According to me net reaction should be at $\ce{C3}$ when Reimer-Tiemann reaction is done on indole (see image), since in the resonance structure, negative charge is on this carbon.

enter image description here

But, the answer given is on $\ce{C2}$, and the solution given is

In pyrolle, the most electrophilic site is $\ce{C2}$. Think in a similar way.

But, I can't either find fallacy in my reasoning, not do I convince with the solution.

TL;DR Where will dichlorocarbene attack on indole (am not interested in ring expansion).


The paper by Order and Lindwall [1] agrees with you that the R-T reaction gives indole-3-carboxaldehyde.

Since its isolation in 1903 by Hopkins and Cole (1), 3-indole aldehyde has been investigated very little. It was first prepared by Ellinger (2) from indole through the use of the Reimer-Tiemann reaction. This method was improved on later by Boyd and Robson (3).


  1. Order, R. B. V.; Lindwall, H. G. 3-Indole Aldehyde and Certain of Its Condensation Products. J. Org. Chem. 1945, 10 (2), 128–133. DOI: 10.1021/jo01178a006.
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  • $\begingroup$ What about the explanation given in the solution? $\endgroup$ – user87840 Jan 14 at 17:52
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    $\begingroup$ Indole is not pyrrole $\endgroup$ – Waylander Jan 14 at 17:54
  • $\begingroup$ Also note that the answer refers to C2 as "most electrophilic", which is true. But in the first step of the R-T reaction, the aromatic ring acts as a nucleophile, so the reaction happens at the most nucleophilic position (C3) rather than the electrophilic position (C2). $\endgroup$ – Andrew Apr 6 at 13:00

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