Nitration

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What is Nitration?

In organic chemistry, nitration definition is given as a chemical process class that introduces the nitro group into an organic chemical compound. The term Nitration is also incorrectly applied to different processes forming the nitrate esters among alcohols and nitric acid, which take place in synthesizing the nitroglycerin. The major difference among the final structure of nitro compounds and nitrates is, the nitrogen atom is bonded directly to a non-oxygen atom, mainly to the carbon or other nitrogen atoms. While in the organic nitrates, the nitrogen shares a bond with an oxygen atom, which in turn bonded to a carbon atom.


Electrophilic Substitution Reaction Between the Benzene And Nitric Acid

Benzene is treated with concentrated sulfuric acid solution and concentrated nitric acid at a temperature that is not more than 50° C. When the temperature rises, there exists a higher chance of obtaining more than one nitro group (-NO₂) being substituted in the ring.

Nitrobenzene is formed.

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

or 

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The sulfuric acid, which is concentrated, acts as a catalyst.


The Formation of Electrophile

An electrophile is the nitryl cation, or nitronium ion, NO2+, formed by the sulphuric and nitric acid reaction.


Electrophilic Substitution Mechanism

Stage 1

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Stage 2

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Nitration is the most studied and researched organic reactions. Both the aromatic and aliphatic compounds can be nitrated by different methods such as heterolytic (electrophilic and nucleophilic) and the radical nitrations. Aromatic nitration is the most frequent, and the Aliphatic is a free radical electrophilic. Nitroaromatic compounds are used as intermediates in plant's synthesis, pharmaceuticals, dyestuffs, and insecticides.


Aromatic Nitration

The typical nitration syntheses apply so-called "mixed acid", a mixture of sulfuric acid and concentrated nitric acids. This mixture forms the nitronium ion (NO2+), the active species in the aromatic nitration. This active ingredient that can be isolated in the nitronium tetrafluoroborate case also affects nitration without the need for the mixed acid. In mixed-acid synthesis, the sulfuric acid is not consumed and thus acts as a catalyst and an absorbent for water as well. In the benzene case's nitration, the reaction is conducted at a warm temperature, which not exceeding 50° C. The process is one of the examples of electrophilic aromatic substitution, which involves the attack using the electron-rich benzene ring:

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There are also some alternative mechanisms have been proposed, including the one involving a Single Electron Transfer (SET).


Nitrating Agents

  • Concentrated, Fuming, and aqueous nitric acid 

  • Mixtures of nitric acid with the sulfuric acid, acetic anhydride, acetic acid, chloroform, and phosphoric acid

  • Nitrogen tetroxide, N204

  • Nitrogen pentoxide, N205 

To make an intelligent choice of nitrating system for the specific nitration, it is desirable to know what species exist in the various systems and understand the reaction mechanism under consideration.


Thermodynamics of Nitration

  • Nitration reaction results are highly exothermic.

  • The thermal properties of nitrating acids study are required for an adequate understanding of this unit process.

  • The nitration reaction must be controlled by the systematic cooling designed to withdraw the evolved energy .

  • When the entire energy set free by an exothermic reaction is forced to appear as heat, it's quantity lost to the cooling mechanism equals the decrease in enthalpy Q = - ΔH.

Applications of Nitration

  • Nitration reactions are notably used in the production of explosives such as converting guanidine to nitroguanidine and converting toluene to trinitrotoluene.

  • They are also widely used as chemical intermediates and precursors.

  • Nitration can also add nitrogen to a benzene ring, which can be used in substitution reactions further.

  • The group of nitro acts as a ring deactivator. Having nitrogen present in a ring is much useful because it can be used as a directing group and a masked amino group.

  • The products of aromatic nitrations are more essential intermediates in industrial chemistry.

Engineering Factors For Nitration


Temperature

The temperature affects the below factors in aromatic nitrations.

  • Kinetic rate constant for various chemical steps increases with the temperature.

  • Solubilities present in the acid phase of both the nitrated and uninitiated aromatics increase with temperature probably.

  • Viscosities decrease, and the diffusivity coefficients increase with temperature.

  • Also, the interfacial area changes with the temperature.

  • Equilibrium constants change with a temperature change.

Agitation

The increased degree of agitation tends to promote reactant transfer in the two-phase system, and as a result, increased agitation causes an increased rate of reaction generally. 


Composition 

The composition of the organic phase and acid affect the concentration of the reactants. It also affects the concentration effect of mutual Solubilities of the two phases to some extent. 


Ratio

The ratio of acid to the organic phase is significant relative to the type of emulsion formed.

FAQ (Frequently Asked Questions)

1. Mention about Ipso Nitration?

With aryl chlorides, nonaflates, and triflates, ipso substitution can occur as well in the so-called ipso nitration. In 1971, firstly, the phrase was used by Perrin and Skinner in an investigation into the chloroanisole nitration. In one protocol, the 4-chloro-n-butylbenzene is reacted with sodium nitrite in a t-butanol in the presence of 0.5 mol% Pd2(dba)3 phase-transfer catalyst and a biaryl phosphine ligand to 4-nitro-n-butylbenzene.

2. What are the Differences Between Nitrites and Nitrates?

  • Nitrates are the salts with a negative ion, called nitrate.

  • Nitrites are the salts with a negative ion, called nitrite.

  • Nitrate ion has the formula, NO3(-)

  • Nitrite ion has the formula NO2(-)

  • For example, Ca(NO3)2 (calcium nitrate) and NaNO3 (sodium nitrate) both are nitrates.

  • And, Ca(NO2)2 (calcium nitrate) and NaNO2 (sodium nitrite) both are nitrites.