By the chance, let’s try to put everything in order!
Vaporization energy Ev follows the Gibbs free energy equation:
Ev = Hv-TS → Hv = Ev + TS
Up to phase transition and if other parameters do not change (e.g. thermal rearrangement, degradation, polymerization, etc.), entropy numerically coincides with the specific heat though they have completely different physical meanings.
On the other hand, vaporization energy is the overall energy needed, in order to destroy all kind of forces and intermolecular interactions in the liquid state, such as:
- Gravitational forces, which are proportional to the molecular mass.
- Energy changes due to changes of heat distribution within atoms and functional groups, depending on the molecular size, shape, as well as on the flexibility/ rigidity of the molecule and thus, indirectly related with entropy.
- Surface energy per active surface area, which coincides with the gas-liquid surface tension and which also changes within temperature (Eötvös rule).
- Temporary van der Waals forces (Keesom forces) = 0.1-0.5 kcal/mole depending on the size and shape of the molecules.
- Permanent Van der Waals forces (London Forces) = 0.3-3.0 kcal/mole depending on the dipole moment.
- Hydrogen bond ≈ 5 kcal/mole but may be higher if fluorine atoms participate.
- Ionization energy (e.g. carboxylic acids), which also follows a Gibbs equation.
Ei,o = 2.303RTpka
- Activation energy of the thermal reactions during evaporation (e.g. peroxides formation during THF distillation, which also explains why never distilling ethers to dryness).
- Ring strain of cyclic molecules, which is negligible for a six member ring but may reach up to 28 kcal/mole for a three member ring (e.g. epoxides).
- Energy difference between conformers and epimers.
But the most important is that neither vaporization enthalpy nor specific heat, are constant within the overall range of temperature changes. They can be considered as real constants within a narrow range of temperature, only.
As a conclusion, the boiling point macroscopically depends on the nature, size and the particular structure of the molecule. Thus, general rules for comparison of boiling points between molecules with the same atom number, may be unsuccessful; regardless if the corresponding literature and software applications already exist.