Aromatic Comp. Lec.2

Dr.M_T The 3rd Vision Academy 01156281369

I

Aromatic Compound

Classes of Aromatic comp.,,,

I. Annulens

-Def;

"completely conjugated monocyclic polyenes."

"cyclic hydrocarbon with continuous alternating of single and double

bond."

Cyclobutadiene, benzene, and cyclooctatetraene are the first members

of a family of annulenes.

-Nomenclature;

[ no. of C-atoms ] + annulene

-Note;

*Aromaticity in the larger annulenes depends on whether the molecule

can adopt the necessary planar conformation.

# carbon atoms must be EVEN (4,6,8 …) to be AROMATIC.

*Geometry of the structure control hybridization concept not vice vesra.

*Polyenes;

" poly-unsaturated organic compounds that contain at least three al-

ternating double and single carbon–carbon bonds, and have conjugated

system."

*The generality and limits of the Hückel rule can be tested by consider-

ing the properties of the annulene series.

e.g.

cyclobutadiene Benzene cyclooctatetraiene

[4]annulene [ 6]annulene [ 8]annulene

n=0.5 n=1 n=1.5

antiaromatic aromatic antiaromatic


II

[10]annulene [12]annulene [14]annulene

n=2 n=2.5 n=3

aromatic antiaromatic aromatic

[16]annulene [18]annulene

n=3.5 n=4

antiaromatic aromatic

N.B.

-The [10]annulene isomer with two trans double bonds cannot adopt a

planar conformation either, because two hydrogen atoms interfere with

each other. Neither of these [10]annulene isomers is aromatic, even

though each has pi electrons, with If the interfering hydrogen atoms in

the partially trans isomer are removed, the molecule can be planar.

[10]annulene [10]annulene [10]annulene

all CIS two trans remved interfering H

n=2 n=2 n=2

nonaromatic nonaromatic aromatic

-Naphthalene is the result when the interfering

hydrogen atoms of [10]annulene are replaced

with a bond.


III

-[18]Annulene offers a particularly significant test of

the Hückel rule. The internal cavity in [18]annulene is

large enough to minimize steric interaction between

the internal hydrogens in a geometry that is free of

angle strain.

[18]annulene

-Hückel’s rule also applies to systems having odd numbers of carbon

atoms and bearing positive or negative charges

II. Aromatic Ions

1. The Cyclopentadienyl Ions

With four 𝝅electrons (a cation), Hückel’s rule predicts this system to be

antiaromatic.

With six 𝝅electrons (an anion), Hückel’s rule predicts aromaticity.

Because the cyclopentadienyl anion (six 𝝅electrons) is aromatic, it is

unusually stable compared with other carbanions. It can be formed by

abstracting a proton from cyclopentadiene, which is unusually acidic

for an alkene.


IV

Hückel’s rule predicts that the cyclopentadienyl cation, with four 𝝅

electrons, is antiaromatic. In agreement with this prediction, the cyclo-

pentadienyl cation is not easily formed. Protonated cyclopenta-2,4-dien-

1-ol does not lose water (to give the cyclopentadienyl cation), even in

concentrated sulfuric acid. The antiaromatic cation is simply too unsta-

ble.

With conjugated cyclic systems such as the shown below, the resonance

approach is a poor predictor of stability. Hückel’s rule, based on MO

theory, is a much better predictor of stability for these aromatic and an-

tiaromatic systems.

2. Cyclopropenyl cation& anion ------- quick study in points

-Resonance structure;


V

cyclopropenyl cation:

# of 𝝅e' =2 apply (4n+2)

cyclopropenyl anion:

# of e' =4 apply (4n)

-MO of both (cation& anion)

cyclopropylenyl cation cyclopropylene anion

Aromatic Antiaromatic

*Applications

1.

-Cyclopentadiene is not aromatic but acidic.

-Cyclopentadiene can be converted to its anion by treatment with

moderately strong bases.

2. Mass spectrometer of Tropyllium cation (cycloheptatrienyl cation)

-Mass spectrometry (MS);

an analytical technique that ionizes chemical species and sorts the ions

based on their mass to charge ratio.

-Information can be gained;

1. The molecular mass.

2. The molecular formula.

3. Structural features of a compound.

4. The exact structure when compared to known structure.


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- The result is a fragment which often found for aromatic compounds

containing a benzyl unit.

-Upon ionization, the benzyl fragment forms a cation (PhCH2+) ,

which rearranges to the highly stable tropylium cation (C7H7+).

III. Polynuclear comp.

1. Fused benzene


VII

2. Linked benzene

IV. Heterocylic comp.

"has a C-atom replaced by heteroatom (N, O, S)"

V. Non benzenoid comp.


VIII

*Application

-Fullerenes;

Around 1985, Kroto, Smalley, and Curl (Rice University) isolated a

molecule of formula from the soot produced by using a laser (or an

electric arc) to vaporize graphite.

The structure of which was named "buckminsterfullerene" in honor of

the American architect R. Buckminster Fuller, whose geodesic domes

used similar five- and six-membered rings to form a curved roof.

The molecules are sometimes called “buckyballs,” and these types of

compounds ( and similar carbon clusters) are called fullerenes.

A soccer ball has the same structure as with each vertex representing a

carbon atom. All the carbon atoms are chemically the same.

Each carbon serves as a bridgehead for two six-membered rings and

one five-membered ring.

There are only two types of bonds:

1.The bonds that are shared by a five-membered ring

2.A six-membered

Nanotubes;

"A cylinder composed of aromatic sixmembered rings similar to graph-

ite. The end of the tube is half of a sphere."

Notice how the fivemembered rings cause the structure to curve at the

end of the tube.


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These structures begin with half of the sphere, fused to a cylinder com-

posed entirely of fused six-membered rings (as in a layer of graphite).

Nanotubes have aroused interest because they are:

1. Electrically conductive only along the length of the tube.

2. They have an enormous strengthto-weight ratio

3. Under the research as a new dosage form of capsules.

Reactions of Aromatic comp.

Like an alkene, benzene has clouds of 𝝅 electrons above and below its

sigma bond framework. Although benzene’s pi electrons are in a stable

aromatic system, they are available to attack a strong electrophile to

give a carbocation. This resonance-stabilized carbocation is called a

sigma complex because the electrophile is joined to the benzene ring by

a new sigma bond.

So,

-Electrophilic reagent because benzene is covered by electron cloud

above and below the plane of the molecule.

-Substitution reaction to allow the aromatic sextet of 𝝅e' to be regener-

ated afire the attack by the electrophile.

General Mechanism of Electrophile Aromatic Substitution

N.B.

-True intermediate not transition state.

-Highly exothermic.

-Two step Rx.;

1st; endothermic& 2nd ; exothermic


X

-1st; RDS: Rate Determining Step ----- formation of sigma complex.

1.Halogenation

Bromination follows the general mechanism for electrophilic aromatic

substitution.

Bromine itself is not sufficiently electrophilic to react with benzene, and

the formation of 𝑩𝒓+ is difficult.

A strong Lewis acid such as 𝑭𝒆𝑩𝒓𝟑 catalyzes the reaction.

Lewis acid; 𝑨𝒍𝑪𝒍𝟑 , 𝑭𝒆𝑩𝒓𝟑 , 𝑭𝒆𝑪𝒍𝟑

Type equation here.


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*Iodine;

-Unreactive (inert)

Iodination of benzene requires an acidic oxidizing agent, such as nitric

acid. Nitric acid is consumed in the reaction, so it is a reagent (an oxi-

dant) rather than a catalyst.

*Fluorine;

-React so rapidly

2.Nitration

using Nitrate Mix. (𝑪. 𝑯𝑵𝑶𝟑) + (𝑪. 𝑯𝟐𝑺𝑶𝟒)

Rx.

Mech.


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Benzene reacts with hot, concentrated nitric acid to give nitrobenzene.

This sluggish reaction is hazardous because a hot mixture of concen-

trated nitric acid with any oxidizable material might explode.

Energy Diagram.


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3. Sulfonation

Using fuming sulfuric acid (𝑺𝑶𝟑+𝑯𝟐𝑺𝑶𝟒)

“Fuming sulfuric acid” is the common name for a solution of 7% in

𝑯𝟐𝑺𝑶𝟒 Sulfur trioxide is the anhydride of sulfuric acid, meaning that

the addition of water to 𝑺𝑶𝟑 gives 𝑯𝟐𝑺𝑶𝟒."

Sulfur trioxide is a strong electrophile, with three sulfonyl 𝑺𝑶𝟐 bonds

drawing electron density away from the sulfur atom.

Benzene attacks sulfur trioxide, forming a sigma complex. Loss of a

proton on the tetrahedral carbon and reprotonation on oxygen gives

benzenesulfonic acid.

𝑺𝑶𝟑 is strong 𝑬+ due to 3-M effect of O

Rx.

Mech.


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N.B. Sulfonation is a protection reaction in synthetic process.

Reverse Rx.

Desulfonation;

Sulfonation is reversible, and a sulfonic acid group may be removed

from an aromatic ring by heating in dilute sulfuric acid. In practice,

steam is often used as a source of both water and heat for desulfona-

tion.

1. Sulfonation (+fuming Sulfuric acid). Used in synthetic work

2. Desulfonation (+Steam& dil.𝑯𝟐𝑺𝑶𝟒).

4. Friedel–Crafts Alkylation

Discovered in 1877 by Charles Friedel and James Crafts.

They discovered that mixing benzene, a haloalkane, and 𝑨𝒍𝑪𝒍𝟑 results

in the formation of an alkylbenzene and HX.

It forms a new carbon–carbon bond between benzene and an alkyl

group.

Rx.


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Mech.

***Limitations;

1.It is practical only with stable carbocations, such as 3° carbocations,

resonance-stabilized carbocations, or 2° carbocations that cannot un-

dergo rearrangement.

Primary carbocations will undergo rearrangement, resulting in multi-

ple products as well as bonding of the benzene ring to unexpected car-

bons in the former haloalkane.


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2.It fails altogether on benzene rings bearing one or more strongly elec-

tron-withdrawing groups.

The following table shows some of these groups:

5. Friedel–Crafts Acylation

Acyl halide;

" A derivative of a carboxylic acid in which the -OH of the carboxyl

group is replaced by a halogen, most commonly chlorine."

Rx.


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Mech.


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N.B.

1. Preparation of Toluene (Friedel-Craft Alkylation)

2. Remember;

preparation of Acid Chloride (Acetyl Cholride, Benzoyl Chloride, Butyl

Chloride) ----- "Acylation"

So, Rx.s summary from energy point of view

Irreversible Reversible

Nitration Sulfonation

FC.Acylation FC.Alkylation

Halogenation ''reversed by dealkylation''


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Aromatic Comp. Lec.2

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