Benzophenone reacts with sodium borohydride in a methanol solution. The result is diphenylmethanol and a secondary reactant. The reduction starts with breaking the benzophenone carbon-oxygen double bond. The carbon attracts a hydrogen atom from borohydride, and the oxygen attracts a hydrogen atom from methanol.
Hydrogen to Central Carbon
Central carbon of benzophenone bonds with one hydrogen from borohydride (BH4), while the benzophenone oxygen briefly exists as an anion, which is a negatively charged atom.
Benzophenone Oxygen to "OH"
The anionic oxygen (O-) attracts a second hydrogen atom from the carbon end of CH3OH. The main product, diphenylmethanol, differs from the original by the presence of an “OH” functional group.
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Other Reaction Products
When benzophenone reduces to diphenylmethanol, leftover products include the CH2OH and NaBH3 species. The energetic CH2OH and NaBH3 quickly bond to give (CH2OH)H3B-Na+. This complex is the main second product of benzophenone reduction.
In life, four benzophenone molecules react with each BH4 complex. Since four benzophenone molecules each attract a hydrogen atom from the “BH4” hydrogen donor, four “CH2OH”s bond with each boron (B) atom. Realistically, the secondary product is (CH2OH)4B-Na+ and four diphenylmethanol molecules. Focusing on one benzophenone molecule at a time is helpful for explaining and understanding reaction steps.