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My teacher taught yesterday that though resonance happens in phenoxide ion, it is less stable because the negative charge gets distributed over the carbons in the ring. And hence, alcohols are more acidic than phenols.

His argument seemed convincing at the time but then I searched the Internet about it and everywhere it says that phenol is more acidic than alcohol.

So which is it?

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    $\begingroup$ Phenol is more acidic than ethanol. Sodium phenate may be obtained in water. Sodium ethanoate is destroyed in water. $\endgroup$
    – Maurice
    Commented Mar 28, 2023 at 12:12
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    $\begingroup$ An old alternative name for phenol is carbolic acid and that is a big clue. Your teacher has it backwards and really should not be teaching chemistry if they get such a fundamental thing wrong. $\endgroup$
    – Waylander
    Commented Mar 28, 2023 at 12:19
  • $\begingroup$ Easy @Waylander. Nobody's perfect! $\endgroup$ Commented Mar 28, 2023 at 12:37
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    $\begingroup$ I'm with @Waylander on this. Fancy hand waving explanations are no good if they fly in the face of experimental facts. $\endgroup$
    – porphyrin
    Commented Mar 28, 2023 at 18:22

2 Answers 2

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The teacher misspoke. It's true that negative charge is relatively unstable in most cases when it's distributed over carbon atoms only, like let us say an allyl anion $(\ce{CH2=CH-\overset{-}{C}H2}).$ But if a more electronegative atom, most commonly oxygen, anchors the bulk of the negative charge then the anion is stabilized; and any delocalization onto carbon atoms only adds to this stability. Phenoxide will in effect concentrate as much of the negative charge on oxygen as it needs to be reasonably stable and delocalize the rest; one oxygen atom plus some carbon atoms beats the oxygen atom alone.

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Is it possible that you misunderstood? If not, your teacher is absolutely wrong. The resonance in phenoxide ions stabilizes the negative charge compared to alkoxide ions.

Phenol has a pKa of 9.95 in water, while methanol has a pKa of 15.5.[1] Whatever explanation we might use to explain it, the simple fact is that phenols are more acidic than alcohols.

[1] https://organicchemistrydata.org/hansreich/resources/pka/pka_data/evans_pKa_table.pdf

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