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During lecture with our chemistry teacher, he randomly asked us to compare solubility of phenol and p-nitrophenol, we expected p-nitrophenol to have greater solubility than phenol due to better dipole moment (5.43 D which is 1.70 D for phenol) and better hydrogen bonding but as it turns out phenol is way more soluble (84 g/L) than p-nitrophenol (10 g/L). Teacher was also confused, he didn’t resolve the problem that day and came to class next day saying he did some research and has found the reason but he hasn’t disclosed it yet and has asked us to do research ourself to get the reason.

I think the reason behind this abnormal behaviour is the strong attractions already existing between p-nitrophenol molecules is a lot more stronger than interactions between water molecules and interactions with water molecules are weaker, I am not sure of my answer and would like a better explanation / ideas to think about solubility. What do you guys think should be the reason for this abnormality.

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I asked my teacher and also this is what i suspected , para nitro phenol is less soluble in water than phenol as para nitro phenol is way more polar than water and so the hydrogen bonding and other interactions formed by interaction between para nitro phenol and water are weaker than formed by interaction between molecules of para nitro phenol because of huge polarity para nitro phenol as compared to water but as dipole moments of water and phenol are almost same so they bond better than being alone and hence making phenol more soluble .

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  • $\begingroup$ The nitro group does not seem to cause water solubility in anything! Nitrobenzene is insoluble, nitromethane and nitroethane are slightly soluble. It must have something to do with the diffuseness of the negative charge about the O atoms in the nitro group and the nitrate ion. Any overt dipole or hydrogen bonding attraction will involve disruption of the resonance lowered molecular orbitals. Add to that the entropy decrease from association and there is no increased intermolecular attraction. $\endgroup$
    – jimchmst
    Oct 6, 2022 at 0:06

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