Benzene & Aromatic Compound

Benzene & Aromatic Compound

1. 1

Benzene & Benzene & Benzene & Benzene &
Aromatic CompoundAromatic CompoundAromatic CompoundAromatic Compound
Jully Tan
School of Engineering
EP101 / EG101

Learning outcomeLearning outcomeLearning outcomeLearning outcome
Structure of benzene
Characteristic of benzene
Naming of benzene
Reaction of benzene
Electrophileaddition
Effect of substituentson reactivity
Relative reactivity of substituentsbenzene
Effect of substituent on orientation
Synthesis of aromatic compound

2. 2
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Structure of BenzeneStructure of BenzeneStructure of BenzeneStructure of Benzene
Definition of ‘Aromatic Compounds’
‘
Cyclic compounds that having some number of conjugated double bonds
and unusually large resonance energy’The opposite of aromatic is aliphatic.
Molecular formula: C 6H6
It is with 6-membered cyclic structure with 3 conjugated double bond.
Benzene is a ring with all of sp
2
hybrid C atoms with unhybridizedorbital P
orbital are aligned perpendicular to the ring.
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The conjugated double bond are constantly moving due to resonances. Hence the
structure is usually drawn as:
The difference is that in A, the double bonds are between C1-C2, C3-C4, and C5-
C6, whereas in B the double bond positions are C2-C3, C4-C5, and C6-C1.
In the real molecule benzene, none of the bonds are double bondsand none are
single, all six bonds are equivalent and are intermediate between double and single.
resonance structures: the possible of moving of pi bonding in benzene.

3. 3
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Physical PropertiesPhysical PropertiesPhysical PropertiesPhysical Properties
Melting points: More symmetrical than corresponding alkane, pack better into
crystals, so higher melting points.
Boiling points: Dependent on dipole moment, so ortho> meta> para, for
disubstitutedbenzenes.
Density: More dense than non-aromatics, less dense than water.
Solubility: Generally insoluble in water.
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Stability of BenzeneStability of BenzeneStability of BenzeneStability of Benzene
Benzene is much less reactive than typical alkene and fails to undergo the usual
alkene reactions. Thus it is a very stable structure.
Cyclohexenereacts rapidly with Br
2
and gives the additionproduct 1,2-
dibromocyclohexane
Benzene reacts only slowly with Br
2
and gives the substitutionproduct C
6
H
5
Br

4. 4
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Characteristic of Aromatic CompoundsCharacteristic of Aromatic CompoundsCharacteristic of Aromatic CompoundsCharacteristic of Aromatic Compounds
It must be cyclic
It must be having the same number of conjugated

bonding
It must be flat so that the porbital overlap can occur. Planar structure
The delocalisationof

electron must lower the electronic energy.
It must fulfill Huckel’srule, having (4n + 2 pi) electrons; whereby n=0 or any
positive number (n=0,1,2,3,…). Hence, with this rule, rings which has pi electrons
=2,6,10,14 are aromatic ring.
4n+2=(4x1)+2
=6
4n+2=(4x1)+2=6
hence, it is not aromatic
as the ring has 8 pi e.
Non-aromatic
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Naming Aromatic Compounds 1. Naming Aromatic Compounds 1. Naming Aromatic Compounds 1. Naming Aromatic Compounds 1. MonosubstitutedMonosubstitutedMonosubstitutedMonosubstitutedBenzenesBenzenesBenzenesBenzenes
Aromatic substances have acquired nonsystematic names
Nonsystematic names are discouraged but allowed by IUPAC.
It is derivative of benzene.
Substituent name is written as though they were attached.
In some case, benzene ring become substituent. Alkyl substituentlarger than the ring (7 or more
carbons), named as a phenyl-substituted alkaneor when the chain is priority.
Aminobenzene
Aniline
NitrobenzeneHydroxybenzeneMethylbenzeneIUPAC
NitrobenzenePhenolTolueneCommon

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DisubstitutedDisubstitutedDisubstitutedDisubstitutedbenzenesbenzenesbenzenesbenzenes
Named using one of the prefixes
1.ortho-(o-)
Ortho-disubstituted benzene has two
substituents in a 1,2 relationship
2.meta-(m-)
Meta-disubstituted benzene has its substituents
in a 1,3 relationship
3.para-(p-)
Para-disubstituted benzene has its substituents
in a 1,4 relationship
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Benzenes with more than two Benzenes with more than two Benzenes with more than two Benzenes with more than two substituentssubstituentssubstituentssubstituents
Named by numbering the position of each so that the lowest possible numbers are used
The substituents are listed alphabetically when writing the name
Any of the monosubstituted aromatic compounds can serve as a parent name, with the
principal substituent (-OH in phenol or –CH
3
in toluene) attached to C1 on the ring.Benzene compounds that contain three or more substituentsare always named by the
number system. In this system, numbers are assigned to substituentsso that the
substituentshave the lowest possible combination of numbers.

6. 6
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Reactions of Aromatic Compounds: Reactions of Aromatic Compounds: Reactions of Aromatic Compounds: Reactions of Aromatic Compounds:
ElectrophilicElectrophilicElectrophilicElectrophilicSubstitutionSubstitutionSubstitutionSubstitution
Electrophilic aromatic substitution
A process in which an electrophile (E
+
) reacts with an aromatic ring and substitutes for one of
the hydrogens.
Aromatic undergo this rxndue to being very stable and has large number of delocalized pie.
This reaction is characteristic of all aromatic rings.
The mechanism involves 2 steps:
1
ST
STEP: attack on E+ by pi e of aromatic ring to form carbocation(benzoniumion)
Benzoniumion has 3 resonance structure which delocalize the +vecharge around the
aromatic ring. (a nucleophilecant add in to form new product as it will lost the
aromaticityof the ring.)
2
ND
STEP: carbocationreact with nucleophileto form an additional product.
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Reactions of Aromatic Compounds: ElectrophilicSubstitution
Many substituents can be introduced onto an aromatic ring through electrophilic
substitution reactions
Halogen (-Cl, -Br, -I)
Nitro group (-NO
2
)
Sulfonicacid
group (-SO
3
H)Hydroxyl group
(-OH)
Alkyl group (-R)
Acylgroup (-COR)

7. 7
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ElectrophilicElectrophilicElectrophilicElectrophilicAromatic Substitution ReactionAromatic Substitution ReactionAromatic Substitution ReactionAromatic Substitution Reaction
i) i) i) i) HalogenationHalogenationHalogenationHalogenation
Electrophilic aromatic substitution reaction begins in a similarway to electrophilic alkene
addition reaction
FeBr
3
catalyst is needed for bromination of benzene to occur
FeBr
3
polarizes Br
2
molecule making it more electrophilic
Polarization makes FeBr
4
-
Br
+
species that reacts as if it were Br
+
The polarized Br
2
molecule reacts with the nucleophilic benzene ring to yield a
nonaromaticcarbocation intermediate which is doubly allylic and has three
resonance forms
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14
•Chlorination proceeds by a similar mechanism.

8. 8
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ElectrophilicElectrophilicElectrophilicElectrophilicSubstitutionSubstitutionSubstitutionSubstitution ii) Nitrationii) Nitrationii) Nitrationii) Nitration
Aromatic rings can be nitrated with a mixture of concentrated nitric and sulfuric acids
The electrophile is the nitroniumion, NO
2+
which is generated from HNO
3by
protonation and loss of water
The nitroniumion reacts with benzene to yield a carbocationintermediate, and loss of
H
+
The product is a neutral substitution product, nitrobenzene
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ElectrophilicElectrophilicElectrophilicElectrophilicSubstitutionSubstitutionSubstitutionSubstitutioniii) iii) iii) iii) SulfonationSulfonationSulfonationSulfonation
Aromatic rings can be sulfonatedin the laboratory by reaction with fuming sulfuric acid, a mixture of
H
2
SO
4
and SO
3The reactive electrophile is either HSO
3
+
or neutral SO
3
Substitution occurs by the same two-step mechanism seen for bromination and nitration
Aromatic sulfonation does not occur naturally
Aromatic sulfonation is widely used in the preparation of dyes and pharmaceutical agents

9. 9
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The electrophile is a carbocation, generated by AlCl
3
-assisted dissociation of an alkyl halide. Only
alkylhalides can be used.
iv) iv) iv) iv) FriedelFriedelFriedelFriedel----Crafts Alkylation of Aromatic RingsCrafts Alkylation of Aromatic RingsCrafts Alkylation of Aromatic RingsCrafts Alkylation of Aromatic Rings
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AlkylationAlkylationAlkylationAlkylation
2.Friedel-Crafts reactions do not succeed on aromatic rings that are substituted either by a
strongly electron-withdrawing group such as carbonyl (C=O) or by an amino group (-NH
2
,
NHR, -NR
2
)
The presence of a substituent group already on a ring can have adramatic effect on that
ring’s subsequent reactivity toward further electrophilic substitution

10. 10
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v) v) v) v) FriedelFriedelFriedelFriedel----Craft Craft Craft Craft AcylationAcylationAcylationAcylationof Aromatic Ringsof Aromatic Ringsof Aromatic Ringsof Aromatic Rings
An aromatic ring is acylatedby reaction with a carboxylic acid chloride, RCOCl, in the presence of AlCl
3
An acylgroup is substituted onto an aromatic ring
The reactive electrophile is a resonance-stabilized acylcation
An acylcation is stabilized by interaction of the vacant orbital on carbon with lone-pair
electrons on the neighboring oxygen
Because of stabilization, no carbocation rearrangement occurs during acylation
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Different substituentsexerts different effect toward the aromatic ring.
2 groups of substituents
E donating
E withdrawing
The Effects of Substituent on ReactivityThe Effects of Substituent on ReactivityThe Effects of Substituent on ReactivityThe Effects of Substituent on Reactivity
> >
X
Reactivity decrease
Y= E donating species
X = E withdrawing species

11. 11
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Electron Donating SpeciesElectron Donating SpeciesElectron Donating SpeciesElectron Donating Species
•An electron-donating resonance effect is observed whenever an atom Z having a lone
pair of electrons is directly bonded to a benzene ring.
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12. 12
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Electron Withdrawing SpeciesElectron Withdrawing SpeciesElectron Withdrawing SpeciesElectron Withdrawing Species
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Example of Electron WithdrawingExample of Electron WithdrawingExample of Electron WithdrawingExample of Electron Withdrawing

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Relative Reactivity of Substituent BenzeneRelative Reactivity of Substituent BenzeneRelative Reactivity of Substituent BenzeneRelative Reactivity of Substituent Benzene
2 type of subs. Which affects reactivity of benzene ring:
(a)Activating subs. –activate ring (donate e)
(b)Deactivating subs –deactivate ring (withdraw e)
Activating subs
Except alkyl group, all activating subs. donate e into the ring by resonance and
withdraw e from the ring inductively.
Example

14. 14
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All substituentsunder e donating group
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•Keep in mind that halogens are in a class by themselves.
•Also note that:

15. 15
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Deactivating subs
Halogen are weakly deactivating subs. (they withdraw e inductively stronger than
they can donate e resonance)
All subs. that are strongly deactivating compare to halogen ableto withdraw e both
inductively and resonance.
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16. 16
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SubstituentsSubstituentsSubstituentsSubstituentsaffect the affect the affect the affect the orientationorientationorientationorientationof the reactionof the reactionof the reactionof the reaction
The three possible
disubstituted products –
ortho, meta, and para–are
usually not formed in equal
amounts
The nature of the substituent
on the ring determines the
position of the second
substitution
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3 Group of Subs. 3 Group of Subs. 3 Group of Subs. 3 Group of Subs.
a.Activating subs.
b.Weakly deactivating subs.
c.Moderate deactivating / strongly deactivating subs.
To understand how substituentsactivate or deactivate the ring, we must consider the first step
in electrophilicaromatic substitution.
The first step involves addition of the electrophile(E+) to form a resonance stabilized
carbocation.
The Hammond postulate makes it possible to predict the relative rate of the reaction by
looking at the stability of the carbocationintermediate.

17. 17
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To evaluate the effects of a given substituent, we can use the To evaluate the effects of a given substituent, we can use the To evaluate the effects of a given substituent, we can use the To evaluate the effects of a given substituent, we can use the
following stepwise procedure:following stepwise procedure:following stepwise procedure:following stepwise procedure:
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•A CH
3
group directs electrophilicattack orthoand parato itself because an
electron-donating inductive effect stabilizes the carbocationintermediate.

18. 18
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•An NH
2 group directs electrophilicattack orthoand parato itself because the
carbocationintermediate has additional resonance stabilization.
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•With the NO
2
group (and all meta directors) meta attack occurs because attack at
the orthoand paraposition gives a destabilized carbocationintermediate.

19. 19
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Orienting Effects: Ortho and Para Directors
The orthoand paraintermediates are more stable than the meta
intermediate because they have more resonance forms
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Orienting Effects: Meta Directors

20. 20
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39
Synthesis of Benzene DerivativesSynthesis of Benzene DerivativesSynthesis of Benzene DerivativesSynthesis of Benzene Derivatives
In a disubstitutedbenzene, the directing effects indicate which substituent must be added to
the ring first.
Let us consider the consequences of brominationfirst followed
by nitration, and nitration first, followed by bromination.
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Pathway I, in which brominationprecedes nitration, yields the desired product. Pathway II
yields the undesired meta isomer.

21. 21
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Note that alkyl benzenes undergo two different reactions depending on the reaction
conditions:
•With Br
2
and FeBr
3
(ionic conditions), electrophilicaromatic substitution occurs,
resulting in replacement of H by Br on the aromatic ring to formorthoand paraisomers.
•With Br
2
and light or heat (radical conditions), substitution of H by Broccurs at the
benzyliccarbon of the alkyl group.