Homework Help: Organic Chemistry

Posted by Toki on Monday, December 6, 2010 at 4:38am.

Hello, I just have a quick question that I would like to have clarified.

According to my textbook definition, NaBH4 (a reducing agent) can be used in solution in alcohols or even solution in water - provided the solution is alkaline."

However, my textbook, in all of the examples only uses ethanol as the solvent. Is there a reason, or is it just a matter of preference?

Organic Chemistry - Dr Russ, Monday, December 6, 2010 at 6:01am
Largely it depends on the solubility of the material to be reduced in the solvent. Many organic compounds are not soluble in water or are hydroylsed in the alkaline conditions

Thus the solvent used is a compromise between availability, cost, and reaction(s) required. Ethanol is a cheap readily available solvent, it is easily purified if needed, and easily recovered. It is also miscible with water so that ethanol containing a small percentage of water can be used.

There are a couple of alternatives. One is to use a two phase system where the compound to be reduced is in an organic phase and the hydride is in an aqueous phase. A better alternative is to use a crown ether (15-Crown-5)to improve the solubility of the borohydride an organic phase (e.g. THF). The resulting reaction mixture is easier to work with. On a very small scale the whole reaction can be carried out in the crown ether.
Organic Chemistry - Toki, Monday, December 6, 2010 at 1:19pm
Ah, I didn't notice that they started to use ethanol once the molecules started getting larger ('less soluble by your definition')

Thank you for that info Dr Russ :)

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