benzene

PHARMACEUTICAL ORGANIC CHEMISTRY –2 A PRESENTATION ON

BENZENE AND ITS DERIVATIVES P R E P A R E D B Y : 19 4 11 R 00 4 2[ S A M R EE N F A T I M A ] U N D E R T H E G U I D A N C E O F M R S . FA R H A N A A Z

CONTENTS ❑Structure Of Benzene ❑Orbital Picture ❑Resonance In Benzene ❑Aromatic Characters ❑Huckel's Rule ❑Reactions Of Benzene; Nitration,Sulphonation,Halogenation,Friedel crafts Alkylation and Friedelcrafts Acylation.

STRUCTURE OF BENZENE

Michael faraday in 1825 isolated the pure material of Benzene. ➢ He did some experiments and named it as CARBURETED HYDROGEN. ➢Later, a German Chemist August Kekule suggested that Benzene is made up of hexagon of 6C atoms. ➢

➢The given cyclic structure of benzene containing 3 conjugated

double bonds is referred to as KEKULES FORMULA. SUPPORTING FACTS OF KEKULES FORMULA: ▪ Highly unsaturated compound ▪ Can form straight chain or closed ring ▪ Open chain structure is not suitable ▪ Does not give oxidation or addition product

LIMITATIONS OF KEKULE'S STRUCTURE ❖It predicts that there should be two different 1,2dibromobenzene. ❖It failed to explain why benzene with three double bonds did not give addition reactions like other alkenes.

For example, benzene did not react with HBr or Br2 in CCl4.

ORBITAL STRUCTURE OF BENZENE

The structure of benzene is best described by Modern Molecular Orbital Theory. ➢ All Carbon atoms in benzene are SP2 hybridised. ➢ The three SP2 hybrid orbitals are lying in one plane and oriented at an angle of 120°.

➢ The fourth unhybridized P-orbital having two lobes is lying

perpendicular to the plane of the hybrid orbital. ➢ The lateral overlap of these P-orbitals produces a π molecular orbital containing six electrons.

RESONANCE IN BENZENE

➢ The phenomenon in which two or more structures can be

written for a substance which involves identical positions of atoms is called "RESONANCE". ➢ Kekule's structure 1 and 2 represent the resonance structures.

➢ The resonance structures 1 and 2 are not the actual

structures of benzene molecule. ➢ Spectroscopic measurements show that benzene is planar and all its C-C bonds are of equal length,1.40A°. ❖ Single bond length: 1.54A° ❖ Double bond length: 1.34A° ❖ Benzene bond length: 1.40A° ➢ Resonance hybrid is more stable than any of its Contributing structures.

RESONANCE ENERGY OF BENZENE Resonance energy is a measure of how much more stable a resonance hybrid structure is than its extreme resonance structures. ➢ The delocalized π orbital provides extra stability to benzene.

This extra stability can be estimated by measuring the changes in Heat of Hydrogenation. ❖ Hydrogenation of cyclohexane evolves 28.6 kcal. ❖ Hydrogenation of both double bonds of 1,3-cyclohexadiene evolves 55.4 kcal. ❖ The molecule 1,3,5-cyclohexatriene on hydrogenation yielded benzene, ➢

But the hydrogenation of benzene gives only 49.8 kcal.

The 36 kcal difference between the heat evolved in hydrogenation of benzene to that estimated is called ADDED STABILITY. This added stability is sometimes called RESONANCE ENERGY.

AROMATIC CHARACTERS OF BENZENE

➢ The aromatic characters are:

▪ It should be planar; ▪ It should be cyclic; ▪ It should contain conjugated π bond system; ▪ It should contain alternate double and sinle bonds.

➢The modern theory of aromaticity was given by Erich Huckel in 1931. ➢He proposed the Huckel's Rule for predicting the aromaticity.

According to this rule,

In a cyclic system of overlapping p-orbitals, the no. Of πe- is 4n+2, then the system will have aromatic characters.

Where n= No. Of benzene rings[1,2,3...]

BENZENE

NAPTHALENE ANTHRACENE

4n+2 4n+2 4n+2 = 4[1]+2 = 4[2]+2 = 4[3]+2 =6e=10e=14eSo the above compounds satisfied the Huckel's Rule and therefore these are said to be aromatic.

REACTIONS OF BENZENE

ELECTROPHILIC SUBSTITION REACTIONS ➢

The substitution reactions of aromatic ring caused by the presence of an active species I.e, the Electrophile are referred to as Electrophilic Substitution Reactions.

It takes place in [3] steps: 1.Generation of electrophile 2.Formation of π complex 3.Elimination of proton

NITRATION Benzene reacts with Nitric acid in the presence of Sulphuric acid to give Nitrobenzene.It follows ESR mechanism.

STEP.1 STEP.2 STEP.3

SULPHONATION Benzene reacts with conc.H2SO4 to form Benzene Sulphonic Acid.It follows ESR mechanism.

STEP.1 STEP.2 STEP.3

HALOGENATION Benzene reacts with Halogens [Br-,Cl-,etc.] to form respective Halobenzenes.Let's take chlorine as an example,

STEP.1 STEP.2 STEP.3

FRIEDAL CRAFTS REACTION It was discovered by Charle's Friedal and M.Crafts in 1877.It is of two Types: 1. Friedal Crafts Alkylation 2.Friedal Crafts Acylation 1.FRIEDAL CRAFTS ALKYLATION: This reaction involves introduction of an Alkyl group in the benzene ring for the synthesis of Alkyl benzene.

2. FRIEDAL CRAFTS ACYLATION: This reaction involves the introduction of Acyl group in the aromatic ring in the presence of AlCl3.