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Answer:
It is well known that unshared electron pairs on
the –OH oxygen in phenol (see figure on left) are
donated to the benzene ring, which makes it stable. This is because
the –OH oxygen forms sp2 orbitals so that –OH is connected to the ring by
a sigma bond and the 2p orbital of the oxygen
is overlapped with the
2p orbitals of the ring. The electrons are delocalized and a resonance
structure is formed. (See picture below: Bonding in phenol, from CHEMISTRY DIAGRAMS with permission,
partially modified)
The –OH oxygen remains as a sp2
orbitals and the stable resonance
structure is formed even when –OH is deprotonated and phenol becomes
phenoxide ion. This causes phenol to be acidic.
It is expected that the orbitals of
the –OH oxygen change from sp3 to sp2
because of the resonance stabilization due to the overlapping of the
2p orbital.
Acknowledgement
We would like to thank an anonymous professor for his careful teaching
and kind support for this answer.
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