Reaction of Anisole with a Mixture of Concentrated H2SO4 and HNO3

When anisole (methoxybenzene) reacts with a mixture of concentrated sulfuric acid (H2SO4) and nitric acid (HNO3), it undergoes a nitration reaction to form 4-nitroanisole as the major product.

Nitration Reaction
The nitration reaction is a type of electrophilic aromatic substitution reaction, where an electrophile (in this case, a nitronium ion, NO2+) substitutes a hydrogen atom in an aromatic compound. The reaction proceeds through the following steps:

1. Formation of Nitronium Ion (NO2+):
Concentrated sulfuric acid (H2SO4) acts as a catalyst in the reaction. It protonates nitric acid (HNO3), producing a strong electrophile, the nitronium ion (NO2+). The reaction can be represented as follows:

H2SO4 + HNO3 → HSO4- + NO2+ + H2O

2. Attack of Nitronium Ion on Anisole:
The nitronium ion (NO2+) acts as the electrophile and attacks the electron-rich benzene ring of anisole. The oxygen atom in the methoxy group (-OCH3) helps in stabilizing the intermediate and facilitating the reaction. The attack occurs at the ortho and para positions to the methoxy group, leading to the formation of two possible products: 2-nitroanisole and 4-nitroanisole. However, due to steric hindrance, 4-nitroanisole is the major product formed.

Reason for the Formation of 4-Nitroanisole as the Major Product
The steric hindrance caused by the methoxy group in anisole makes the attack of the nitronium ion at the ortho position less favorable compared to the para position. This is because the ortho position is sterically hindered by the methoxy group, making it more difficult for the nitronium ion to approach and attack the benzene ring. On the other hand, the para position is relatively more accessible and allows for an easier attack by the nitronium ion. Therefore, the major product formed in this reaction is 4-nitroanisole.

In summary, when anisole reacts with a mixture of concentrated sulfuric acid and nitric acid, 4-nitroanisole is obtained as the major product due to steric hindrance, which favors the attack at the para position of the methoxy group.