All the atoms of these compounds are in a plane.

o-phthalic acid can show extensive intra-molecular hydrogen bonding as well as inter-molecular hydrogen bonding.

m-phthalic acid can show intra-molecular hydrogen bonding (though, in lesser extent) as well as inter-molecular hydrogen bonding.

p-phthalic acid can not show intra-molecular hydrogen bonding but can show extensive (more than ortho & meta isomers, due to no steric hindrance) inter-molecular hydrogen bonding.

Now, intra-molecular hydrogen bonding increases volatility whereas, inter-molecular hydrogen bonding decreases volatility (I am not comparing boiling points because these molecules decompose at temperature below their melting point).

What can we say about the order of volatility of these compounds?

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    $\begingroup$ How does m-phthalic acid show intramolecular bonding? Seems unlikely. $\endgroup$ – user55119 Sep 14 '19 at 16:34
  • $\begingroup$ Do you have a temperature in mind? Because all of three compounds are solid at room temperature. $\endgroup$ – Mathew Mahindaratne Sep 14 '19 at 17:03
  • $\begingroup$ @MathewMahindaratne Melting point: o-isomer 207 *C, m-isomer 300 *C (sublimes without melting) & p-isomer 345 *C $\endgroup$ – Apurvium Sep 16 '19 at 13:44

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