Phenols: Phenols contain a hydroxyl, -OH, directly bonded to an aromatic ring. Simplest member
called phenol C6H5OH, same name as the group. Another example is 2- hydroxybenzoic acid
below.
Aromatic Alcohol: Remember an alcohol is –OH bonded to carbon chain. An alcohol can still
have a benzene ring but not directly bonded to benzene ring so not a phenol, but an aromatic
alcohol. E.g. 2- phenylethanol below.
Phenol used for plastics, antiseptics and disinfectants.
Name things from phenol being in position 1, with a few
exceptions.
Weak Acid: Phenol is less soluble in water than alcohols due to presence of non- polar benzene
ring. When dissolved in water, phenol partially dissociates forming phenoxide ion and proton, so
phenol a weak acid.
Comparing Acidity: Phenols more acidic than alcohol but less acidic than carboxylic acid. Alcohol
does not react with weak or strong base. Phenols and COOH react with strong bases e.g.
aqueous NaOH. Only carboxylic reacts with weak base e.g. sodium carbonate, other carbonates.
These reactions can differentiate the three. All three react with metals.
NaOH does not react with alcohols, only COOH.
1) Reaction of Phenol with NaOH: Phenol reacts with sodium
hydroxide to form salt, sodium phenoxide, and water. A neutralisation reaction. If excess, all OH
groups in compound form salt.
Electrophilic Substitution Reactions of Phenol: Phenols are aromatic compounds so undergo
electrophilic substitution reactions in similar mechanism to alkenes. But because benzene,
always have horseshoe bit.
2) Bromination of Phenol: Phenol reacts with bromine water to form white
precipitate of 2, 4, 6- tribromophenol. Decolourises bromine water- always give both
observations. Halogen carrier catalyst not required and room temperature. 2,4 and 6 Br
positions because these are most reactive hydrogens due to position of highly electronegative
oxygen.
If another group already bonded to 2, 4 or 6, can’t have a bromine there, but still add bromine to
the other available positions.
3) Nitration of Phenol: Phenol reacts with dilute nitric acid. At room
temperature. A mixture of 2- nitrophenol and 4- nitrophenol formed.
Comparing Reactivity Phenol and Benzene: Bromine and nitric acid react more readily with
phenol than benzene. Phenol- dilute nitric acid and room temperature. Benzene- concentrated
nitric and sulphuric acid and 50C.
Increased reactivity is caused by lone pairs of electrons from the oxygen of –OH group is partially
delocalised into ring. Phenol has higher electron density, pi region in benzene ring activated,
electrophile more polarised and attracts electrophiles more strongly than benzene. So
electrophile more easily induced. So no halogen carrier.
Same thing for occurs with nitrogen.