Organic synthesis reduces many organic carbonyls, depending on the conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. Nevertheless, an alcohol, often methanol or ethanol, is generally the solvent of choice for sodium borohydride reductions of ketones and aldehydes. The mechanism of ketone and aldehyde reduction has been scrutinized by kinetic studies, and contrary to popular depictions in textbooks, the mechanism does not involve a 4-membered transition state like alkene hydroboration, or a six-membered transition state involving a molecule of the alcohol solvent. Hydrogen-bonding activation is required, as no reduction occurs in an aprotic solvent like
diglyme. However, the rate order in alcohol is 1.5, while carbonyl compound and borohydride are both first order, suggesting a mechanism more complex than one involving a six-membered transition state that includes only a single alcohol molecule. It was suggested that the simultaneous activation of the carbonyl compound and borohydride occurs, via interaction with the alcohol and alkoxide ion, respectively, and that the reaction proceeds through an open transition state. α,β-Unsaturated ketones tend to be reduced by in a 1,4-sense, although mixtures are often formed. Addition of cerium chloride improves the
selectivity for 1,2-reduction of unsaturated ketones (
Luche reduction). α,β-Unsaturated esters also undergo 1,4-reduction in the presence of . Mixing water or an alcohol with the borohydride converts some of it into unstable hydride ester, which is more efficient at reduction, but the reductant eventually decomposes spontaneously to produce hydrogen gas and borates. The same reaction can also occur intramolecularly: an α-ketoester converts into a diol, since the alcohol produced attacks the borohydride to produce an ester of the borohydride, which then reduces the neighboring ester. The reactivity of can be enhanced or augmented by a variety of compounds. Many additives for modifying the reactivity of sodium borohydride have been developed as indicated by the following incomplete listing.
Oxidation Oxidation with
iodine in
tetrahydrofuran gives
borane–tetrahydrofuran, which can reduce carboxylic acids to alcohols. Partial oxidation of
borohydride with iodine gives
octahydrotriborate: :
Coordination chemistry is a
ligand for metal ions. Such borohydride complexes are often prepared by the action of (or the ) on the corresponding metal halide. One example is the
titanocene derivative: :
Protonolysis and hydrolysis reacts with water and alcohols, with evolution of hydrogen gas and formation of the corresponding borate, the reaction being especially fast at low pH. Exploiting this reactivity, sodium borohydride has been studied as a prototypes of the
direct borohydride fuel cell. : (ΔH < 0) ==Applications==