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# Benzenesulfonic Acid

Benzene is an aromatic compound with hexagonal planer structure. There are six carbon atoms in benzene with $sp^2$ hybridization and planer geometry. The pi-bonds are arranged in alternate manner in benzene ring which delocalized the pi-electrons all over six carbons. The delocalization of pi-electrons in benzene ring makes it more stable compare to simple alkenes.

This characteristic of benzene that provides extra stability to molecule is called as aromaticity. We know that alkenes readily give addition reactions because of un-saturation in the molecules. Some common examples of addition reactions of alkenes are hydrogenation, hydration, ozonolysis etc. Unlike alkenes, benzene cannot undergo addition reactions readily but exhibit electrophilic substitution reactions. The extra stability of benzene is due to resonance in molecule.

Due to presence of high electron density in the molecule, an electrophile can easily attack over benzene ring. The electrophile (E+) contains positive charge and attacks on benzene ring to form an arenium ion which is stabilized by resonance. An electrophilic substitution reaction involves the substitution of one of the hydrogen atom of aromatic ring with an electrophile. Last step involves the loss of proton to form the substituted product.

Halogenation, nitration, sulfonation, Friedel craft alkylation and Friedel craft acylation reactions are some common examples of electrophilic substitution reactions of benzene molecule. All these types of electrophilic substitution reactions of benzene involve three steps. First step is formation of electrophile in the presence of some Lewis base. Second step involves the attack of electrophile over aromatic ring to form an intermediate which is stabilized by resonance. This intermediate is called as arenium ion or sigma complex. Last step is the formation of product in the presence of base with elimination of hydrogen ion. Since benzene is a stable aromatic compound, the substitution of hydrogen atom from aromatic ring requires a strong electrophile and also some favorable reaction conditions.

Nitration of benzene occurs in the presence of concentrated nitric acid and concentrated sulfuric acid. It results the formation of nitrobenzene. Here concentrated nitric acid provides nitronium ions with the help concentrated sulfuric acid. Nitronium ion reacts with benzene ring to form an intermediate. This intermediate further reacts with base to form final product.

## Alkyl Benzenesulfonic Acid

As name suggested, alkyl benzenesulfonic acid is an aromatic compound in which one long chain alkyl group and –SO$_3$H group is bonded with benzene ring.

It is synthetic surfactant. Because of low cost and good performance it is the largest volume synthetic surfactant. Since it contains a large straight chain, it is a stable powder and the biodegradable in nature. It is mainly used to produce household detergents such as laundry powders, liquids, dishwashing liquids and other household cleaners. It is also involved in the manufacturing of various substances like emulsifier for agricultural herbicides and in emulsion polymerization. The sulfonic group of molecule exhibits a distinctive tendency to bind the proteins and carbohydrates. Therefore it is an important part of most washable dyes.

## Benzenesulfonic Acid Reactions

The sulfonation reaction of benzene occurs in the presence of concentrated sulfuric acid. Overall reaction can be shown as given below.

Let’s discuss the mechanism of sulfonation of benzene.

The mechanism of sulfonation completes in four steps. First step involves the formation of electrophile that is sulfur trioxide molecule which is formed by the reaction of two molecules of sulfuric acid. Other two products of first step are hydronium ion and bisulphate ion which involve in last two steps. Next step is the reaction of sulfur trioxide molecule with benzene ring to form an intermediate. It is a slow and reversible step. Here intermediate is overall neutral in nature.

Like other electrophilic reactions of benzene, this intermediate also losses proton in the presence of base. Here bisulphate ion acts as base and form benzenesulfonate ion. Later protonation on the presence of $H_3O^+$ ion results the formation of benzenesulfonic acid.

## Benzenesulfonic Acid to Phenol

The conversion of bezenesulfonic acid to phenol is an important synthetic reaction for the preparation of phenols. It is one of the industrial methods for the preparation of phenols.  Benenesulfonic acid reacts with sodium hydroxide (NaOH) at 300-350 $^{\circ}$C results the formation of phenoxide ion which further acidified to form phenol. The reaction follows addition-elimination mechanism. Here $SO_3^{-2}$ acts as the leaving group. The pyrolysis of the sodium salt of benzene sulfonic acid is also known as the Dow process. The reaction involves reaction of benzenesulfonic acid with aqueous sodium hydroxide to form sodium salt of benzenesulfonic acid. Further fusion with solid sodium hydroxide at a high temperature forms sodium phenoxide. Acidification of sodium phenoxide forms phenol.

## Nitration of Benzenesulfonic Acid

The orientation effect of a substituent on benzene ring determines the position of coming electrophile in substitution reactions. The electrophile can attach at ortho, para and meta-position which depends on inductive and mesomeric effect of substituent.

The substituents with positive inductive effect (+I) and positive mesomeric (+M) effect are ortho-para directing groups as coming electrophile attached on ortho and para position of aromatic compound. They are electron donating groups like –$OH$, -$NH_2$, -$CH_3$ etc. It can be explained with the help of resonance structure in which ortho and para-positions will more electronegative compare to meta-position.

Another type of orientation effect is meta-directing group. They are electron withdrawing groups which induced positive charge over ortho and meta-positions therefore coming electrophile will attack over meta-position of aromatic ring.

Some common examples of meta-directing groups are –$SO_3H$, -$COOH$, -$CHO$, -$NO_2$ etc. Let’s discuss one of the examples of such reaction. In benzenesulfonic acid, -$SO_3H$ group is a meta-directing group therefore coming electrophile attacks at meta-position of aromatic ring.

Because of presence of three oxygen atoms in –$SO_3H$ group, it is a strong acidic group which shows –M effect on aromatic ring. The negative mesomeric effect results the depletion of electron cloud over aromatic ring and makes it highly deactivated for electrophilic substitution reactions. Still a strong electrophile can exhibit electrophilic substitution reaction at meta-position because of positive charge over ortho and para position in resonating structures. For example, nitration of benzenesulfonic acid forms 3-nitrobenzene sulfonic acid. Reaction occurs in the presence of concentrated nitric acid and concentrated sulfuric acid at high temperature. Further nitration results the formation of 3, 4-dinitrobenzenesulfonic acid.