Chapter 6: Nucleophilic Addition to the Carbonyl Group

The carbonyl functional group exhibits characteristic polarization of its carbon-oxygen double bond, rendering the carbon electrophilic and vulnerable to attack from nucleophilic species. Using molecular orbital theory, the discussion reveals how the carbonyl's lowest unoccupied molecular orbital is positioned to accept electron density from nucleophile highest occupied molecular orbitals, facilitating productive C-O bond formation. The Bürgi-Dunitz trajectory provides a geometric framework for understanding the preferred approach angle of approximately 107 degrees at which nucleophiles attack carbonyl carbons most effectively. Steric factors, including the size of substituents adjacent to the carbonyl, significantly modulate reaction rates, while electronic effects from electron-withdrawing groups enhance the electrophilicity of the target carbon. Cyanide addition exemplifies this reactivity pattern, yielding cyanohydrin products through nucleophilic assault followed by protonation. Hydride-based reagents including sodium borohydride and lithium aluminum hydride serve as practical nucleophilic sources, reducing aldehydes and ketones to primary and secondary alcohols respectively, with mechanistic differences reflecting the distinct steric environments of these carbonyl substrates. Organometallic nucleophiles such as Grignard reagents and alkyllithium compounds enable carbon-carbon bond formation, expanding the synthetic utility of carbonyl additions. The chapter addresses reversible equilibria governing cyanohydrin formation, acetal genesis from alcohol addition, and hydrate generation from water nucleophiles, emphasizing how these equilibria shift under varying conditions. Acid and base catalysis accelerate many nucleophilic addition pathways by activating electrophiles or nucleophiles through protonation or deprotonation respectively. Bisulfite addition compounds emerge as valuable intermediates for aldehyde purification and protection in synthesis, with pharmaceutical applications including drug delivery formulations. Throughout this treatment, curly arrow mechanisms and orbital diagrams provide mechanistic insight into how electron flow drives these transformations across synthetic, biological, and medicinal contexts.