CAIE Chemistry A-level
32: Hydroxy Compounds
(A-level only)
Notes
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Alcohols
Formation of Esters by Acylation with Acyl Chlorides
Esters can also be produced from alcohols and acyl chlorides. Alcohol reacts vigorously
with acyl chlorides, releasing steamy fumes of hydrochloric acid.
Phenol
Phenol is an aromatic hydrocarbon consisting of one alcohol group bonded
to the benzene ring:
Phenyl benzoate:
Benzoyl chloride is very unreactive because the -COCl group is directly attached to a
stable benzene ring. To produce the ester phenyl benzoate, phenol is first converted into
an ionic compound by dissolving it in sodium hydroxide. This produces the phenoxide
ion:
The phenoxide ion is more reactive than the phenol so this ion can react with benzoyl
chloride to produce phenyl benzoate:
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Reactions with Bases
Phenol is a weak acid. A hydrogen ion can be removed from the hydroxyl group and
transferred to a base. The position of equilibrium for the reaction below lies far to the left
which means that phenol is a weak acid.
Phenol is able to donate a hydrogen ion because the phenoxide ion is relatively stable.
The lone pair on the oxygen atom is delocalised into the pi system which is above and
below the benzene ring. This means that the negative charge is dispersed among the
carbon atoms so the compound is more stable.
Reaction with sodium hydroxide
When phenol reacts with sodium hydroxide, colourless sodium phenoxide is formed.
Reaction with sodium carbonate
Phenol isn’t acidic enough to react with sodium carbonate.
Reactions with Sodium
Phenol reacts with metals in the same way that most acids. Hydrogen gas and a salt are
produced. The reaction occurs slower than comparable acid-metal reactions because
phenol is a weak acid.
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Reactions with diazonium salts
Diazonium salts contain the diazonium ion, R-N2+ (where R is
any organic group). In the case of the following example, we will
be looking at the diazonium ion when it is attached to a benzene
ring.
Before a diazonium salt is reacted with phenol, phenol is dissolved in sodium hydroxide to
give the phenoxide ion:
Cold benzenediazonium chloride is then added to the sodium phenoxide solution. The
diazonium and phenoxide ions react together to form an azo compound. An azo compound
contains two benzene rings that are joined together with a nitrogen bridge. The product can
be identified as a yellow solution or precipitate.
Nitration and Bromination of the Aromatic Ring
Phenol is more reactive than benzene because it contains an -OH functional group. The
lone pair on the oxygen atom is delocalised into the pi system. This increases the electron
density, making phenol more likely to be attacked by electrophiles.
The effect of the OH group is sometimes referred to as the 2,4-directing effect. This is
because the incoming groups tend to bond to the second and fourth carbons from the
hydroxyl group.
Nitration
Phenol reacts with dilute and concentrated nitric acid:
● Dilute nitric acid
The 2,4-directing effect of the hydroxyl group causes a mixture of 2-nitrophenol and
4-nitrophenol to be produced. The reaction for 2-nitrophenol is shown below.
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● Concentrated nitric acid
The compound 2,4,6-trinitrophenol is formed.
Bromination
When bromine water is added to a solution of phenol, the orange bromine water is
decolourised. 2,4,6-tribromophenol (white precipitate) and hydrogen bromide are formed.
Relative Acidities of Water, Phenol and Ethanol
The relative acidities are as follows: phenol > water > ethanol
● Phenol is the most acidic because the phenoxide ion (formed when phenol
donates a proton) is relatively stable. The lone pair on the oxygen atom is
delocalised into the pi system meaning the negative charge is dispersed among the
carbon atoms. This means that phenol is more likely to donate a hydrogen ion
than water or ethanol.
● Ethanol and water have similar acidities but ethanol is the least acidic. This is
because of the positive inductive effect. The alkyl group in the ethoxide ion
“pushes” electrons away from itself, towards the oxygen. This increases the
electron density of the oxygen, making it more likely to bond to a hydrogen ion and
reform ethanol.
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