I am stuck on a question (again) and would like some help, as always I have done my research and come to the point where I got stuck. The question goes as followed:
A simple ketone (e.g. Propanone) is readily reduced by borohydride, e.g. NaBH4, but an ester (e.g. methyl methanoate) is not. Show the mechanism of the reactions and based on this offer an explanation for the difference in reactivity.
The mechanism for the Ketone reduction:
I think it’s the second one because in the question it’s not mentioned that ethanol is present as a reagent.
As for what I know is that NaBH4 will reduce many organic carbonyls, which include ketones and aldehydes. However it is not a strong enough reducing agent to reduce esters. For that usually LiAlH4 (lithium aluminium hydride) is used. Therefore I am not sure what the mechanism of the ester is with BH4.
So I thought let’s look at the mechanism of ester with LiAlH4, which goes as followed:
What I see is that there are two reduction processes to receive the final product. While the reduction of an ketone only has one. Does this explain the difference in reactivity? So why isn’t it possible, or is it less reactive, to use NaBH4 to reduce an ester?