Benzene Reactions

Benzene is a colorless liquid that was first discovered by Michael Faraday in 1825. The molecular formula of benzene is C6H6. It is evident from the molecular formula that the organic compound is highly unsaturated. Due to its high degree of unsaturation, it is highly reactive. Unlike alkenes, it never participates in addition, oxidation, and reduction reactions. For example, benzene won't react with Br, HCl or other reagents to result in carbon-carbon double bonds formation. In most of its reaction, benzene undergoes substitution reaction that replaces one or more hydrogen atom with another atom or radical.

Benzene belongs to the category of aromatic compounds. The term aromatic was, in fact, preliminary used to describe benzene and its derivatives due to its multiple aroma or odor. Later the classification of benzene was done based on their structure and chemical reactivity and not on the basis of their aroma. So, now the term aromatic compounds are used to classify those compounds that are extremely unsaturated and peculiarly stable towards reagents that actively react with alkenes.

Nowadays, the term arene is used to refer to the aromatic hydrocarbons, by analogy with alkane and alkene. Benzene is considered as parent arene. Like alkyl group compounds are denoted by the symbol RJ, similarly, if one hydrogen is removed from the arene then the combination of the aryl group with the new atom or group is denoted as ArJ. 

Structure of Benzene 

Substitution reactions are the characteristic reactions of the benzene and it hardly undergoes addition reaction. Benzene is treated with bromine in the presence of ferric chloride as a catalyst then the compound called bromobenzene is formed and that is the compound generated from this product. The reaction is as follows:

But during these years the chemists were not very sure about the structure of the benzene and how the structure could impact the chemical reactivity. It was not until the 1930s that chemists gradually found a general understanding of the unique structure and chemical properties of benzene and its derivatives.

Various Chemical Reactions of Benzene  

1. Halogenation of Benzene

By the means of electrophilic aromatic substitution reaction, one hydrogen atom of the arene is substituted by one halogen atom. The reactions mentioned above belong to the category of halogenation reaction. Here we will try to understand the mechanism of the reaction. This reaction is carried out in the presence of a Lewis acid catalyst. Lewis acid is nothing but an electron pair acceptor and the electrons are essentially nonbonding ones.

2. Nitration of Benzene

In the reaction nitration of benzene, benzene is treated with a mixture of concentrated nitric acid and concentrated sulfuric acid at a temperature which is not more than 50°C. With the increase in temperature, there is are more chances of producing more than one nitro group, -NO₂, that gets substituted onto the ring and results in the formation of Nitrobenzene. The concentrated sulfuric acid acts as a catalyst in this reaction. The "nitronium ion" or the "nitryl cation", NO₂⁺ is the electrophile here. This is produced by the reaction between the nitric acid and the sulphuric acid.

Applications of Nitrobenzene

Nitrobenzene is widely used in manufacturing of the chemical, aniline. It finds its application in lubricating oils used in motors and machines. It is also used in manufacturing of dyes, drugs, pesticides, and synthetic rubber. 

3. Sulfonation of Benzene

Sulfonation of benzene includes an electrophilic substitution reaction that occurs between benzene and sulfuric acid. There are two equivalent ways of sulfonating benzene:

The first way involves heating of benzene under reflux of concentrated fuming sulfuric acid for several hours at 40°C. The product formed is benzenesulfonic acid. The electrophile here is actually sulfur trioxide, SO₃. The sulfur trioxide electrophile can be manufactured in one of the two ways depending on which sort of acid is being used. It can be produced from slight dissociation of concentrated sulfuric acid containing traces of SO₃.

H₂SO₄ ⇌ H₂O + SO₃

Fuming sulfuric acid, H₂S₂O₇, can be considered as a solution of SO₃ in sulfuric acid - and thus it is a much richer source of the SO₃. Sulfur trioxide is electrophilic in nature because it is a highly polar molecule with a fair amount of positive charge on the sulfur atom. It is this that gets attracted to the ring electrons. The reaction that occurs can be shown as:

Applications of Halogenation of Benzene

Benzenesulfonic acid is used as an acid catalyst and to standardise dyes. A variety of pharmaceutical drugs are prepared as benzenesulfonate salts and are known as besilates or besylates. 

4. Alkylation and Acylation of Benzene

This reaction is popularly known as Friedel-Crafts reaction. The reactivity of haloalkanes gradually increases as you move up the periodic table and polarity also increases. This means that the reactivity of an RF haloalkane is maximum followed by the reactivity of RCl then RBr and finally RI. This denotes that the Lewis acids used as catalysts in Friedel-Crafts Alkylation reactions tend to have similar halogen combinations such as BF3, SbCl₅, AlCl₃, SbCl₅, and AlBr₃, all of which are commonly used in these reactions.

In 1877, the below-mentioned procedure was used to produce alkyl halide but was accompanied by some unwanted supplemental activity that hampered its effectiveness.

As a remedy to these limitations, a new and improved reaction was devised: The Friedel-Crafts Acylation, also known as Friedel-Crafts Alkanoylation.

The very first step begins with the formation of the acylium ion that reacts with benzene in the consequent stage. The second step is about the attack of the acylium ion on benzene as a new electrophile that results in one complex structure. The third step involves the removal of the proton in order to ensure that aromaticity returns to benzene. During the third step, AlCl₄ comes back to remove a proton from the benzene ring, thus enabling the ring to return to its aromaticity. In doing so, the original AlCl₃ is regenerated for re-use, along with HCl. Ketone is produced as the first final product of the reaction. This first part of the product is a complex one with aluminum chloride. The final step includes the addition of water to release the final product as the acylbenzene:

Because the acylium ion (as was shown in the first step) is stabilized due to resonance, no rearrangement takes place here (Limitation of this reaction). Also, due to the deactivation of the product, it is no longer prone to electrophilic attack and hence, no longer further reactions will be initiated (another Limitation). However, Friedel-Crafts Acylation may fail with strong deactivating rings.

But this alkylation has a couple of drawbacks. These drawbacks include:

1. There are chances of rearrangements

2. The probability of multiple additions can’t also be ignored

3. This is not applicable for benzenes with multiple electron-withdrawing groups.

To counter these issues, a Friedel Craft Acylation was introduced. This method of acylation solves the first two problems. 

5. Nucleophilic Aromatic Substitution

A nucleophilic aromatic substitution involves the substitution reaction where the nucleophile relocates a strong leaving group, like a halide, on an aromatic ring. This reaction mainly follows either of the two mechanisms:

1. Addition-elimination reaction or

2. Elimination-addition reaction

The basic principle for this reaction can be stated in the manner where the substituted H atoms "leaves" in the form of a proton, and formally the electrons in the C-H bond are "left behind" and there is a need to complete the bonding with the electron deficient electrophile.

When a nucleophile substitutes, fails to do the replacement for a hydrogen, since the electrons are also supposed to "leave" (the nucleophile brings its own electrons in the form of the hydride anion H-) there is a need for a better leaving group that can "take" the electrons, need a conventional leaving group such as halide. Electrons are very poor leaving groups.

Benzylic Position and Its Impact on Benzene Reactivity 

The benzene’s aromaticity is responsible for its resistance towards many of the reactions that alkenes typically can take part. However, chemists have found out ways to react benzene following various other methodologies. We begin our discussion of benzene reactions with processes that occur not on the ring directly, but at the carbon immediately bonded to the benzene ring, more accurately called the benzylic carbon.

The strong oxidizing agents, such as H₂CrO₄ and KMnO₄ can’t even make an impact on benzene. When toluene is treated with these oxidizing agents under extreme conditions, the side-chain methyl group is oxidized to a carboxyl group producing the main byproduct benzoic acid.

The oxidization of the methyl group keeping the aromatic ring unaffected makes it evident that the aromatic ring is extremely stable. Halogen and nitro substituents on an aromatic ring even remain unaffected by these oxidations. For example, chromic acid oxidizes 2-chloro-4-nitrotoluene to produce 2-chloro-4-nitrobenzoic acid. But here as well the nitro and chloro group remain unaffected.

These conditions also allow the oxidation of Ethylbenzene and isopropylbenzene to benzoic acid. Again, the side chain of tert-butylbenzene, devoid of benzylic hydrogen, is unaffected by these oxidizing conditions.

If benzylic hydrogen exists, then the benzylic carbon gets oxidized to a carboxyl group and there is the removal of all other carbons of the side chain. If no benzylic hydrogen is present, as in the case of tert-butylbenzene, then the oxidation of the side chain also does not happen.