When benzene is heated with concentrated sulfuric acid ( H2SO4) and concentrated nitric (HNO3) acid, nitrobenzene is given as the product. A hydrogen atom of benzene is replaced by a nitro group (-NO2) in this reaction.
In this tutorial, we will learn followings.
- Benzene nitration and characteristics of the reaction
- Nitration at different temperatures
- Mechanism of benzene nitration and uses of benzene nitration in chemical industries
- Nitration of other benzene compounds
Nitration of benzene
The replacement of a hydrogen atom in the benzene ring by a nitro group (-NO2) is called nitration. Benzene is heated with concentrated nitric acid and sulfuric acid about 330K temperature to obtain nitrobenzene. Water is given as a by-product.
This reaction is a very dangerous reaction because benzene nitration is an exothermic reaction (reaction releases large heat to the outside environment. . Heat of the reaction is -117 kJ/mol.
Concentrated H2SO4 and HNO3 acids mixture is called as a nitrating mixture.
Here we study some characteristics of nitrobenzene.
- A yellowish, oily, aromatic nitro-compound with an almond-like odor with C6H5NO2 molecular formula.
- -NO2 is a deactivating group
- Nitrobenzene is used to prepare aniline.
Concentrated H2SO4 and HNO3 acids
Concentrated H2SO4 acid behaves as a proton donator and HNO3 acid accepts that electron. After the reaction, H2SO4 is generated again to show catalyst properties.
- Sulfuric acid shows acidic properties while nitric acid behaves as a base.
Benzene nitration at different temperatures
At higher temperature, there is a chance to give different substitutes of nitro (can join more than two -NO2 groups).
Nitrobenzene is a deactivator. So furthermore nitration is hard. So we have to heat the nitration mixture more than benzene nitration. For nitration of nitrobenzene, temperature should be around, 650C.
Mechanism of benzene nitration
Nitronium ion (NO2+) is formed as first step in the mechanism, this formation is facilitated by the presence of concentrated sulfuric acid which behaves as a catalyst. (NO2+) is a electrophile.
The electrophile attacks benzene ring to form an intermediate carbocation.
Bisulfite ion ( HSO4-) takes a proton from intermediate carbocation (σ complex) to give nitrobenzene.
Uses of benzene nitration
Major use of nitrobenzene is production of aniline. Also nitrobenzene is used as a solvent in the laboratory.
Nitration of benzene compounds
Other benzene compounds such as toluene, aniline, phenol and more can be nitrated. But conditions may vary with given compounds.
Example: Nitration of phenol is much easier than nitration of toluene because phenol is a strong activator while toluene is a weak activator.
Nitration with dilute HNO3
Which benzene substitutes will be nitrated with dilute HNO3? This means , there are some benzene substituted compounds, which shows nitration without concentrated H2SO4 acid as a catalyst.
Nitration of activators
Nitration of activators such as phenol or aniline is easier than benzene and de-activators. Activators can be nitrated with dilute HNO3 acid. Concentrated H2SO4 acid is not required to catalysis the reaction.
Aniline and dilute HNO3
A product mixture is given as the product. Aniline is an ortho para director. So nitration can be occurred at ortho or para positions with dilute HNO3 acids.
As ortho product, 2-nitroaniline is given. As para product, 4-nitroaniline is given.
Phenol and dilute HNO3
A product mixture (ortho and para product) is given as the product. Phenol is an ortho para director. So nitration can be occurred at ortho or para positions with dilute HNO3 acids.
As ortho product, 2-nitrophenol is given. As para product, 4-nitrophenol is given.
Uses of nitration
- Benzene nitration is useful to prepare more organic compounds such as aniline, phenol, benzene diazonium salt. So nitrobenzene is used as an intermediate chemical in preparations of those compounds.
C6H6 + HNO3 (con H2SO4
>C6H6 and HNO3 are the raw materials of benzene nitration. con H2SO4 is used as a catalyst in this reaction. Also, reactant mixture should be heated to occur the nitration.
What happens when benzene is treated with concentrated H2SO4 at 330k?
The sulphonation of benzene can be done when benzene is treated with concentrated H2SO4 acid and heat the mixture.
C6H6 (benzene) + conc. H2SO4 → C6H5-SO3H
Nitration of nitrobenzene
Nitrobenzene is a deactivator. So electron density of benzene ring of nitrobenzene is reduced than benzene. This results, ability of showing electrophilic substitution reactions of benzene decreases. Therefore substituting one more -NO2 group to nitrebenzene is not easy like nitration of benzene. For the nitration of nitrobenzene, higher temperature than used in the benzene nitration is required.
Reaction of benzene with h2so4 and hno3 to form nitrobenzene, why h2so4 is used?
H2SO4 is used as a catalyst to increase the reaction rate by produce nitronium ion (NO2+). When nitronium ion prouction rate increases, reaction rate increases to give the product in a short time.
C6H6 (benzene) + H2SO4 + HNO3 , what is this reaction and what are the prouducts?
This reaction is nitration of benzene. A -NO2 group is substituted to benzene ring instead of hydrogen atom.
H2SO4 is used in concentrated state and it behaves as an catalyst in the reaction.
What will happen to anhydrous AlCl3, after the reaction?
The amount (mol) of anhydrous AlCl3 is not changed in the reaction because AlCl3 is a catalysy in this reaction.
In the benzene nitration, is it an oxidation – reduction reaction?
If you want to know whether this is an oxidation – reduction reaction, you have to find oxidation numbers of each element in the right side and left side of the reaction.
Oxidation number of carbon which is joint with nitrogen atom is oxidized. With that, nitrogen atom was in the nitric acid is reduced.
toluene reaction with h2so4 ?
Tolune cannot be oxidized by H2SO4 alone. Need a strong oxidizing agent such as potassium permanganate to oxidiz toluene.