JEE Advanced 2023 Revision Notes for Reactions of Benzene

Aromatic Compounds

Aroma-pleasant odour-burns with sooty flames, with Benzene in the aromatic compounds are called Benzenoid compounds.

• Without benzene ring-non benzenoid compounds.

• Aromatic compounds with Heteroatom like B, N, O, S, etc are called heterocyclic compounds.

• Benzenoid compounds include benzene and its derivatives and polynuclear hydrocarbons such as naphthalene, anthracene, biphenyl, etc.

Structure of Benzene:

The stability and the structure of benzene have been explained on the basis of two modern theories.

1. Resonance or valence bond theory.

2. Molecular orbital theory.

Huckel’s Rule:

According to Huckel’s rule, monocyclic, Planar conjugated systems with $\left( {4n + 2} \right)\pi {e^ - }$ are aromatic those with ${\text{4n} \pi }{{\text{e}}^{\text{ - }}}$ are Anti-Aromatic, n=integer.

Reaction of Benzene: 

Benzene preferably undergoes electrophilic substitution

Mechanism of Aromatic Electrophilic Substitution (EAS)

Aromatic Electrophilic Substitution.

All electrophilic substitution reaction follows similar mechanism. The reaction was found to take place in three steps.

1. Generation of electrophile.

$Nu - E \to N{u^ - } + {E^ + }$

2. Formation of the carbocation.

Formation of Carbocation

The carbocation formed in the above structure is stabilized due to resonance.

3. Removal of Proton

The carbocation loses a proton from $s{p^3}$ hybridized carbon to regain its aromatic character.

Removal of Proton

Energy Level Diagram from EAS

• The number of transition states in the process determines the Energy level diagram.

• Electrophilic aromatic substitution has two steps (attack of electrophile, and deprotonation) which each have their own transition state. There is also a carbocation intermediate. This means that we should have a “double-humped” reaction energy diagram.

• Second, the relative heights of the “peaks” should reflect the rate-limiting step.

• The attack on the (step 1) electrophile is the rate determining step.

• So, it is represented with high jump in energy level diagram by representing higher activation energy.

Energy Level Diagram

Halogenation:

• Electrophile is halonium ion $\left( {{X^ + }} \right)$

• Order of reactivity of halogen is ${F_2} > C{l_2} > B{r_2} > {I_2}$.

• Fluorination is not carried out directly as ${F_2}$ is highly reactive. Fluorination is carried out indirectly by reactive. Fluorination is carried out indirectly by the decomposition of benzene diazonium fluoroborate.

Halogenation 1

Chlorination and bromination may be carried out by $\frac{{{{\text{X}}_{\text{2}}}}}{{{\text{halogen carrier}}}}$ (Lewis’s acid, Fe-dust, ${I_2}$etc.),

${\text{HOCl Or HOBr,B}}{{\text{r}}_{\text{2}}}{\text{/}}{\left( {{\text{C}}{{\text{H}}_{\text{3}}}{\text{COO}}} \right)_{\text{3}}}Ti,C{l_2}O/{H_2}S{O_4}$, N-chloro or N-bromosuccinamide.

Halogenation 2

Nitration:

• Benzene, when reacted with concentrated nitric acid at 323-333 K in the presence of concentrated sulphuric acid form nitrobenzene.

• This reaction is known as nitration of benzene.

Nitration of Nitrobenzene

On heating the reaction mixture to about 90-100℃, m-dinitro benzene is obtained.

Nitration of Nitrobenzene 2

• If fuming nitric acid is used along with Conc.${H_2}S{O_4}$at above 100℃ 1,3,5-tri nitro benzene is formed.

Nitration of nitrobenzene-3

Sulphonation:

• Benzene undergoes sulphonation with the following reagents.

Sulfonation of Benzene

• The attacking electrophilic substitutions, sulphuric acid i.e., ${H_2}S{O_4} + S{O_3}$.

Addition Reaction:

Benzene undergoes an addition reaction under special conditions.

Hydrogenation:

• At high temperature and pressure, in the presence of finely divided nickel, benzene undergoes hydrogenation to give cyclohexane.

Hydrogenation of Benzene

In the presence of Pt, a reaction occurs at room temperature.

Addition of Chlorine

• Benzene reacts with chlorine in the presence of sunlight to give benzene hexachloride (BHC) or hexachlorocyclohexane or Gammaxene or 666 or Lindane.

• Benzene reacts with chlorine in the presence of sunlight and in the absence of sunlight or ultraviolet light and in the absence of halogen carriers (such as only $FeC{l_3}$ and $AlC{l_3}$ etc) to produce crystalline hexachloride and hexabromide respectively.

Hydrogenation of Benzene 1

• BHC is a powerful insecticide.

Birch Reduction

Brich Reduction

Oxidation:

Ozonolysis

• One mole of benzene reacts with three moles of ozone to give a triozonide, which on hydrolysis in presence of zinc gives three moles of glyoxal. 

Ozonolysis of Benzene

• Zn dust destroys ${H_2}{O_2}$ which may otherwise oxidize glyoxal to oxalic acid.

Ozonolysis of Benzene

The above addition reaction of benzene proves the presence of three double bonds in benzene but they are different from aliphatic double bonds in the following reaction:

• Benzene does not decolorize $\frac{{B{r_2}}}{{{H_2}O}}$ or cold $alk.KMn{O_4}$.

• Benzene does not give additional reaction with HX or HOX.

Combustion:

${C_6}{H_6} + \frac{{15}}{2}{O_2} \to 6C{O_2} + 3{H_2}O$

Oxidation with $KMn{O_4}$

Oxidation with $KMn{O_4}$

Note: Alkyl benzenes with at least one benzylic hydrogen on oxidation with $KMn{O_4}$ give benzoic acids.

Uses of Benzene:

• It is used as a solvent for fats and resins.

• It is used in dry-cleaning.

• It is used in the synthesis of phenol, styrene, aniline, and insecticide like BHC.

• It is used as a motor fuel.

Importance of Reactions of Benzene

This chapter focuses on explaining the chemical properties of benzene with the help of different chemical reactions. For example, it describes how a molecule of benzene behaves in electrophilic substitution reactions. In fact, it also covers how benzene is less prone to addition reactions.

The presence of delocalized electrons is discussed with proper illustration and explanation. Due to the presence of these unique sets of electrons distributed equally among the carbon atoms in a benzene ring, it can create an electrophile very easily.

All the reactions of benzene will be described in three steps. They are:

• The formation of an electrophile

• An intermediate formation of carbocation

• The process of removal of a proton from this intermediate carbocation

The mechanism of the electrophilic substitution reactions has been explained using pictorial Illustrations of the reaction steps. The use of various reagents and intermediate products will be explained with diagrams and elaborated reaction steps.

The related reactions are described to explain the mechanism elaborately. Students will understand how electrophilic substitution reactions occur in organic chemistry and can answer questions accordingly.

Benefits of Reactions of Benzene JEE Advanced Notes PDF

• These revision notes are designed by the experts as per the previous years’ JEE Advanced question papers. They have simplified the crucial concepts of this chapter so that every aspirant can easily comprehend them in no time.

• The time taken to learn this chapter will reduce considerably with the help of these revision notes. It will help aspirants to understand and correlate the mechanism explained in this chapter with the questions asked. Hence, you will be able to answer questions accurately and score well in the exam.

• The Reactions of Benzene JEE Advanced notes will help you revise the topics covered in this chapter quickly.

• These notes will help you solve the sample questions for this chapter effectively in the JEE Advanced exam.

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