organic chemistry chm 207 chapter 4: aromatic compounds (benzene and toluene) nor akmalazura jani
TRANSCRIPT
ORGANIC CHEMISTRY CHM 207
CHAPTER 4:AROMATIC COMPOUNDS
(BENZENE AND TOLUENE)
NOR AKMALAZURA JANI
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.
• The corner of each hexagon represents a carbon and a hydrogen atom.
• Can be represented in two abbreviated ways.
Structure of Benzene
Kekulé Structure of Benzene
Each carbon atom must have four covalent bonds.
Molecular formula is C6H6
All the hydrogen atoms are equivalent
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
• Hexagonal ring – 6 carbon-carbon bonds are equal.
• Circle – delocalised electrons of the benzene ring
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.
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.
Naming Aromatic Compounds
Naming Aromatic Compounds
• 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.
Monosubstituted Benzenes
Monosubstituted Benzenes
• 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
• Certain monosubstituted benzenes have special names.
• These are parent names for further substituted compounds.
methyl group
toluene
hydroxy group
phenol
carboxyl group
benzoic acid
aniline
amino group
Disubstituted BenzenesDisubstituted Benzenes
• 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.
ortho-dichlorobenzene(1,2-dichlorobenzene)mp –17.2oC, bp 180.4oC
ortho disubstituted benzene
substituents on adjacent carbons
meta-dichlorobenzene(1,3-dichlorobenzene)mp –24.82oC, bp 172oC
meta disubstituted benzene
substituents on adjacent carbons
para-dichlorobenzene(1,4-dichlorobenzene)mp 53.1, bp 174.4oC
para disubstituted benzene
substituents are on opposite sides of the benzene ring
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.
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
Tri- and Polysubstituted Benzenes
Tri- and Polysubstituted Benzenes
• 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.
4
6
5
2
3
1
clockwise numbering
1,4,6-trichlorobenzene
4-chloro
1-chloro
6-chloro
4
2
3
6
5
1
counterclockwise numbering
1,2,4-trichlorobenzene
4-chloro
1-chloro
2-chloro
chlorine substituents have lower numbers
• 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.
toluene
5
16
34
2 5
16
34
2
2,4,6-trinitrotoluene
(TNT)
• When the hydrocarbon chain attached to the benzene ring is small, the compound is named as benzene derivative.
• Example:
CH2CH3
ethylbenzene
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.
The phenyl group, C6H5-
CH=CH2 NH2 CH2Cl
CH2
phenylethene phenylamine benzyl chloridecommonname
phenyl benzyl
• 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
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.
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.
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
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
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
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
HALOGENATION OF BENZENE
Cl2
Br2
AlCl3
FeBr3
Cl
Br
HCl
HBr
a)Chlorination
b)Brominationchlorobenzene
bromobenzene
1/2I2
I
NO2
c) Iodination
iodobenzene
HNO3H2O
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
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
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
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
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
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.
• 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
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
CH2CH3
Br2
FeBr3
CH2CH3Br
CH2CH3
Br
CH2CH3
Br
Example:
ortho position para position meta position
major products minor product
-CH2CH3 = ortho and para directors
NO2
Br2
FeBr3
NO2
Br
NO2Br
NO2
Br
Example:
ortho position para positionmeta position
minor productsmajor product
-NO2 = meta director
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.
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
SIDE-CHAIN REACTIONS
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
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.
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.
NO2
Cl
HNO3
H2SO4
NO2
NO2
Cl
Cl2
FeCl3
NO2
Cl
HNO3, H2SO4
Cl2, FeCl3 m-chloronitrobenzene
chlorobenzene
nitrobenzene
TWO STEPS:
nitrobenzene m-chloronitrobenzenebenzene
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
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
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
Br2
FeBr3
CH3Cl
AlCl3
Br
CH3
AlCl3
CH3Cl
Br2
FeBr3
Br
CH3KMnO4
Br
COOH
benzene
TWO WORKABLE ROUTES FROM BENZENE TO p-BROMOBENZOIC ACID
• 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
• 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