Benzene

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Benzene

• Part 1

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Starter name the functional group.
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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

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Arene and aromatic

• Look at page 4
• Write your definition of Arene and Aromatic

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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

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• Calculate the empirical formula
• 92.3 Carbon, 7.7 hydrogen.

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• Element C H
• 92.3 7.7
• Ar 12.0 1.0
• Moles 7.69 7.7
• Ratio 7.69 7.69
• 1 1 empirical formula CH

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• 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?

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• The relative molecular mass of the sample is 78
• The molecular formula78/13 6
• Therefore the molecular formula is C6H6

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Molymods

• Find as many structures as possible for C6H6
• Draw them in your notes

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Problem

• Doesnt react like an alkene- no reaction with
  HBr
• Doesnt undergo electrophilic addition
• Enthalpy change should be about 800 kJmol-1,
  where as it is only 207kJmol-1

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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.
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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 CC bond) is reduced to
cyclohexane, 120kJ of energy is released per
mole. C6H10(l) H2(g) gt C6H12(l)
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- 120 kJ mol-1
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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 CC bond) is reduced to
cyclohexane, 120kJ of energy is released per
mole. C6H10(l) H2(g) gt
C6H12(l) Theoretically, if benzene contained
three separate CC bonds it would release 360kJ
per mole when reduced to cyclohexane C6H6(l)
3H2(g) gt C6H12(l)
Theoretical - 360 kJ mol-1 (3 x -120)
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- 120 kJ mol-1
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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 CC bond) is reduced to
cyclohexane, 120kJ of energy is released per
mole. C6H10(l) H2(g) gt
C6H12(l) Theoretically, if benzene contained
three separate CC bonds it would release 360kJ
per mole when reduced to cyclohexane C6H6(l)
3H2(g) gt C6H12(l) Actual benzene releases
only 208kJ per mole when reduced, putting it
lower down the energy scale
Theoretical - 360 kJ mol-1 (3 x -120)
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Experimental - 208 kJ mol-1
- 120 kJ mol-1
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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 CC bond) is reduced to
cyclohexane, 120kJ of energy is released per
mole. C6H10(l) H2(g) gt
C6H12(l) Theoretically, if benzene contained
three separate CC bonds it would release 360kJ
per mole when reduced to cyclohexane C6H6(l)
3H2(g) gt C6H12(l) 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.
MORE STABLE THAN EXPECTED by 152 kJ mol-1
Theoretical - 360 kJ mol-1 (3 x -120)
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Experimental - 208 kJ mol-1
- 120 kJ mol-1
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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 CC bond) is reduced to
cyclohexane, 120kJ of energy is released per
mole. C6H10(l) H2(g) gt
C6H12(l) Theoretically, if benzene contained
three separate CC bonds it would release 360kJ
per mole when reduced to cyclohexane C6H6(l)
3H2(g) gt C6H12(l) 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.
MORE STABLE THAN EXPECTED by 152 kJ mol-1
Theoretical - 360 kJ mol-1 (3 x -120)
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Experimental - 208 kJ mol-1
- 120 kJ mol-1
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Kekulé 1865
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STRUCTURE OF BENZENE
HOWEVER... it did not readily undergo
electrophilic addition - no true CC bond
only one 1,2 disubstituted product existed
all six CC bond lengths were similar CC 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.
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Next X-ray diffraction
Bond type Structure Bond length
C C Cyclohexane 0.15
C C Benzene 0.14
C C cyclohexene 0.13
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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
delocalised pi orbital system
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.
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STRUCTURE OF BENZENE
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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.

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(No Transcript)
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(No Transcript)
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Homework

• Produce a leaflet or a poster showing where
  benzene is used and hence why it is important.
  Due Friday 10th September

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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

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Naming aromatic compounds
Phenol
Chlorobenzene
Methylbenzene
Many of these compounds are foul smelling and
toxic, they are still called aromatic
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• 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.

2-Chloro-3-phenylbutane
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Naming dominos
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Identify the following molecules as alkene, arene
or cycloaklane

• CH3CH2CH2CH2CH3
• C6H5CH3
• CH3CHCHCH2CH3
• CH3CH(CH3)CH2CH3

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Identify the following molecules as alkene,
arene, alkane or cycloalkane

• CH3CH2CH2CH2CH3 alkane
• C6H5CH3 arene
• CH3CHCHCH2CH3 alkene
• CH3CH(CH3)CH2CH3 alkane
• cycloalkane

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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

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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
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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
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ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric
acid (catalyst) Conditions reflux at
55C Equation C6H6 HNO3 gt
C6H5NO2 H2O
nitrobenzene
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ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric
acid (catalyst) Conditions reflux at
55C Equation C6H6 HNO3 gt
C6H5NO2 H2O
nitrobenzene Mechanism
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ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric
acid (catalyst) Conditions reflux at
55C Equation C6H6 HNO3 gt
C6H5NO2 H2O
nitrobenzene Mechanism Electrophile
NO2 , nitronium ion or nitryl cation it is
generated in an acid-base reaction...
2H2SO4 HNO3 2HSO4
H3O NO2 acid base
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ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric
acid (catalyst) Conditions reflux at
55C Equation C6H6 HNO3 gt
C6H5NO2 H2O
nitrobenzene Mechanism Electrophile
NO2 , nitronium ion or nitryl cation it is
generated in an acid-base reaction...
2H2SO4 HNO3 2HSO4
H3O NO2 acid base Use
The nitration of benzene is the first step in
an historically important chain of
reactions. These lead to the formation of dyes,
and explosives.
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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 gt
C6H5Cl HCl Mechanism Electrophile
Cl it is generated as follows...
Cl2 FeCl3 FeCl4
Cl a
Lewis Acid
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Now your turn

• Write the mechanisims for the following
  electrophiles
• CH3
• CH3CO

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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
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I effect
- I effect
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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

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Diploma students only
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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
gt 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 CCl
bond.
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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
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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 (an ethyl
carbonium ion) Equation C6H6 C2H4
gt C6H5C2H5 (ethyl benzene) 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
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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 50C dry inert
solvent (ether) Electrophile RC O ( e.g.
CH3CO ) Equation C6H6 CH3COCl gt
C6H5COCH3 HCl Mechanism Product
A carbonyl compound (aldehyde or ketone)
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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 where the next substituent
goes Group ELECTRON DONATING
ELECTRON WITHDRAWING Example(s) OH, CH3
NO2 Electron density of ring
Increases Decreases Ease of substitution
Easier Harder Position of
substitution 2,4,and 6 3 and 5