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Phenol, also known under an older name of carbolic acid, is a colourless crystalline solid with a typical sweet tarry odor. Its chemical formula is C6H5OH and its structure is that of a hydroxyl group (-OH) bonded to a phenyl ring; it is thus an aromatic compound.
The word phenol is also used to refer to any compound which contains a six-membered aromatic ring, bonded directly to a hydroxyl group (-OH). In effect, phenols are a class of organic compounds of which the phenol discussed in this article is the simplest member.
Phenol has a limited solubility in water (8.3 g/100 ml). It is slightly acidic: the phenol molecule has weak tendencies to lose the H+ ion from the hydroxyl group, resulting in the highly water-soluble phenoxide anion C6H5O-. Compared to aliphatic alcohols, phenol shows much higher acidity; it even reacts with NaOH to lose H+ whereas aliphatic alcohols do not. Contrary to popular belief, this has little to do with orbital overlap between the oxygen's lone pairs and the aromatic system.
While the aromaticity of the benzene ring allows the delocalization the negative charge throughout the ring to stabilizes the anion, the dominant effect is the induction from the sp2 hybridized carbons (the comparatively more powerful inductive withdrawal of electron density that is provided by the sp2 system compared to an sp3 system allows for great stabilization of the oxyanion). In making this conclusion, we cite the pKa of acetone enol, which is 10.9 (in comparison to phenol with a pKa of 10.0). For further reading on the fine points of this topic, see David A. Evans's explanation.
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