Bedlasky
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Charge-transfer complexes of nitroaromatics with aromatic amines
Hi.
I few times noticed that when drop of waste containing nitrobenzene get mixed with a drop of waste containing aniline on paper towel, it make yellow
spot. So few months back I decide to investigate this strange behavior. I add little bit of nitrobenzene in to the small vial and mix it with a few
drops of aniline. Nitrobenzene instantly turned orange. I tried similar experiment with N-ethylaniline and N,N-diethylaniline. This time colour change
was even more apparent, both vials turned red (N-ethylaniline/nitrobenzene gave the most intense colour). I tried mix ethylaniline with
cyclohexylamine, if aliphatic amines act the same, but no colour change at all. I conducted the same series, replacing nitrobenzene with
o-nitrotoluene. Reaction lead to formation of three orange solutions. Nitrobenzene with o-toluidine formed dark red solution.
I found online that nitroaromatics form charge-transfer complexes with various aromatic compounds. Electron-poor nitroaromatic is electron acceptor,
electron-rich aromatic is donor.
https://sci-hub.se/https://pubmed.ncbi.nlm.nih.gov/29850035/
From left to right nitrobenzene + aniline, N-ethylaniline, N, N-diethylaniline, cyclohexylamine
From left to right o-nitrotoluene + aniline, N-ethylaniline, N, N-diethylaniline
Nitrobenzene + o-toluidine, on the right o-toluidine (brown colour probably from aerial oxidation).
[Edited on 28-9-2022 by Bedlasky]
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Lionel Spanner
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This can also be seen in o-nitrophenyleneamines, which carry both groups on the same ring. Many of these are highly coloured compounds, and some are
used for temporary/semi-permanent hair dyeing. Compounds with one amino group tend to be yellow, while those with two amino groups tend to be red.
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Bedlasky
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That's really interesting info! Few months ago I came across some paper about role of transition metal cations in hair coloring, but I didn't have
time to look at it yet. But by quicky search I find mention about these o-nitrophenyleneamines.
https://etheses.whiterose.ac.uk/6955/1/Final%20thesis%20KRN%...
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Lionel Spanner
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The commercially available dyes are all minor variations on the same basic framework, the variable element being the substituent(s) on the amino group
next to the nitro group - they're sold as HC Red 1, 2, 3 etc. and HC Yellow 1, 2, 3 etc.
Having worked with HC Red 2 and HC Yellow 3 in a previous job, I have to say they're not very wash-fast or effective compared to basic or disperse
dyes.
There are HC Blues as well, but they're based on completely different chemistry.
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