MODULE 6
CH 25 AROMATIC COMPOUNDS
25.1 – INTRODUCING BENZENE
BENZENE (C6H6) – the simplest aromatic hydrocarbon
BENZENE is….
~ A colourless, sweet smelling, highly flammable liquid.
~ Found naturally in crude oil, is a component of petrol, and also found in cigarette
smoke.
~ Classified as a carcinogen - it can cause cancer
~ A benzene molecule consists of a hexagonal ring of 6 carbon atoms, with each carbon
atom joined to two other carbon atoms and to one hydrogen atom.
~ Benzene is classed as an aromatic hydrocarbon or arene.
STRUCTIRE OF BENZENE
THE KEKULÉ AND DELOCALISED
MODELS OF BENZENE
~ Benzene contains many double
bonds or triple bonds.
~ Compounds containing multiple bonds were known to be very reactive, however
benzene appeared unreactive.
THE KEKULÉ MODEL
~ It was suggested that the structure of benzene was based on a 6 membered ring of
carbon atoms joined together by alternate single and double bonds.
SKELETAL AND DISPLAYED STRUCTURE OF KEKULÉ’S BENZENE
EVIDENCE TO DISPROVE KEKULÉ’S MODEL
1. THE LACK OF REACTIVITY OF BENZENE
Benzene doesn’t undergo electrophilic addition reactions /decolourise bromine under
normal conditions.
So this suggests benzene cannot have any C=C bonds.
2. THE LENGTHS OF THE CARBON-CARBON BONDS IN BENZENE
Saw all bonds were the same length using X-ray diffraction, in between length of
single/double bond.
3. HYDROGENATION ENTHALPIES
Hydrogenation enthalpies (less exothermic than predicted compared to cyclohexene)
The actual enthalpy change of hydrogenation is -208 KJmol -1 so the actual structure of
benzene is more stable than the theoretical kekulé model of benzene.
THE DELOCALISED MODEL OF BEZENE
Developed to disprove kekulé structure of benzene.
MAIN FEATURES OF DELOCALISED MODEL:
,~ Benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6
hydrogen atoms.
~ Each carbon uses 3 of it’s available 4 electrons in bonding to 2 other carbons and 1
hydrogen.
~ Each carbon atom has 1 electron in a p-orbitals right angles to the plane of the
bonded carbon and hydrogen atoms
~ Adjacent p-orbital electrons overlap sideways above and below the plane of the
carbon atoms to form a ring of electron density.
~ This overlapping creates a system of π -bonds.
~ The 6 electrons occupying this system of π - bonds are said to be delocalised.
NAMING AROMATIC COMPOUNDS
MONOSUBSTITUTED - compounds with one substituent group
Alkyl groups, halogens and nitro groups are all considered the prefixes to bezene.
e.g. ethylbenzene, chlorobenzene, nitrobenzene
EXEPTIONS: benzoic acid, phenylamine,
benzaldehyde
25.2 – ELECTROPHILIC SUBSTITUTION REACTIONS OF BENZENE
REACTIVITY OF BENZENE
~ Benzene undergoes substitution reactions in which a hydrogen atom on the benzene
ring is replaced by another atom or group of atoms.
~ Benzene reacts with electrophiles and most of the
reactions proceed by electrophilic substitution.
NITRATION OF BENZENE
~ Benzene reacts slowly with nitric acid to form
nitrobenzene
~ It’s catalysed by sulfuric acid and heated to 50° C to obtain a good rate of reaction.
~ Water bath is used to maintain the steady temp.
~ One of the H atoms is replaced by a nitro – NO2 group.
If temp
rises
above 50°
C, further substitution reactions may occur leading to production of dinitrobenzene.
~ Nitrobenzene is an important starting material in the preparation of dyes,
pharmaceuticals, and pesticides.
REACTION MECHANISM FOR THE NITRATION OF BENZENE
Electrophilic substitution
1. The electrophile is the nitronium ion, NO2+ produced by the reaction of conc nitric acid
with conc sulfuric acid.
2. The electrophile accepts a pair of electrons from the benzene ring to form a dative
covalent bond.
The organic intermediate formed is unstable and breaks down to form the organic
product nitrobenzene and the H+ io.
, A stable benzene ring is reformed.
3. The H+ ion formed react with the HSO4- ion from step 1 to regenerate the catalyst
H2SO4
HALOGENATION OF BENZENE
The halogens don’t react with benzene unless a catalyst called a halogen carrier is
present
Common halogen carriers: AlCl3, FeCl3, AlBr3, FeBr3
BROMINATION OF BENZENE
Conditions: room temp and pressure, a halogen carrier
present
Electrophilic substitution reaction
One of the H atoms is replaced by a bromine
1. Benzene is too stable to react with a non-polar
bromine molecule. The electrophile is the
bromonium ion Br+ which is generated when the halogen carrier catalyst reacts with
bromine.
2. The bromonium ion accepts a pair of electrons from the benzene ring to form a dative
covalent bond. The organic intermediate is unstable and breaks down to form the
organic product, bromobenzene and a H+ ion.
3. The H+ formed reacts with FeBr-4 ion from step 1 to regenerate the FeBr3 catalyst
CHLORINATION
Reacts in the same way as bromine but the
halogen carrier used is FeCl3, AlCl3 or iron
metal and chloride which react to make FeCl3.
ALKYLATION REACTIONS – it’s the
substitution of a H atom by an alkyl group.
~ React benzene with a haloalkane in the presence of AlCl3, which acts as a halogen
carrier catalyst, generating the electrophile.
~ Alkylation increases the number of carbon atoms in a
compound by forming C-C bonds.
~ Reaction can be called Friedel-Craft alkylation
ACYLATION REACTIONS
~ Benzene reacts with an acyl chloride in the presence
of an AlCl3 catalyst to form an aromatic ketone.
~ Electrophilic substitution
~ Useful in organic synthesis
~ First member of acyl chlorides: ethanoyl chloride,
CH3COCl
COMPARING THE REACTIVITY OF ALKENES WITH ARENES
Alkenes decolourise bromine by an electrophilic addition
reaction.
CH 25 AROMATIC COMPOUNDS
25.1 – INTRODUCING BENZENE
BENZENE (C6H6) – the simplest aromatic hydrocarbon
BENZENE is….
~ A colourless, sweet smelling, highly flammable liquid.
~ Found naturally in crude oil, is a component of petrol, and also found in cigarette
smoke.
~ Classified as a carcinogen - it can cause cancer
~ A benzene molecule consists of a hexagonal ring of 6 carbon atoms, with each carbon
atom joined to two other carbon atoms and to one hydrogen atom.
~ Benzene is classed as an aromatic hydrocarbon or arene.
STRUCTIRE OF BENZENE
THE KEKULÉ AND DELOCALISED
MODELS OF BENZENE
~ Benzene contains many double
bonds or triple bonds.
~ Compounds containing multiple bonds were known to be very reactive, however
benzene appeared unreactive.
THE KEKULÉ MODEL
~ It was suggested that the structure of benzene was based on a 6 membered ring of
carbon atoms joined together by alternate single and double bonds.
SKELETAL AND DISPLAYED STRUCTURE OF KEKULÉ’S BENZENE
EVIDENCE TO DISPROVE KEKULÉ’S MODEL
1. THE LACK OF REACTIVITY OF BENZENE
Benzene doesn’t undergo electrophilic addition reactions /decolourise bromine under
normal conditions.
So this suggests benzene cannot have any C=C bonds.
2. THE LENGTHS OF THE CARBON-CARBON BONDS IN BENZENE
Saw all bonds were the same length using X-ray diffraction, in between length of
single/double bond.
3. HYDROGENATION ENTHALPIES
Hydrogenation enthalpies (less exothermic than predicted compared to cyclohexene)
The actual enthalpy change of hydrogenation is -208 KJmol -1 so the actual structure of
benzene is more stable than the theoretical kekulé model of benzene.
THE DELOCALISED MODEL OF BEZENE
Developed to disprove kekulé structure of benzene.
MAIN FEATURES OF DELOCALISED MODEL:
,~ Benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6
hydrogen atoms.
~ Each carbon uses 3 of it’s available 4 electrons in bonding to 2 other carbons and 1
hydrogen.
~ Each carbon atom has 1 electron in a p-orbitals right angles to the plane of the
bonded carbon and hydrogen atoms
~ Adjacent p-orbital electrons overlap sideways above and below the plane of the
carbon atoms to form a ring of electron density.
~ This overlapping creates a system of π -bonds.
~ The 6 electrons occupying this system of π - bonds are said to be delocalised.
NAMING AROMATIC COMPOUNDS
MONOSUBSTITUTED - compounds with one substituent group
Alkyl groups, halogens and nitro groups are all considered the prefixes to bezene.
e.g. ethylbenzene, chlorobenzene, nitrobenzene
EXEPTIONS: benzoic acid, phenylamine,
benzaldehyde
25.2 – ELECTROPHILIC SUBSTITUTION REACTIONS OF BENZENE
REACTIVITY OF BENZENE
~ Benzene undergoes substitution reactions in which a hydrogen atom on the benzene
ring is replaced by another atom or group of atoms.
~ Benzene reacts with electrophiles and most of the
reactions proceed by electrophilic substitution.
NITRATION OF BENZENE
~ Benzene reacts slowly with nitric acid to form
nitrobenzene
~ It’s catalysed by sulfuric acid and heated to 50° C to obtain a good rate of reaction.
~ Water bath is used to maintain the steady temp.
~ One of the H atoms is replaced by a nitro – NO2 group.
If temp
rises
above 50°
C, further substitution reactions may occur leading to production of dinitrobenzene.
~ Nitrobenzene is an important starting material in the preparation of dyes,
pharmaceuticals, and pesticides.
REACTION MECHANISM FOR THE NITRATION OF BENZENE
Electrophilic substitution
1. The electrophile is the nitronium ion, NO2+ produced by the reaction of conc nitric acid
with conc sulfuric acid.
2. The electrophile accepts a pair of electrons from the benzene ring to form a dative
covalent bond.
The organic intermediate formed is unstable and breaks down to form the organic
product nitrobenzene and the H+ io.
, A stable benzene ring is reformed.
3. The H+ ion formed react with the HSO4- ion from step 1 to regenerate the catalyst
H2SO4
HALOGENATION OF BENZENE
The halogens don’t react with benzene unless a catalyst called a halogen carrier is
present
Common halogen carriers: AlCl3, FeCl3, AlBr3, FeBr3
BROMINATION OF BENZENE
Conditions: room temp and pressure, a halogen carrier
present
Electrophilic substitution reaction
One of the H atoms is replaced by a bromine
1. Benzene is too stable to react with a non-polar
bromine molecule. The electrophile is the
bromonium ion Br+ which is generated when the halogen carrier catalyst reacts with
bromine.
2. The bromonium ion accepts a pair of electrons from the benzene ring to form a dative
covalent bond. The organic intermediate is unstable and breaks down to form the
organic product, bromobenzene and a H+ ion.
3. The H+ formed reacts with FeBr-4 ion from step 1 to regenerate the FeBr3 catalyst
CHLORINATION
Reacts in the same way as bromine but the
halogen carrier used is FeCl3, AlCl3 or iron
metal and chloride which react to make FeCl3.
ALKYLATION REACTIONS – it’s the
substitution of a H atom by an alkyl group.
~ React benzene with a haloalkane in the presence of AlCl3, which acts as a halogen
carrier catalyst, generating the electrophile.
~ Alkylation increases the number of carbon atoms in a
compound by forming C-C bonds.
~ Reaction can be called Friedel-Craft alkylation
ACYLATION REACTIONS
~ Benzene reacts with an acyl chloride in the presence
of an AlCl3 catalyst to form an aromatic ketone.
~ Electrophilic substitution
~ Useful in organic synthesis
~ First member of acyl chlorides: ethanoyl chloride,
CH3COCl
COMPARING THE REACTIVITY OF ALKENES WITH ARENES
Alkenes decolourise bromine by an electrophilic addition
reaction.
