Question

P-fluoroanisole reacts with sulfur trioxide and sulfuric acid. Draw the major product of this substitution reaction; if applicable

Answer 1

Answer: (Structure attached).

Explanation:

This type of reaction is an aromatic electrophilic substitution. The overall reaction is the replacement of a proton (H +) with an electrophile (E +) in the aromatic ring.

The aromatic ring in p-fluoroanisole has two sustituents, an halogen and a methoxy group, which are ortho-para directing substituents.

Aryl sulfonic acids are easily synthesized by an electrophilic substitution reaction aromatic using sulfur trioxide as an electrophile (very reactive).

The reaction occurs in three steps:

1. The attack on the electrophile forms the sigma complex.
2. The loss of a proton regenerates an aromatic ring.
3. The sulfonate group can be protonated in the presence of a strong acid (H₂SO₄).

Normally, a mixture of ortho-para substituted products would be obtained. However, since both para positions are occupied, only the ortho substituted product is obtained here.