Answer
🔬 Identification of the Oxygen Atom from the Nucleophile
In nucleophilic addition to a carbonyl compound (e.g., aldehyde or ketone), the nucleophile (often a negatively charged or neutral oxygen-containing species like alcohol or water) attacks the electrophilic carbon of the carbonyl group.
The oxygen atom introduced by the nucleophile becomes the hydroxyl (–OH) group attached to the carbon formerly double bonded to the carbonyl oxygen.
⚙️ Correct Mechanistic Stepwise Order
- Step 1: Protonation of the carbonyl oxygen by an acid (H⁺), increasing the electrophilicity of the carbon.
- Step 2: Nucleophilic attack by the nucleophile on the carbonyl carbon.
- Step 3: Formation of a tetrahedral intermediate.
- Step 4: Proton transfer to stabilize the intermediate, usually yielding an alcohol or related functional group.
This sequence ensures proper charge stabilization and makes the carbon more reactive toward nucleophiles.
💡 Why Protonate the C=O Oxygen?
- The carbon in a carbonyl group is partially positive due to the difference in electronegativity between carbon and oxygen.
- Protonation of the oxygen increases the polarity of the C=O bond.
- This enhances the electrophilicity of the carbonyl carbon, making it more susceptible to nucleophilic attack.
- Without this protonation (in acidic conditions), the reaction would proceed more slowly or require a stronger nucleophile.
Protonating the oxygen makes the carbonyl carbon a much better electrophile, facilitating nucleophilic addition efficiently.