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The origin of August Kekulé’s view of benzene structure:
“There I sat and wrote mytextbook, but things did notgo well; my mind was occupiedwith other matters, I turned thechair towards the fireplace andbegan to doze.
“Once again the atoms dancedbefore my eyes. This time smaller groups modestly remained in the background.My mental eye, sharpened byrepeated apparitions of similarkind, now distinguished largerunits of various shapes.”
“Long rows, frequently joined more densely; everything in motion, twisting and turninglike snakes. And behold, whatwas that? One of the snakescaught hold of its own tail andmockingly whirled around myeyes.
“1 awoke, as if by lightening; this time, too, I spent the rest of the night working out theconsequences of this hypothesis”
“Let us learn to dream, gentlemen, then perhaps we shall find the truth” AUGUST KEKULÉ
Edison, Einstein and many others have used the subconscious mind to give them the insight and the “know-how” to bring about their great achievements.
August Kekulé
“In variably, my device works as I imagined it should. In twenty years there has not been single exception.” NIKOLA TESLA
“Imagination is more important than knowledge.” ALBERT EINSTEIN
AROMATIC COMPOUNDS
Introduction to aromatic Compounds14.1
Arenes = aromatic compounds:the word aromatic has nothing to do with odour.
Kekulé structure
resonance structure
aromatic
ortho meta para
Friedel-Craft acylationelectrophilic substitution
activating and deactivating group
ortho-para and meta director
oxidation ofalkylbenzene
substitution oftoluene
KEYWORDS
1825 Michael Faraday (British)
Isolated a pure compound of boiling point 80oC
Empirical formula = CH
Named “bicarburet of hydrogen”
DISCOVERY OF BENZENE
1834 Eilhard Mitscherlich (German)
Prepared benzene from benzoic acid
C6H5CO2H + CaO C6H6 + CaCO3
heat
Molar mass = 78, molecular formula = C6H6
named “benzin”
benzene benzin benzoin
benjoin(French)
gum benzoin Luban Jawi(Arabic)
1866 Friedrich August Kekulé (German)
Proposed a cyclic structure for benzene.
AROMATIC COMPOUNDS
In earlier time, compounds are called aromatic because of their pleasant odours.
Today, we use the word aromatic to refer to benzene and its structural relatives.
Benzene has strong pleasant odour.
AROMATIC COMPOUNDS
Aromatic compound is a cyclic conjugated molecule or ion that is stabilized by ∏ electron delocalisation.
it is characterised by substitution reactions.
C
CC
CC
C
H
H
H
H
H
H
or
KEKULÉ’S STRUCTURE
Kekule was the first to formulate a reasonable representation of benzene
The Kekule structure suggests alternating double and single carbon-carbon bonds
Benzene is actually a resonance hybrid of the two Kekulé structures.
All C–C bond length equal = 139 pm
Shorter than typical C–C (148 pm)
Longer than typical C=C (134 pm)
resonance hybrid
equivalent to
RESONANCE STRUCTURE
The six electrons completely delocalized around the ring
All six C atoms and six p orbitals are equivalent
The circle represents the six electrons, distributed over the six atoms of the ring!
THE CRITERIA FOR AROMATICITY
4 structural criteria must be satisfied for compound to be aromatic
Completely
conjugated
CyclicPlanar
Contain particular number of ∏ electron obeys HÜckel’s Rule
THE CRITERIA FOR AROMATICITY
HÜckel’s Rule
* cyclic, planar and completely conjugated compounds that contain [4n+2] ∏ electron (n=0,1,2…..) are said to be aromatic
planar monocyclic rings with 2,6,10,14 and so forth ∏ electrons are aromatic
Erich HÜckel (1896-1980)
THE CRITERIA FOR AROMATICITY
EXAMPLE OF AROMATIC COMPOUNDS
Benzene
1. Aromatic compounds with a single ring
[4n+2] ∏ = [4(1) + 2]∏ = 6 ∏ electrons
aromatic
Benzene is aromatic because:
contains 6∏ electrons (obeys HÜckels Rule)
cyclic, planar and has double bond in the ring
THE CRITERIA FOR AROMATICITY
2. Aromatic compounds with more than one ring
naphthalene
4n+2= 4(2)+ 2 10 ∏ electrons
Aromatic
* Two benzene rings joined together forms naphthalene
EXAMPLE
AROMATIC COMPOUNDS
Nomenclature of benzene and its derivatives14.2
Arenes = aromatic compounds:the word aromatic has nothing to do with odour.
NAMING BENZENE DERIVATIVES
Many organic molecules contain a benzene ring with one or more substituents.
Many common name are recognized by the IUPAC system
CH3
Common: tolueneIUPAC: methylbenzene
EXAMPLE:
Benzene is the parent name and the substituent is indicated by a prefix.
F
fluorobenzene
Cl
chlorobenzene
Br
bromobenzene
NO2
nitrobenzene
CH2CH3
ethylbenzene
MONO SUBSTITUTED BENZENE
IUPAC rules allow some common names to be retained.
CH3
toluene
OH
phenol
NH2
aniline
COOH
benzoic acid
CHO
benzaldehyde
Relative position of subsituents are indicated by prefixes ortho, meta, and para ( o–, m–, and p–) or by the use of number.
Br
Br12
1,2–dibromobenzeneor
o–dibromobenzene
Br
Br
12
3
1,3–dibromobenzeneor
m–dibromobenzene
Br
Br
1,4–dibromobenzeneor
p–dibromobenzene
12
34
DISUBSTITUTED BENZENE
Two Same Substituents
NO2
NO21
2
1,2–dinitrobenzeneor
o–dinitrobenzene
12
3
1,3–dinitrobenzeneor
m–dinitrobenzene
NO2
NO2
NO2
NO2
1,4–dinitrobenzeneor
p–dinitrobenzene
12
34
COOH
NO21
2
2–nitrobenzoic acidor
o–nitrobenzoic acid
12
3
3–nitrobenzoic acidor
m–nitrobenzoic acid
COOH
NO2
4–nitrobenzoic acidor
p–nitrobenzoic acid
12
34
COOH
NO2
Select one of the substituent that give new parent name and numbered as C1.
DISUBSTITUTED BENZENE
Two Different Substituents
Position of substituents must be indicated by numbers.
Br
Br
Br
1 2
34
1,2,4–tribromobenzene
The substituents are listed alphabetically when writing the name.
Cl
Br
I2–bromo–1–chloro–3–iodobenzene
THREE OR MORE SUBSTITUENTS
C atom bearing the subtituent that define the new parent name is numbered as C1.
OH
NO2
NO2
2,4–dinitrophenol
HO OH
COOH1
2
34
12
3
4
5
6
3,5–dihydroxybenzoic acid
Br
CH3
CH3
1
23
4
4–bromo–1,2–dimethylbenzene
Br
CH3
CH3
1
23
4
4–bromo–1,2–dimethylbenzene
o–, m– and p– naming system is used for arenes with 2 substituents only!
4–bromo–o–dimethylbenzene
correct
If alkyl substituent is larger than the ring (more than 6 C), the compound is named as phenyl-substituted alkane.
CH–CH2–CH2–CH2–CH2–CH3
CH21
23 4 5 6 7
2–phenylheptane
14.2-11
PHENYL GROUP
Benzene ring as substituent.
Phenyl = C6H5– = Ph
If the chain is unsaturated (have C═C or C≡C) or contains important functional group, the benzene ring is considered as phenyl substituent.
CH2–C C–CH3
1 2 3 4
1–phenyl–2–butene
CH2–CH2–OH
2–phenylethanol
12
14.2-12
phenyl group
CH2—
benzyl group
CH2Br
benzyl bromide
CH2OH
benzyl alcohol
14.2-13
BENZYL GROUP
AROMATIC COMPOUNDS
Chemical properties of benzene and its derivatives14.3
Arenes = aromatic compounds:the word aromatic has nothing to do with odour.
BENZENE
Br2 / CCl4 no addition of Br2
(no decolorization)
KMnO4 / H2O no oxidation(no decolorization)
H2 / Nislow addition at hightemperature and pressure
UNUSUAL REACTIONS OF BENZENE14.3-01
Alkenes readily undergo addition reaction, benzene does not!
Involves substituents attached to the ring
Electrophilic aromatic substitution
+ Br2
FeBr3
Br
benzene bromobenzene
Involves the benzene ring itself
CH3CH2CH2CH3 KMnO4
H2O
COOH
butylbenzene benzoic acid
REACTION OF ARENES14.3-03
Most characteristic reaction of benzene.
ELECTROPHILIC SUBSTITUTION14.3-04
A H atom is replaced by an electrophile.
H+ E+
E+ H+
electrophile
XX2, FeX3
(X = Cl Br) + HX halogenation
HONO2
H2SO4
NO2
+ H2O nitration
RCl , AlCl3
(R can rearrange)
R+ HCl
Friedel-CraftsAlkylation
RCCl , AlCl3
O
C–RO
+ HClFriedel-CraftsAcylation
14.3-05
STEP 1 Formation of arenium ion
H
H
H
H
H
H E+
H
H
H
H
+
E
H
H
H
H
H
H
E
H
+H
H
H
H
H
H
E
H+
GENERAL MECHANISM 14.3-07
arenium ion
E+
Electrophile takes two electrons of six–electrons system to form bond.
benzene ring
This interrupts of cyclic system of electrons.
Benzene ring arenium ion (aromatic) (nonaromatic)
The four electrons delocalized through these the five C atom (p orbitals)
E
H
+
arenium ion
ARENIUM ION 14.3-08
STEP 2 Loss of H+
H
H
H
H
H
EH
H
H
H
H
E
H+
EH
+E
6 electrons 4 electrons
E+ – H+
6 electrons
Substitution reaction allowaromatic six electrons tobe regenerated.
14.3-09
Kekulé structures are more appropriate for writing mechanisms.
For simplicity, however, we can show the mechanism in the following way:
STEP 1
+ E+ +
E
H
arenium ion
STEP 2 E
+ H+
14.3-10
E
H+
Br
+ Br2no reaction(decolorization not observed)
+ Br2
FeBr3
bromobenzene
+ HBr
HALOGENATION 14.3-11
Reactants: benzene and halogens (Cl2 or Br2).
Conditions:
Lewis acid such as FeCl3 and FeBr3
STEP 1 Formation of Br+
Br–Br + FeBr3 Br—Br–FeBr3
+ –Br+ + FeBr4
–
complex
+ Br+
STEP 2 Formation of arenium ion
STEP 3 Loss of H+
Br
+ HBr
MECHANISM 14.2-13
FeBr4–
+ FeBr3
+
Br
H
+
Br
H
Increase polarity of halogen molecules.
Produce positive halogen ions (Br+ or Cl+).
FUNCTION OF LEWIS ACIDS 14.3-14
NO2HNO3
H2SO4nitrobenzene
+ H+ + HSO4–
NITRATION 14.3-15
Reactants: benzene and concd. HNO3.
Conditions:
Concd. H2SO4
STEP 1 Formation of nitronium ion (NO2+)
H2O + NO2+
MECHANISM 12.5-49
H
..
..
H-O-NO2 + H-OSO3H H-O+-NO2 + HSO4–
..
nitronium ion
+ NO2+
STEP 2 Formation of arenium ion
STEP 3 Loss of H+
NO2
+ H2SO4
HSO4–
+
NO2
H
+
NO2
H
R+ R–X
AlCl3
alkylbenzene
+ HCl
FRIEDEL–CRAFT ALKYLATION 12.5-51
Reactants: benzene and haloalkane.
Conditions:
Catalyst: Lewis acid such as AlCl3.
Alkylation = transfer an alkyl group to benzene
12.5-52
EXAMPLE:
+ CH3CHCH3
ClCH(CH3)2AlCl3
2–chloropropane+ HCl
isopropylbenzene
+ (CH3)3C–ClC(CH3)3AlCl3
2–chloro–2–methylpropane+ HCl
tert–butylbenzene
12.5-53
STEP 1 Formation of carbocation
(CH3)2CHCl + AlCl3 (CH3)2CH–Cl–AlCl3
carbocation
(CH3)2CH+ + AlCl4–
MECHANISM 12.5-54
:
:
:
:
:
+ -
+ +CH(CH3)2
STEP 2 Formation of arenium ion
STEP 3 Loss of H+
CH(CH3)2
+ HCl
AlCl4–
+
CH(CH3)2
H
+
CH(CH3)2
H
Rearrangement can occur, especially when 1o haloalkanes are used.
EXAMPLE:
CH3CH2CH2CH2Cl
CH2CH2CH2CH3
+
butylbenzene
CHCH2CH3
CH3
sec–butylbenzene
65%
35%
Rearrangement:
CH3—CH2—CH—CH2
H+
1o carbocationCH3—CH2—CH—CH3
+
2o carbocation
AlCl3
OTHER FACTS ABOUTFRIEDEL–CRAFT ALKYLATION
12.5-55
EXAMPLE:
+ CH3C—Cl
O
acetyl chloride
AlCl3CCH3
O
acetophenone
+ HCl
FRIEDEL–CRAFT ACYLATION 12.5-56
Reactants: benzene and acid chloride.
Conditions:
Catalyst: Lewis acid such as AlCl3.
Product: ketone.
EXAMPLE:
CH3C—
O
acyl group
benzoyl group
–C—
O
RC—
O
acetyl group
ACYL GROUP 12.5-57
Acylation = transfer an acyl group to benzene
STEP 1 Formation of acylium ion
acylium ion
R–C–Cl + AlCl3 R–C–Cl–AlCl3
:O: :O:
R–C+═O R–C≡O+ + AlCl4–
MECHANISM 12.5-59
: :
+ R–C+═O
STEP 2 Formation of arenium ion
STEP 3 Loss of H+
+ HCl
C═O
R
+
C=O
H
R
+
C=O
H
R AlCl4–
But, what would happen if we were to carry out a reactionon aromatic ring that already has a substituent?
SUBSTITUENT EFFECT 12.5-61
Activating groups:
Substituents that activate the ring, making it more reactive than benzene.
NO2 Cl H OH
relative rateof nitration
EXAMPLE:
6 x 10–8 0.033 1 1000 reactivity
EFFECT ON REACTIVITY 12.5-62
Deactivating groups:
Substituents that deactivate the ring, making it less reactive than benzene.
EXAMPLE:
CH3
HNO3
H2SO4
CH3
NO2
+
CH3
NO2
CH3
NO2(59%) (37%)
+
(4%)
ortho-para director
Ortho–para directors
EFFECT ON ORIENTATION 12.5-63
Meta directors
Tend to direct the incoming group into ortho and para positions.
Tend to direct the incoming group into meta position.
CLASSIFICATION OF SUBSTITUENTS
ortho– , para– directingactivators
ortho– , para– directingdeactivators
meta– directing deactivators
12.5-64
ORTHO– , PARA– DIRECTING ACTIVATORS
12.5-65
SMH, 1 ed, p.625*WAD, 4 ed, p.217CRY: 5 ed., p.651MCM: 6 ed., p.300SOL: 8 ed., p. 300
—NH2
●●
—R
—NHCOR●●
—OR●●
●●
—OH●●
●●
—NHR●●
—NR2
●●
Increasing activation
General structure:—R or —Z●
●
Alkyl groups or havenonbonded electron pairon the atom bonded to
benzene ring
EXAMPLE:
CH2CH3
Br2
FeBr3
CH2CH3
Br
+
CH2CH3
Br(38%)ethylbenzene (62%)
12.5-66
ORTHO– , PARA– DIRECTING DEACTIVATORS
12.5-67
SMH, 1 ed, p.625*WAD, 4 ed, p.217CRY: 5 ed., p.651MCM: 6 ed., p.300SOL: 8 ed., p. 300
—F●●
●●
●●—Cl
●●●●
●●
—Br●●
●●
●●— I
●●●●
●●
General structure
—X (halogens)
●●●●
●●
EXAMPLE:
Cl
HNO3
H2SO4
Cl
NO2
+
Cl
NO2(35%)chlorobenzene (64%)
12.5-68
META– DIRECTING ACTIVATORS 12.5-69
SMH, 1 ed, p.625*WAD, 4 ed, p.217CRY: 5 ed., p.651MCM: 6 ed., p.300SOL: 8 ed., p. 300
—CHO
Increasing deactivation
General structure:—Y (+ or –)
—COR
—COOR
—COOH
—CN
—SO3H
—NO2
—NR3
+
Have a full or partial positive charge on the
atom bonded to benzene ring
EXAMPLE:
NO2
HNO3
H2SO4
nitrobenzene
NO2
NO2
(93%)
12.5-69
Due to:
INDUCTIVE EFFECT 12.5-70
Electronegativity of the atoms in the substituent.
Polarisability of the substituent.
EXAMPLE:
Cl
Cl— electron–withdrawing
CH3
CH3— electron–donating
Activating Groups
Release electrons to the ring
Stabilise arenium ion
Form faster
12.5-71
CH3
Deactivating Groups
Withdraw electrons from the ring
Destabilise arenium ion
Form slower
+
NO2
Cl
+
NO2
EXAMPLE:
CF3
(trifluoromethyl)benzene benzene
CH3
toluene increasing rate of nitration
CF3 withdraws e-, arenium ion less stable ring less reactive
CH3 releases e-, arenium ion more stable ring more reactive
12.5-72
CF3
+
NO2
+
NO2CH3
+
NO2
OXIDATION OF SIDE CHAIN 12.5-75
Reactants: arene with benzylic H.
Conditions:
Strong oxidizing agent such as KMnO4
and Na2Cr2O7.
Heat.
CH3
CH(CH3)2CH3
benzylic H
CH3
toluene
KMnO4
heat
COOH
CH3O2N
p–nitrotoluene
Na2Cr2O7
heatCOOHO2N
p–nitrobenzoic acid
CH(CH3)2CH3 COOHHOOC
KMnO4
heat
EXAMPLE:
benzoic acid
12.5-76
isopropyl toluene terepththalic acid
Alkyl group, regardless their chain length are converted to –COOH.
12.5-77
CH(CH3)2CH3 COOHHOOC
KMnO4
heat
Compounds without a benzylic H are inert to oxidation.
C(CH3)3CH3 C(CH3)3HOOC
KMnO4
heat
Take place at high temperature or in the presence of uv light.
Mechanism: free–radical substitution
Cl or Br replaces H atom of alkyl group
CH3
Cl2
heat or light
CH2Cl
EXAMPLE:
Cl2
heat or light
CHCl2
Cl2
heat or light
CCl3
toluene benzyl chloride
(dichloromethyl)benzene
(trichloromethyl)benzene
HALOGENATION OF TOLUENE 12.5-78
Free radical substitution reaction
HALOGENATION OF TOLUENE
Electrophilic aromatic substitution reaction
CH3
toluene
Br2
FeBr3
CH3
Br2 +
CH3
Br2
Benzylic radicals are even more stable than 3o radicals!
CH2
benzylicradical
●12.5-79
Many aromatic compounds are carcinorgenic and toxic.
CH3
toluene benzene
CARCINOGENIC EFFECT12.5-34
Example: benzene, benzo[a]pyrene.
At one time, benzene was widely used as solvent.
Studies revealed benzene is carcinorgenic (can cause cancer).
Replaced by toluene.
12.5-35
Benzo[a]pyrene is found in cigarette smoke, automobile exhaust, and the fumes from charcoal grills.
benzo[a]pyrene
When ingested or inhaled, it oxidised to carcinogenic products.
12.5-36
12.5-21
Benzoic acid, the simplest organic acid, prevent the growth of many organism.
12.5-20
widely used as a food preservative.
12.5-22
Fresh wild berries
12.5-19
END OF SLIDE SHOW