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?
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.
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 :)
The choice of solvent in a chemical reaction can indeed have an impact on the reaction itself. In the case of using NaBH4 as a reducing agent, the choice of solvent may depend on various factors including reaction conditions, reaction kinetics, and solubility of the reactants.
NaBH4 is commonly used in alcohol solvents because it is soluble in alcohols and reacts readily with carbonyl compounds. Ethanol, being a commonly available alcohol, is often used as a solvent in these reactions. It provides a suitable environment for the reaction to occur by allowing the NaBH4 to dissolve and interact with the carbonyl compound effectively.
The textbook might have used ethanol as a solvent in its examples to demonstrate the typical conditions in which NaBH4 is used. Additionally, the choice of ethanol as a solvent could be due to its comparative simplicity and availability.
However, it is important to note that NaBH4 can also be dissolved in water under alkaline conditions. The alkaline environment helps to stabilize the reducing agent, increasing its reactivity. This makes NaBH4 in alkaline solution suitable for reducing carbonyl compounds, even in aqueous solutions. So, while ethanol is commonly used as a solvent, it is not the only option, and depending on the specific reaction requirements, other solvents like water can also be used.
In conclusion, the choice of solvent in using NaBH4 as a reducing agent can vary depending on factors like reaction conditions and solubility of the reactants. Ethanol is often used as a solvent in textbook examples due to its availability and compatibility with NaBH4, but other solvents such as water can also be used, especially when the solution is alkaline.