organic chemistry chm 207 chapter 4: aromatic compounds (benzene and toluene) nor akmalazura jani
TRANSCRIPT
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ORGANIC CHEMISTRY CHM 207
CHAPTER 4:AROMATIC COMPOUNDS
(BENZENE AND TOLUENE)
NOR AKMALAZURA JANI
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Aromatic compounds
• Organic compound that contains a benzene ring in its molecule is known as an aromatic compounds.
• Sometimes called arenes.• Molecular formula: C6H6
• Represented as a regular hexagon containing an inscribed circle.
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• The corner of each hexagon represents a carbon and a hydrogen atom.
• Can be represented in two abbreviated ways.
Structure of Benzene
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Kekulé Structure of Benzene
Each carbon atom must have four covalent bonds.
Molecular formula is C6H6
All the hydrogen atoms are equivalent
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Resonance Structure
• Resonance theory: the structure of benzene is a resonance hybrid structure of two Kekulé cononical forms.
• The hybrid structure is often represented by a hexagon containing an inscribed circle.
represents a resonance hybrid between the two
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• Hexagonal ring – 6 carbon-carbon bonds are equal.
• Circle – delocalised electrons of the benzene ring
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CRITERIA OF AROMATIC COMPOUNDS
• Structure must be cyclic, containing some number of conjugated pi bonds.
• Each atom in the ring must have an unhybridized p orbital. (The ring atoms are usually sp2 hybridized or occasionally sp hybridized).
• The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. The structure must be planar (or nearly planar) for effective overlap to occur.
• Delocalization of the pi electrons over the ring must lower the electronic energy.
* Antiaromatic compound: fulfills the first three criteria, but delocalization of the pi electrons over the ring increase the electronic energy.
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Huckel’s rule
• Used to determine aromaticity for planar, cyclic organic compounds with a continous ring of overlapping p-orbitals.
• If the number of pi (π) electrons in the monocyclic system is (4N+2), the system is aromatic. N is 0, 1, 2, 3…..
• Systems that have 2, 6 and 10 pi electrons for N = 0, 1, 2 is a aromatic.
• Systems that have 4, 8, and 12 pi electrons for N = 1, 2, 3 are antiaromatic.
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Naming Aromatic Compounds
Naming Aromatic Compounds
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• A substituted benzene is derived by replacing one or more of benzene’s hydrogen atoms with an atom or group of atoms.
• A monosubstituted benzene has the formula C6H5G where G is the group that replaces a hydrogen atom.
• All hydrogens in benzene are equivalent.
• It does not matter which hydrogen is replaced by G.
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Monosubstituted Benzenes
Monosubstituted Benzenes
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• Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene.
• The name is written as one word.
nitrobenzene
nitro group
ethylbenzene
ethyl group
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• Certain monosubstituted benzenes have special names.
• These are parent names for further substituted compounds.
methyl group
toluene
hydroxy group
phenol
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carboxyl group
benzoic acid
aniline
amino group
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Disubstituted BenzenesDisubstituted Benzenes
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• Three isomers are possible when two substituents replace hydrogen in a benzene molecule.
• The prefixes ortho-, meta- and para- (o-, m- and p-) are used to name these disubstituted benzenes.
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ortho-dichlorobenzene(1,2-dichlorobenzene)mp –17.2oC, bp 180.4oC
ortho disubstituted benzene
substituents on adjacent carbons
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meta-dichlorobenzene(1,3-dichlorobenzene)mp –24.82oC, bp 172oC
meta disubstituted benzene
substituents on adjacent carbons
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para-dichlorobenzene(1,4-dichlorobenzene)mp 53.1, bp 174.4oC
para disubstituted benzene
substituents are on opposite sides of the benzene ring
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phenol 3-nitrophenol
When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.
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When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.
toluene 3-nitrotoluene
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Tri- and Polysubstituted Benzenes
Tri- and Polysubstituted Benzenes
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• When a benzene ring has three or more substituents, the carbon atoms in the ring are numbered.
• Numbering starts at one of the substituent groups.• The numbering direction can be clockwise or
counterclockwise.• Numbering must be in the direction that gives the
substituent groups the lowest numbers.
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4
6
5
2
3
1
clockwise numbering
1,4,6-trichlorobenzene
4-chloro
1-chloro
6-chloro
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4
2
3
6
5
1
counterclockwise numbering
1,2,4-trichlorobenzene
4-chloro
1-chloro
2-chloro
chlorine substituents have lower numbers
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• When a compound is named as a derivative of the special parent compound, the substituent of the parent compound is considered to be C-1 of the ring.
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toluene
5
16
34
2 5
16
34
2
2,4,6-trinitrotoluene
(TNT)
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• When the hydrocarbon chain attached to the benzene ring is small, the compound is named as benzene derivative.
• Example:
CH2CH3
ethylbenzene
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Naming compounds that cannot be easily named as benzene derivatives
diphenylmethane4-phenyl-2-pentene
Benzene named as a substituent on a molecule with another functional group as its root by the prefix phenyl.
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The phenyl group, C6H5-
CH=CH2 NH2 CH2Cl
CH2
phenylethene phenylamine benzyl chloridecommonname
phenyl benzyl
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• If the hydrocarbon chain contains more than three carbon atoms, phenyl is used as part of the name.
• Examples:
CH2(CH2)5CH3
1-phenylheptane
C
Br
CH3
CH2 CH3
2-bromo-2-phenylbutane
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PHYSICAL PROPERTIES OF BENZENE AND ITS DERIVATIVES
• Benzene derivatives tend to be more symmetrical than similar aliphatic compounds, and pack better into crystals and have higher melting points.
• Density:- Slightly dense than non-aromatic analogues, but still less dense than water.- halogenated benzenes are denser than water.
• Insoluble in water• Boiling points depends on the dipole moments of
compounds.
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REACTION OF BENZENEELECTROPHILIC SUBSTITUTION REACTIONS OF
BENZENE
stability of π-electron system is lost when benzene undergoes addition reactions.
benzene and its derivatives undergo substitution reaction rather than addition reactions.
product of substitution reactions: aromatic compounds and not saturated compounds.
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Mechanism of electrophilic substitution Mechanism of electrophilic substitution of benzeneof benzene
Step 1: Electrophilic addition of the benzene ring
E+E
H
slow
arenium ion (a carbocation)
Step 2: Deprotonation of the arenium ion
EH
Nu- fast
nucleophile
E
H Nu
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ELECTROPHILIC SUBSTITUTION REACTIONS
H
H
H
X2
HNO3
SO3
H2SO4
H2SO4
H2SO4
X
NO2
SO3H
HX
2H2O
a) Halogenation
or FeX3
b) Nitration
halobenzene
nitrobenzene
c) Sulphonation
benzenesulphonic acid
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H
H
CH3Cl
CH3CCl
O
AlCl3
AlCl3
CH3
C CH3
O
HCl
HCl
d) Friedel-Crafts alkylation
e) Friedel-Crafts acylation
toluene
acetophenone
ELECTROPHILIC SUBSTITUTION REACTIONS
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Reagents, electrophiles and catalysts in electrophilic substitution reactions
Reactions Reagents Catalysts Electrophiles
Halogenation Cl2 or Br2 AlCl3, AlBr3, FeCl3 or FeBr3
Cl , Br
Nitration HNO3 H2SO4 NO2
Alkylation RCl
RCH=CH2
AlCl3
H2SO4
R
RCH-CH3
Acylation RCOCl AlCl3
RCO
Sulphonation SO3 H2SO4 SO3H
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HALOGENATION OF BENZENE
Cl2
Br2
AlCl3
FeBr3
Cl
Br
HCl
HBr
a)Chlorination
b)Brominationchlorobenzene
bromobenzene
1/2I2
I
NO2
c) Iodination
iodobenzene
HNO3H2O
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MECHANISM: BROMINATION OF BENZENE
H
H
H
H
H
H
Br Br
H
H
H
H
H
HBr
FeBr3
FeBr4-
Br Br FeBr3
Br Br FeBr3
H
Br
H
H
H
H
H
H
H
H
H
HBr
FeBr4-
HBr
H
H
H
H
H
HBr
FeBr3
H
H
H
H
H
HBr
Step 1: Formation of a stronger electrophile
Br2.FeBr3 intermediate(a stronger electrophile than Br2)
Step 2: Electrophilic attack and formation of the sigma complex
sigma complex
Step 3: Loss of a proton gives the products
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Step 1: Formation of the nitronium ion, NO2+
Step 2: Formation of an arenium ion as a result of electrophilic addition
Step 3: Loss of a proton gives the products
HO SO3 H HO NO2 H2O + NO2+ + HSO4
-
NO2+
H NO2
arenium ionnironium ion
slow
H NO2
HSO4-
fast
NO2
H2SO4
MECHANISM: NITRATION OF BENZENE
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C ClH
CH3CH3
AlCl3
C CH3
H
CH3
C
H
CH3
CH3
H CH(CH3)2
AlCl4-
CH(CH3)2
H CH(CH3)2
HCl + AlCl3
AlCl4-
Step 1: Formation of electrophile
Step 2: Formation of an arenium ion
Step 3: Loss of a proton
arenium ion
carbocation (electrophile)
MECHANISM: FRIEDEL-CRAFTS ALKYLATION
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CH3 C Cl
O
AlCl3
CH3 C
O
AlCl4-
H C
O
CH3
CH3 C
O
C
O
CH3
H C
O
CH3
AlCl4-
HCl + AlCl3
Step 1: Formation of electrophile
Step 2: Formation of an arenium ion
Step 3: Loss of a proton
MECHANISM: FRIEDEL-CRAFTS ACYLATION
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Ortho-Para and Meta Directing Substituents
• When substituted benzenes undergo further substituents, the substituent group present in the benzene derivative will influence electrophilic substitution in 2 ways which are:i) Reactivityii)Orientation
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EFFECTS OF SUBSTITUENTS ON THE REACTIVITY OF ELECTROPHILIC
AROMATIC SUBSTITUTION
• Substituent group present in the benzene ring can influence the rate of reaction of further substitutions.
• Electron-donating groups make the ring more reactive (called activating groups) thus influence the reaction become faster.
• Electron-withdrawing groups make the ring less reactive (called deactivating groups) thus influence the reaction become slower.
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• A substituents group already in the ring influences the position of further electrophilic substitution whether at ortho, meta or para position.
• Ortho-para directors: the groups that tend to direct electrophilic substitution to the C2 and C4 positions.
• Meta directors: the groups that tend to direct electrophilic substitution to the C3 position.
EFFECTS OF SUBSTITUENTS ON THE ORIENTATION OF
ELECTROPHILIC AROMATIC SUBSTITUTION
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Effetcs of substituent groups on the benzene ring
Activating groups (electron donating)
Deactivating groups
(electron-withdrawing)
-NH2 -R
-OH
-OR
-NHCOCH3
-F
-Cl
-Br
-I
ortho-para directors ortho-para directors
meta directors
C
O
R
C
O
OH
C
O
OR
SO3H
C N
NO2
NR3
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CH2CH3
Br2
FeBr3
CH2CH3Br
CH2CH3
Br
CH2CH3
Br
Example:
ortho position para position meta position
major products minor product
-CH2CH3 = ortho and para directors
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NO2
Br2
FeBr3
NO2
Br
NO2Br
NO2
Br
Example:
ortho position para positionmeta position
minor productsmajor product
-NO2 = meta director
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REACTIONS OF BENZENE DERIVATIVES
• Alkylbenzene such as toluene (methylbenzene) resembles benzene in many of its chemical properties.
• It is preferable to use toluene because it is less toxic.
• The methyl group activates the benzene nucleus.• Toluene reacts faster than benzene in all
electrophilic substitutions.
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Reactions of toluene
Reactions of the methyl group
Reactions of the benzene ring
Substitution-halogenation
Oxidation
Electrophilic substitutions- Halogenation- Nitration- Friedel-Crafts reactions- Sulfonation
Addition reaction-hydrogenation
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SIDE-CHAIN REACTIONS
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OXIDATION REACTION OF ALKYLBENZENE
CH2 R C
O
OHhot, conc., KMnO4/H+
reflux
examples:
CH3 C
O
OHhot, conc., KMnO4/H+
reflux
CH2 CH3 C
O
OHhot, conc., KMnO4/H+
reflux
CH3hot, conc., KMnO4/H+
refluxCH3 COOH
COOH
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HALOGENATION OF TOLUENE
CH3
Cl2
CH2 Cl
Cl2
CHCl2
Cl2
CCl3
CHCl2
CH2 Cl
HCl
HCl
HCluv light
(chloromethyl)benzene
uv light
(dichloromethyl)benzene
uv light
(trichloromethyl)benzene
Side chain substitution
* Bromination of toluene takes place under similar conditions to yield corresponding bromine derivatives.
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SYNTHESIZING A SUBSTITUTED AROMATIC COMPOUNDS
NO2
Cl
?
Synthesis m-chloronitrobenzene starting from benzene
• Two substituents: -NO2 (meta-directing) and –Cl (ortho- and para-directing)• Cannot nitrate chlorobenzene because the wrong isomer (o- and p-chloronitrobenzenes) would formed.
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NO2
Cl
HNO3
H2SO4
NO2
NO2
Cl
Cl2
FeCl3
NO2
Cl
HNO3, H2SO4
Cl2, FeCl3 m-chloronitrobenzene
chlorobenzene
nitrobenzene
TWO STEPS:
nitrobenzene m-chloronitrobenzenebenzene
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SYNTHESIZING A SUBSTITUTED AROMATIC COMPOUNDS
COOH
Br
?
Synthesis p-bromobenzoic acid starting from benzene
• Two substituents: -COOH (meta-directing) and –Br (ortho- and para-directing)• Cannot brominated benzioc acid because the wrong isomer (m-bromobenzoic acid) would formed.• Oxidation of alkylbenzene side chains yields benzoic acids.• Intermediate precursor is p-bromotoluene
COOH
BrBr
CH3KMnO4
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Immediate precursor of p-bromotoluene:i)Bromination of toluene
orii) Methylation of bromobenzene
CH3Br2
FeCl3
CH3
Br
CH3
Br
separate the isomeror
CH3Cl
AlCl3
CH3
Br
CH3
Br
separate the isomer
Br
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Immediate precursor of toluene:i)Benzene was methylated in a Friedel-Crafts reaction
CH3CH3Cl
AlCl3
toluenebenzene
Immediate precursor of bromobenzene:i)Bromination of benzene
Br2
FeBr3
bromobenzenebenzene
Br
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Br2
FeBr3
CH3Cl
AlCl3
Br
CH3
AlCl3
CH3Cl
Br2
FeBr3
Br
CH3KMnO4
Br
COOH
benzene
TWO WORKABLE ROUTES FROM BENZENE TO p-BROMOBENZOIC ACID
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• Benzene:Benzene:- as solvent for oils and fats - starting material for making other chemicals. For example, benzene is used in the cumene process to produce phenol.- making organic compounds such as phenylethene (styrene) and nitrobenzene. These organic compounds are then used to make plastics (polystyrene), dyes and nylon.
USES OF BENZENE AND TOLUENE
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• Toluene:Toluene:
- A common solvent, able to dissolve paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives (glues), lacquers, leather tanners and disinfectants.- As a solvent to create a solution of carbon nanotubes.- Dealkylation to benzene (industrial uses).- As an octane booster in gasoline fuels used in internal combustion engines.-As a coolant in nuclear reactor system loops.
USES OF BENZENE AND TOLUENE