Download Aromatic Compounds

Aromatic Compounds
• In 1825 Michael Faraday isolated a compound which boils
at 80o and had a H:C ratio of 1:1
• It was later synthesized from PhCO2H isolated from gum
benzoin, it was found to have a MW of 78 amu (C6H6) and
hence called benzene.
• Numerous compounds related to benzene with low C:H
ratio and pleasant aroma has been discovered in the 19th
century; hence called AROMATIC.
• In 1866 Kekulé propose a cyclic structure with three
double bonds.
1.397 Ao
=
Resonance representation
1.34 Ao
1.43 Ao
Benzene show some unusual reactions
OH
KMnO4 / H2O
+
MnO2
OH
cyclohexene
KMnO4 / H2O
No reaction
Benzene
Br2 / CCl4
Br
Br
cyclohexene
Br
Br2.FeBr3
Br2 / CCl4
Benzene
No reaction
Benzene
+
HBr
Unusual stability of benzene
Molar heat of hydration
H2
catalyst
∆ Hobs = -120.1 KJmol-1
cyclohexene
H2
catalyst
∆ Hobs = -232.7 KJmol-1
∆ Hcal = -240.2 KJmol-1
1,3 cyclohexadiene
H2
catalyst
benzene
∆ Hobs = -209.2 KJmol-1
∆ Hcal = -360.3 KJmol-1
151.4 KJmol-1
∆Hobs is much less (151.4 KJmol-1) than predicted. That is benzene is
more stable by 151.4 KJmol-1 than the hypothetical “cyclohexatriene”
This energy difference is called RESONANCE ENERGY
Proposed theories for the explanation of benzene
unusual stability
• Resonance or valence bond theory (VBT)
• Molecular orbital theory (MOT)
Valence Bond Theory
When two or more structures for the same molecule can be
drawn that differ only in the position of the electrons, then
none of these structures will truly indicate the physical and
chemical properties of that molecule.
Resonance or Connonical forms
“All aromatic compounds obey the V.B.T. However not all
compounds that obey the V.B.T. are aromatic”.
Failure of V.B.T
Not all ANNULENES show similar stability to benzene
[4] annulene
[6] annulene
[8] annulene
Cyclooctatetraene~: Readily decolourize Br2
Oxidize by MnO4
Cyclobutadiene~: Very difficult to isolate pure.
Molecular Orbital Theory
Structure of benzene
Six sp2 hybrid carbon atoms each with an unhybridized porbital that overlaps with the p-orbital of its neighbours to form
a continuous ring of orbital above & below the plane of carbon
atoms.
The six π electrons are contained in this ring of overlapping
orbitals.
H
H
C
C
H
C
C
H
C
H
120O
H
Apply the principles of MOT
• σ bonds form the basic framework of the molecule.
• The π electrons are placed in MO obeying the following
rules.
(i) There are as many MO as carbon atoms in the system.
(ii) There is always an orbital of lowest energy which is
single and can accommodate 2π electrons.
(iii) The remaining orbitals occur in pairs which are
degenerate and can accommodate 4π electrons.
(iv) For even # of carbon atoms, the last pair of orbitals is
followed by a single orbital of highest energy.
• There is a certain number of π electron to impart stability.
MO diagram of benzene
E
antibonding
(node) nonbonding
bonding
NOTE
(i) All bonding orbitals are full.
(ii) No electron in anti-bonding orbitals
(iii) All electrons have their spin paired.
This configuration where the molecule having a closed
bonding shell is energetically very favorable.
Consider cyclobutadiene
E
antibonding
(node) nonbonding
bonding
There are unpaired π electrons in non-bonding orbitals.
Therefore do not have a closed bonding shell, hence not
aromatic
Requirements for aromaticity
Compound must have a planar cyclic structure with each atom
in the ring having an unhybridized p-orbital which form a
continuous ring of parallel orbital in which the π electrons are
delocalized resulting in a lowering of the electronic energy.
Predicting aromaticity
Hückel Rule
If the number of π electrons in the cyclic system is 4N+2,
where N is an integer, the system is aromatic.
ie. N = 0, 1, 2, 3……
πe- = 2, 6, 10, 14…….
Non-aromatic~: Does not have a continuous ring of porbitals. eg.
Antiaromatic~: contain 4N π electrons ie 4, 8, 12…...
O
aromatic
antiaromatic
aromatic
aromatic
antiaromatic
NOMENCLATURE
Two systems are used in naming monosubstituted benzenes
(i) Benzene is used as the parent name and the substituent is
simply indicated by a prefix.
NO2
Br
Bromobenzene
Nitrobenzene
Cl
Chlorobenzene
(ii) The substituent & benzene ring taken together to form a
new parent
NH2
CH3
OH
Methylbenzene
Hydroxybenzene
Aminobenzene
Toluene
Phenol
Aniline
O
OCH3
C
Methoxybenzene
Anisole
COOH
SO3H
CH3
Methyl phenyl ketone
Benzoic acid
Benzenesulfonic
acid
Acetophenone
When two substituents are present, there relative positions are
indicated by the prefixes ORTHO, META & PARA
(abbreviated o-, m-, p- or by use of numbers)
x
ortho
1,2-
meta
1,3-
y
para
1,4-
Br
I
NO2
Br
Cl
o-dibromobenzene
m-chloronitrobenzene
CH3
p-iodotoluene
When three or more substituents are on the benzene ring,
numbers are used to give their positions. Assign the numbers
exactely as you would with a substituted cyclohexane. The
carbon atom bearing the functional group that defines the base
name is assumed to be C-1.
Br
CH3
OH
NO2
Br
NO2
Br
1,3,5-tribromobenzene
NO2
2,4-dinitrotoluene
I
4-iodo-2-nitrophenol
REACTIONS OF BENZENE
Benzene is an electron rich molecule and hence is a center of
high electron density, it therefore undergo electrophilic
aromatic substitution (E. A. S)
E
H
+
E
+
electrophile
H
Mechanism of E. A. S.
(i)
+
E
R. D. S.
slow
H
H
E
E
H
E
(ii) Rearomatisation
H
E
fast
E
+
H
Reaction Energy Profile
δ+
PE
H
E
TS 1
δ+
E
TS2
H
+
H
E
σ-complex
+ E
EAct1
EAct2
E
+
Reaction Progress
H
Some reactions of benzene
Nitration
NO2
+
H2SO4
HNO3
+
H2O
(i) Generation of the electrophile; nitronium ion
(a)
H2SO4
(b)
H2O
(c)
H2SO4
+
H2O
2 H2SO4
+
HNO3
OVERALL
+
HO
NO2
NO2
H2O
NO2
+
H2O
+
NO2
+
HSO4
+
H3O
H3O
NO2
O
HSO4
+
2 HSO4
N
O
(ii) Addition of electrophile (+NO2)
H
O
R. D. S.
N
NO2
O
(iii) Rearomatization
The σ-complex, once formed will only react in a forward
direction since the activation energy for the expulsion of +NO2
is much higher than for loss of the proton.
H
NO2
HSO4
fast
NO2
+
H2SO4
Sulfonation of benzene
SO3H
fuming H2SO4
+
H2O
O
(i) Generation of the electrophile; sulfur trioxide
O
SO3
2 H2SO4
+
H3O
+
HSO4
(ii) Addition of electrophile (SO3)
H
O
S
O
O
R. D. S.
O
S
O
O
S
O
(iii) Rearomatization
H
O
O
S
HSO4
O
S
fast
O
O
O
O
S
O
O
H
O
SO3H
SO3H
+
HSO4
Halogenation of benzene
E.A.S. on benzene by halides requires the assistance of
Lewis acid catalyst.
X
+
X2
FeX3
+
HX
X = Cl or Br
e.g. bromonation of benzene
(i) Generation of the electrophile
Br
Br
+
FeBr3
Br
Br
FeBr3
Br
BrFeBr3
(ii) Addition of electrophile
H
+
Br
Br
Br
FeBr3
+
FeBr4
(iii) Rearomatization
H
Br
FeBr3
Br
fast
Br
+
HBr
+
FeBr3
Alkylation of benzene
The Friedel-Crafts Reaction
Chloro and bromoalkanes react with benzene in the
presence of Lewis acid catalysts to give alkylbenzenes
R
+
RX
AlX3
HX
+
X = Cl or Br
e.g. Ethylation of benzene
CH2CH3
+
CH3CH2Cl
AlCl3
+
HCl
(i) Generation of the electrophile
CH3CH2 Cl
AlCl3
+
CH3CH2
Cl
AlCl3
CH3CH2 AlCl4
(ii) Addition of electrophile
H
+
CH2CH3
Cl
CH2CH3
AlCl3
+
AlCl4
(iii) Rearomatization
H
CH2CH3
Cl
AlCl3
fast
CH2CH3
+
HCl
+
AlCl3
Alkylation with other reagents
The electrophile is essentially a carbocation (R+). Compounds
capable of generating such species may be used in the
Friedel-Crafts alkylation.
e.g. alcohols
alkenes
OH
H
OH2
-H2O
H
Once electrophile is formed, then proceed as in previous slide.
Limitations of Friedel-Craft rxn.
(i) When R+ formed from an from alkylhalides, alkenes or
alcohols, can rearrange to a more stable R+ it usually does
so. e.g.
CH3
CH
CH2OH
CH3
CH
CH2
CH3
CH3
CH3
C
CH3
(ii) Reaction is difficult to stop at monoalkylation, i.e.
R
polyalkylation often occur.
R
R
R
e.g.
RX
Lewis acid
+
+
R
(i) Aryl and vinylic halides cannot be used as the halide
component because they do not form R+ readily.
e.g.
RCH
CHX
RCH
CH
CH3
O
Friedel-Craft acylation of benzene
R
C
Similar to the Friedel-Craft alkylation except that the
haloalkane is replaced by an acyl halide to produce the
corresponding ketone.
O
C
+
AlCl3
RCOCl
R
+
(i) Generation of the electrophile
O
R
C
O
Cl
+
AlCl3
R
C
R
O
C
Cl
R
C
acylium ion
AlCl3
O
+
AlCl4
HCl
(ii) Addition of electrophile
R
H
C
C
O
R
O
(iii) Rearomatization
O
H
C
O
C
R
Cl
AlCl3
R
+
HCl
+
AlCl3
Advantages of acylation vs. alkylation
(i) Rearrangement of the acyl group does not occur.
(ii) Acylation produce only monosubstituted product.
(iii) Easily reduced products.
O
C
R
CH2R
alkylbenzene