Part II: Reactions of Phenols
Compounds in which a hydroxyl group is bonded to an aromatic ring are called phenols. Alcohols and phenols are similar in some ways, but there are enough differences so that they are considered different functional groups. One major difference is that phenols are typically about a million time more acidic than alcohols. We shall focus on chemical reactions that can help to distinguish phenols from alcohols.
A) Solubility of Phenols
The presence of a hydroxyl group in phenols permits hydrogen bonding between them and the similar substance water H-OH. This leads to appreciable water solubility of phenols. If non polar groups like alkyl groups are attached to the aromatic ring, the water solubility of the phenols decreases.
B) Acidity of Phenols
Most phenols are weaker acids than carboxylic acids and stronger than alcohols. When phenols react with a base, the phenol is converted phenoxide anion. The phenoxide anion is more soluble in water than the corresponding phenol. Consequently, if a water-insoluble phenol is treated with an aqueous solution of a base that is strong enough to convert most of the phenol to phenoloxide anion, that phenol will dissolve in the aqueous base as the phenoxide salt. None of the above-mentioned bases is strong enough to convert a substantial amount of a typical alcohol into an alkoxide anion which would cause a water-soluble alcohol to dissolve as its alkoxide anion.
C) Reaction with Bromine Water
The hydroxyl groups of phenols activate the ring to electrophilic substitution, so that reaction occurs under very mild conditions. With bromine water and phenol, the product is 2,4,6-tribromophenol, which has such a low solubility in water and appears as a white precipitate.
D) Ferric Chloride Test
Phenols give a colouration (pink, green or violet depending on the structure of the phenol) with ferric chloride. This is due to the formation of certain coordination complexes with the iron. Ordinary alcohols do not react. This test may be used to distinguish most phenols from alcohols.
3. ason--
in aqueous solution ,phenol ionizes to form phenoxide ion ...due to the presence of negative charge ,the oxygen of phenoxide ion donates electron on benzene ring to a large extent...as a result the ring gets highly activated and hence trisubstituion occurs.....
on the other hand in non-polar solvents the ionization of phenols is greatly supprressed ...as a result oxygen of OH group donates electrons to benzene ring only to a small extent ....consequently the ring is activated only slighlty and hence only monosubstitution occurs.....furthur because of steric hindrance at ortho-position the para substittuion product usually predominates.....
see this link--
4. ResultsThe dissipation of the purple color of the permanganate reagent to a yellow color indicates that the testedcompound contains an unsaturated hydrocarbon, that is not aromatic. Benzylic carbon or nitrogen atoms thatare either primary or secondary will also react with permanganate and cause a dissipation of the purple color.Some alcohols are also susceptible to oxidation with this reagent.
