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I have a question about reaction between aniline and $\ce{FeCl3}$. I supposed that the following reaction proceeds with formation of brown precipitate:

$$\ce{3C6H5NH2 + FeCl3 + 3H2O -> \underset{brown ppt.}{Fe(OH)3} \downarrow + 3C6H5NH3^+Cl^-}$$

But if we firstly acidify the solution, it’s colorless:

$$\ce{C6H5NH2 + HCl -> C6H5NH3^+Cl^-}$$

If we then add $\ce{FeCl3}$, solution gives pale green color. What substance determines the color if anilinium chloride is colorless?

Here’s the video of experiment. After addition of aqueous solution of $\ce{FeCl3}$ to aniline, there’s no formation of precipitate ($\ce{Fe(OH)3}$ or polymer), only color changes.

Also in book Practical Organic Chemistry for biology students, there’s the following information: o-toludine in such reaction gives green color, p-toludine gives brown color, α-Naphthyl amine gives green ppt.

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  • $\begingroup$ This looks more like an inorganic reaction. It would be an organic reaction, for instance, if it were a $\ce{FeCl3}$ catalyzed Friedel-Crafts reaction. $\endgroup$
    – Josiah_H
    Commented Jan 1, 2022 at 17:24
  • $\begingroup$ Note the other precipitate. Iron hydroxide or iron oxide hydrate is brown in color. On the other hand, in the second reaction, aniline is oxidised to polyaniline and iron(III) chloride is reduced to iron(II) chloride giving pale green solution. Here is the reaction performed electrolytically: researchgate.net/figure/… $\endgroup$
    – Nilay Ghosh
    Commented Jan 2, 2022 at 3:15
  • $\begingroup$ @NilayGhosh here’s the video of experiment in a test tube. I’m not sure, maybe precipitate will fall out in other conditions, different from this video? $\endgroup$
    – Ann Ivanov
    Commented Jan 2, 2022 at 7:59

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The answer is FeCl2, which per Wikipedia:

FeCl2 crystallizes from water as the greenish tetrahydrate, which is the form that is most commonly encountered in commerce and the laboratory. There is also a dihydrate. The compound is highly soluble in water, giving pale green solutions.

You apparently have a reverse reaction observed in the normal REDOX reaction system (think Photo-Fenton with oxygen exposure acting as a stand-in for $\ce{H2O2}$, or more precisely, via the light assisted formation of $\ce{.HO2}$ or Hydroperoxyl radical):

$\ce{ Fe+++ + e- -> Fe++}$

In particular, I point to Eq (3.4) per this Science Direct source:

$\ce{Fe+++ + .HO2 -> Fe++ + O2 + H+}$

This is just part of classic Fenton chemistry, albeit, I suspect that the organic here also actually nicely promotes solvated electron activity as well.

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  • $\begingroup$ What is oxidized, if Fe is reduced from 3+ to 2+? Thanks. $\endgroup$
    – Josiah_H
    Commented Jan 1, 2022 at 17:32
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    $\begingroup$ According to Wikipedia, aniline is oxidized by $\ce{FeCl3}$, producing the polymer $\ce{-(C6H5-NH)_n -}$ $\endgroup$
    – Maurice
    Commented Jan 1, 2022 at 18:29
  • $\begingroup$ This the reverse reaction in a REDOX system, recycling ferric to ferrous. It occurs normally in a REDOX system in a so-called Photo-Fenton reaction. I agree with the comment from Quantum namely: "This looks more like an inorganic reaction". Is it possible that light is a causative agent here? Note, for clarity, I have added my comments here into my answer. $\endgroup$
    – AJKOER
    Commented Jan 2, 2022 at 0:58
  • $\begingroup$ youtu.be/xZZpGSpGHSg - here’s the video of experiment. After addition of aqueous solution of FeCl3 to acidified aniline, there’s no formation of precipitate (Fe(OH)3 or insoluble polymer —(C6H5–NH)n—), only color changes. Maybe it’s caused by acidified medium? $\endgroup$
    – Ann Ivanov
    Commented Jan 2, 2022 at 7:13
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    $\begingroup$ @AJKOER, Also in book Practical Organic Chemistry for biology students there’s the following information: o-toludine in such reaction gives green color, p-toludine - brown color, α-Naphthyl amine - green ppt, that’s results are different. I’m not sure, but I think, not only Photo-Fenton is observed. What’s your opinion? $\endgroup$
    – Ann Ivanov
    Commented Jan 2, 2022 at 7:54

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