# Why can't sodium borohydride reduce carboxylic acid and ester groups? [duplicate]

Sodium borohydride ($$\ce{NaBH_4}$$) is a reducing agent used in various organic reaction to reduce aldehydes, ketones and acyl chlorides.

Why can't sodium borohydride reduce carboxylic acid groups and esters? Why can it reduce acyl chlorides but not esters, both which are derivatives of the carboxylic acid group?

I read somewhere that it had to do with the oxidizing property of sodium borohydride, but I'm not sure if it is correct or how it explains the same.

• It forms the acyloxyborohydride which is a stable species, and indeed a reducing agent cf sodium triacetoxyborohydride. Apr 12, 2021 at 11:55
• Think about the way the borohydride ion can react with carbonyls, mechanistically. This should make you think about the electrophilicity of carbonyls, and how bad carboxylic acids (and esters, to some extent) behave as electrophyles. Now, for the carboxylic acid, try to think about the acidity of this species and how this could affect the borohydride ion. Apr 12, 2021 at 12:43
• NaBH4 is very slow to reduce esters but LiBH4 (NaBH4 + LiCl in solution) will get the job done. Apr 12, 2021 at 13:19
• Thank you. I understand what all of you mean. Apr 13, 2021 at 0:29
• @NilayGhosh Yes, it does. That answer makes more sense, thanks. Apr 13, 2021 at 10:19