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I find that there are two contradicting factors at play here:

1) On removing proton, negative charge on oxygen is more stable than it is on nitrogen (because of greater electronegativity of oxygen). Therefore, phenol is more acidic than aniline.

2) +M effect of nitrogen is more than oxygen, so negative charge (after deprotonation) in the case of aniline will be more delocalized making the anion more stable. Therefore, aniline should be more acidic than phenol.

So is that a factual question or am I missing something?

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  • $\begingroup$ What does point 2 have to do with this? I don’t see the link. $\endgroup$ – orthocresol Dec 8 '17 at 10:27
  • $\begingroup$ @orthocresol After deprotonation, the negative charge will be more delocalized in the case of aniline because of greater +M effect of nitrogen. Since the anion is more stable, aniline should be more acidic. $\endgroup$ – Arishta Dec 8 '17 at 10:43
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    $\begingroup$ Ok, I see now. Electronegativity is usually the main factor. The pKa difference between alcohols and amines is ~20 which is quite large; a difference in the extent of resonance delocalisation isn’t likely to affect pKas by that much. (Compare for example methane (pKa 50) vs toluene (41). In your example both molecules are resonance stabilised so the difference due to this should be much smaller.) $\endgroup$ – orthocresol Dec 8 '17 at 10:52
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    $\begingroup$ The electronegativity scale is essentially arbitrarily constructed, there’s little reason to expect that there should be a quantitative correlation. Maybe there is, I don’t know, but you can’t say that an electronegativity difference of 0.5 “looks small to me” so the pKa difference “shouldn’t be so large”, or anything along those lines. $\endgroup$ – orthocresol Dec 8 '17 at 10:57
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    $\begingroup$ Same reason that water is more acidic then ammonia. $\endgroup$ – Mithoron Dec 8 '17 at 18:14

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