Sulfonation of Benzene

Michael Faraday discovered benzene in 1825, and it is a colourless, odourless liquid. C6H6 is the benzene molecule’s molecular formula. The organic compound’s molecular formula reveals that it is extremely unsaturated. It is very reactive because of its high degree of unsaturation. 

As opposed to alkenes, it does not take part in reactions such as addition, oxidation, or reduction. Benzene, for example, does not react with Br, HCl, or other reagents to produce carbon-carbon double bonds.  

Benzene and its derivatives were formerly referred to as aromatic because of their numerous aromas or odours. It wasn’t until later that benzene’s classification was based on its structure and chemical reactivity, rather than its scent. 

This new classification of unsaturated compounds that are particularly stable to alkene-reactive reagents, previously known as aromatic compounds, is now utilised to describe this class of compounds. The aromatic hydrocarbons are now referred to as arene, as opposed to alkane and alkene, by analogy. The parent arene is benzene. Similar to alkyl group compounds, the combination of the aryl group with a new atom or group is indicated as ArX when one hydrogen is removed from the arena. 

Sulfonation of benzene 

It is sulfonation, also known as Sulphonation, that produces sulfonic acids in any one of various ways. An important sulfonation technique is the reaction of aromatic hydrocarbons with sulfuric acid, chlorosulfonic acid, or sulphur trioxide; the reaction of organic halogen compounds with inorganic sulfites; and the oxidation of disulfides and thiols. 

Sulfonation of benzene is the process of forming benzenesulfonic acid by heating benzene with fuming sulphuric acid (H2SO4 + SO3). This reaction is reversible. 

Sulfonation of benzene mechanism 

Electrophilic substitution happens between sulfuric acid and benzene during the sulfonation of benzene. There are two sulfonation methods for benzene. Both methods of sulfonating benzene are equally effective.

At 40°C for several hours, benzene is heated under the reflux of intense fuming sulfuric acid. Benzenesulfonic acid is the end product. Sulphur trioxide (SO3) is the electrophile in this case. Based on the type of acid being utilised, there are two approaches to making sulphur trioxide electrophile. Dissociation of concentrated sulfuric acid with traces of SO3 can yield it. 

Because it is a solution of SO3 in sulfuric acid (H2S2O7). Fuming sulfuric acid, or H2S2O7 is a much richer source of sulfuric acid. Due to its polarity and positive charge on the sulphur atom, sulphur trioxide is electrophilic. This is what attracts the electrons of the ring. 

2H₂SO₄ → H3O+ + SO₃  + HSO4–

Reverse sulfonation of benzene 

Benzene sulfonation is a chemical process that can be undone. In the presence of water, sulphur trioxide quickly produces sulfuric acid and heat. The reaction is reversed by heating benzenesulfonic acid in a solution of sulfuric acid diluted in water. 

It is possible to make deuterated benzene because the sulfonation reaction is so reversible. To better understand reaction pathways, because the C-D bond is more powerful than the C-H bond, isotopically labelled reagents can be helpful. 

Conclusion  

Sulfonation of benzene is the process of forming benzenesulfonic acid by heating benzene with fuming sulphuric acid (H2SO4 + SO3). This reaction is reversible. It can also be used as a directed blocking off group in other substitution processes because it can be easily removed. The sulfonic group protects the carbon against attack by other substituents, and it can be removed after the process is complete by utilising reverse sulfonation. Sulfonamides, which may be used in chemotherapy, are made from benzene sulfonyl chloride.