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Q)Compare the melting points of the following?

  1. o-Hydroxybenzaldehyde
  2. o-Methoxybenzaldehyde
  3. p-Hydroxybenzaldehyde
  4. p-Methoxybenzaldehyde

My attempt: Intermolecular H-bonding increases melting point and intramolecular H-bonding decreases melting point. Also, the more the molar mass more the Van der Waals forces and the melting point.

So, based on these arguments 3) will have the highest melting point, and 1) will have the lowest.

How can we compare between 2) and 4)?

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    $\begingroup$ These arguments are suitable for boiling points, not melting points. Melting points depend more on molecular geometry. $\endgroup$
    – Paul Kolk
    Commented May 18 at 11:42
  • $\begingroup$ okay, so what will be the order of melting points? $\endgroup$
    – Chetan
    Commented May 18 at 13:14
  • $\begingroup$ You can look them up from Wikipedia. $\endgroup$
    – Paul Kolk
    Commented May 18 at 13:19
  • $\begingroup$ @Paul Kolk: You are correct on molecular geometry because it increase the packing in crystal structure. Also increase $\pi$-interaction between molecules. $\endgroup$
    – Mathew Mahindaratne
    Commented May 18 at 13:50
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    $\begingroup$ @Chetan If an ordered structure exists where oxygens on adjacent molecules can be further apart, without increasing the distance between the molecules, this structure melts at higher temperature. Charge density spreads out compared to more disordered liquid. Be aware that this only applies in simple cases when everything else is almost equal. $\endgroup$
    – Paul Kolk
    Commented May 19 at 8:42

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