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I'm trying to reduce an aryl nitro group. I understand that these reactions are radical reactions by the metal, but I don't understand why alcohols are used as the solvent. Are they used as a source of protons, or just because they are good solvents?

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LHM...Look at this mechanism that I posted as an answer to an earlier SE Chem question/answer relating to the reduction of nitrobenzene by metals.

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In most reactions that occur on metal surfaces, many of the mechanistic details are not fully understood (see @Martin's comments in the earlier post). Nonetheless, this mechanistic scheme shows several steps where a proton ($\ce{H+}$) is required. These proton transfers are necessary to complete the reduction process. This is why these reactions are typically run in solvents, like ethanol, that have available protons that can be donated in order to complete the reaction...ron

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  • $\begingroup$ Thanks for your answering my question. If you know mechanism of ethanol give a proton to nitro? In condition, it's react in RT so i think O-H bond is hard to break. or nitrogen's lonepair electons catch the O-H proton? $\endgroup$ – LHM Nov 19 '14 at 5:56
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    $\begingroup$ Notice that in some of the steps requiring proton transfer there is a negative charge on oxygen or nitrogen. This makes the oxygen or nitrogen more basic, and makes it easier for the oxygen or nitrogen to remove a proton from the solvent. Notice too that acid is usually involved in these reductions and can also serve as a source of protons. $\endgroup$ – ron Nov 19 '14 at 13:12
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I do not believe that ethanol is mandatory. You surely need a proton source, but that's why those reactions are performed in protic acidic media, like your first example, with HCl, or what I think were the original conditions of Bechamp reduction, using acetic acid.

Probably ethanol makes a good solvent for phenol rings.

Edit: this is the Bechamp original paper. I couldn't manage to find it in English. I'm not French speaker either, but just in case you are curious :-)

As far as I'm concerned, nowadays catalytic hydrogenation is used to perform this reaction (at least at industrial scale)

About the mechanism, I'm sorry but I can't give you a solid statement on this.

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