Phenol, \mathrm{C_6H_5OH}, is a colourless solid with a characteristic smell that melts at 41 °C. Compounds like phenol, in which a hydroxy group is bonded directly to a benzene ring, are called phenols.
Like alcohols, phenol reacts with sodium and gives off hydrogen. Unlike alcohols, however, its solution in water is weakly acidic, and it reacts with sodium hydroxide.
Phenol kills bacteria and was once used as a disinfectant. Today it is used to make plastics such as phenolic resins, and medicines.
At room temperature, phenol dissolves only a little in water, but it dissolves in sodium hydroxide solution, forming sodium phenoxide.
\mathrm{C_6H_5OH} + \mathrm{NaOH} \longrightarrow \mathrm{C_6H_5ONa} + \mathrm{H_2O}
When carbon dioxide is blown into a solution of sodium phenoxide, phenol is set free and the solution turns cloudy. This happens because phenol is a weaker acid than carbonic acid.
\mathrm{C_6H_5ONa} + \mathrm{CO_2} + \mathrm{H_2O} \longrightarrow \mathrm{C_6H_5OH} + \mathrm{NaHCO_3}
From the strongest, these acids are in the order hydrochloric acid, carboxylic acids, carbonic acid and phenol. For this reason, unlike carboxylic acids, phenol does not give off carbon dioxide with sodium hydrogencarbonate.
When iron(III) chloride solution is added to a solution of phenol, the solution turns violet.
Other phenols also give a colour, such as violet or blue, with iron(III) chloride solution, so this reaction is used to detect phenols. Alcohols do not give this colour.
When bromine water is added to a solution of phenol, the orange-brown colour of the bromine disappears at once, and a white precipitate of 2,4,6-tribromophenol forms.
\mathrm{C_6H_5OH} + 3\mathrm{Br_2} \longrightarrow \mathrm{C_6H_2Br_3OH} + 3\mathrm{HBr}
Here, three hydrogen atoms of the benzene ring are replaced by bromine atoms without a catalyst. The hydroxy group makes the benzene ring of phenol much more reactive than that of benzene. This reaction is used to detect phenol.
When sodium phenoxide is heated with carbon dioxide under high pressure and an acid is then added, salicylic acid forms. Salicylic acid has both a hydroxy group and a carboxy group on its benzene ring.
When salicylic acid reacts with ethanoic anhydride, the hydrogen atom of its hydroxy group is replaced by an acetyl group, -\mathrm{COCH_3}, and acetylsalicylic acid forms. This reaction is called acetylation, and acetylsalicylic acid is used as aspirin, a medicine that brings down fever and eases pain.
\mathrm{C_6H_4(OH)COOH} + (\mathrm{CH_3CO})_2\mathrm{O} \longrightarrow \mathrm{C_6H_4(OCOCH_3)COOH} + \mathrm{CH_3COOH}
When salicylic acid is heated with methanol and a little concentrated sulfuric acid, its carboxy group undergoes esterification, and methyl salicylate forms. It has a strong smell and is used in creams and patches for muscle pain.
Acetylsalicylic acid has no hydroxy group on its benzene ring, so it does not give a violet colour with iron(III) chloride, but methyl salicylate does.