The white solid $ Na^+ BH_4^- $ is an acid-efficient reducer in the chemistry of carbohydrates: $BH_4^-\; -\;2e^-\;$ $\leftrightarrows$ $\; BH_3\; +\;H^+$ $...C=O$ (Aldose or Ketose)$ +2e^- +2H^+$ $\leftrightarrows$ $..CHOH $ ____________________________________________________________ $BH_4^-\;+\; ...C=O+\;H^+$ $\leftrightarrows$ $\; BH_3\; +\;..CHOH $
Reduction of the ketone function of the oses takes place only when hot, while that of the aldehyde function is also done in the cold. It is not stereospecific and thus can lead to the two corresponding epimers:
