The mechanism behind the nucleophilic addition reaction of an aldehyde or ketone with a nucleophile involves the following steps:1. Nucleophilic attack: The nucleophile, which is a species with a lone pair of electrons, attacks the electrophilic carbonyl carbon of the aldehyde or ketone. This results in the formation of a tetrahedral intermediate. The carbonyl carbon's double bond with the oxygen atom is broken, and the oxygen atom becomes negatively charged.2. Protonation: The negatively charged oxygen atom in the tetrahedral intermediate abstracts a proton H+ from a suitable proton donor usually a solvent or an acid . This step results in the formation of the final product, which is an alcohol.Example:A simple example of a nucleophilic addition reaction is the reaction between an aldehyde e.g., acetaldehyde, CH3CHO and a nucleophile e.g., a hydride ion, H- . The reaction can be represented as follows:CH3CHO + H- CH3CH2O- CH3CH2OHIn this example, the nucleophile hydride ion attacks the carbonyl carbon of acetaldehyde, forming a tetrahedral intermediate CH3CH2O- . This intermediate then abstracts a proton from a suitable proton donor to form the final product, ethanol CH3CH2OH .