NaBH4 does not reduce carboxylic acids while LiAlH4 does due to differences in reducing agent strength
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Chemical reference literature reports that sodium borohydride (NaBH4) does not reduce carboxylic acids, whereas lithium aluminum hydride (LiAlH4) is a stronger reducing agent capable of reducing them, owing to differences in bond polarity and electronegativity.
Reduction to a 1º-alcohol takes place rapidly on treatment with the powerful metal hydride reagent, lithium aluminum hydride, as shown by the following equation. One third of the hydride is lost as hydrogen gas, and the initial product consists of metal salts which must be hydrolyzed to generate the alcohol. These reductions take place by the addition of hydride to the carbonyl carbon, in the same manner noted earlier for aldehydes and ketones. The resulting salt of a carbonyl hydrate then breaks down to an aldehyde that undergoes further reduction. | 4 RCO2H + 3 LiAlH4 | ether | 4 H2 + 4 RCH2OM + metal oxides | H2O | 4 RCH2OH + metal hydroxides |
Diborane, B2H6, reduces the carboxyl group in a similar fashion. Sodium borohydride, NaBH4, does not reduce carboxylic acids; however, hydrogen gas is liberated and salts of the acid are formed. Partial reduction of carboxylic acids directly to aldehydes is not possible, but such conversions have been achieved in two steps by way of certain carboxyl derivatives. These will be described later. Oxidation
Because it is already in a high oxidation state, further oxidation removes the carboxyl carbon as carbon dioxide.
(NaBH4). Note! The hydride anion is not present during this reaction; rather, these reagents serve as a source of hydride due to the presence of a polar metal-hydrogen bond. Because aluminum is less electronegative than boron, the Al-H bond in LiAlH4 is more polar, thereby, making LiAlH4 a stronger reducing agent. Addition of a hydride anion (H:-) to an aldehyde or ketone gives an alkoxide anion, which on protonation yields the corresponding alcohol. Aldehydes produce 1º-alcohols and ketones produce 2º-alcohols. In metal hydrides reductions the resulting alkoxide salts are insoluble and need to be hydrolyzed (with care) before the alcohol product can be isolated. In the sodium borohydride reduction the methanol solvent system achieves this hydrolysis automatically. In the lithium aluminum hydride reduction water is usually added in a second step. The lithium, sodium, boron and aluminum end up as soluble inorganic salts at the end of either reaction. Note! LiAlH4 and NaBH4 are both capable of reducing aldehydes and ketones to the corresponding alcohol. Mechanism
This mechanism is for a LiAlH4 reduction.
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