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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.

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    $\begingroup$ It forms the acyloxyborohydride which is a stable species, and indeed a reducing agent cf sodium triacetoxyborohydride. $\endgroup$
    – Waylander
    Apr 12, 2021 at 11:55
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    $\begingroup$ 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. $\endgroup$ Apr 12, 2021 at 12:43
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    $\begingroup$ NaBH4 is very slow to reduce esters but LiBH4 (NaBH4 + LiCl in solution) will get the job done. $\endgroup$
    – user55119
    Apr 12, 2021 at 13:19
  • $\begingroup$ Thank you. I understand what all of you mean. $\endgroup$
    – m-Xylene
    Apr 13, 2021 at 0:29
  • $\begingroup$ @NilayGhosh Yes, it does. That answer makes more sense, thanks. $\endgroup$
    – m-Xylene
    Apr 13, 2021 at 10:19

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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.

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  • $\begingroup$ But won't that effect be the same regardless of the reducing agent? I think chemistry.stackexchange.com/questions/38747/… might be right... $\endgroup$
    – m-Xylene
    Apr 13, 2021 at 10:20
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    $\begingroup$ Yes LiAlH4 can reduce carboxylic and esters because it is stronger than NaBH4. I just answered why NaBH4 can reduce aldehydes and ketones but not carboxylic acids or esters $\endgroup$
    – M.L
    Apr 13, 2021 at 16:18

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