Struttura del benzene Il benzene è più trieneconiugato Gentilucci/Gentilucci_cap9...Struttura del...
Transcript of Struttura del benzene Il benzene è più trieneconiugato Gentilucci/Gentilucci_cap9...Struttura del...
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Struttura del benzene
Il benzene è più stabile di un triene coniugato
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Aromaticity (Huckel).
All compounds having
planar, conjugated
systems containing
4n + 2 π electrons
Delocalizzazione
elettronica nel
benzene
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6 pz orbitals 3 bonding orbitals +
3 antibonding
The in phase overlap of the six
p-orbitals forms the
continuous π-system
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Aromaticity. It is also essential that the carbon skeleton be
planar.
Cyclodecapentaene has 10 π-electrons, but is not aromatic
because the ring hydrogens force the π-system out of
planarity.
In [18]annulene (18 π-electrons) there is sufficient
room for the central six hydrogen atoms to fit
within the middle of the ring system.
anthracen phenantren
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Heteroaromatic
compounds
Purinic and
Pyrimidinic bases
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Arenes: Nomenclature
1. Benzene is the parent
name; when a benzene ring
is a substituent: "phenyl"
group.
2. Disubstituted benzenes are named using ortho-, para- and meta- to describe the
substitution pattern (1,2 1,4 and 1,3 respectively) or simply by numbering the
substituents.
3. Substituents are numbered to give the lowest possible number sequence at the
first point of difference, assigning priorities alphabetically if there is a "tie".
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4. There are also a large number of common (or "trivial") names for arenes
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The role of the FeBr3 is to complex the bromine to form a bromonium cation-like
species which is the actual electrophilic agent.
Bromination
Br2 + benzene,
catalyzed by FeBr3.
In general, these reactions require a Lewis acid catalyst.
Electrophilic aromatic substitution,
An electrophile reacts with the ring, forming a new bond to a ring carbon with the
loss of one hydrogen.
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Since the ring is a conjugated system, the cationic charge which forms on the
adjacent carbon is delocalized over the ring, ortho- and para- to the position
where the electrophile bonded.
Loss of H+ from the sigma-complex regenerates the aromatic π-system.
This electrophile first
forms a loose complex
with the π-cloud, which
rearranges to a cationic
σ-complex, in which the
electrophile is directly
bonded to a ring carbon.
σ-complex
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σ-complex
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Chlorination
proceeds by a similar
mechanism.
Iodination
I2/CuCl2 is typically
utilized to generate the
electrophilic I+ cation
Nitration
using a mixture of
nitric and sulfuric acids
to generate the
electrophile NO2+.
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Sulfonation
Fuming sulfuric acid
(H2SO4 saturated with
SO3) to generate SO3H+.
Friedel-Crafts
alkylation.
A Lewis acid complexes
with an alkyl halide to
give a species with
electrophilic character
on the carbon of the
alkyl halide.
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(this means that an alkyl halide such as 1-bromopropane is not
suitable in this reaction, since it would be prone to rearrange to
the more stable isopropyl carbocation)
These limitations can be circumvented by using A derivative
of the Friedel-Crafts alkylation is the Friedel-Crafts acylation
5.
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Friedel-Crafts acylation: the arene is converted to an aryl ketone. An acylium ion-
like species is formed by reaction of an acid halide, or an acid anhydride, with the
Lewis acid.
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Substitution of substitutedbenzenes
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fast
slow
slow
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Kinetic and orientation effects
When electrophilic aromatic substitution occurs on a ring already bearing one or
more substituent, the nature of that substituent will impact both the rate of the
reaction and the regiochemistry of the reaction.
Arenes with activating
substituents will react faster
than benzene itself,
Arenes with deactivating
substituents will react more
slowly.
Further, substituents are
grouped into two categories;
ortho- para- directing,
meta-directing.
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An activating substituent releases electron density into the ring either inductively,
or through resonance (the electrophile is, after all, looking for electrons).
Ortho-and para- orientation effect is seen by considering the family of resonance
forms; these clearly show enhanced electron density localized ortho-and para- to the
point of attachment.
CH3
Inductive effect Conjugative effect
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Deactivating, meta-directing substituents such as the nitro group can be seen
to function by removing electron density from the ring ortho- and para- to
themselves, leaving only the meta-positions with sufficient electron density to
support the electrophilic (electron-seeking) reaction.
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Halogens: deactivating, ortho-and para- directing
Halogens are somewhat unique in that they deactivate inductively (and are
therefore less reactive than benzene), but they direct ortho-and para- since they
enhance the electron density at these positions by resonance, as shown below
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meta-directing
substituents don't
really activate the
meta-positions,
they deactivate
everywhere else.
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Multiple substituents on a ring.
The effects are generally either
cumulative, or the most strongly
activating substituent ultimately
directs the regiochemistry.
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Exercise: electrophilic substitution of toluene
Pyridoxal-phosphate (PLP, pyridoxal-5'-phosphate) is a coenzyme in all
transaminations, and in some decarboxylation and dehydration reactions of amino
acids.
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Reactions of Aryl Side-Chains
Oxidation. Arenes having an alkyl side-chain with at least one benzylic hydrogen
will undergo oxidation by neutral MnO4- to give the corresponding benzoic acid.
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As with all reactions
of MnO4-, the
reaction involves
radical
intermediates and
side reactions are
common.
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Radical helogenation.
Since benzyl radicals are
quite stable, radical
bromination occurs quite
rapidly on alkyl benzenes.
The reaction often utilize
NBS (N-bromosuccinimide)
in CCl4 in the presence of a
"radical initiator" to
generate the bromine
radical.
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Reduction
Since arenes are resistant to
catalytic reduction, alkene side-
chains can be specifically
reduced to the alkane without
reducing the ring.
The ring can be reduced with Pt
or Pd at high temperatures and
pressure, although Rh will
catalyze the reduction under
very mild conditions.
Nitro groups are reduced to
the corresponding amine.
Aryl ketones are smoothly
reduced catalytically to the
corresponding alkane
(alternative to the Friedel-
Crafts alkylation).
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Nucleophilic Aromatic Substitution
Arenes having strongly electron-
withdrawing groups on the ring, and at
least one potential leaving group (e.g.
an halogen) can undergo substitution by
strong nucleophiles,
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This substitution cannot be
occurring by a simple SN1 or
SN2.
The pathway for SN2 is blocked
by the ring, and the SN1
mechanism would involve the
generation of an unstable aryl
cation on a ring which is
already extremely electron
deficient.
The reaction involves an addition-elimination pathway.
The negative charge
is delocalized