Nitration is a general chemical process in organic chemistry. During this process, the nitro group is introduced into an organic compound. Here, one or more nitro groups are presented in a reacting molecule called a substrate.
It is misused in various processes between alcohols and nitric acids to form nitrate esters. As a result, the nitrogen present in these nitro compounds is bonded directly to a non-oxygenated atom. In contrast, the nitrogen present in the nitrate esters is bonded to an oxygenated atom.
The process of nitration has many industrial applications. Most importantly, it is used to produce nitro-aromatic compounds (nitrobenzene). These are also used to make explosives. For instance, glycerin can be converted into nitroglycerin by using nitric acid in the process.
Applications of nitrated compounds
The nitrated compounds can be used in the production of the following:
- Solvents
- Explosives
- Dyes
- Pharmaceuticals
- Compounds such as amines
Nitration agents
To make an excellent nitration system, understanding the nitrating agents is essential. The compounds included in the nitration process are as follows:
- Nitric acid, i.e., HNO3(fuming, concentrated, and aqueous)
- Nitrogen pentoxide, i.e., N2O5
- Nitrogen tetroxide, i.e., N2O4
- Mixed acids (Mixtures of nitric acid with sulphuric acid, acetic anhydride, phosphoric acid, chloroform, and acetic acid).
Aromatic nitration
The typical nitration combinations use a mixture of concentrated nitric acid and sulfuric acid to complete the process. The nitronium ion (NO2+) produced in this process is active in aromatic nitration. In the case of nitronium tetrafluoroborate, it is isolated and affects nitration. Here, it doesn’t need mixed acids.
Here, sulfuric acid performs as a catalyst and an absorbent for H2O. Hence, it is impossible to consume it here. While nitrating benzene, the reaction occurs at a temperature that does not exceed 50 °C. This forms the aromatic ring.
The following equation represents the nitration of aromatic compounds:
ArH + HNO3 → NO2 + H2O
Nitronium ion is an electrophilic reactant. Therefore, there is a strong electron density in the carbon atom present in the aromatic ring. Depending on the density, nitro groups can get attached to different positions.
Kinetics of aromatic nitration
The kinetics of the aromatic nitration reaction depends on the reacting mixture. The nitration reaction and its mechanism with different nitrating agents are given below.
Mixed Acids
Here, a mixture of sulphuric acid and nitric acid is formed. In this system, vital (-I) and (-M) components are included. It is a kind of heterogeneous reaction where only liquids react with each other. The reaction process here is slow and steady. For example, nitrobenzene and ethyl benzoate.
Organic solvents
Here, nitromethane or acetic acid mixes with nitric acid to form a mixture. The kinetics of this process depends on the compounds being nitrated. Components with strong deactivating power are nitrated to be proportional to substrate concentration. However, the more reactive components than benzene react with the proportion of an independent substrate.
Aqueous Nitric Acid
When the substrate is highly reactive, it shows zero-order kinetics. However, the lowly-reactive compounds show first-order kinetics.
The nitrous acid causes inhibiting effects in the nitration compounds with no activating groups. This is why the reaction is carried out with strong acids or mixed acids. The nitration compounds with a reactive group experience a catalytic effect. Hence, these compounds can easily be nitrated in a weak nitric acid. Oxy nitration is the last process included in the kinetics of the reaction. It occurs between benzene and 50% nitric acid (with 0.2 molar mercuric nitrate).
Ipso Nitration
Ipso Nitration occurs when the components such as aryl chlorides, triflates, and nonaflates undergo the Ipso substitution. In Ipso Nitration, Y+ is easily formed and changes the prediction of different substitution products. The Ipso Nitration reaction becomes easier as Y+ becomes more stable. Benzene reacts with nitric acid and mercury nitrate to form picric acid in this process. The following equation favours ipso substitution reaction:
Y+ = (CH3)2 C+H, (CH3)3C+
In short, when two substituents share the same ring position in an intermediate compound, it forms Ipso Nitration. Sometimes, this process can also occur in an electrophilic aromatic ring substitution.
The orientation of the benzene ring (with one or more substituents) can be successfully discovered in the following ways:
- At first, the electrophiles attack the methyl bearing carbons (ortho and para dimethyl benzenes). Here, the methyl group’s electron-donating activates the ring by stabilising the intermediate ion.
- Next, the substitution of the ipso carbon occurs indirectly.
- The nitro group changes its position and eliminates a proton for the substitution process.
- As the ipso substitution occurs indirectly in this reaction, the total amount of the final product cannot be predicted.
Ipso substitution and Ortho or Para Ratio
Generally, in the presence of ortho or para directing groups, the ideal substitution product ratio both theoretically and statistically should be 2:1. But, practically, it is impossible. Hence, the ratio is determined based on different factors such as
- The steric effect,
- electronic effect of the substitution,
- effect of temperature and solvent, and
- interaction between substituent and electrophiles. For instance, the ratio decreases when the steric repulsion between the substituent and electrophiles increases.
Conclusion
Nitration Chemistry is studied a lot in the industries. Different methods can undertake the nitration of aromatics, such as heterolytic nitration and radical nitration.
Generally, the process of aromatic nitration is electrophilic. However, nitro-aromatic compounds are also used as substantial intermediates in the syntheses of plastics, insecticides, dyes, explosives, and pharmaceuticals. In addition, nitro-aliphatic compounds are used as solvents in different organic syntheses.
Nitrating mixtures of nitric acid and sulfuric acid combinations are the most commonly used. Hence, nowadays, the nitration of different compounds has become a topic of interest in different industries.
