Question

Isoquinoline on treatment with oleum at 90°C yields majorly:

• A. Isoquinoline-3-sulfonic acid
• B. Isoquinoline-5-sulfonic acid
• C. Isoquinoline-6-sulfonic acid
• D. Isoquinoline-7-sulfonic acid

Answer 1

The Correct Option is B

The reaction of isoquinoline with oleum at elevated temperatures is a sulfonation reaction. Isoquinoline, like quinoline, can undergo electrophilic aromatic substitution. The sulfonation of aromatic compounds usually involves the addition of a sulfonic acid group (\text{-SO_3H}) onto the aromatic ring.

In isoquinoline, the most reactive site for electrophilic substitution is determined by the electron density on different positions of the isoquinoline ring. The electrophile \text{SO}_3 can attack these positions to produce sulfonated derivatives.

The following factors influence the position of sulfonation in isoquinoline:

• Resonance Effect: The lone pairs on the nitrogen atom of isoquinoline can delocalize, affecting the electron density distribution in the ring.
• Inductive Effect: The nitrogen atom can exhibit an inductive effect, affecting electron density in the heterocycle.

Considering these effects, position 5 on the isoquinoline ring becomes one of the more favorable sites for electrophilic attack due to the balance between these effects. Therefore, the sulfonation primarily yields isoquinoline-5-sulfonic acid as the major product.

Thus, the correct answer to the question is:

• Isoquinoline-5-sulfonic acid