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 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.
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:
|Electrophile(E+)||Name||Source||Name of reaction|
|SO3||Sulphur trioxide||Fuming H2SO4||Sulphonation|
X = Cl, Br
The substitution reactions are classified as the characteristic reactions for benzene.
Arenes reacts with halogens in the presence of a Lewis acid like anhydrous FeCl3, FeBr3 or AlCl3 to yield haloarenes.
C6H6 +3F2 6HF +6C
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:
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.
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.
It forms acetophenone when acetyl chloride interacts with benzene in the presence of anhydrous aluminium chloride.
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.
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.
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.
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
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.
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:
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.
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
2C6H6+9O2 → 12CO+36O
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.
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:
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 ______ .
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-
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?
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.
|Preparation of Benzene||Structure of Benzene|
|Breaking of Bonds||Oxidative Reactions involving Peroxy acids|