Aromáticos
Michael Faraday (1791-1867)
•Mais conhecido por suas descobertas sobre
fenômenos elétricos. Contudo, iniciou sua carreira
com estudos na área da química.
• Isolou o benzeno do resíduo oleoso encontrado na
postes de iluminação à gás das ruas de Londres e
determinou que a razão C:H era de 1:1.
•O nome do grupamento fenila, usado quando o uma unidade de benzeno
encontra-se como um substituinte, deriva de sua origem, pois a palavra
grega pheno significa “Eu tenho a luz”.
•O benzeno foi sintetizado em 1834 por Eilhard Mitscherlich, o qual
determinou sua fórmula molecular como C6H6.
•Outros compostos com uma baixa razão C:H apresentavam um aroma
agradável, desta maneira foram classificados como aromáticos.
Aromatic Compounds
• Aromatic was used to described some fragrant compounds in early 19th century
– Not correct: later they are grouped by chemical behaviour
– Current: distinguished from aliphatic compounds by electronic configuration
CHO
OMe
CH CH CHO
OH
OH
CH2
OMe
CH CH2
CHO
anisaldehyde
(anise)
cinnamaldehyde
(cinnamon)
thymol
(thyme)eugenol
(cloves)cuminaldehyde
(cumin)
Sources of Aromatic Hydrocarbons
• From high temperature distillation of coal tar
• Heating petroleum at high temperature and pressure
over a catalyst
Naming Aromatic Compounds
• Many common names (toluene = methylbenzene; aniline = aminobenzene)
• Monosubstituted benzenes systematic names as hydrocarbons with –benzene
– C6H5Br = bromobenzene
– C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 is propylbenzene
SOME SPECIAL NAMES
CH3
NH2
toluene aniline anisole
O CH3
CH3
CH3
o-xylene m-xylene
p-xylene
phenol
CH3
CH3
CH3
CH3
OH
COOH
benzoic
acid
The Phenyl Group
• When a benzene ring is a substituent, the term phenyl is
used (for C6H5 )
– You may also see “Ph” or “f” in place of “C6H5”
• “Benzyl” refers to “C6H5CH2”
Ripso
ortho
meta
para
ortho, meta and para Positions
CH3
NO2
m-nitrotoluene
3-nitrotoluene
1-methyl-3-nitrobenzeneo-
m-
p-
1
2
3
4
5
6
Cl
Cl
p-dichlorobenzene
1,4-dichlorobenzene
Disubstituted Benzenes
Naming Benzenes With More Than Two
Substituents
• Choose numbers to get lowest possible values
• List substituents alphabetically with hyphenated numbers
• Common names, such as “toluene” can serve as root name (as in TNT)
AROMATICITY
THE HUCKEL RULE
HUCKEL 4n+2 RULE..
Prediction: Compounds that have 4n+2 pi electrons in
a cyclic array will be aromatic.
AROMATICITY
POLYCYCLIC AROMATIC COMPOUNDS
benzene naphthalene anthracene
6 10 14
1814
4n+2 series = 2, 6, 10, 14, 18, 22, 26, 30 …….. etc.
The rule was derived by observation of
BENZENE
6p electrons
E
N
E
R
G
Y
closed*
shell
AROMATIC
Aromatic
compounds
will have all
of the occupied
p M.O. levels
completely
filled with no
unpaired
electrons.
(*completed
level)
BENZENEIsodensity surfaces - electron potential mapped in color.(van der Waal’s)
Color adjusted
to enhance the
pi system.
Highest
electron density
is red.
Note the symmetry.
MO Derivation of
Hückel’s Rule • Lowest energy MO has 2 electrons.
• Each filled shell has 4 electrons.
=>
The “Double Bonds” in a Benzene Ring Do Not React Like Others
Alkene Benzene
RClH
R
H
Cl
ClH+ +no
reaction
RCl
2
R
Cl
Cl
Cl2+ +
no
reaction
RBr
2
R
Br
Br
Br2+ +
no
reaction
R R ORCO
3H RCO
3H+ +
no
reaction
Tipos de Reações Orgânicas
• Reações de :
– Adição – duas moléculas se combinam
– Eliminação – uma molécula quebra em duas
– Substituição – partes de duas moléculas
trocam
– Rearranjo – a molécula sofre mudanças no
modo como seus átomos são conectados.
Electrophilic
Aromatic SubstitutionElectrophile substitutes for a hydrogen on
the benzene ring.
=>
Mechanism
=>
ENERGY PROFILE FOR AROMATIC SUBSTITUTION
H
E
E
H
E+
+
Ea
H+
benzenium
intermediate(+)
(+)
Transition
state 1
Transition
state 2
STEP 1 STEP 2
slow fast
activation
energy
intermediate
Bromination of Benzene• Requires a stronger electrophile than Br2.
• Use a strong Lewis acid catalyst, FeBr3.
Br Br FeBr3 Br Br FeBr3
Br Br FeBr3
H
H
H
H
H
H
H
H
H
H
HH
Br+ + FeBr4
_
Br
HBr+
Energy Diagram
for Bromination
=>
Chlorination
and Iodination• Chlorination is similar to bromination. Use
AlCl3 as the Lewis acid catalyst.
• Iodination requires an acidic oxidizing
agent, like nitric acid, which oxidizes the
iodine to an iodonium ion.
H+
HNO3 I21/2 I
+NO2 H2O+ ++ +
=>
Formation of the Chloronium Ion Complex
Cl Cl Al Cl
Cl
Cl
Cl Cl Al Cl
Cl
Cl
Al Cl
Cl
Cl
ClCl
:: :
:
:
:
:
: :
:
:
:
:
:: :
:
: :
:
.. ..
.. ..
..
..
..
.. d+
.. ..
..
..
..
..
..
..
..
..
..
..
..
+ -
d-
Al
Cl
Cl
Cl
chloronium
ion complex
sp2
..
..
..
Electrophilic aromatic halogenations occur in
the biosynthesis of numerous naturally
occurring molecules, particularly those
produced by marine organisms
– Thyroxine, synthesized in the
thyroid gland in humans, is a
thyroid hormone involved in
regulating growth and
metabolism
Reactions of Aromatic Compounds:
Electrophilic SubstitutionAromatic Nitration
• Aromatic rings can be nitrated
with a mixture of concentrated
nitric and sulfuric acids
– The electrophile is the
nitronium ion, NO2+ which is
generated from HNO3 by
protonation and loss of water
– The nitronium ion reacts with benzene to yield a carbocation intermediate, and loss of H+
– The product is a neutral substitution product, nitrobenzene
Aromatic nitration
• Does not occur naturally
• Important in the laboratory
– The nitro-substituted product can be reduced by
reagents such as iron or tin metal or to yield an
arylamine, ArNH2
– Attachment of an amino group to an aromatic ring
by the two-step nitration-reduction sequence is a
key part of the industrial synthesis of many dyes
and pharmaceutical agents
• Sulfur trioxide, SO3, in fuming sulfuric acid is the
electrophile.
The mechanism of electrophilic
sulfonation of an aromatic ring
Desulfonation
• All steps are reversible, so sulfonic acid
group can be removed by heating in
dilute sulfuric acid.
• This process is used to place deuterium
in place of hydrogen on benzene ring.
Benzene-d6
=>
D
D
D
D
D
D
D2SO4/D2O
large excess
H
H
H
H
H
H
O CH3
Nitration of Anisole
NO2
O CH3
NO2
O CH3
Reacts faster
than benzene
+
ortho para
= “ACTIVATED”
The -OCH3 group when it preexists on the ring gives only
ortho and para products, and no meta.
Substituents that cause this result are called o,p directors
HNO3
H2SO4
and they usually activate the ring.
anisole
CO
OMe
Nitration of Methyl Benzoate
C
O
NO2
OMe
Reacts slower
than benzene
meta
HNO3
H2SO4
= “DEACTIVATED”
methyl benzoate
The -COOMe group when it preexists on the ring gives only
meta, and no ortho or para products.
Substituents that cause this result are called m directors
and they usually deactivate the ring.
DEACTIVATED RING
Most ring substituents fall into one of these two categories:
o,p - directors m- directors
activate the ring deactivate the ring
SUBSTITUENT CATEGORIES
We will look at one of each kind in order to
understand the difference…..
G
NITRATION OF ANISOLE
H
HNO
2
O CH3
+
H
H
NO2
O CH3
+
H NO2
H
O CH3
+
O CH3
+ N
O
O
+
Nitration of Anisole
NO2
O CH3
NO2
O CH3
BENZENIUM ION INTERMEDIATES
actual
products
activated
ring
ortho meta para
ortho para+
H NO2
H
O CH3
+
H NO2
H
O CH3
+
H NO2
H
O CH3
+
H NO2
H
O CH3
+
H
H
NO2
O CH3
+
H
H
NO2
O CH3
+
H
H
NO2
O CH3
+H
H
NO2
O CH3
+
H
HNO
2
O CH3
+
H
HNO
2
O CH3
+
H
HNO
2
O CH3
+
ortho
meta
para :
:
EXTRA!
EXTRA!
Energy Profiles
meta
ortho
para
NITRATION OF ANISOLEbenzenium
intermediateRECALL:
HAMMOND
POSTULATE
Ea
benzenium
intermediates
have more
resonance
ortho-paradirector
The Hammond Postulate
• If carbocation intermediate is more stable than another, why is the reaction through the more stable one faster?
– The relative stability of the intermediate is related to an equilibrium constant (DGº)
– The relative stability of the transition state (which describes the size of the rate constant) is the activation energy (DG‡)
– The transition state is transient and cannot be examined
Transition State Structures
• A transition state is the highest energy species in a reaction step
• By definition, its structure is not stable enough to exist for one vibration
• But the structure controls the rate of reaction
• So we need to be able to guess about its properties in an informed way
• We classify them in general ways and look for trends in reactivity – the conclusions are in the Hammond Postulate
Statement of the Hammond
Postulate
• A transition state should be similar to an
intermediate that is close in energy
• Sequential states on a reaction path that are close
in energy are likely to be close in structure - G. S.
Hammond
carbocation
G
Reaction
In a reaction
involving a
carbocation, the
transition states look
like the intermediate
Energy Profiles
meta
ortho
para
NITRATION OF ANISOLEbenzenium
intermediateRECALL:
HAMMOND
POSTULATE
Ea
benzenium
intermediates
have more
resonance
ortho-paradirector
H
HNO
2
O CH3
+
:B elimination_
H
NO2
O CH3
H
HNO
2
O CH3
+:B
addition
_
H
HNO
2
B
O CH3
doesn’t happen
resonance would be lost
restores aromatic ring
resonance
ADDITION REACTION
ELIMINATION REACTION
BENZENIUM IONS GIVE ELIMINATION INSTEAD OF ADDITION
( 36 Kcal / mole )
X
The Amino Group
Aniline reacts with bromine water (without a
catalyst) to yield the tribromide. Sodium
bicarbonate is added to neutralize the
HBr that’s also formed.
NH2
Br23
H2O, NaHCO3
NH2
Br
Br
Br
=>
Summary of
Activators
=>
Deactivating Meta-
Directing Substituents• Electrophilic substitution reactions for
nitrobenzene are 100,000 times slower than for benzene.
• The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers.
• Meta-directors deactivate all positions on the ring, but the meta position is lessdeactivated.
=>
Ortho Substitution
on Nitrobenzene
=>
Para Substitution
on Nitrobenzene
=>
Meta Substitution
on Nitrobenzene
=>
Energy Diagram
=>
Structure of Meta-
Directing Deactivators
• The atom attached to the aromatic ring
will have a partial positive charge.
• Electron density is withdrawn inductively
along the sigma bond, so the ring is less
electron-rich than benzene.
=>
Summary of Deactivators
=>
More Deactivators
=>
Halobenzenes
• Halogens are deactivating toward
electrophilic substitution, but are ortho,
para-directing!
• Since halogens are very electronegative,
they withdraw electron density from the
ring inductively along the sigma bond.
• But halogens have lone pairs of electrons
that can stabilize the sigma complex by
resonance. =>
Sigma Complex
for Bromobenzene
Br
E+
Br
H
E
(+)
(+)(+)
Ortho attack
+ Br
E+
Br
H E
+
(+)
(+)(+)
Para attack
Ortho and para attacks produce a bromonium ion
and other resonance structures.
=>
Meta attack
Br
E+
Br
H
H
E
+
(+)
(+)No bromonium ion
possible with meta attack.
Energy Diagram
=>
Summary of
Directing Effects
=>
Substituent Effects in
Electrophilic SubstitutionsElectrostatic potential maps of benzene, phenol (activated),
chlorobenzene (weakly deactivated), and benzaldehyde (more strongly deactivated)
• The –OH substituent makes the ring more negative (red)
• The –Cl makes the ring less negative (orange)
• The –CHO makes the ring still less negative (yellow)
DIRECTIVITY OF SINGLE GROUPS
ortho, para - Directing Groups
X
Groups that donate
electron density
to the ring.XX :
+I Substituent +R Substituent
CH3-
R-
CH3-O-
CH3-N-
-NH2
-O-H
These groups also
“activate” the ring, or
make it more reactive.
E+
The +R groups activate
the ring more strongly
than +I groups.
..
..
..
..
..
..
increased
reactivity
PROFILE:
X YY
meta - Directing Groups
X
Groups that withdraw
electron density from
the ring.
These groups also
“deactivate” the ring,
or make it less reactive.
E+
-I Substituent -R Substituent
d+ d-
C
O
R
C
O
OR
C
O
OH
C N
N
O
O
N
R
R
R
CCl3
-SO3H
+
decreased
reactivity
+
-
PROFILE:
Halides - o,p Directors / Deactivating
X
E+
: :..
Halides represent a special case:
They are o,p directors (+R effect )
They are deactivating ( -I effect )
Most other other substituents fall
into one of these four categories:
1) +R / o,p / activating
2) +I / o,p / activating
3) -R / m / deactivating
4) -I / m / deactivating
+R / -I / o,p / deactivating
They are o,p directing groups
that are deactivating
-F
-Cl
-Br
-I
THE EXCEPTION
CH3
O CH3
NO2
C
O
O CH3
PREDICT !
o,p m
o,p m
DIRECTIVITY OF MULTIPLE GROUPS
GROUPS ACTING IN CONCERT
O CH3
NO2
m-director
o,p director
HNO3
H2SO4 O CH3
NO2
NO2
major
product
very
little
formed
O CH3
NO2
O2N
steric
crowding
When groups direct to the
same positions it is easy to
predict the product.
GROUPS COMPETING
o,p-directing groups win
over m-directing groups
HNO3
H2SO4
O CH3
NO2
NO2
O CH3
NO2
O2N
O CH3
NO2
too
crowded
X+
HNO3
H2SO4
RESONANCE VERSUS INDUCTIVE EFFECT
O CH3
CH3
NO2
O CH3
CH3
+R
+I
resonance effects are more
important than inductive effects
major
product
SOME GENERAL RULES
1) Activating (o,p) groups (+R, +I) win over
deactivating (m) groups (-R,-I).
2) Resonance groups (+R) win over inductive (+I)
groups.
3) 1,2,3-Trisubstituted products rarely form due to
excessive steric crowding.
4) With bulky directing groups, there will usually be
more p-substitution than o-substitution.
5) The incoming group replaces a hydrogen, it will not
usually displace a substituent already in place.
HOW CAN YOU MAKE ...
C
O
O CH3
NO2
CH3
NO2
NO2
NO2
O2N
CH2CH
2CH
2CH
3
only,
no para
BROMINE - WATER REAGENT
PHENOLS AND ANILINES
H O
H
Br Br H O
H
Br Br
OMe
Br O
H
H
H
Br
OMe
..
.... .. ..
.. ..
..
..
..
..
..
..
:: : : :
:
+
+
-
BROMINE IN WATER
+
This reagent works only with highly-activated rings
such as phenols, anisoles and anilines.
bromonium
ion
etc
OH
Br2
H2O
OH
Br
BrBr
All available
positions are
bromiated.NH
2
CH3
NH2
CH3
Br
BrBr2
H2O
PHENOLS AND ANILINES REACT
Friedel-Crafts Alkylation
• Synthesis of alkyl benzenes from alkyl
halides and a Lewis acid, usually AlCl3.
• Reactions of alkyl halide with Lewis acid
produces a carbocation which is the
electrophile.
• Other sources of carbocations:
alkenes + HF or alcohols + BF3.
=>
Examples of
Carbocation Formation
CH3 CH CH3
Cl
+ AlCl3
CH3
C
H3C H
Cl AlCl3+ _
H2C CH CH3
HFH3C CH CH3
F+
_
H3C CH CH3
OHBF3
H3C CH CH3
OH BF3+
H3C CH CH3
++ HOBF3
_
=>
Formation of
Alkyl Benzene
C
CH3
CH3
H+
H
H
CH(CH3)2+
H
H
CH(CH3)2
B
F
F
F
OH
CH
CH3
CH3
+
HF
BF
OHF
=>
+
-
Limitations of
Friedel-Crafts
• Reaction fails if benzene has a substituent
that is more deactivating than halogen.
• Carbocations rearrange. Reaction of
benzene with n-propyl chloride and AlCl3produces isopropylbenzene.
• The alkylbenzene product is more reactive
than benzene, so polyalkylation occurs.
=>
Friedel-Crafts
Acylation
• Acyl chloride is used in place of alkyl
chloride.
• The acylium ion intermediate is
resonance stabilized and does not
rearrange like a carbocation.
• The product is a phenyl ketone that is
less reactive than benzene.
=>
Mechanism of Acylation
R C
O
Cl AlCl3 R C
O
AlCl3Cl+ _
R C
O
AlCl3Cl+ _
AlCl4 +
_ +R C O R C O
+
C
O
R
+
H
C
H
O
R
+
Cl AlCl3
_C
O
R +
HCl
AlCl3
=>
Clemmensen ReductionAcylbenzenes can be converted to
alkylbenzenes by treatment with aqueous
HCl and amalgamated zinc.
+ CH3CH2C
O
Cl1) AlCl3
2) H2O
C
O
CH2CH3Zn(Hg)
aq. HCl
CH2CH2CH3
=>
Gatterman-Koch
Formylation
• Formyl chloride is unstable. Use a high
pressure mixture of CO, HCl, and catalyst.
• Product is benzaldehyde.
CO + HCl H C
O
ClAlCl3/CuCl
H C O+
AlCl4
_
C
O
H
+ C
O
H+ HCl+
=>
Chlorination of Benzene
• Addition to the benzene ring may occur
with high heat and pressure (or light).
• The first Cl2 addition is difficult, but the
next 2 moles add rapidly.
• The product, benzene hexachloride, is
an insecticide.
=>
Catalytic Hydrogenation
• Elevated heat and pressure is required.
• Possible catalysts: Pt, Pd, Ni, Ru, Rh.
• Reduction cannot be stopped at an
intermediate stage.
=>
CH3
CH3
Ru, 100°C
1000 psi3H2,
CH3
CH3
Birch Reduction:
Regiospecific
• A carbon with an e--withdrawing group
is reduced.
• A carbon with an e--releasing group
is not reduced.
C
O
OH Na, NH3
CH3CH2OH
C
O
O
H
_
OCH3 Li, NH3
(CH3)3COH, THF
OCH3
=>
Birch Mechanism
=>
Side-Chain OxidationAlkylbenzenes are oxidized to benzoic acid
by hot KMnO4 or Na2Cr2O7/H2SO4.
CH(CH3)2
CH CH2
KMnO4, OH-
H2O, heat
COO
COO
_
_
=>
Side-Chain Halogenation
• Benzylic position is the most reactive.
• Chlorination is not as selective as
bromination, results in mixtures.
• Br2 reacts only at the benzylic position.
=>
CHCH2CH3
Br
hBr2,
CH2CH2CH3
Top Related