of liquids are discussed from a thermodynamic point of view.