Nitration and Sulphonation

Nitration and Sulphonation are both types of electrophilic substitution reactions. When compared to addition reactions, benzene is more susceptible to electrophilic substitution interactions because it sheds its aromaticity during the addition process. Electrophiles are drawn to benzene since it has delocalized electrons which span the ring’s carbon atoms. It is also extremely stable to electrophilic replacements.

Electrophilic aromatic substitutions include nitration and sulfonation of benzene. The electrophiles are nitronium ion (NO2+) and sulphur trioxide (SO3), which combine with benzene to produce nitrobenzene and benzene sulphonic acid, respectively. The electrophilic substitution process of benzene seems to be a three-step procedure that begins with the generation of the electrophile. A proton is removed from a carbocation intermediate.

Michael Faraday discovered benzene in 1825, and benzene is a colourless liquid. The organic molecule is extremely unsaturated, as seen by the chemical formula. It is very reactive due to the high degree of unsaturation. It never engages in addition, oxidation, or reduction processes, unlike alkenes.

Nitration

Other than aromatic rings, nitrating functional groups produce other fairly explosive compounds. For example, nitro-glycerine is produced by nitration of triol glycerol, which has three good releasing groups (ONO2; precisely, they are “nitrate” groups, not nitric groups). The tiniest disruption to this liquid sets off a chain reaction that produces a large amount of hot gas quickly. 6 moles of Nitrogen, 12 moles of Carbon, 10 moles of Pure water, & 7 moles of Oxygen gas are produced by 4 moles of fluid nitro-glycerine.

Furthermore, nitration on cellulose (the polymer of sugar glucose) creates ‘nitrocellulose’, which is a polymer utilised for film stock in the initial periods of the film industry, in addition to being employed in smokeless powder. Although ‘Celluloid’ as it was known, is not a direct explosive, it does have the terrible trait of spontaneous combustion.

Nitration of benzene

In the presence of sulphuric acid, benzene combines with concentrated nitric acid to generate nitrobenzene at 323-333K.  A nitronium ion is formed when sulfuric acid protonates nitric acid, resulting in the loss of a molecule of water and the production of a nitronium ion.

Nitric acid absorbs a proton via sulphuric acid before dissociating to generate the nitronium ion. In the process, the nitronium ion functions as an electrophile, reacting with benzene to generate the arenium ion. The proton of the arenium ion is subsequently lost to the Lewis base, resulting in the formation of nitrobenzene.

Benzene nitration yields pretty steady nitro compounds which are far more challenging to explode than nitration of alcohols. Triple nitration of toluene, for example, produces the high-explosive TNT (2,4,6-trinitrotoluene). RDX is made by the nitration of trihydric-1,3,5-triazine, another explosive. 

The protonation of OH upon nitric acid, which transforms it to H2O, is essential to the process. Because H2O is a far stronger acceptor than HO-, it is rapidly removed from nitric acid, resulting in the extremely reactive “nitronium ion,” NO2+.

Applications of Nitration

Nitrogen is added towards a benzene ring by nitration, which may then be employed in substitution processes. The nitro group deactivates the ring. The presence of nitrogen in a ring is advantageous since it may serve as both a directing and a concealed amino group. In industrial chemistry, aromatic nitration products are particularly significant intermediates. Nitrobenzene is commonly used in the production of aniline, a chemical. It’s utilised in lubricating lubricants for motors and machinery. It’s also used to make colours, pharmaceuticals, insecticides, and synthetic rubber.

Sulphonation

Electrophilic aromatic substitution can also be used to introduce the sulfonyl group, SO3H, to an aromatic ring. The electrophilic reagent in this example is sulphur trioxide, SO3 (a gas), which may be introduced into the solvent by bubbling. SO3 isn’t extremely reactive to aromatic rings on its own, but as we’ve seen, adding an acid boosts electrophilicity (& reaction rate) dramatically.

Sulphur trioxide, like nitric acid, is “activated” by adding a proton to sulfuric acid. [Note: this mixture of SO3 & H2SO4 is known as “fuming sulphuric acid,” or “oleum” if you wish to use old-school terminology.] In the rate-determining phase, the aromatic ring attacks a highly electrophilic SO3H+ to produce the carbocation intermediate, creating C-S and breaking C-C (pi). The C–H bond is subsequently deprotonated with a weak base, as with other electrophilic aromatic substitutions, to regenerate the C–C pi link and restore aromaticity, yielding the sulphonic acid product. 

Sulphonation of Benzene 

Sulphonation of benzene is the process of converting benzene to benzenesulfonic acid by heating it with boiling sulphuric acid (H2SO4 +SO3). In nature, the reaction is reversible.

Sulphur trioxide and fuming sulphuric acid are used to make benzene sulphonic acid from benzene. An intense solution of dissolved sulphur trioxide with sulfuric acid is known as fuming sulphuric acid, or oleum. Because oxygen is relatively electronegative, the oxygens in sulphur trioxide draw electrons away from it, making the sulphur electrophilic. To make benzenesulfonic acid, benzene attacks sulphur (and additional proton transfers occur).

Benzene sulfonation is a reversible process. When sulphur trioxide combines with water, sulfuric acid & heat are produced. As a result, heating benzenesulfonic acid with dilute aqueous sulphuric acid reverses the process.

Application of Sulphonation 

Benzenesulfonic acids can also be used to make detergents, pigments, and sulfa medications. Sulphonamides, which are employed in chemotherapy, are made from benzene sulphonic chloride. Benzenesulfonic acid is being used to standardise colours and as an acid catalyst. Basilides or besylates are benzene sulfonate salts that are used to make a range of medicinal medicines.

Conclusion 

If you compare reactions, you’ll notice that the main element for electrophilic aromatic substitution remains the same save for the electrophile’s identity. The way the electrophile is triggered, whether by Lewis’s acid treatment (chlorination, bromination) or Bronsted acid catalysis, is what gives each reaction its distinct taste (nitration, sulfonation).