Nitration of 1, 4-dimethylbenzene (p-xylene) gives a single product having the molecular formula C8H9NO2 in high yield. What is this product?
The correct answer and explanation is :
The nitration of 1,4-dimethylbenzene (p-xylene) gives 2-nitro-1,4-dimethylbenzene as the major product. Its molecular formula is C₈H₉NO₂.
Explanation (300 words):
p-Xylene is a benzene ring substituted with two methyl groups at the 1 and 4 positions. Methyl groups are electron-donating via hyperconjugation and inductive effects, which means they activate the aromatic ring toward electrophilic substitution and direct incoming electrophiles to the ortho and para positions relative to themselves.
During nitration, the reaction mixture typically involves concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄). The nitronium ion (NO₂⁺), generated in situ, acts as the electrophile, attacking the aromatic ring.
Given that both methyl groups are already in para positions relative to each other (1,4), each methyl group directs the incoming nitro group to positions ortho and para relative to itself. However, due to the symmetry of the p-xylene molecule, there is only one unique position (position 2 or 6) that is ortho to one methyl group and meta to the other, making the nitro substitution selective.
Therefore, the nitration occurs at either position 2 or 6 — both are equivalent due to symmetry — resulting in a single mononitrated product:
2-nitro-1,4-dimethylbenzene (also called 2-nitro-p-xylene).
This product is favored both electronically and sterically:
- Electronically, both methyl groups enhance the reactivity of the ring.
- Sterically, positions 2 and 6 are less hindered compared to position 3 (which is between the two methyl groups).
This explains why only one product is formed in high yield. The structure of the product is:
CH3
|
NO2—⟶⟶ ⟶⟶—CH3Summary:
- Reactant: 1,4-dimethylbenzene (p-xylene)
- Reagents: HNO₃/H₂SO₄
- Product: 2-nitro-1,4-dimethylbenzene
- Molecular formula: C₈H₉NO₂