What is the basis of comparison between two aromatic compounds having different ring structures Like in propyl and heptyl rings

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    $\begingroup$ There is no point in comparing the stability of two entirely different things. $\endgroup$ – Ivan Neretin Aug 8 '19 at 12:19
  • $\begingroup$ Do you mean comparing how much "aromaticity" it has? The degree of aromaticity can be evaluated either through computations or hydrogenations; it represents how much more stable a compound is due to aromaticity (and thus how much more energy it takes to hydrogenate it compared to an "equivalent" non aromatic compound) $\endgroup$ – Raphaël Aug 8 '19 at 12:34
  • $\begingroup$ To put Ivan's comment differently, how do you compare the stability of an electron and the stability of nothing? $\endgroup$ – Zhe Aug 8 '19 at 13:29

You have to take ring strain into account. If you analyze the energy of an organic cycle as a function of the number of atoms in the ring, you find out that cyclohexane and cyclopentane rings are most stable, because the internal angles of an hexagon (120°) and a pentagon (108°) are closer to the angle in a tetrahedron (109°28').

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