Why can't sodium borohydride reduce carboxylic acid and ester groups? 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.
The reason for this is because OH and OR groups are much more electron-donating. Typically, when you have an aldehyde or ketone, the electronegative electron is slightly more negative and thus the carbonyl carbon is more positive. However, with an OH or OR group attached (i.e in carboxylic acid and esters), the group can push electrons towards the carbon since they are electron-donating groups. This makes the carbonyl carbon less positive and less open to nucleophilic attack.