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According to my textbook, Clemmensen reduction reduces only aldehydes and ketones to alkanes. But in this question:

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$\ce{Zn-Hg/HCl}$ has reduced both the ketonic and nitro group. Correct answer given is D.

So, what exactly is this reduction's mechanism? Is it an exception? If not, why isn't it stated in the books alongside the carbonyl groups?

Gaurang Tandon
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Akash Goel
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    The paper - Diez-Cecilia, E., Kelly, B., Rozas, I., One-step double reduction of aryl nitro and carbonyl groups using hydrazine, Tetrahedron Letters (2011), doi: 10.1016/j.tetlet.2011.09.112 - discusses Wolff Kishner reduction of nitro compounds. I still find it surprising, though, that Wikipedia doesn't mention even once at least the possibility of reduction of nitro compounds by Clemenssen reduction, let alone their industrial use or whatever. – Gaurang Tandon Mar 15 '18 at 15:41
  • @AshishAhuja This may be of use https://www.sciencedirect.com/topics/chemistry/clemmensen-reduction and this as well https://www.organic-chemistry.org/namedreactions/clemmensen-reduction.shtm – Rishi Jul 04 '21 at 14:26
  • @AshishAhuja Seems like the reduction here is specific to aryl nitro groups, see here https://www.masterorganicchemistry.com/2018/10/08/nitration-baeyer-villiger/ and you might also want to take a look here https://www.researchgate.net/publication/257908880_Reduction_of_aromatic_nitro_compounds_to_amines_using_zinc_and_aqueous_chelating_ethers_Mild_and_efficient_method_for_zinc_activation maybe one of the references of this paper might have some related information. – Rishi Jul 04 '21 at 17:35
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    @AshishAhuja Found it! https://onlinelibrary.wiley.com/doi/10.1002/9780470638859.conrr063 the abstract is enough and here is Bechamp reduction https://en.wikipedia.org/wiki/Bechamp_reduction. – Rishi Jul 04 '21 at 17:45
  • @AshishAhuja This as well confirms the doubt. https://pubs.acs.org/doi/pdf/10.1021/ie50513a019?src=recsys – Rishi Jul 06 '21 at 11:43
  • @Rishi thanks, I had a look at the papers you linked. I also found this and this. I don't think it is specifically called Bechamp reduction, since that seems to be using iron. It looks like such reductions only apply to nitroarenes, albeit none of these seem to be using exactly the same Zn-Hg/HCl mixture. You can post an answer quoting the papers if you'd like (you can find them on SciHub). – Box Box Box Box Jul 06 '21 at 12:54
  • @AshishAhuja Yes, I think I made a mistake, it may not necessarily follow a mechanism similar to Béchamp. But as mentioned in this Zinc and acid has been mentioned and even in this the same has been mentioned. I also forgot the fact that Clemmensen uses the amalgam :/ . Also, I don’t think I will be able to write an answer. Sorry! :( – Rishi Jul 06 '21 at 15:49
  • @AshishAhuja You want source for what? That Clemmensen isn't magical reduction that has perfect selectivity? On contrary, it's an old method using aggressive reagent and reactions like Wolff–Kishner and Mozingo reduction tend to be used instead. – Mithoron Jul 07 '21 at 20:35
  • @Mithoron I was looking for a source for the reduction of a nitroarene to aniline using Zn-Hg/HCl, and maybe it's mechanism. I've found papers (after the bounty) which reference the same reaction with zinc + mild acid, so I'm satisfied with that. You say that Clemmensen is an aggressive reagent, but I am yet to find a source which uses the classic clemmensen reduction Zn-Hg (or zinc dust) with a conc. acid to reduce nitroarenes. – Box Box Box Box Jul 08 '21 at 12:15
  • https://chemistry.stackexchange.com/questions/29192/how-can-an-aromatic-ketone-be-reduced-in-the-presence-of-a-nitro-group https://chemistry.stackexchange.com/questions/140426/how-does-al-hg-react-with-carbonyl-in-presence-of-concentrated-hcl\ – Mithoron Jul 08 '21 at 13:30
  • @Mithoron the first question talks about clemmensen reducing the nitro group but without a reference, I don't see how the second question is relevant. Also (second paragraph): "Moreover, when attempted in the present work, the concurrent reduction of carbonyl and nitro groups with Zn, using Clemmensen conditions (as described in reference 6), was not achieved", which is exactly what this question is asking about. – Box Box Box Box Jul 08 '21 at 14:19
  • *https://chemistry.stackexchange.com/questions/60959/are-groups-other-than-carbonyl-reduced-by-clemmensen-reduction* –  Jul 09 '21 at 11:50

1 Answers1

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I would look at it in the following way: The oxidation state of the carbonyl carbon is +II, while the oxidation state of the nitrogen in the nitro group is +III. So nitrogen is in a higher oxidation state and it is also more electronegative. Thus, isn't it sensible that it gets reduced first before the carbonyl group? In fact, reduction of a nitro group is also possible using much milder reducing agents (e.g. only zinc dust).

In my experience, most questions of chemoselectivity can be answered by "thinking about it". However, if you would still like to have a good book for that, I would recommend Warren "Organic Synthesis: Strategy and Control" which contains a very nice treatment of chemoselectivity problems.

Gaurang Tandon
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logical x 2
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    Just a correction: " The oxidation state of the carbonyl carbon is +II" oxidation state of carbon in aldehyde is +I instead; +II occurs for ketones only. Doesn't change the fundamental reason of your answer though. – Gaurang Tandon Mar 15 '18 at 15:36
  • @GaurangTandon I don't get that. The oxidation state of the carbonyl carbon in the above reaction is +II. This wasn't meant as any general statement. – logical x 2 Mar 16 '18 at 21:50