Final answer:
In nucleophilic addition at a carbonyl carbon, a nucleophile donates electrons to carbonyl carbon, causing a shift from double to single bonding with oxygen and forming a tetrahedral intermediate. The oxygen temporarily bears a negative charge before being protonated. The mechanism depends on whether the nucleophile is neutral or negatively charged.
Step-by-step explanation:
During nucleophilic addition at a carbonyl carbon, a nucleophile donates a pair of electrons to the electrophilic carbon of the carbonyl group. In these reactions, the double bond (π-bond) of the carbonyl is attacked by the nucleophile, causing the carbon to switch from sp² hybridization to sp³ hybridization and form a tetrahedral intermediate. This change creates a new sigma (σ) bond between the nucleophile and the carbonyl carbon. The carbonyl oxygen, now bearing a negative charge due to the shift of electron density, is typically protonated to restore its neutral state. The reaction mechanism varies depending on the nature of the nucleophile. If the nucleophile is neutral, it gains a positive charge, which is later neutralized by deprotonation. In an anionic nucleophile, no positive charge is formed on the nucleophile but the extra electrons push the electron density onto the oxygen, forming a negative charge that is then neutralized by protonation.