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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?

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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.

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    $\begingroup$ 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. $\endgroup$
    – Gaurang Tandon
    Commented Mar 15, 2018 at 15:36
  • $\begingroup$ @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. $\endgroup$
    – logical x 2
    Commented Mar 16, 2018 at 21:50

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