Nucleophilic Addition to Carbonyl Compounds

• 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)).