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ROUTES
TO PHENOL
A guide for A level students
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
KNOCKHARDY PUBLISHING
PHENOL
INTRODUCTION
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selected topics at AS and A2 level Chemistry. It is based on the requirements of
the AQA and OCR specifications but is suitable for other examination boards.
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be used for classroom teaching if an interactive white board is available.
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www.knockhardy.org.uk/sci.htm
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PHENOL
CONTENTS
• Prior knowledge
• Synthesis from benzene
• Reactions of the OH group
• Reactions of the ring
• Benzene diazonium chloride - preparation
• Benzene diazonium chloride - reactions
PHENOL
Before you start it would be helpful to…
• know the functional groups found in organic chemistry
• know the arrangement of bonds around atoms
• recall and explain electrophilic substitution of aromatic rings
PHENOL
Structure
phenol is an aromatic alcohol with formula C6H5OH
the OH group is attached directly to the benzene ring
it is an almost colourless crystalline solid
PHENOL
Structure
phenol is an aromatic alcohol with formula C6H5OH
the OH group is attached directly to the benzene ring
it is an almost colourless crystalline solid
Preparation
You cannot put an OH group directly onto a benzene ring by
electrophilic substitution, so phenol has to be synthesised in
a multi-stage process
PHENOL
Structure
phenol is an aromatic alcohol with formula C6H5OH
the OH group is attached directly to the benzene ring
it is an almost colourless crystalline solid
Preparation
You cannot put an OH group directly onto a benzene ring by
electrophilic substitution, so phenol has to be synthesised in
a multi-stage process
1
Step 1 - Nitration of benzene
reagents
conc. nitric acid and conc. sulphuric acid (catalyst)
conditions
reflux at 55°C
equation
C6H6 + HNO3
——>
C6H5NO2 + H2O
mechanism
electrophilic substitution
PHENOL
Structure
phenol is an aromatic alcohol with formula C6H5OH
the OH group is attached directly to the benzene ring
it is an almost colourless crystalline solid
Preparation
You cannot put an OH group directly onto a benzene ring by
electrophilic substitution, so phenol has to be synthesised in
a multi-stage process
2
Step 2 - Reduction of nitrobenzene
reagents
tin and conc. hydrochloric acid
conditions
reflux
equation
C6H5NO2 + 6 [H] ——> C6H5NH2
mechanism
reduction
+ 2H2O
PHENOL
Structure
phenol is an aromatic alcohol with formula C6H5OH
the OH group is attached directly to the benzene ring
it is an almost colourless crystalline solid
Preparation
You cannot put an OH group directly onto a benzene ring by
electrophilic substitution, so phenol has to be synthesised in
a multi-stage process
3
Step 3 - Diazotisation of phenylamine
reagents
nitrous acid and hydrochloric acid (use sodium nitrite)
conditions
keep below 10°C
equation
C6H5NH2 + HNO2 + HCl ——> C6H5N2+ Cl¯ + 2H2O
reaction type
diazotisation
PHENOL
Structure
phenol is an aromatic alcohol with formula C6H5OH
the OH group is attached directly to the benzene ring
it is an almost colourless crystalline solid
Preparation
You cannot put an OH group directly onto a benzene ring by
electrophilic substitution, so phenol has to be synthesised in
a multi-stage process
4
Step 4 - Substitution of diazo group by OH
reagents
water
conditions
warm above 10°C
equation
C6H5N2+ Cl¯ + H2O ——> C6H5OH
reaction type
hydrolysis / substitution
+
N2 + HCl
PHENOL - REACTIONS OF THE OH GROUP
Water
phenol is a weak acid
it dissolves very slightly in water to form a weak acidic solution
it is a stronger acid than aliphatic alcohols
the ring helps weaken the O-H bond and stabilises the resulting anion
C6H5OH(aq)
C6H5O¯(aq)
+
H+(aq)
PHENOL - REACTIONS OF THE OH GROUP
Water
phenol is a weak acid
it dissolves very slightly in water to form a weak acidic solution
it is a stronger acid than aliphatic alcohols
the ring helps weaken the O-H bond and stabilises the resulting anion
C6H5OH(aq)
NaOH
C6H5O¯(aq)
+
H+(aq)
phenol reacts with sodium hydroxide to form a salt - sodium phenoxide
it is ionic and water soluble
C6H5OH(aq) + NaOH(aq) ——> C6H5O¯ Na+(aq) + H2O(l)
PHENOL - REACTIONS OF THE OH GROUP
Water
phenol is a weak acid
it dissolves very slightly in water to form a weak acidic solution
it is a stronger acid than aliphatic alcohols
the ring helps weaken the O-H bond and stabilises the resulting anion
C6H5OH(aq)
NaOH
C6H5O¯(aq)
+
H+(aq)
phenol reacts with sodium hydroxide to form a salt - sodium phenoxide
it is ionic and water soluble
C6H5OH(aq) + NaOH(aq) ——> C6H5O¯ Na+(aq) + H2O(l)
Sodium phenol reacts with sodium to form an ionic salt - sodium phenoxide
hydrogen is also produced
this reaction is similar to that with aliphatic alcohols such as ethanol
2C6H5OH(s)
+
2Na(s)
——>
2C6H5O¯ Na+(s)
+
H2(g)
PHENOL - REACTIONS OF THE AROMATIC RING
ELECTROPHILIC SUBSTITUTION
Ease
the OH group is electron releasing
it increases the electron density of the delocalised system
it makes substitution much easier compared to benzene
a p orbital on the oxygen overlaps with the p orbitals in benzene
p orbitals in the system
the p orbital on the O overlaps
with the p orbitals in the ring
PHENOL - REACTIONS OF THE AROMATIC RING
ELECTROPHILIC SUBSTITUTION
Bromine
the OH group is electron releasing
it increases the electron density of the delocalised system
it makes substitution much easier compared to benzene
the electron density is greatest at the 2,4 and 6 positions
substitution takes place at the 2,4 and 6 positions
phenol reacts readily with bromine water WITHOUT A CATALYST
it is so easy that multiple substitution takes place
other electrophiles such as NO2+ react in a similar way
BENZENE DIAZONIUM CHLORIDE
Structure
has the formula C6H5N2+ Cl¯
a diazonium group is attached to the ring
the aromatic ring helps stabilise the ion
BENZENE DIAZONIUM CHLORIDE
Structure
has the formula C6H5N2+ Cl¯
a diazonium group is attached to the ring
the aromatic ring helps stabilise the ion
PREPARATION
reagents
phenylamine, nitrous acid and hydrochloric acid
conditions
keep BELOW 10°C
equation
C6H5NH2
notes
nitrous acid is unstable and is made in situ from sodium nitrite
+
HNO2 + HCl ——> C6H5N2+ Cl¯ + 2H2O
C6H5NH2 + NaNO2 + 2HCl ——> C6H5N2+ Cl¯ + NaCl + 2H2O
the solution is kept cold to slow down decomposition
of the diazonium salt
C6H5N2+ Cl¯ +
H2O ——>
C6H5OH
+
HCl
+
N2
DIAZONIUM SALTS - REACTIONS
Benzene diazonium chloride undergoes two main types of reaction
SUBSTITUTION OF
THE DIAZONIUM GROUP
nitrogen (N2) expelled
COUPLING REACTIONS
nitrogen atoms are retained
DIAZONIUM SALTS - SUBSTITUTION
OH
reagents
water (HYDROLYSIS)
conditions
warm above 10°C
equation
C6H5N2+ Cl¯ +
use
H2O ——>
C6H5OH
+
HCl
+
the only reasonably simple way to substitute OH
phenol is an antiseptic and is used to make polymers
N2
DIAZONIUM SALTS - SUBSTITUTION
OH
reagents
water (HYDROLYSIS)
conditions
warm above 10°C
equation
C6H5N2+ Cl¯ +
use
I
reagents
conditions
equation
H2O ——>
C6H5OH
+
HCl
+
the only reasonably simple way to substitute OH
phenol is an antiseptic and is used to make polymers
potassium iodide solution
warm
C6H5N2+ Cl¯ + KI ——>
C6H5I
+ KCl +
N2
N2
DIAZONIUM SALTS - COUPLING
reagents
phenol and sodium hydroxide
conditions
alkaline solution below 10°C
equation
C6H5N2+Cl¯ + C6H5OH + NaOH ——> C6H5-N=N-C6H4OH + NaCl + H2O
(4-hydroxyphenol)azobenzene
YELLOW
use
making azo dyes
THE AZO GROUP
SOME AZO DYES
(4-hydroxyphenol)azobenzene
YELLOW
4-phenylazophenylamine
YELLOW
The first azo dye to be made
methyl orange
YELLOW IN ALKALI
RED IN ACID
ROUTES
TO PHENOL
THE END
© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING