Nucleophilic addition is a fundamental reaction mechanism in organic chemistry, particularly for the reactivity of carbonyl compounds like aldehydes and ketones.
This process involves a nucleophile attacking an electrophilic center to form a new chemical bond.
For carbonyl compounds, the electrophilic center is the carbon atom of the carbonyl group (C=O).
Mechanism of Nucleophilic Addition to Carbonyl Compounds
1.Nucleophilic Attack:
The nucleophile, with a lone pair of electrons, attacks the electrophilic carbonyl carbon.
The carbon is electrophilic due to the polarization of the C=O bond, where the oxygen atom pulls electron density away, making the carbon partially positive (δ+).
Equation: Nu⁻ + R-CHO → Nu-R-CHO⁻
2.Formation of a Tetrahedral Intermediate:
The attack breaks the π bond of the C=O group, pushing electrons onto the oxygen atom.
This forms a tetrahedral intermediate with a negatively charged oxygen (O⁻).
Intermediate Formation: Nu-R-CHO⁻
3.Proton Transfer:
The negatively charged oxygen in the intermediate abstracts a proton (H⁺) from a proton source (solvent, acid, or conjugate acid of the nucleophile).
This step neutralizes the charge on the oxygen, resulting in the formation of an alcohol.
Protonation: Nu-R-CHO⁻ + H⁺ → Nu-R-CH(OH)
Example Reaction
For a generic aldehyde (R−CHO):
Step 1: The nucleophile (Nu⁻) attacks the carbonyl carbon, forming a tetrahedral intermediate.
Step 2: The intermediate captures a proton (H⁺), yielding an alcohol (Nu−R−CH(OH)).
This mechanism illustrates how nucleophilic addition transforms aldehydes and ketones into alcohols, emphasizing the reactivity of the carbonyl carbon, a key site for chemical synthesis in organic chemistry.