Benzene
Part 1
Starter: name the functional group.
1
2
4
3
5
Learning outcomes
•
•
•
•
•
Define arene and aromatic
Name aromatic compounds
Describe the structure of benzene
Review the evidence for this structure
Show how electrophilic substitutions
occurs with different electrophiles
Arene and aromatic
• Look at page 4
• Write your definition of Arene and
Aromatic
Benzene
• Isolated by Faraday in 1825
Physical properties:
• Clear, colourless, hydrocarbon
• 92.3% Carbon, 7.7% hydrogen.
• 0.250g was vaporised at 100oC had a
volume of 98cm3
• Boiling point 80oC, melting point 5oC
Calculate the empirical formula
• 92.3% Carbon, 7.7% hydrogen.
Element
%
Ar
Moles
Ratio
empirical formula CH
C
92.3
12.0
7.69
7.69
1
H
7.7
1.0
7.7
7.69
1
Calculate the molecular formula
1 mole of gas is 24dm3 at RTP
• 0.250g was vaporised at 100oC had a
volume of 98cm3.
• From this it was calculated that the
molecular mass was 78g
• So what is the molecular formula?
The relative molecular mass of the sample is
78
The molecular formula78/13 = 6
Therefore the molecular formula is C6H6
Molymods
Find as many structures as possible for C6H6
Draw them in your notes
Problem
• Doesn’t react like an alkene- no reaction
with HBr
• Doesn’t undergo electrophilic addition
• Enthalpy change should be about 800
kJmol-1, where as it is only 207kJmol-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.
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
Kekulé 1865
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.
Next: X-ray diffraction
Bond
type
Structure
Bond
length
C
C Cyclohexane 0.15
C
C Benzene
0.14
C
C cyclohexene
0.13
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
Exam question
• In this question, one mark is available for
the quality of spelling, punctuation and
grammar.
•
Describe with the aid of suitable
diagrams the bonding and structure of a
benzene molecule.
Discussion of the π-bonding
p-orbitals overlap (1)
above and below the ring (1)
(to form) π-bonds / orbitals (1)
any of the first three marks are available from a labelled
diagram
eg
bonds
(π-bonds / electrons) are delocalised (1)
4 marks
Other valid points – any two of:
•
ring is planar /
•
C-C bonds are equal length / have intermediate length/strength
between C=C and C-C /
•
σ-bonds are between C-C and/or C-H
•
bond angles are 120°
6
MAX 2 out of 4 marks (1)(1)
Quality of written communication
two or more sentences with correct spelling, punctuation and grammar
1
[7]
Homework
• Produce a leaflet or a poster showing
where benzene is used and hence why it
is important. Due Friday 10th September
Learning outcomes
•
•
•
•
•
Define arene and aromatic
Name aromatic compounds
Describe the structure of benzene
Review the evidence for this structure
Show how electrophilic substitutions
occurs with different electrophiles
Naming aromatic compounds
Phenol
Chlorobenzene
Methylbenzene
Many of these compounds are foul smelling and toxic, they are still called
aromatic
• When a long alkyl chain with other substitutions is
present, think of the benzene as substituted onto the
chain, using phenyl and a number to position the chain.
Cl
CH3– CH – CH – CH3
2-Chloro-3-phenylbutane
Naming dominos
Identify the following molecules as alkene,
arene or cycloaklane
1.
2.
3.
4.
5.
CH3CH2CH2CH2CH3
C6H5CH3
CH3CH=CHCH2CH3
CH3CH(CH3)CH2CH3
Identify the following molecules as alkene,
arene, alkane or cycloalkane
1.
2.
3.
4.
5.
CH3CH2CH2CH2CH3 alkane
C6H5CH3 arene
CH3CH=CHCH2CH3 alkene
CH3CH(CH3)CH2CH3 alkane
cycloalkane
Learning outcomes
•
•
•
•
•
Define arene and aromatic
Name aromatic compounds
Describe the structure of benzene
Review the evidence for this structure
Show how electrophilic substitutions
occurs with different electrophiles
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
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+
Now your turn
Write the mechanisims for the following
electrophiles:
1. CH3+
2. CH3CO+
Where does the electrophile end up?
Groups with +I
Groups with a - I
Mostly 2, 4 and 6 positions
Mostly with 3 and 5 positions
OH
Cl
CH3
COOH
NH2
NO2
+ I effect
- I effect
Learning outcomes
•
•
•
•
•
Define arene and aromatic
Name aromatic compounds
Describe the structure of benzene
Review the evidence for this structure
Show how electrophilic substitutions
occurs with different electrophiles
Diploma students only
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+
FRIEDEL-CRAFTS REACTIONS - INDUSTRIAL ALKYLATION
Industrial
Alkenes are used instead of haloalkanes but an acid must be present
Phenylethane, C6H5C2H5 is made by this method
Reagents
ethene, anhydrous AlCl3 , conc. HCl
Electrophile
C2H5+
Equation
C6H6 + C2H4
Mechanism
the HCl reacts with the alkene to generate a carbonium ion
electrophilic substitution then takes place as the C2H5+ attacks the ring
Use
ethyl benzene is dehydrogenated to produce phenylethene (styrene);
this is used to make poly(phenylethene) - also known as polystyrene
(an ethyl carbonium ion)
———>
C6H5C2H5
(ethyl benzene)
FRIEDEL-CRAFTS REACTIONS OF BENZENE - ACYLATION
Overview
Acylation involves substituting an acyl (methanoyl, ethanoyl) group
Reagents
an acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3
Conditions
reflux 50°C; dry inert solvent (ether)
Electrophile
RC+= O
Equation
C6H6 + CH3COCl
( e.g. CH3C+O )
———>
C6H5COCH3 + HCl
Mechanism
Product
A carbonyl compound (aldehyde or ketone)
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