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I was trying to synthesise indigo via the Heumann indigo synthesis (II.), which involves heating antrhanilic acid and chloroacetic acid, both dissolved in isopropanol. After heating up, a hydroxide or another strong base is added to form indigo

I have slightly altered the synthesis process, added trichloroacetic acid instead of chloroacetic, and also added more reactants to tilt the equilibrium towards the product.

The problem is, that any signs of reaction have taken place after more than 3 hours

I have discussed the issue with my chemistry teacher, and he said that the trichloroacetic acid had been of poor quality (made 1990, majority of the original crystals dissolved, turned to brownish liquid). I also think could be simply stopped by the forming of salt (sodium trichloroacetate) and that being unable to interact with anthranilic acid.

I'm very desperate about this. Do you have any thoughts on how to get that reaction going?

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  • $\begingroup$ chemexplore.net/indigo.htm $\endgroup$
    – Jan Mičan
    Mar 22, 2016 at 18:53
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    $\begingroup$ If you do not want to do the Heumann synthesis for some reason or you are having trouble, the page you link gives several friendlier alternatives. The von Baeyer synthesis from 2-nitrobenzaldehyde and acetone is easy (but the yields are low). $\endgroup$
    – Ben Norris
    Mar 22, 2016 at 19:15
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    $\begingroup$ Chemistry is not about playing with chemicals in the lab. Read carefully the link that you provided and it will be clear why you cannot replace chloroacetic with trichloroacetic acid. I'm surprised that your instructor thought this was a good idea. $\endgroup$
    – K_P
    Mar 22, 2016 at 23:23

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Trichloroacetic acid was definitely the wrong way to go.

The reason is the chloroacetic acid is alkylating your anthranilic acid, adding the missing carbons Ben Norris comments on. Ttrichloroacetic acid is not a great alyklating agent, owing to steric bulk.

If you are dead set against working it up and separating your anthranilic acid, you might try just adding the chloroacetic acid.

Otherwise clean it up and start over.

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    $\begingroup$ I concur. Anthranilic acid has 7 carbon atoms. Indigo has 16, which is more than can be accounted for by two equivalents of anthranilic acid. The extra carbon atoms come from chloroacetic acid (but cannot come from trichloroacetic acid). $\endgroup$
    – Ben Norris
    Mar 22, 2016 at 19:09
  • $\begingroup$ Doesn't the indigo precursor need a reaction temperature in the region of 300 degrees C to convert to indigo? $\endgroup$
    – Beerhunter
    Mar 25, 2016 at 19:23

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