Sulfonation, of benzene

Benzene reacts with fuming sulfuric add at room temperature to produce benzenesulfonic acid. Fuming sulfuric add is sulfuric add that contains added sulfur trioxide (SO3). Sulfonation also takes place in concentrated sulfuric acid alone, but more slowly. Under either condition, the electrophile appears to be sulfur trioxide. 

In concentrated sulfuric acid, sulfur trioxide is produced in an equilibrium in which H2SO4 acts as both an acid and a base (see step 1 of the following mechanism). 

SO3 is the electrophile that reacts with benzene to form an arenium ion. 

A proton is removed from the arenium ion to form the benzenesulfonate ion. 

The benzenesulfonate ion accepts a proton to become benzenesulfonic acid. 

Electrostatic potential map of sulfur trioxide. The region of greatest positive charge surrounds sulfur. 

The reaction of benzene with sulfuric acid to produce benzenesulfonic acid is reversible and can be driven to completion by several techniques. Removing the water formed in the reaction, for example, allows benzenesulfonic acid to be obtained in virtually quantitative yield. When a solution of sulfur trioxide in sulfuric acid is used as the sulfonating agent, the rate of sulfonation is much faster and the equilibrium is displaced entirely to the side of products, according to the equation 

On being heated with sulfur trioxide in sulfuric acid, 1,2,4,5-tetramethylbenzene was converted to a product of molecular formula C10H14O3S in 94% yield. Suggest a reasonable structure for this product. 

Iron(lll) bromide (FeBra) is also called ferric bromide. 

The reaction of benzene with sulfuric acid to produce benzenesulfonic acid, 

Step 1 The active electrophile in the sulfonation of aromatic compounds is sulfur trioxide and reacts with benzene in the rate-determining step. 

Step 2 A proton is abstracted from the sp -hybridized carbon of the intermediate to restore the aromaticity of the ring. The species shown that abstracts the proton is a hydrogen sulfate ion formed by ionization of sulfuric acid. 

This reaction appears extensively in synthesis problems. Keep this reaction in mind when dealing with any synthesis problem involving an aromatic system. 

Sulfonation is easily reversible. Simply diluting the fuming sulfuric acid leads to the removal of the -SO3H. This is an important synthetic technique for protecting certain sites from reaction. Sulfonation can act as a placeholder while other reactions are performed, and then the easy removal of the sulfonic acid group makes the site available for reaction in a later step in a series of reaction steps. 

Step 1 Attack on the electrophile forms the sigma complex. 

Step 2 Loss of a proton regenerates an aromatic ring. 

Step 3 The sulfonic acid group may become protonated in strong acid. 

Sulfonation is economically important because alkylbenzene sulfonates are widely used as detergents. Sulfonation of an alkylbenzene (R = unbranched C10 — C14) gives an alkylbenzenesulfonic acid, which is neutralized with base to give an alkylbenzene sulfonate detergent. Detergents are covered in more detail in Section 25-4. 

Use resonance forms to show that the dipolar sigma complex shown in the sulfonation of benzene has its positive charge delocalized over three carbon atoms and its negative charge delocalized over three oxygen atoms. 

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Among the variety of electrophilic species present m concentrated sulfuric acid sulfur tnoxide (Figure 12 4) is probably the actual electrophile m aromatic sulfonation We can represent the mechanism of sulfonation of benzene by sulfur tnoxide by the sequence of steps shown m Figure 12 5... 

Phenol was originally recovered during the coking of coal, essentially being a by-product. Eventually, commercial routes were developed based on benzene (from coal or petroleum) for example, sulfonation of benzene to ben-zenesulfonic acid followed by reaction with water to phenol plus regenerated sulfuric acid. Phenol is used to make plastics (phenol-formaldehyde and epoxy resins) and textile fibers (nylon). Phenol is also used in solution as a general disinfectant for cleaning toilets, stables, floors, drains, etc. and is used both internally and externally as a disinfectant for animals. 

Aromatic aminosulfonic acids are synthesized by a sequence of important industrial processes, including sulfonation of benzene. This is followed, wherever necessary, by chlorination, nitration, and reduction, or by aniline sulfonation, possibly involving subsequent baking [7,8]. 

In the sulfonation of benzene, the electrophile, HOSO , is generated from the concentrated sulfuric acid ... 

A historically important method, first used about 1900, is sulfonation of benzene followed by desulfonation with caustic. This is classic aromatic chemistry. In 1924 a chlorination route was discovered. Both the sulfonation and chlorination reactions are good examples of electrophilic aromatic substitution on an aromatic ring. Know the mechanism of these reactions. These routes are no longer used commercially. 

Phenol was prepared before World War I through the distillation of coal tar. The first synthetic process involved the sulfonation of benzene followed by desulfonation with a base. In this process, benzene sulfonic acid is prepared from the reaction of benzene and sulfuric acid ... 

Sulfonation of Benzene and Alkylbenzenes. Since the main utilization of ben-zenesulfonic acid was its transformation to phenol, the importance of the sulfonation of benzene has diminished. The process, however, is still occasionally utilized since it is a simple and economical procedure even on a small scale. Excess sulfuric acid or oleum is used at 110-150°C to produce benzenesulfonic acid.97,102 Sulfonation of toluene under similar conditions yields a mixture of isomeric toluenesul-fonic acids rich in the para isomer. This mixture is transformed directly to cresols by alkali fusion. 

Phenol has been made, over the years, by a variety of processes. Historically, an important method was the sulfonation of benzene followed by desulfonation with caustic soda ... 

The first example is an accident (August, 1970) which occurred during the process of coproducing m-benzenedisulfonic acid and toluenesulfonic acid by the simultaneous sulfonation of benzene and toluene in the same reactor. One person was killed and 6 wounded 9). ... 

Benzene-m>disulfonic acid is used in the preparation of phenol-m-sulfonic acid and of resorcinol. Further sulfonation of benzene-m-disulfonic acid to the trisulfonic acid is accomplished by heating the sodium salt for several hours at about 250°, in the presence of mercury, with 66 per cent oleum. 

Nitration and sulfonation of benzene introduce two different functional groups on an aromatic ring. Nitration is an especially useful reaction because a nitro group can then be reduced to an NH2 group, a common benzene substituent, in a reaction discussed in Section 18.14. 

Sulfonation is a bimolecular electrophilic substitution reaction (SE2) and the electrophile is sulfur trioxide.3a Sulfur trioxide is a powerful electrophile because of the electron-withdrawing effect of the three double-bonded oxygen atoms. Consequently, oleum (fuming sulfuric acid), which contains approximately 10% of excess sulfur trioxide, is a much more powerful sulfonating agent than concentrated sulfuric acid. Sulfur trioxide is a sufficiently powerful electrophile to attack benzene (23) directly. The mechanism of the sulfonation of benzene by hot concentrated sulfuric acid to give benzenesulfonic acid (24) is shown in Scheme 15.4a... 

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