Reduction of propanone with NaBH₄

Sodium borohydride is a source of hydride ions (H⁻). Hydride adds to the carbonyl carbon of aldehydes and ketones, and the resulting alkoxide is protonated by water or the alcohol solvent. Ketones give secondary alcohols.

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Nucleophilic addition (reduction)

A hydride from BH₄⁻ attacks the δ+ carbonyl carbon of propanone and the C=O π electrons move onto oxygen. Water then protonates the alkoxide to give propan-2-ol.

Overall reaction

1. 1Hydride attacks the carbonyl carbonnucleophilic addition

   borohydride ion

   +

   propanone

   alkoxide

   +

   borane (BH₃)

   Electron movement

   1. The electrons of a B–H bond carry the hydrogen, as hydride, to the carbonyl carbon.
   2. The C=O π electrons move onto oxygen.

   Boron is less electronegative than hydrogen, so the B–H bonds deliver hydride (H⁻) to the δ+ carbonyl carbon. (In practice the BH₃ goes on to reduce more ketone molecules; this is simplified.)

2. 2Protonation by waterproton transfer

   alkoxide

   +

   water

   propan-2-ol

   +

   hydroxide ion

   Electron movement

   1. The alkoxide oxygen takes a proton from water.
   2. The O–H bond breaks, leaving hydroxide.

   Protonating the alkoxide gives the neutral alcohol.