Understanding Selectivity of Reducing Agents in Organic Chemistry

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Discussion Overview

The discussion revolves around the selectivity of reducing agents in organic chemistry, specifically focusing on the behavior of sodium borohydride and tin in HCl when reducing 3-nitroacetophenone. Participants explore the mechanisms behind these reactions and the reasons for the observed selectivity towards nitro groups versus carbonyl groups.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that sodium borohydride is understood to reduce only carbonyl groups due to its mechanism, but questions why tin in HCl preferentially reduces the nitro group instead.
  • Another participant suggests that the answer lies in the reaction mechanisms, although they do not provide specific details.
  • A participant mentions their familiarity with carbonyl reduction but expresses confusion about the lack of reduction of nitro groups by sodium borohydride.
  • One contributor reflects on their learning experience, stating that the selectivity of reducing agents was often presented as empirical observations rather than mechanistic explanations, citing historical publications that document varying results with borohydrides.
  • This participant speculates that redox potentials and the nature of the electron transfer processes may play a role in the selectivity of tin in HCl, though they admit to lacking evidence for this claim.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the mechanistic reasons behind the selectivity of reducing agents, indicating that there is no consensus on the explanations for the observed behaviors of sodium borohydride and tin in HCl.

Contextual Notes

Limitations include the lack of detailed mechanisms for the reactions discussed and the reliance on empirical observations rather than theoretical justifications. Participants acknowledge that organic chemistry may not always provide clear mechanistic rationalizations for selectivity.

kyletheskater
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I need some help with understanding reducing agent selectivity. I recently did a lab where we used two reducing agents (separately) for 3-nitroacetophenone (benzene with a ketone group on an ethyl branch and a nitro group) sodium borohydride and tin in HCl. I know that sodium borohydride can only reduce carbonyl groups due to the mechanism, but I don't understand why tin in HCl would reduce the nitro group over the carbonyl group. Any help would be greatly appreciated

/e on second thought I actually can't figure out why BH4+ can only reduce carbonyl. Please help!
 
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I can't help you much, but I am sure it is in the reaction mechanisms.
 
Darn I really need help!
 
Have you checked reaction mechanism? I was able to google mechanism of the borohydride reduction in less then half minute. That's half of what you need (perhaps even more).
 
Borek said:
Have you checked reaction mechanism? I was able to google mechanism of the borohydride reduction in less then half minute. That's half of what you need (perhaps even more).

Thank you for your response, but I am very familiar with carbonyl reduction with sodium borohydride...I just don't understand how it cannot reduce nitro groups. I have played with the mechanism extensively trying to figure out why but I just don't get it

But to the second part of my question I don't understand why tin in HCl selects the nitro group over the carbonyl and I couldn't find a mechanism for that one
 
When I was taught this part of Organic Chem, there was no clear cut rationalization as to why these things happened from a mechanistic stand point. It was presented as experimental observation, in fact a google scholar search will reveal some old ACS (~1950's) publications where they used many borohydride salts with many ketones, aldehydes, carboxylic acids (and derivatives) and found that borohydrides reduced some but not others etc. Organic chemistry does not always have a mechanistic reason behind why some things happen and some don't. Sometimes its really just the fact that they have ran thousands of these reactions and found that 99% of the time you get A and not B.

The case of Tin + HCl is much the same. It was presented as a reaction to memorize and use, if necessary, for synthesis problems.

I suspect it has to do with Redox potentials (aka thermodynamics) and possibly the mechanism as well (IE one or two electron processes), but I have no evidence to back it up.
 

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