Understanding Selectivity of Reducing Agents in Organic Chemistry

[kyletheskater]

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!

 

[Borek]

I can't help you much, but I am sure it is in the reaction mechanisms.

 

[kyletheskater]

Darn I really need help!

 

[Borek]

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).

 

[kyletheskater]

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

 

[Yanick]

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.