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Phenol
• Aim:
• To introduce phenol and its properties
• To describe the reactions of phenol
• (i) With aqueous alkalis and sodium to
form salts.
• (i) With bromine to form 2,4,6tribromophenol.
• To explain the relative ease of
bromination of phenol compared with
benzene.
What is phenol?
• Phenol is an aromatic alcohol.
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The formula is C6H5OH
It has a sweet smell.
It is sparingly soluble in water.
It is extremely corrosive and causes white
blisters on the skin.
Solubility?
• The OH group hydrogen bonds to water.
• The ring can only form weak Van der
Waal’s forces since it is a large
hydrocarbon section of the molecule.
• When a large mass of phenol is added to
water 2 liquid layers form, one , the
lower layer is a solution of water in
phenol and the other, top layer is a
solution of phenol in water.
Reactions of Phenol
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Phenol was originally called carbolic acid.
It is a weak acid- a proton donor.
Strength of acid?
Compare ethanol, water and phenol
reactions with sodium metal.
• All react to give off hydrogen gas in a
similar reaction where the OH bond
breaks.
Comparing alcohols as acids
Water
Melts, fizzes,
floats.
Hydrogen gas
will burn if
ignited.
Ethanol
Solution of
phenol in
ethanol
Sinks. Steady Sinks, rapid
evolution of
production of
hydrogen gas. hydrogen gas.
Does not melt. Does not melt.
Order of acidity?
• Phenol>water>ethanol
• Why?
• Phenol a weak solution in ethanol so does
not melt.
• Wouldn’t want it to because if it was
hot enough to melt it would ignite the
ethanol.
• Explain the observations.
Water as an acid.
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In the reaction :
2Na +2H2O → 2NaOH + H2
The water is behaving as H-OH.
The H-O bond breaks, producing H+ ions
and OH- ions.
• Reduction takes place:
• 2Na + 2H+ → 2Na+ + H2
• When evaporated the OH- ions and Na+
ions form the ionic sodium hydroxide.
Ethanol as an acid
• Since ethanol also liberates hydrogen
the reaction is the same.
•
HH
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• H-C-C-O-H The O-H bond breaks .
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HH
• This liberates H+ as before.
• 2Na + 2C2H5OH→ 2C2H5O-Na+ + H2
Why is ethanol a weaker acid than water?
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This is written as an equilibrium:
C2H5OH + H2O D C2H5O- + H3O+
C2H5OH D C2H5O- + H+
is the simpler alternative.
This equilibrium lies LEFT because
there is a high electron density on the
O- which attracts the proton back to
recombine.
• The ethoxide ion is a good base due to
the +I effect of the chain.
Phenol as an acid
• Phenol is a stronger acid than water because,
in the equilibrium the phenoxide ion is a
weaker base than the OH- ion because it is a –
I group.
• The ion is stabilised by the partial
delocalisation of the negative charge over the
ring by the oxygen p orbitals interacting with
the ring Π cloud.
• This causes the equilibrium to lie RIGHT so
that the H+ ion does not recombine.
• The phenoxide ion is a weak base.
2C6H5OH + 2Na → 2C6H5O-Na+ +H2
• C6H5OH D C6H5O- + H+
• However all three are weak acids
compared with other weak acids ( e.g.
ethanoic acid).
• Only phenol will affect UI – but even
then it has a pH of only 6.6.
Reaction with alkali
• Only phenol is a strong enough acid (of the
three) to react with sodium hydroxide
solution.
• C6H5OH + NaOH → C6H5O-Na+ + H2O
• The salt dissolves completely in water (why?)
so a colourless solution of sodium phenoxide
forms.
• If strong acid is added hydrolysis occurs and
phenol reforms as a milky emulsion (why?).
• C6H5O-Na+ + H+ → C6H5OH + Na+
Reactions of the benzene ring
• Undergoes electrophilic substitution
reactions MORE READILY than
benzene.
• The ring is said to be activated.
• It has a higher electron density in the
ring than benzene due to partial
delocalisation of the lone pair electrons
on the O atom into the ring via the py
orbitals.
Wheland Intermediates
(not real but an attempt to explain activation)
OH
OH
OH
Electrons delocalise into the ring and repel electron density away
progressively around the ring as shown, leading to extra electron density at
2,4 and 6 positions of the ring which are therefore more available for
electrophilic attack by bromine which does not need a halogen carrier as a
result.