Hydroxy Compounds - Reactivity of Phenols

Phenol like benzene undergoes electrophilic substitution reaction. The presence of the -OH substitutent enriches the electron density of the pi electron cloud. Hence, phenol undergoes electrophilic substitution readily.

The O atom of phenol lies on the same plane as the C atoms, this enables the p-orbital of O to overlap with the p-orbital of carbon. This results in a formation of tunnel where O lone pair of electron can flow despite O being electron-withdrawing.

If we were to draw the lewis structure of phenol, we can show the lone pair of electron on O can indeed be donated to the pi electron cloud, thus enriching the latter electron density. However, the converse is impossible (i.e. pi electron cloud donating an electron pair to O atom) as that would result in O to violate the octet rule.

In conclusion, I would like to highlight two reactions of phenol which helps to reiterate the concept that the -OH substitutent results in the pi electron cloud to be enriched hence enabling phenol to be more susceptible to attacks of electrophiles. Therefore, a milder reaction condition is required.

(1) Electrophilic substitution reaction - Nitration
Since the pi electron cloud is enriched, the electrophilic substitution of phenol do not require a halogen carrier. In fact, phenol's reaction are generally milder. Using of dilute HNO₃ results in the formation of the mono-substituted product (I showed the 4-substitution as it is favoured).

(2) Electrophilic substitution reaction - Bromination
While in the reaction between Br₂ and phenol, the use of Br₂ in CCl₄ results in the mono-substituted product. While the usage of aqueous Br₂ results in the formation of the tribromophenol product to be formed, which is is a useful reaction as it produces a white precipitate. This reaction helps distinguish benzene and phenol.

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Article written by Kwok YL 2009.}

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