Transcript Arenes: Benzene - Miller, Jonathan

THE CHEMISTRY
OF ARENES
A guide for A level students
ARENES
CONTENTS
• Prior knowledge
• Structure of benzene
• Thermodynamic stability
• Delocalisation
• Electrophilic substitution
• Nitration
• Chlorination
• Friedel-Crafts reactions
• Further substitution
• The chemistry of phenol
ARENES
Before you start it would be helpful to…
• know the functional groups found in organic chemistry
• know the arrangement of bonds around carbon atoms
• recall and explain electrophilic addition reactions of alkenes
STRUCTURE OF BENZENE
Primary analysis revealed benzene had...
an
a
a
empirical formula of CH
molecular mass of 78
formula of C6H6
and
and
STRUCTURE OF BENZENE
Primary analysis revealed benzene had...
an
a
a
empirical formula of CH
molecular mass of 78
formula of C6H6
Kekulé
and
suggested that benzene was...
PLANAR
CYCLIC and
HAD ALTERNATING DOUBLE AND SINGLE BONDS
STRUCTURE OF BENZENE
HOWEVER...
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• all six C—C bond lengths were similar; C=C bonds are shorter than C-C
• the ring was thermodynamically more stable than expected
STRUCTURE OF BENZENE
HOWEVER...
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• all six C—C bond lengths were similar; C=C bonds are shorter than C-C
• the ring was thermodynamically more stable than expected
To explain the above, it was suggested that the structure oscillated
between the two Kekulé forms but was represented by neither of
them. It was a RESONANCE HYBRID.
THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated
compound, energy is released. The amount of heat liberated per mole (enthalpy of
hydrogenation) can be measured.
THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated
compound, energy is released. The amount of heat liberated per mole (enthalpy of
hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to
cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
2
3
- 120 kJ mol-1
THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated
compound, energy is released. The amount of heat liberated per mole (enthalpy of
hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to
cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate
C=C bonds it would release 360kJ per mole when
reduced to cyclohexane
Theoretical
- 360 kJ mol-1
(3 x -120)
C6H6(l) + 3H2(g) ——> C6H12(l)
2
3
- 120 kJ mol-1
THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated
compound, energy is released. The amount of heat liberated per mole (enthalpy of
hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to
cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate
C=C bonds it would release 360kJ per mole when
reduced to cyclohexane
Theoretical
- 360 kJ mol-1
(3 x -120)
C6H6(l) + 3H2(g) ——> C6H12(l)
2
Actual benzene releases only 208kJ per mole when
reduced, putting it lower down the energy scale
3
- 120 kJ mol-1
Experimental
- 208 kJ mol-1
THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated
compound, energy is released. The amount of heat liberated per mole (enthalpy of
hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to
cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate
C=C bonds it would release 360kJ per mole when
reduced to cyclohexane
Theoretical
- 360 kJ mol-1
MORE STABLE
THAN EXPECTED
by 152 kJ mol-1
(3 x -120)
C6H6(l) + 3H2(g) ——> C6H12(l)
2
Actual benzene releases only 208kJ per mole when
reduced, putting it lower down the energy scale
It is 152kJ per mole more stable than expected.
This value is known as the RESONANCE ENERGY.
3
- 120 kJ mol-1
Experimental
- 208 kJ mol-1
THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated
compound, energy is released. The amount of heat liberated per mole (enthalpy of
hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to
cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate
C=C bonds it would release 360kJ per mole when
reduced to cyclohexane
Theoretical
- 360 kJ mol-1
MORE STABLE
THAN EXPECTED
by 152 kJ mol-1
(3 x -120)
C6H6(l) + 3H2(g) ——> C6H12(l)
2
Actual benzene releases only 208kJ per mole when
reduced, putting it lower down the energy scale
It is 152kJ per mole more stable than expected.
This value is known as the RESONANCE ENERGY.
3
- 120 kJ mol-1
Experimental
- 208 kJ mol-1
HYBRIDISATION OF ORBITALS - REVISION
The electronic configuration of a
carbon atom is 1s22s22p2
2p
2
2s
1
1s
HYBRIDISATION OF ORBITALS - REVISION
The electronic configuration of a
carbon atom is 1s22s22p2
2p
2
2s
1
If you provide a bit of energy you
can promote (lift) one of the s
electrons into a p orbital. The
configuration is now 1s22s12p3
1s
2p
2
2s
1
1s
The process is favourable because of the arrangement of
electrons; four unpaired and with less repulsion is more stable
HYBRIDISATION OF ORBITALS - REVISION
The four orbitals (an s and three p’s) combine or HYBRIDISE
to give four new orbitals. All four orbitals are equivalent.
2s22p2
2s12p3
4 x sp3
HYBRIDISE
sp3
HYBRIDISATION
HYBRIDISATION OF ORBITALS - REVISION
Alternatively, only three orbitals (an s and two p’s) combine or
HYBRIDISE to give three new orbitals. All three orbitals are
equivalent. The remaining 2p orbital is unchanged.
2s22p2
2s12p3
3 x sp2
HYBRIDISE
sp2
HYBRIDISATION
2p
STRUCTURE OF ALKENES - REVISION
In ALKANES, the four sp3 orbitals
repel each other into a tetrahedral
arrangement.
In ALKENES, the three
sp2 orbitals repel each
other into a planar
arrangement and the
2p orbital lies at right
angles to them
STRUCTURE OF ALKENES - REVISION
Covalent bonds are formed
by overlap of orbitals.
The resulting bond is called
a SIGMA (δ) bond.
An sp2 orbital from each carbon
overlaps to form a single C-C bond.
STRUCTURE OF ALKENES - REVISION
The two 2p orbitals also overlap. This forms a second bond; it
is known as a PI (π) bond.
For maximum overlap and hence the strongest bond, the 2p
orbitals are in line.
This gives rise to the planar arrangement around C=C bonds.
ORBITAL OVERLAP IN ETHENE - REVIEW
two sp2 orbitals overlap to form a sigma
bond between the two carbon atoms
s orbitals in hydrogen overlap with the
sp2 orbitals in carbon to form C-H bonds
two 2p orbitals overlap to form a pi
bond between the two carbon atoms
the resulting shape is planar
with bond angles of 120º
STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility
STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility
delocalised pi
orbital system
STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility
This final structure was particularly stable and
resisted attempts to break it down through normal
electrophilic addition. However, substitution of any
hydrogen atoms would not affect the delocalisation.
delocalised pi
orbital system
STRUCTURE OF BENZENE
STRUCTURE OF BENZENE
ANIMATION
The animation doesn’t work on early versions of Powerpoint
WHY ELECTROPHILIC ATTACK?
Theory
The high electron density of the ring makes it open to attack by electrophiles
HOWEVER...
Because the mechanism involves an initial disruption to the ring
electrophiles will have to be more powerful than those which react
with alkenes.
A fully delocalised ring is stable so will resist attack.
WHY SUBSTITUTION?
Theory
Addition to the ring would upset the delocalised electron system
STABLE DELOCALISED SYSTEM
ELECTRONS ARE NOT DELOCALISED
AROUND THE WHOLE RING - LESS STABLE
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Overall there is ELECTROPHILIC SUBSTITUTION
ELECTROPHILIC SUBSTITUTION
Theory
The high electron density of the ring makes it open to attack by electrophiles
Addition to the ring would upset the delocalised electron system
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Because the mechanism involves an initial disruption to the ring,
electrophiles must be more powerful than those which react with alkenes
Overall there is ELECTROPHILIC SUBSTITUTION
ELECTROPHILIC SUBSTITUTION
Theory
The high electron density of the ring makes it open to attack by electrophiles
Addition to the ring would upset the delocalised electron system
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Because the mechanism involves an initial disruption to the ring,
electrophiles must be more powerful than those which react with alkenes
Overall there is ELECTROPHILIC SUBSTITUTION
Mechanism
• a pair of electrons leaves the delocalised system to form a bond to the electrophile
• this disrupts the stable delocalised system and forms an unstable intermediate
• to restore stability, the pair of electrons in the C-H bond moves back into the ring
• overall there is substitution of hydrogen ... ELECTROPHILIC SUBSTITUTION
H E
H
H
+
H
H
H
H
H + E+
H
H
H
H
H
H
E + H+
H
H
H
ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents
conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions
reflux at 55°C
Equation
C6H6
+
HNO3
———>
C6H5NO2 + H2O
nitrobenzene
ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents
conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions
reflux at 55°C
Equation
C6H6
Mechanism
+
HNO3
———>
C6H5NO2 + H2O
nitrobenzene
ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents
conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions
reflux at 55°C
Equation
C6H6
+
HNO3
———>
C6H5NO2 + H2O
nitrobenzene
Mechanism
Electrophile
NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...
2H2SO4 +
acid
HNO3
base
2HSO4¯ + H3O+ + NO2+
ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents
conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions
reflux at 55°C
Equation
C6H6
+
HNO3
———>
C6H5NO2 + H2O
nitrobenzene
Mechanism
Electrophile
NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...
2H2SO4 +
acid
Use
HNO3
base
2HSO4¯ + H3O+ + NO2+
The nitration of benzene is the first step in an historically important chain of
reactions. These lead to the formation of dyes, and explosives.
ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION
Reagents
chlorine and a halogen carrier (catalyst)
Conditions
reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3)
chlorine is non polar so is not a good electrophile
the halogen carrier is required to polarise the halogen
Equation
C6H6
+
Cl2
———>
C6H5Cl + HCl
Mechanism
Electrophile
Cl+
it is generated as follows...
Cl2 +
FeCl3
a
Lewis Acid
FeCl4¯ +
Cl+
ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION
Reactivity of Halogenarenes
Compared with halogenalkanes (e.g. chloroethane), the C-X bond
In halogenarenes is shorter and thus stronger:
Bond
length
CH3Cl
0.177nm
C6H5Cl
0.169nm
This is because the lone-pair on the halogen overlap with the p
System of the aromatic ring and delocalisation occurs between the
C-Cl atoms too; thus partial double bond character.
Nucleophilic substitution is thus more difficult [requires 600K and
2 x 104 kPa pressure with NaOH(aq)].
FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION
Overview
Alkylation involves substituting an alkyl (methyl, ethyl) group
Reagents
a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3
Conditions
room temperature; dry inert solvent (ether)
Electrophile
a carbocation ion R+ (e.g. CH3+)
Equation
C6H6 + C2H5Cl
———>
C6H5C2H5 + HCl
FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION
Overview
Alkylation involves substituting an alkyl (methyl, ethyl) group
Reagents
a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3
Conditions
room temperature; dry inert solvent (ether)
Electrophile
a carbocation ion R+ (e.g. CH3+)
Equation
C6H6 + C2H5Cl
———>
C6H5C2H5 + HCl
Mechanism
General
A catalyst is used to increase the positive nature of the electrophile
and make it better at attacking benzene rings.
AlCl3 acts as a Lewis Acid and helps break the C—Cl bond.
FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION
Catalyst
anhydrous aluminium chloride acts as the catalyst
the Al in AlCl3 has only 6 electrons in its outer shell; a LEWIS ACID
it increases the polarisation of the C-Cl bond in the haloalkane
this makes the charge on C more positive and the following occurs
RCl
+
AlCl3
AlCl4¯ + R+
FURTHER SUBSTITUTION OF ARENES
Theory
It is possible to substitute more than one functional group.
But, the functional group already on the ring affects...
• how easy it can be done
Group
• where the next substituent goes
ELECTRON DONATING
Example(s)
Electron density of ring
Ease of substitution
Position of substitution
OH, CH3
Increases
Easier
2,4,and 6
ELECTRON WITHDRAWING
NO2
Decreases
Harder
3 and 5
FURTHER SUBSTITUTION OF ARENES
Examples
Substitution of nitrobenzene is...
• more difficult than with benzene
• produces a 1,3 disubstituted product
Substitution of methylbenzene is…
• easier than with benzene
• produces a mixture of 1,2 and 1,4
isomeric products
Some groups (OH) make substitution so much
easier that multiple substitution takes place
FURTHER SUBSTITUTION OF ARENES
Methylbenzene undergoes (a) electrophilic substitution reactions in the ring or (b)
free-radical substitution in the methyl group (the alkyl substituent).
(a) Electrophilic Substitution reaction: chlorination reaction
CH3
CH3
CH3
Cl
2
+ 2Cl2
AlCl3
+ 2HCl
+
Cl
2-chloromethylbenzene 4-chloromethylbenzene
With methylbenzene, a mixture of 2-chloromethylbenzene and 4chloromethylbenzene is mostly; only 5% is 3-chloromethylbenzene.
The methyl group is electron releasing (+I effect) and helps to stabilize the
positive charge further during the reaction.
FURTHER SUBSTITUTION OF ARENES
Methylbenzene undergoes (a) electrophilic substitution reactions in the ring or (b)
free-radical substitution in the methyl group (the alkyl substituent).
(b) Free Radical Substitution reaction: chlorination reaction
Similar to methane, methylbenzene with chlorine in the presence of U.V. light will
give mostly chloromethylbenzene and a mixture of further substitution products:
CH2Cl
CH3
+ Cl2
U.V.
+ HCl + further substitution
chloromethylbenzene
STRUCTURAL ISOMERISM
RELATIVE POSITIONS ON A BENZENE RING
1,2-DICHLOROBENZENE
ortho dichlorobenzene
1,3-DICHLOROBENZENE
meta dichlorobenzene
1,4-DICHLOROBENZENE
para dichlorobenzene
Compounds have similar chemical properties but different physical properties
OXIDATION OF THE SIDE CHAIN
In addition to substitution reactions, the methyl group can be oxidised to
benzoic acid
In the presence of hot alkaline potassium manganate(VII), the methyl side-chain
can be oxidised to benzoic acid:
COOH
CH3
a) alkaline KMnO4
b) H2SO4(aq)
benzoic acid
a)
The KMnO4 is made alkaline using aqueous sodium carbonate. The purple colour of aqueous
manganate (VII) becomes a cloudy brown as MnO2 is formed (and can be removed by filtration).
b) The mixture is acidified to form benzoic acid which is only slightly soluble in aqueous solution.
C6H5COO-(aq) + H+(aq)
C6H5COOH(S)
CHEMISTRY OF PHENOL
(d) Recall the chemistry of phenol, as exemplified by the following reactions:
(i)
With bases
(ii)
With sodium
(iii) Bromination of, and nitration of, the aromatic ring
(e) Explain the relative acidities of water, phenol and ethanol
OH
Phenol
CHEMISTRY OF PHENOL
Extra delocalisation in phenol
CHEMISTRY OF PHENOL
Physical properties of phenol
Melting point Boiling point
Phenol
40-43 oC
182
Methylbenzene
-95 oC
111
The solubility of phenol in water is about 8g per 100 ml of water
CHEMISTRY OF PHENOL
Reaction of phenol with bases: Phenol is a white, crystalline solid with a
pungent odour and is water soluble (due to hydrogen bonding). It is a weak
acid (pH 5-6) and reacts with bases including sodium carbonate.
C6H5O-(aq) + H3O+(aq)
C6H5OH(aq) + H2O(l)
O-Na+
OH
+ NaOH(aq)
+ H2O
Phenol
sodium phenoxide
C6H5O-Na+(aq) + H2O(l)
C6H5OH(aq) + NaOH(aq)
O-
O
O
-
O
O
-
negative charge delocalised
around benzene ring
The phenoxide ion stabilised by the aromatic ring, and easier to form than alkoxides (e.g. sodium
ethoxide), where there is no delocalisation of the negative charge.
CHEMISTRY OF PHENOL
Reaction with sodium:
2C6H5O-Na+(s) + H2(g)
2C6H5OH(s) + 2Na(s)
Phenol is dissolved in a dry solvent such as diethyl ether.
Bromination of phenol: Phenol reacts with electrophiles by undergoing
electrophilic substitution reactions. Due to the electron releasing effect of
the –OH group, it is much more reactive than benzene and will react with
elemental bromine:
OH
OH
Br
Br
+ 3Br2(aq)
+ 3HBr
Br
2,4,6-tribromophenol
CHEMISTRY OF PHENOL
interaction of the lone pair on oxygen
with delocalised p-bonding: stabilises positive charge
Mechanism:
+
OH
..
OH
OH
OH
-Br+
+
H
Br
Br
+
H
Br
H
Br

bromine electrophile
OH
+
H
OH
+ Br-
Br
In a similar manner, trisubstitution occurs.
+ HBr
Br
H
Br
CHEMISTRY OF PHENOL
TCP (2,4,6-trichlorophenol) is used as an anti-bacterial disinfectant
(消毒剂) and can be made by reacting phenol with chlorine.
Dettol (4-chloro-3,5-dimethylphenol) is a more popular disinfectant
used around the home. www.dettol.co.uk.
Dettol
Only 4.8% of dettol consists of this molecule (C8H9ClO) with
CAS number 88-04-0, the rest being made up of Pine Oil, isopropyl alcohol,
castor oil soap, caramel (= brown colour) and water.
CHEMISTRY OF PHENOL
Nitration of phenol: Phenol reacts with dilute nitric acid to give a mixture of 2and 4-nitrophenols:
OH
OH
OH
NO2
2
+ 2HNO3(aq)
+
+ 2H2O
NO2
4-nitrophenol
2-nitrophenol
With boiling concentrated nitric acid and sulphuric acid, the thermally unstable
(explosive) and tri-substituted product, 2,4,6-trinitrophenol (picric acid) is produced:
OH
OH
NO2
NO2
+ 3HNO3(aq)
+ 3H2O
NO2
2,4,6-trinitrophenol
CHEMISTRY OF PHENOL
Test for phenol: we have seen some simple reactions of
Phenol. We have two tests for phenol:
1. It will decolourise Br2(aq) as above.
2. It forms a violet complex with aqueous iron(III)
solution [e.g. FeCl3(aq)].
THE CHEMISTRY
OF ARENES
THE END