Greetings,
Most alcohols have no measurable acidic properties. Phenol goes against that rule. In order for a compound to have appreciable acidity the negative ion must be stable. Alkane alcohols, if behaving as an acid, would produce an alkoxide anion. An alkoxide ion will tend to be unstable because the negative charge will only be allowed to reside on the oxygen atom. We say that the charge is localized and this is not a stable state.
With Phenol, valence electrons on oxygen become part of the benzene ring conjugated bond system. This has the effect of distributing the negative charge around the benzyl ring which is a delocalized state: The anion is stabilized and the alcohol takes on some acidic characteristics. The following graphic provides a visual aid.
Another interesting property of phenol in water is its higher pKa of 10. Many compounds with a pKa that high tend to produce aqueous solutions with pH's greater than 7. The delocalized charged state of phenol likely promotes its acidity by way of a series of complex interactions between the roaming negative charge and the surrounding water molecules.
That's all for now. Once again, as always, thank you for reading!
A Publication of http://ExcellenceInLearning.biz
Most alcohols have no measurable acidic properties. Phenol goes against that rule. In order for a compound to have appreciable acidity the negative ion must be stable. Alkane alcohols, if behaving as an acid, would produce an alkoxide anion. An alkoxide ion will tend to be unstable because the negative charge will only be allowed to reside on the oxygen atom. We say that the charge is localized and this is not a stable state.
With Phenol, valence electrons on oxygen become part of the benzene ring conjugated bond system. This has the effect of distributing the negative charge around the benzyl ring which is a delocalized state: The anion is stabilized and the alcohol takes on some acidic characteristics. The following graphic provides a visual aid.
Another interesting property of phenol in water is its higher pKa of 10. Many compounds with a pKa that high tend to produce aqueous solutions with pH's greater than 7. The delocalized charged state of phenol likely promotes its acidity by way of a series of complex interactions between the roaming negative charge and the surrounding water molecules.
That's all for now. Once again, as always, thank you for reading!
A Publication of http://ExcellenceInLearning.biz
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