Bouveault–Blanc reduction

This reaction is used commercially although for laboratory scale reactions it was made obsolete by the introduction of lithium aluminium hydride.

Reaction mechanism

Sodium metal is a one-electron reducing agent. Four equivalents of sodium are required to fully reduce each ester. The electron-transfer reaction occurs at the surface of the metal, thus finely divided sodium is employed. Ethanol serves as a proton source. The reaction produces sodium alkoxides, according to the following stoichiometry:

RCOOR' + 4 Na + 2 CH₃CH₂OH → RCH₂ONa + R'ONa + 2 CH₃CH₂ONa

In practice, considerable sodium is consumed by the formation of hydrogen. For this reason, an excess of sodium is often required. Because the hydrolysis of sodium is rapid, not to mention dangerous, the Bouveault–Blanc reaction requires anhydrous ethanol and can give low yields with insufficiently dry ethanol. The mechanism of the reaction follows:

Consistent with this mechanism, sodium-ethanol mixtures will also reduce ketones to alcohols.

This approach to reducing esters was widely used prior to the availability of hydride reducing agents such as lithium aluminium hydride and related reagents. It requires vigorous reaction conditions and has a significant risk of fires, explaining its relative unpopularity. One modification involves encapsulating the alkali metal into a silica gel, which has a safety and yield profile similar to that of hydride reagents. Another modification uses a sodium dispersion.