Boiling points of the three series of compounds were graphed and analyzed. The nitro group has two resonance forms, the combined representation of which results in a positive charge on the nitrogen atom, one and one half bonds from the nitrogen atom to each oxygen atom, and a charge of negative on half on each oxygen atom. Nitro compounds are polar, with the dipole running from the nitrogen atom out between the two oxygen atoms. Polar molecules are strongly attracted by dipolar association between molecules, which stabilizes the liquid phase and requires higher temperatures for boiling. What is interesting is the difference between the three types of compounds.
Branched alkanes boil at a lower temperature than the n-alkane with same molecular weight.
This difference disappears as the number of carbon atoms increases to seven, because the 2-nitro series of compounds still has one chain getting increasingly longer. This is not the expected result. This larger difference in compounds of seven carbons or more is also apparent in the higher boiling points of the nitrocycloalkanes. Conclusions I hypothesize that the reason that the the nitrocycloalkanes have the highest boiling points of the three series is that the cyclic carbon chains are held in a fairly rigid, almost flat conformation, allowing the atoms in the ring to align with atoms in another compound's ring and be held by the temporary dipoles of the London dispersion forces, and also the dipole-dipole interactions due to the nitro group. Since the influence then of these forces is less in the acyclic compounds, their molecules will be less tightly held together, will be less stable, and will require lower temperatures for boiling.