Question

How is acetophenone phenylhydrazone catalyzed into 2-phenylindole?

Answer 1

Here's my proposal for the mechanism for the Fischer indole synthesis of 2-phenylindole.

Explanation:

Note: For convenience, I am using `"HCl"` as the acid and `"Cl"^"-"` as the conjugate base, but you should use the acid and base that were used in your synthesis.

Step 1. Protonation of the phenylhydrazone

Step 2. Tautomerization to an enamine

Step 3. Cyclic [3,3]-sigmatropic rearrangement

This forms a diimine and results in the loss of aromaticity.

Step 4. Re-aromatization

Step 5. Protonation of the imine

Step 6. Cyclization

Step 7. Deprotonation of the ammonium ion

Step 8. Protonation of the primary amine

Step 9. Elimination of ammonia

This forms a stable, aromatic indole.

And that is my proposed mechanism for the formation of 2-phenylindole.