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Reactions of Benzene– Electrophilic Substitution Reactions, Halogenation, Nitration, Sulphonation, Friedel-Craft Reactions, Practice Problems & FAQs

Benzene is like the fresh essence that truly enchants the young minds when they first get introduced to organic chemistry. Proudly so, benzene, the simplest aromatic organic hydrocarbon, is very effective at manufacturing several indispensable chemicals that encompass in all ways. Let’s take a look at some really cool applications of benzene where a special reaction of benzene is so significant!

Many components of perfumes have benzene as a key starting point. 2-phenylethanol is one of the most significant benzene-derived fragrance compounds. A significant component of rose oils, phenylethanol is utilised frequently in fragrance for its mixing properties and artificial essence.

In the production of fragrances, 2-phenylethanol esters such as acetate, isobutyrate, and phenylacetate are often employed. Alcohol is converted to an ester to create these.

Benzene serves as the starting point for the manufacturing of a wide range of downstream petrochemicals, including Caprolactam, Linear alkyl benzene (LAB), Styrene, Maleic Anhydride, Insecticides, Pesticides, Phenol, and others. Additionally, benzene is produced as solvents and thinners for the paint industry.

But how do we transform the simplest aromatic hydrocarbon benzene into all of these? I have an answer! Absorb within you all the knowledge about the reactions that benzene undergoes (here on this page!). And you will know the answers too!!

TABLE OF CONTENTS

  • Benzene Reactions - Introduction
  • Chemical Properties of Benzene
  • Electrophilic Substitution Reactions of Benzene (SEAr)
  • Halogenation of Benzene
  • Sulphonation of Benzene
  • Nitration of Benzene
  • Friedel-Craft Alkylation Reaction
  • Friedel-Craft Acylation Reaction
  • Gattermann-Koch Synthesis
  • Gattermann-Aldehyde Reaction
  • Mercuration of Benzene
  • Blanc-Chloromethylation Reaction 
  • Addition Reactions of Benzene 
  • Ozonolysis of Benzene
  • Combustion Reaction of Benzene
  • Benzene with Metal Complexes
  • Practice Problems
  • Frequently Asked Questions–FAQs

Benzene Reactions- Introduction

Benzene is the simplest aromatic hydrocarbon. In essence, benzene is an organic chemical molecule that is colourless but liquid at room temperature. Its chemical structure is C6H6. Six carbon atoms are connected in a planar ring to create the benzene molecule, and one hydrogen atom is joined to each carbon atom. It qualifies as a hydrocarbon because it only has carbon and hydrogen atoms in it. It is inflammable and has a specific characteristic aroma.

Benzene evaporates relatively quickly into the atmosphere and is heavier than air. The benzene vapour as a result sinks into low-lying places. In the water, the liquid benzene barely dissolves, but it floats on top of the water.

The major reactions undergone by aromatic hydrocarbons are: Electrophilic Substitution reactions, Addition reactions and Combustion reactions. 

Benzene is far more responsive when subjected to electrophilic substitution reactions rather than addition reactions. The reason for this is that during the addition process, benzene loses its aromaticity. Due to its set of delocalized electrons covering every carbon atom in the ring, benzene exhibits a high level of electrophile affinity and is also particularly stable to electrophilic substitution.

Chemical Properties of Benzene

  • Benzene is a rather stable compound. Although its molecular formula C6H6, indicates a high degree of unsaturation, it is not as reactive as alkenes or alkynes. It does not show the usual reactions of unsaturated hydrocarbons. 
  • Benzene is an electron-rich species due to its conjugated -electron cloud.
  • Benzene does not form any addition. product with halogen acids and hypochlorous acid. It resists oxidation by alkaline KMnO4 (i.e., no decolourisation) and does not decolourise bromine solution. In many respects benzene behaves like a saturated hydrocarbon as it gives substitution reactions, i.e., hydrogen atoms are replaced by other atoms or groups (benzene is more reactive than alkanes). 
  • Benzene is not affected by alkalies. 
  • General reactions shown by benzene are Electrophilic Aromatic Substitution Reactions, Addition Reactions, Combustion and Oxidation Reactions. 

Electrophilic Substitution Reactions of Benzene (SEAr)

The most frequent reactions of benzene involve the substitution of an H+ of benzene by other groups. The most common way to derivatize benzene is through electrophilic aromatic substitution. Because it is so nucleophilic, benzene can be substituted by alkyl carbocations and acylium ions to produce substituted derivatives.

Electrophilic aromatic substitution reactions are organic reactions in which an atom connected to an aromatic ring is replaced by an electrophile. In these reactions, an electrophile normally takes the place of a hydrogen atom from a benzene ring.

The aromaticity of the aromatic system is preserved by an electrophilic aromatic substitution process.

The electrophilic substitution reaction involves three major steps when benzene is involved. These steps are listed as follows:

  • Generation of the electrophile. In the first step, different electrophiles are generated in different electrophilic substitution reactions. E.g. X+, R+, SO3,NO2+ etc. In the generation of an electrophile, a lewis acid is used. Anhydrous aluminium chloride is a very useful Lewis acid in the generation of electrophiles from aromatic ring chlorination, alkylation, and acylation. Some electrophiles used in the electrophilic aromatic substitution reaction are:
Electrophile(E+) Name Source Name of reaction
Cl+ Chloronium Cl2+AlCl3  Chlorination
Br+ Bromonium Br2+AlBr3  Bromination
NO2+ Nitronium HNO3+H2SO4 Nitration
SO3 Sulphur trioxide Fuming H2SO4 Sulphonation
R+

Alkyl

Carbocation

RX+AlX3

X = Cl, Br

Friedel-Crafts Alkylation
R-CO+

Acyl

Carbocation

RCOCl+AlCl3  Friedel-Crafts Acylation
  • Formation of an Intermediate (arenium ion ) carbocation. Attack of an electrophile results in the formation of arenium ion. This is also known as the sigma () complex or Wheland’s intermediate. George Williard Wheland (American chemist) discovered this intermediate i,e also known as Wheland's intermediate.

  • Removal of proton (H+) from the arenium ion in order to achieve stability. To restore the aromatic character, 𝜎-complex loses H+ from the sp3 hybridised carbon.

The substitution reactions are classified as the characteristic reactions for benzene.

Halogenation of Benzene

Arenes reacts with halogens in the presence of a Lewis acid like anhydrous FeCl3, FeBr3 or AlCl3 to yield haloarenes. 

  • The Lewis acid combines with the attacking reagent to form an electrophile (X+). 
  • Benzene undergoes chlorination when it is treated with chlorine in presence of Lewis catalysts such as AlCl3 or FeCl3 and in absence of light.

  • For example, Electrophile (Cl+) is formed during the first step by the action of Lewis acid AlCl3 on Cl2, which then attacks the benzene ring to form an intermediate carbocation which is resonance stabilised. This formation of the carbocation step is a slow and rate-determining step of halogenation reaction. The mechanism is shown below, consisting of the three basic steps for any aromatic electrophilic substitution reaction:
  1. Generation of Electrophile
  2. Formation of Arenium Ion
  3. Removal of proton from Arenium ion

  • With an excess of halogens in presence of anhydrous AlCl3 (catalyst) and dark, all the hydrogen atoms of the benzene ring may be successively replaced.
  • Similar to this, bromination of benzene can also occur in this manner. During bromination, Br2/FeBr3 is used wherein FeBr3 acts as the Lewis acid responsible for forming the electrophile Br+. Here is the mechanism for Bromination of Benzene: Mechanism follows the same three steps: 
  1. Generation of Electrophile
  2. Formation of Arenium Ion
  3. Removal of proton from Arenium ion

  • Iodination is done only in presence of strong oxidising agents like HNO3. Fluorination can be explosive as F2 reacts so rapidly with benzene that aromatic fluorination requires special conditions and apparatus. Even then, it is difficult to limit the reaction to monofluorination.

C6H6 +3F2 6HF +6C

  • During SEAr halogenation of benzene, the intermediate arenium ion obtained is resonance stabilised and so the reaction is extremely spontaneous.

Sulphonation of Benzene

Benzene and sulfuric acid are mixed in the electrophilic substitution reaction known as the sulfonation of benzene. Benzene reacts with fuming sulfuric acid at room temperature to produce Benzenesulfonic acid. Fuming sulfuric acid is sulfuric acid that contains added sulphur trioxide (SO3).

The mechanism follows the same three steps as any other electrophilic aromatic substitution reaction that are: 

  1. Generation of Electrophile: SO3 is the electrophile generated. Because sulfuric acid is strongly oxidising, the oxygen in sulfuric acid attracts an electron to itself, creating an electrophile. This damages the benzene ring and produces benzenesulfonic acid as a result.

  1. Formation of the arenium ion: Electrophile (SO3) is formed in the first step and attacks the benzene ring to form an intermediate carbocation on which is resonance stabilised. This formation of the carbocation step is a slow and rate-determining step of sulphonation reaction.

  1. Removal of proton from the arenium ion: In the third step, by the attack of nucleophile the sigma complex releases a proton from the sp3 hybridised carbon in order to restore the aromatic character. In the last step, the anion formed in the third step undergoes hydrolysis to yield benzene sulphonic acid as a product.

  • When benzene is heated at a high temperature in the presence of oleum, m-Benzenedisulfonic acid is formed.

  • Sulphonation of benzene can also take place at room temperatures in the presence of BF3 a catalyst.

Nitration of Benzene

When benzene is treated with concentrated HNO3 in the presence of conc.H2SO4, at 40-50C temperature, a hydrogen atom of the benzene ring is replaced by a nitro group, and nitrobenzene is formed as a product. This reaction follows the same electrophilic substitution reaction mechanism.

Mechanism of Nitration:

Step 1: Formation of an electrophile: The electrophile here is NO2+ obtained from conc. Sulphuric and conc. Nitric acid.

Step 2: Formation of arenium ion:

Electrophile (NO2+) is formed in the first step and attacks the benzene ring to form an intermediate carbocation which is resonance stabilised. This formation of the carbocation step is a slow and rate-determining step of sulphonation reaction.

Step 3: Removal of proton and Formation of product: In the third step, by the attack of nucleophile the sigma complex releases a proton from the sp3 hybridised carbon in order to restore the aromatic character, and nitrobenzene is formed as a product.

  • If nitrobenzene is further nitrated or benzene is treated with a nitrating mixture at 90C m-Dinitrobenzene is formed as a product.

  • If benzene is treated with fuming nitric acid and concentrated sulphuric acid, 1,3,5-trinitrobenzene is formed as a product that is used as a powerful explosive.

Friedel-Craft Alkylation Reaction

In the presence of anhydrous aluminium halides, alkyl halides react with benzene to form alkylbenzene.

Friedel-Crafts alkylation of benzene with propylene is the method used to produce cumene for commercial use. 20% of the world's benzene demand is from the producers of cumene.

Friedel-Craft Acylation Reaction

It forms acetophenone when acetyl chloride interacts with benzene in the presence of anhydrous aluminium chloride.

Gattermann-Koch Synthesis

This is the formylation of benzene, also known as Gattermann-Koch synthesis. In this synthesis reaction, the aldehyde group is directly introduced into the benzene ring. When benzene is heated with a mixture of carbon monoxide and dry HCl gas in the presence of anhydrous aluminium chloride benzaldehyde is formed as a product.


Gattermann-Aldehyde Reaction

When benzene is heated with H-CN and hydrochloric acid in the presence of anhydrous aluminium chloride followed by hydrolysis benzaldehyde is formed as a product.

H-C+=NH is used as an electrophile in this reaction.

Mercuration of Benzene 

Benzene on being heated with an alcoholic solution of mercuric acetate at elevated temperature undergoes aromatic electrophilic substitution, wherein a hydrogen atom of the benzene ring is replaced by the acetoxy mercuric acid group and acetoxy mercuric benzene is formed as a product.

This reaction is known as mercuration of benzene and it is used in the preparation of various medicines in the pharmaceutical industry.

Blanc-Chloromethylation Reaction

Chloromethylation of benzene is known as the Blanc reaction. When benzene is heated with formaldehyde and hydrochloric acid in the presence of anhydrous aluminium chloride, a chloromethyl group is introduced into the benzene ring as a result benzyl chloride is formed as a product. This reaction is known as chloromethylation.

Addition Reactions of Benzene

Catalytic Reduction of Benzene:

The addition of 2, 4, or 6 hydrogen atoms to benzene's carbon atoms is known as the hydrogenation of benzene, also known as the hydrogen addition reaction of benzene. When benzene is fully hydrogenated, cyclohexane is created. A helpful solvent and a starting point for the creation of additional compounds, cyclohexane.

The reaction below demonstrates how benzene is successively hydrogenated. A nickel, palladium, or platinum catalyst is used in conjunction with moderate temperatures and pressures to carry out these reactions. not less than 70oC and in between 150oC-190oC:

Benzene is reduced to cyclohexane at high temperature and pressure conditions. It is cyclohexane and its derivatives are created through the hydrogenation of benzene and its derivatives. High hydrogen pressures are used in conjunction with heterogeneous catalysts, such as finely split nickel, to produce this reaction. Alkenes can be hydrogenated at temperatures close to room temperature, whereas benzene and similar chemicals are less willing substrates and need temperatures above 100 °C.

Addition of Halogen:

In the presence of UV light, chlorine and bromine react with benzene through an addition mechanism. Heat and UV light alone is adequate for full halogenation in the absence of a catalyst.

Below is a diagram illustrating the entire addition chlorination & bromination of benzene:

Ozonolysis of Benzene

In ozonolysis, an alkene or alkyne is broken down into organic molecules with a double bond to oxygen in place of the numerous carbon-carbon bonds. Benzene Triozonide is produced as an intermediate during the ozonolysis of benzene to finally produce glyoxal.

Combustion Reaction of Benzene

Complete combustion of benzene produces carbon dioxide and water. Incomplete combustion occurs in a limited supply of oxygen and produces CO.

Here is the reaction of complete combustion of benzene:

C6H6+152O2 → 6CO2+3H2O

Incomplete combustion:

2C6H6+9O2 → 12CO+36O

Benzene with Metal Complexes

In the organometallic chemistry of low-valent metals, benzene makes an effective ligand. The sandwich and half-sandwich complexes, Cr(η6-C6H6)2and [RuCl2(η6-C6H6)]2, are two notable examples.

Recommended Video

https://www.youtube.com/watch?v=8E8rAHRDcy0

https://www.youtube.com/watch?v=8POOptnCX3Q

https://youtu.be/5PzeW2muE38

Practice Problems

Q.1. Why is fluorination of benzene not possible?
Answer:
It is not possible because an explosion takes place when fluorine reacts with benzene as this reaction happens rapidly. Aromatic fluorination requires special conditions and apparatus. Even then, it is difficult to limit the reaction to monofluorination.

Q.2. Direct iodination of benzene is not possible. State the reason.
Answer: 
Because the reaction is reversible and the HI produced therein is a very potent reducing agent that converts iodobenzene back to benzene, direct iodination of benzene is not possible.

C6H6+I2 ⇌ C6H5I+HI

Therefore, for the reaction to proceed, it must be carried out in the presence of a potent oxidising agent such as iodic acid or nitric acid, which oxidises the HI produced in iodine.

6HI+2HNO3 → 4H2O+3I2+2NO

Q.3. The electrophile involved in the nitration of benzene:

  1. NO2-
  2. NO2
  3. NO2+
  4. NO
     

Answer: (C)

Solution: The electrophile used in the nitration of benzene is nitronium ion or nitryl cation (NO2+). It is obtained from the reaction of nitric acid and sulphuric acid. 

HNO3+2H2SO4 → NO2++ H3O++2HSO4-

So, the correct answer is option (C).

Q.4. Benzene reacts with ICl in the presence of lewis acid AlCl3 to give ______ .

  1. C6H5I
  2. C6I6
  3. C6I12
  4. C6H12
     

Answer: (A)

Solution: In the presence of anhydrous aluminium chloride, benzene and iodine monochloride react. Since iodine is less electronegative than chlorine, AlCl3 reacts with ICl to form iodonium ion (I+) as the electrophile which attacks the benzene ring.

ICl +AlCl3 I+ + AlCl4- 

C6H6+I+→C6H5I+H+

So, the correct answer is option (A).

Q.5. During nitration of benzene using concentrated H2SO4 and concentrated HNO3 if we add a large quantity of potassium hydrogen sulphate to the mixture, what shall be the consequence to the rate of the reaction?

  1. Doubled
  2. Faster
  3. Slower
  4. No change in rate of reaction
     

Answer: (C)

Solution: The concentration of HSO4- ions rises when a lot of KHSO4 is added, shifting the process in the opposite direction and slowing down nitration. This is because, the formation of the electrophile NO2+ (which is actually responsible for nitration) will decrease, as the reaction goes toward the backward direction due to the common ion effect.

KHSO4 K+ +HSO4- 

H2SO4+HNO3 ⇌ NO2+ +HSO4-+H2O

So, the correct answer is option (C).

Frequently Asked Questions–FAQs

Q.1 What is the difference between benzene hexachloride and hexachlorobenzene?
Answer: 
Under sunlight (h), when three moles of chlorine combine with one mole of benzene, it produces benzene hexachloride. It is also known as Lindane or hexachlorane.

Whereas, when all of the hydrogen atoms in benzene are substituted for chlorine atoms, it creates the substituted chemical known as hexachlorobenzene. So, it is obtained as a product of electrophilic substitution when benzene reacts with an excess of halogens in presence of anhydrous AlCl3 (catalyst) and in dark.

Q.2. What is the significance of an electrophilic aromatic substitution reaction?
Answer:
Electrophilic aromatic substitution is one of the most significant processes in synthetic organic chemistry. Important intermediates that can be employed as precursors in the development of medicinal, agrochemical, and industrial products are created by these processes.

For example, Derivatives of sulfonated benzene make good detergents. In the process of nitration, nitronium ions (NO2+), a potent electrophile created by mixing sulfuric and nitric acids, react with benzene. Aniline is produced from nitrobenzene.

Q.3. What is a Wheland complex?
Answer: 
The second step of the electrophilic aromatic substitution process produces an intermediate carbocation known as an arenium ion, which is also known as a Wheland complex or a Meisenheimer complex.

Q.4. Is sigma complex/arenium ion aromatic?
Answer: 
Sigma () complex/arenium ion loses its aromatic character because the cyclic delocalisation of electrons stops at the sp3 hybridised carbon. And there are 4𝛑-electrons, which do not follow Huckel’s rule. Hence, the arenium ion is a non-aromatic resonance stabilised carbocation.

Related Topics

Preparation of Benzene Structure of Benzene
Toluene Wurtz reaction
Breaking of Bonds Oxidative Reactions involving Peroxy acids







 

 

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