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Struttura del benzene

Il benzene è più stabile di un triene coniugato

Gentilucci- Composti aromatici e reattività

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


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


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.


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


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.



(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.


Substitution of substitutedbenzenes


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.


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.


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.


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.

Gentilucc i- Composti aromatici e reattività

As with all reactions

of MnO4-, the

reaction involves

radical

intermediates and

side reactions are

common.


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).


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,


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

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