The nucleophilic addition reaction of sodium borohydride NaBH4 to benzaldehyde and acetone involves the reduction of the carbonyl group C=O to an alcohol C-OH group. Sodium borohydride is a mild reducing agent that is selective for aldehydes and ketones. In this case, benzaldehyde is an aldehyde, and acetone is a ketone. The mechanism of the reaction can be broken down into the following steps:1. Formation of the nucleophile: Sodium borohydride NaBH4 dissociates into Na+ and BH4- ions in the solution. The BH4- ion acts as a nucleophile, with the hydride ion H- being the nucleophilic center.2. Nucleophilic attack: The hydride ion H- from the BH4- ion attacks the electrophilic carbonyl carbon C=O of benzaldehyde or acetone. This results in the formation of an alkoxide intermediate, with a new C-H bond and a negatively charged oxygen atom.3. Protonation: The alkoxide intermediate is protonated by a solvent molecule usually an alcohol or water , resulting in the formation of the alcohol product.For benzaldehyde, the product is benzyl alcohol, and for acetone, the product is isopropanol 2-propanol .In terms of stereoselectivity, the reaction with benzaldehyde does not involve any stereocenters, so there is no stereoselectivity issue. However, for acetone, the reaction is not stereoselective because the carbonyl carbon is not a stereocenter, and the product, isopropanol, has only one stereoisomer due to the presence of two equivalent methyl groups. Therefore, the reaction does not produce any stereoisomers.In summary, the nucleophilic addition reaction of NaBH4 to benzaldehyde and acetone involves the reduction of the carbonyl group to an alcohol group through a nucleophilic attack by the hydride ion from the BH4- ion. The reaction products are benzyl alcohol and isopropanol, and there is no observed stereoselectivity due to the absence of stereocenters in the reactants and products.