It is one of the most important electrophilic aromatic substitution reaction. The substitution of nitro group at benzene ring is called nitration. Actually, one of the Hydrogen atoms of benzene ring or aromatic nucleus is replaced with nitro group(-NO2). This reaction takes place as normal practice. because aromatic substrate or benzene is electron rich and nitronium ion generated is the strong electrophile and there occurs normal interaction between them and substitution takes place.
Mechanism of Reaction:
Step 1:- Generation of Electrophile:
In nitration reaction, electrophile used is Nitronium ion(NO2+). Nitronium ion can be generated by using different reagents. It depends upon the reactivity of substrate molecule.
- For simple benzene, alkyl benzene and some less reactive aromatic compounds we use strong nitrating mixture(concentrated nitric acid and concentrated sulphuric acid) for nitration. i.e.
The quality of this nitrating mixture is that it promotes the formation of nitronium ion upon which the entire Nitration process depends. Actually we use two moles of sulphuric acid 1 mole is used in first step for the generation of nitronium ion and second mole is used for promoting the forward reaction. According to Li-chatelier’s principle, the reaction moves in the direction in which it can minimize the stress applied. Since, we are removing water(one of the product) in the form of hydronium ion and thus, reaction moves in forward direction to minimize this stress applied and more nitronium ion is generated and increase the the rate of reaction.
- For active substrates like aniline, phenol, pyrrole etc. strong nitrating mixture cannot be used since it is also a strong oxidizing agent and oxidises the them. i.e.
So, mild nitrating agents are used such as dilute HNO3, HNO3/H2O, HNO3/CH3COOH, HNO3/Acetic anhydride, HNO3/HNO3 for their nitration.
- If anhydros conditions are required, then we use N205 in the presence of non-polar, non-protic solvent(CCl4) and phosphorus pentaoxide (P2O5) which is a dehydrating agent. i.e.
- For polycyclic aromatic compounds harsh conditions cannot be used because they are oxidized at high temperature which is generated in the presence of HNO3/H2SO4. i.e.
So, alternatively we use mild nitrating salts such as
These nitrating salts give good yield at low temperature also.
- For heterocyclic aromatic compound such as pyridine, quinoline, isoquinoline etc. we use N2O5/SO3 as a nitrating agent as it generates nitronium ion and nitration occurs. i.e.
Evidences for Generation of Nitronium ion:
We can determine the the number and type of iron generated in a reaction by using Raman spectroscopy in the Raman spectroscopic analysis of Nitration reaction we get two peaks at 1400cm-1 and 1050cm-1 for nitronium Ion and sulfonate Ion respectively. This gives clear indication for the generation of nitronium ion.
Freezing Point Depression:
Number of of folds of depression in freezing point is directly proportional to the number of ions formed in the reaction. In Nitration reaction freezing point depression is 4 folds which means 4 ions are produced in this reaction. i.e. 1 mole nitronium ion+2 mole sulphonate ion+1 mole hydronium ion=4 ions
X-ray crystallography of nitronium salts such as(nitronium tetrafloro borate, nitronium hexamethyl phoshate, nitronium trifloromethyl sulphonate) indicates the clear presence of nitronium ion in them.
Step 2:- Formation of Arenium ion Intermediate
It is the slowest step of the reaction. So, it is the rate determining step. Actually, aromatic substrates are not so good nucleophilic in nature as compared to simple alkenes so it is slowest step. Firstly, there occurs a simple interaction between nitronium ion and nucleophilic aromatic species and this is is also known as Pi complex. After this, there is formation of sigma bond between electrophile(nitronium ion) and nucleophilic aromatic substrate and it is called arenium ion. Arenium ion is also known as Sigma complex or Whealand intermediate. i.e.
Step 3:- Aromatization:
It is the last step of reaction. Sulfate ion acts as a base and abstracts a proton from arenium ion to retain the aromaticity of aromatic substrate. This step is the driving force of overall Nitration reaction. i.e.
Mono Nitration products of some aromatic compounds:
It depends upon the existence of group at the ring before nitration. This group may be electron donating or electron withdrawing.
- If there is electron donating group already present at benzene ring before nitration we get ortho or para product. For example:
- If there is an electron withdrawing group already present at benzene ring before nitration we get meta product.
- 3. Mono nitration products of some heterocyclic compounds are:
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