Mechanism for Ketone and aldehyde reduction by NaBH4 to alcohol

Sodium borohydride is a salt which is made up of a sodium cation (Na+) and a borohydride anion (BH4-) The sodium ion plays no important role in the reaction, so we will ignore it (ions like sodium and potassium are seldom directly involved in reactions. They are present merely to maintain charge balance so that stable compounds can be added to reaction mixtures. They are often called spectator ions.) The borohydride ion is the important player in this process, and it is the B-H bond that we want to examine.

From periodic it is understood that both boron and aluminum are metals with relatively low electronegativities. Each is less electronegative than carbon, and since hydrogen has about the same electronegativity as carbon we can conclude that the B-H bond is polarized with the boron positive and the hydrogen negative. So B-H bond effectively serves as a source of hydride ion (:H-). This is given below.

 Mechanism of aldehyde to alcohol conversion

 Below attack by a nucleophile at the carbonyl carbon, followed by protonation of the carbonyl oxygen.

The same mechanism applies to the reduction of ketones by sodium borohydride and to reactions of carbonyl groups with lithium aluminum hydride. In case of lithium aluminum hydride, the reagent itself highly reactive with water, so the water is added after the lithium aluminum hydride has reacted.