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A common reaction used to identify resorcinol (benzene-1,3-diol) in the laboratory is to react it with chloroform in presence of 30% sodium hydroxide (NaOH). A red coloured solution (or precipitate) is produced. I have searched many books and websites, but none of them give any clue about the actual reaction, its products, far less the reaction mechanism.

The reaction is as follows:

$$\ce{Resorcinol + Chloroform + 30\% NaOH -> Red Colour}$$

So any ideas about what reaction is taking place?

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Well, turns out to be, the reaction proceeds to product a red colored species.

Reaction with chloroform in the alkaline medium

Dissolve 0.1 g in 1 ml of water R, add 1 ml of strong sodium hydroxide solution R and 0.1 ml of chloroform R, heat and allow to cool. An intense, deep-red colour develops which becomes pale yellow on the addition of a slight excess of hydrochloric acid R.

enter image description here

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    $\begingroup$ @OscarLanzi probably. I could not find enough evidence to prove that. $\endgroup$ – Nilay Ghosh Aug 23 '18 at 18:55
  • $\begingroup$ Do you know the mechanism of this reaction? I cannot figure it out. $\endgroup$ – Shoubhik Raj Maiti Aug 25 '18 at 3:26
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    $\begingroup$ You're probably looking at a two-part reaction. First chloroform+sodium hydroxide imparts a carbonyl group ortho to a phenolic hydroxyl group (see here). Then this carbonyl group electrophilically attacks a second resorcinol molecule to give the dimeric structure. $\endgroup$ – Oscar Lanzi Aug 25 '18 at 10:19

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