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The correct decreasing order of melting point for the given organic compounds is: substituted benzaldehydes

3 has maximum melting point because of intermolecular hydrogen bonding.

I am unable to predict rest of the order correctly. 4 should have higher melting point as it shows greater intermolecular hydrogen bonding than the rest, but this contradicts the experimental melting point (mp) data [1]:

$$ \begin{array}{llr} \hline \text{#} & \text{Compound} &\text{mp}/\pu{°C} \\ \hline \textbf{1} & \textit{o}\text{-hydroxybenzaldehyde} & −7(2) \\ \textbf{2} & \textit{o}\text{-methoxybenzaldehyde} & 37.5(0.5) \\ \textbf{3} & \textit{p}\text{-hydroxybenzaldehyde} & 116.0(0.2) \\ \textbf{4} & \textit{p}\text{-methoxybenzaldehyde} & 0 \\ \hline \end{array} $$

Reference

  1. Haynes, W. M.; Lide, D. R.; Bruno, T. J. CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data, 97th ed.; Taylor & Francis Group (CRC Press): Boca Raton, FL, 2016. ISBN 978-1-4987-5429-3.
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    $\begingroup$ melting point depends on how well the molecule packs in a lattice, not neccessarily on strength of H-bond $\endgroup$ Sep 14 '20 at 3:07

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