Benzene Reactions - Sulfonation, Nitration and Halogenation

Think about the bright colors of dyes on fabrics, perfume odor, or highly energized explosives used in construction. All of them fit under one general title: all of them are products of benzene chemistry. Though common, the aromatic hydrocarbon represented by benzene actually forms a foundation stone behind several chemical reactions that result in the synthesis of various compounds one encounters in life. The understanding of the various changes that benzene can undergo is tantamount to appreciating its modern chemistry role. Of these several reactions, sulfonation, nitration, and halogenation have emerged as prominent reactions due to their importance industrially and further applicability generally. Sulfonation of benzene is the process of the introduction of a sulfonic acid group into the ring. This process is conducted as a step in detergent, dye, and sulfa drug synthesis. Nitration is a process intended to introduce nitro groups into benzene, mainly in the production of such explosives as TNT and in the synthesis of important chemicals in the pharmaceutical and dye industries. In the formation of a great variety of aromatic halides applied in many chemical industries, halogenation introduces the halogen atoms into the benzene ring at its core.

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This Story also Contains

1. General Concept and Definitions
2. Detailed Analysis of Reactions
3. Summary

General Concept and Definitions

The unique molecular structure and electronic configuration in benzene, a hexagonal ring with delocalized π electrons, resulting in so many substitution reactions where hydrogen atoms are substituted by different functional groups. These reactions are made easy by the aromatic nature of benzene because it is able to build up sufficient stability to the intermediate produced in the reaction process.

Benzene undergoes chlorination when it is treated with chlorine in the presence of a Lewis catalyst such as $\mathrm{AlCl}_3$ or Fe or $\mathrm{FeCl}_3$ and in the absence of light.

For example:

Mechanism
The mechanism of this reaction follows two-step:

NOTE: The hydrogen is removed by the AlCl⁻₄ ion which was formed in the first stage. The aluminum chloride catalyst is re-generated in this second stage.

Nitration of Benzene: The nitro group, -NO₂ is inserted into the ring. This is done with a mixture of concentrated nitric acid and sulfuric acid. In this, the sulfuric acid will act as a catalyst to generate nitronium ion NO₂⁺; then this becomes the active electrophile that attacks the benzene ring.

Sulfonation of Benzene is a method of putting a sulfonyl group$\left(-\mathrm{SO}_3 \mathrm{H}\right)$ into the benzene ring. This reaction consists of fuming sulfuric acid, which contains sulfur trioxide, $\mathrm{SO}_3$. The $\mathrm{SO}_3$ acts as the electrophile which reacts with the benzene to form benzenesulfonic acid.

Nitration
Benzene undergoes nitration when treated with concentrated nitric acid in the presence of concentrated sulphuric acid, i.e, nitrobenzene is formed. The reaction is carried out at 313-323 K when one of the H atoms from the benzene ring is replaced by the nitro group.

For example:

Sulphonation
Benzene forms benzene sulphonic acid with hot concentrated sulphuric acid or with fuming sulphuric acid (oleum). The attacking electrophile in the reaction is Sulphur trioxide SO₃

For example:

Detailed Analysis of Reactions

Nitration

The mechanism of the nitration reaction starts with the formation of nitronium ion:${ }^{\mathrm{TM}} \mathrm{HNO}_3+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{NO}_2^{+}+\mathrm{HSO}_4^{-}+\mathrm{H}_2 \mathrm{O}$

Next, the nitronium ion attacks the benzene ring to form a resonance-stabilized carbocation intermediate. This promptly loses a proton to re-establish the aromatic ring, to form nitrobenzene.

Example:$\mathrm{npCHH}_6+\mathrm{NO}_2^{+} \rightarrow \mathrm{CaH}_5 \mathrm{NO}_2+\mathrm{H}^{+}]$

Sulfonation

Sulfonation is a process in which sulfur trioxide reacts with benzene as follows:$\mathrm{nSO}_3+\mathrm{C}_6 \mathrm{H}_6 \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{SO}_3 \mathrm{H}$

Here, an electrophilic SO₃ molecule attacks the benzene ring to form a sulfonium ion intermediate that further reacts with water to obtain benzenesulfonic acid. All these reactions demonstrate the power of benzene as a substrate and how efficient these methods are in installing functional groups into the aromatic ring.

Applications and Relevance

The practical applications of nitration and sulfonation of benzene are very huge and critical in various industries. Nitration processes are utilized in the manufacture of such explosives as TNT and in the production of fine pharmaceuticals and dyes. Nitrobenzene, for example, is applied as a precursor to aniline, a component in the manufacture of polyurethane and other important industrial chemicals.

On the other hand, sulfonation in detergent, dye, and sulfa drug manufacturing is quite important. Sulfonic acid and its derivatives are surfactants. They raise washability by developing the interaction of the product with water and oils. Furthermore, sulfonation is also one of the significant processes in synthesizing a wide range of organic compounds used in medical chemistry.

These reactions are very important in the academic aspect of research, especially in the understanding by students and professionals alike in the field of Organic Chemistry. They are very good examples of electrophilic substitution reactions on the aromatic ring, very clearly showing the general principles for reaction mechanisms, reactivity, and substituent effects on an aromatic ring.

Recommended topic video on(Benzene Reactions - Sulfonation, Nitration and Halogenation)

Some Solved Examples

Example 1
Question: Benzyl chloride is formed by treating toluene with Cl\(_2\) in which condition?

1. Presence of light
2. Absence of light
3. Treating benzene with anhydrous $AlCl(_3)$
4. Treating benzene with $As(_2)S(_3)$

Solution:
Benzyl chloride is formed when toluene reacts with chlorine in the presence of light. The presence of light facilitates the chlorination at the benzylic position.

Therefore, option (1) is correct.

Example 2
Question: Which of the following properties is not shown by NO?

1) (correct)It is diamagnetic in a gaseous state

2)It is a neutral oxide

3)It combines with oxygen to form nitrogen dioxide

4)It's bond order is 2.5

Solution

As we have learned,

Magnetic behavior of molecule -

If all the molecular orbitals in a molecule are doubly occupied, the substance is diamagnetic

- wherein

However, if one or more molecular orbital are singly occupied it is paramagnetic e.g. O₂

NO in the gaseous state is paramagnetic due to the presence of unpaired electrons in its $\pi^*-$ orbital

Hence, the answer is the option (1).

Therefore, option (3) is correct.

Example 3
Question: The reaction of toluene with $Cl(_2)$ in the presence of $FeCl(_3)$ gives predominantly:

1. m-chlorobenzene
2. benzoyl chloride
3. benzyl chloride
4. o- and p-chlorotoluene

Solution:
In the presence of$\mathrm{FeCl}(3)$ a Lewis acid, toluene undergoes chlorination primarily at the ortho and para positions relative to the methyl group. This is due to the activating effect of the methyl group, which directs the incoming chlorine atoms to the ortho and para positions.

Therefore, option (4) is correct.

Summary

Nitration and sulfonation form the basis of some very important transformations in the chemistry of benzene. Such are those that eventually lead to the production of a great many industrial and pharmaceutical products. In particular, nitration introduces the nitro group into the ring, of paramount importance for the production of explosives and dyes. Sulfonation introduces the sulfonyl group, very important in detergents and in the manufacture of drugs. These reactions not only indicate the reactivity of the benzene ring but also tell us that electrophilic substitution on the aromatic ring is important to both industries and, in a broader circle, chemical studies. If understood, they show potential to shed volumes of useful insight into the chemical behavior of aromatic compounds and their place in everyday life. Only some of the examples concerning the colors of synthetic dyes or components of pharmaceuticals are cases where the transformations of benzene lie at the heart of much of modern chemistry and industry.