AROMATIC HYDROCARBONS

AROMATIC HYDROCARBONS

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MODERN CONCEPT OF STRUCTURE OF BENZENE

1. Molecular formula: Elemental analysis and molecular mass determination confirms the molecular

formula of benzene as C6H6.

2. Open Chain structure: The formula of benzene suggests that the molecule to be highly unsaturated. But

benzene is very stable. It does not decolourise alkaline potassium permanganate or bromine water. This

indicates that benzene is unlike alkenes or alkynes but almost behaves like a saturated molecule. Thus

an open chain structure will be unable to explain the properties of benzene. Therefore it cannot be

represented by a open chain structure.

3. Closed chain structure:

a. Benzene adds three molecules of hydrogen on catalytic hydrogenation to give cyclohexane. This

indicates the presence of three double bonds. As the addition product is cyclic compound, benzene also

must have a ring structure.

C6H6 + 3H2 Ni / 1800C C6H12

Cyclohexane

b. Benzene reacts with bromine in presence of ferric bromide catalyst to form monobromobenzene. The

formation of monoproduct indicates that all the six hydrogen atoms in benzene are same.

C6H6 + Br2 FeBr3 C6H5Br + HBr

The above reactions confirm the cyclic structure of benzene and also the presence of three double

bonds.

4. Kekule’s structure: In1865, August Kekule suggested a cyclic planar structure for benzene. According to

him benzene is made up of hexagonal planar ring of six carbon atoms with alternate single and double

bonds. Each carbon is attached to a hydrogen atom with a single bond. It is a structure of 1, 3, 5-

cyclohexatriene.

H

H C H

C C

or

C C

H C H

H

A structure like this should give two ortho isomers of dibromobenzene (given below). In one

isomer, the two bromine atoms should be attached to carbon atoms connected by a double bond and in

the other isomer, the bromine atoms should be attached to the carbon atoms connected by a single

bond. But benzene gives only one ortho product.

Br Br

Br Br

To overcome this drawback, Kekule suggested that benzene is a mixture of two forms, 1 and 2which

are in rapid equilibrium.

Aromatic Hydrocarbons

2

1 2

The main drawbacks of Kekule’s structures are:

a) The normal bond length of carbon-carbon single bond is 154 pm while that of double bond is

134 pm. According to Kekule’s structures, 1 and 2, there should be three single bonds and three double

bonds. In fact, in benzene all the six carbon-carbon bonds are equal with the bond length of 139 pm.

Therefore the concept of equilibrium between two structures is ruled out.

b) Kekule’s structures failed to explain the stability of benzene.

5. Resonance structure: According to valence bond theory the structure of benzenes is explained by the

concept of resonance. Resonance is a phenomenon in which a molecule can be represented by two or

more structures which involves identical position of atoms. The actual structure of the molecule is said

to be a Resonance hybrid of all the possible structures. Each of the individual structure is known as

canonical structures or contributing forms. These canonical structures are not the actual structures and

only exist in theory. According to resonance theory, the actual structure of benzene is a resonance

hybrid of two Kekule’s structures.

≡

1 2 Resonance hybrid

The resonance hybrid is associated with lesser energy and is therefore stable than any of its

contributing forms. The difference in energy between the most stable contributing form and the

resonance hybrid is called resonance energy. Benzene has a resonance energy of 36 kcal/mol. Higher

stability of benzene is attributed to the high value of resonance energy.

6. Molecular orbital theory:

Modern approach to benzene structure is on the basis of M.O. theory. In benzene all the six carbon

atoms are sp2 hybridized. The three hybrid orbitals of each of such a carbon atom are in the same plane

and form two sigma bonds with two other such carbon atoms on either side by coaxial overlap of hybrid

orbitals and one sigma bond with s orbital of hydrogen. The usual valency angle among the sp2 hybrid

orbitals is 1200 which is also the observed angle in benzene.

Formation of σ bonds in benzene


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Sigma bonds of benzene lying in one plane

Also each carbon atom in benzene possesses an unhybridized p-orbital containing one electron.

These p-orbital are perpendicular to the plane of σ-bonds. The lateral overlap of these p-orbitals

produces a molecular orbital containing six electrons. One-half of this molecular orbital lies above

and the other half lies below the plane of the σ-bonds.

Formation of π molecular orbital in benzene

The six electrons of the p orbitals cover all six carbon atoms, and are said to delocalized. As a result

of delocalization is formed a stronger -bond and a more stable molecule. Thermochemical data has

actually shown that stabilization energy of a delocalized molecular orbital, as in benzene. Is 36

kcal/mole compared to the p-orbitals forming three ordinary -bonds as in 1, 3, 5-cyclohehatriene

(Kekule Structure). Thus benzene gives substitution reaction in which the stability of the benzene ring is

retained. There are three ways in which benzene can be represented.

AROMATICITY: It is the property of the sp2 hybridized planar rings in which the p orbitals, one on each atom,

allow cyclic delocalization of electrons.

Criteria for Aromaticity: The following conditions determine whether a molecule is Aromatic or non-aromatic.

1. An aromatic compound is cyclic and planar.

2. Each atom in an aromatic ring has a p orbital. These p orbitals must be parallel so that a continuous

overlap is possible around the ring.

3. The cyclic molecular orbital formed by overlap of p orbitals must contain (4n + 2) electrons, where n

= integer (0, 1, 2, 3, . . .) This is known as Huckel rule.

Determination of Aromaticity for some molecules:

1. Benzene: It is cyclic planar compound. It has a p orbital on each carbon of the ring involved in a double

bond. It has three double bonds and six electrons, which is in accordance with Huckel rule.

4n + 2 =6 or 4n =6-2.

4n = 4 and n =1.


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2. Cycloheptatriene: It is cyclic and planar. It has three double bonds and six electrons. But one of the

carbons is saturated and does not possess a p orbital. Hence a continuous overlap around the ring is not

possible. This compound is therefore non-aromatic.

3. Cyclooctatetraene: It is cyclic and has a p orbital on each atom of the ring. The Huckel rule is not

satisfied, since there are 8 electrons.

4n + 2 = 8 or 4n = 6

i.e. n = 1.5

4. Naphthalene: It is cyclic planar compound. It has a p orbital on each carbon of the ring involved in a

double bond. It has five double bonds and ten electrons, which is in accordance with Huckel rule.

4n + 2 = 10 or 4n = 10 -2.

4n = 8 and n = 2

Resonance energy of Benzene:

Resonance Energy: It is the difference between the actual energy of the molecule and that of the most stable

form of the resonating structures.

Benzene’s special stability is due to the formation of the delocalized molecular orbital. The

magnitude of this extra stability can be estimated by measuring the changes in heat of hydrogenation that are

associated with reactions. Hydrogenation of cyclohexane evolves 28.6 kcal/mole, a value typical for

hydrogenation of alkenes.

+ H2 → + 28.6 kcal

Hydrogenation of both double bonds of 1.3-cyclohexadiene evolves 54.4 kcal/mole, approximately double

the amount observed for cycloheene.

+ H2 → + 55.4 kcal

The molecule of 1, 3, 5-cyclohehatriene (Kekule structure), containing three ordinary double bonds, is

hypothetical, any efforts to produce it yields benzene. It would be expected that the hydrogenation of the

unknown 1, 3, 5-cyclohehatriene to evolve approximately 3 x 28.6 = 85.8 kcal/mole.

+ H2 → + 85.8 kcal

Hypothetical Cyclohexane

1, 3, 5-cyclohehatriene Hydrogenation of benzene gives only 49.8 kcal/mole.

+ H2 → + 49.8 kcal


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The 36 kcal difference between the heat evolved in the hydrogenation of benzene and that estimated for

hydrogenation of a compound with three ordinary double bonds is the added stability. This added stability is

sometimes called ‘Resonance energy’.

ELECTROPHILIC SUBSTITUTION REACTIONS OF BENZENE WITH MECHANISM

1. CHLORINATION OF BENZENE: Benzene reacts with chlorine in presence of anhydrous FeCl3 or AlCl3 at room temperature to form

chlorobenzene. Iron powder can be used in place of ferric chloride.

Cl

+ Cl2 → + HCl

chlorobenzene

Mechanism: The mechanism consists of the following steps:

Step 1. Formation of the electrophile (Cl+).

Cl Cl + FeCl3 → Cl+ + FeCl4-

Step 2. The electrophile attacks the benzene ring to form a carbonium ion.

+ H

Cl

+ Cl+ →

Benzene Carbonium ion

Step 3. Loss of proton yields chlorobenzene.

+ H

Cl Cl

+ FeCl4- → + HCl + FeCl3

Chlorobenzene

2. NIRATION OF BENZENE: Benzene reacts with concentrated nitric acid in the presence of concentrated sulphuric acid at 600C to

form nitrobenzene.

NO2

+ HNO3 H2SO4 + H2O

Nitrobenzene


Step 1. Formation of electrophile (NO2+)

HNO3 + 2H2SO4 → NO2+ + 2HSO4

- + H3O+

Step 2. The electrophile attacks the benzene ring to form a carbonium ion.

+ H


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NO2

+ NO2+ →


Step 3. Loss of proton yields nitrobenzene.

+ H

NO2 NO2

+ HSO4- → + H2SO4

Nitro benzene

3. SULPHONATION OF BENZENE:

Benzene reacts with concentrated sulphuric acid at 1200C or fuming sulphuric acid at room

temperature to give benzene sulphonic acid.

SO3H

+ H2SO4 1200C + H2O


Step 1: Formation of electrophile, sulphur trioxide. In concentrated sulphuric acid, SO3 is produced as

follows. In fuming sulphuric acid, this step is unimportant because the dissolved SO3 reacts directly.

2H2SO4 ⇌ SO3 + H3O+ + HSO4

-

Step 2: The electrophile attacks the benzene ring to form a carbonium ion.

+ H

SO3-

+ SO3 →


Step 3: Loss of proton.

+ H

SO3- SO3

-

+ HSO4- → + H2SO4

Step 4: Addition of Proton gives benzene sulphonic acid.

SO3- SO3H

+ H3O+ → + H2O

Benzene sulphonic acid

4. FRIEDEL-CRAFT’S ALKYLATION:


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Benzene reacts with alkyl halides in the presence of anhydrous aluminium chloride to form alkyl

benzenes.

eg.

CH3

+ CH3Cl anhy. AlCl3 + HCl

Benzene Toluene


Step 1: Formation of electrophile.

CH3 Cl + AlCl3 → CH3+ + AlCl4

-

Step 2: The electrophile attacks the benzene ring to form carbonium ion.

+ H

CH3

+ CH3+ →


Step 3: Loss of proton gives alkyl benzene.

+ H

CH3 CH3

+ AlCl4- → + HCl + AlCl3

Toluene

5. FRIEDEL-CRAFT’S ACYLATION:

Benzene reacts with acid chloride or anhydride in the presence of anhydrous aluminium chloride to

give aromatic ketone.

e.g.

COCH3

+ CH3COCl anhy. AlCl3 + HCl

Benzene Acetophenone


Step 1: Formation of electrophile (CH3CO+).

O

CH3 – C – Cl + AlCl3 → CH3CO+ + AlCl4-

Step 2: The electrophile attacks the benzene ring to form carbonium ion.

+ H

COCH3

+ CH3CO+ →


Step 3: Loss of proton gives aromatic ketone.


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+ H

COCH3 COCH3

+ AlCl4- → + HCl + AlCl3

Acetophenone

DIRECTIVE EFFECTS

Orientation effect or Directive effect: The first substituent present in the benzene ring may direct the next

incoming substituent to ortho, meta or para position, depending on the nature of the first substituent. This is

called Directive or the Orientation effect.

1. Ortho - Para Directing Effect: Certain substituents, in benzene, which direct the second substituent to the

ortho and para position simultaneously. These are called Ortho – Para Director. The ortho and para directing

groups, in general, increase the electron density at these positions and therefore electrophilic substitution

preferentially takes place in these positions. They are, hence, also called as electron donating groups.

e.g. When phenol is nitrated, the only products obtained are o-nitrophenol and p-nitrophenol, and no

meta isomer is formed.

OH OH OH OH

NO2

+ HNO3 H2SO4

NO2 Phenol NO2

o-Nitrophenol (53%) p-Nitrophenol(47%) m-Nitrophenol (0%)

Some common ortho-para directing groups are –Cl, -Br, -I, -CH3, -NH2, -OH.

2. Meta – Directing effect: Certain substituents, present in benzene, which direct the second incoming

substituent primarily to the meta position, are referred as Meta-Directors. The meta directing groups, in

general, decrease the electron density in the ortho and para position and thus indirectly increases the electron

density in the meta position and therefore electrophilic substitution preferentially takes place in meta position.

They are hence, also called as electron withdrawing groups.

e.g. Nitration of Nitrobenzene gives 94% of meta dinitrobenzene and only 5% of ortho and 1% of para

dinitrobenzene.

NO2 NO2 NO2 NO2

NO2

+ HNO3 H2SO4

NO2

Phenol NO2

m-dinitrophenol (94% ) o- dinitropheno (5%) m- dinitrophenol (1%)

Some common meta directing groups are: -NO2, -SO3H, -COOH, -CHO.

Electronic interpretation of orienting influence of electron donating and withdrawing groups:


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1. Ortho-Para Directors: The resonance theory clearly explains why certain substituents are ortho- para

directing.

a) –OH group:

b) –Cl group:

c) –NH2 group:

d) –CH3 group

In ortho and para directing groups with the exception of alkyl groups (like –CH3group) possess atleast

one lone pair of electrons at the atom adjacent (key atom) to the benzene ring. One of the lone pairs is

distributed into the benzene ring by interaction with the π electron system, thus increasing the overall electron

density of the ring. However, the relative increase of electron density is greater at the ortho and para position

due to the nature of the conjugation and hence the electrophilic substitution occurs at these positions.

The only exception to the above rule is the methyl group or alkyl group. It is ortho-para directing, although

it has no non-bonding electron pair on the key atom, carbon. The ortho-para directive influence of CH3 group is

on the basis of a special type of resonance called Hyper conjugation or No Bond Resonance. It postulates the

interaction of σ electrons instead of the usual non-bonding electron pair of the key atom.

2. Meta directors: The meta directing nature of certain groups is explained by resonance theory.


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a) -NO2 group:

b) –SO3H group:

c) –COOH group:

d) –CHO group:

C C C C C

H H H H HO O O OO

In these the key atom of the substituents is bonded to another highly electronegative atom

by a double or triple bond. The electronegative atom pulls the electron pair of the multiple bonds and thus

places a positive charge on the key atom. The key atom in turn withdraws electrons from the benzene ring

causing resonance and thus deactivating the benzene ring. However, the effect is more pronounced at ortho

and para position leaving the meta position as a point of relatively high electron density and is therefore

attacked by electrophiles resulting in meta substitution.

1. Resonance structures of Naphthalene:

2. Resonance structure of Anthracene:


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3. Resonance structure of Phenanthrene:

AROMATIC HYDROCARBONS

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