Sodium borohydride is ineffective in reducing the carboxyl group in carboxylic acids. Diborane is preferred for reduction as it selectively targets the carboxyl group without affecting other functional groups like ester, nitro, or halo groups.
Why doesn’t sodium borohydride reduce the carboxyl group in carboxylic acids, and what is the significance of using diborane for reduction?
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Sodium borohydride (NaBH4) is ineffective in directly reducing carboxylic acids due to the carboxyl group’s poor electrophilicity. The acidic proton on the carboxyl group impedes direct hydride ion (H-) transfer. In contrast, diborane (B₂H₆) is effective for carboxylic acid reduction as it can form a boron-carbonyl complex, facilitating hydride attack. Diborane’s versatility lies in its ability to reduce a variety of functional groups, including carboxylic acids, by providing hydride ions in a controlled manner. This makes diborane a valuable reagent for selective and efficient reduction of carboxylic acids to alcohols, overcoming the limitations associated with NaBH₄.