Review the key concepts, formulae, and examples before starting your quiz.
πConcepts
The carbonyl carbon in aldehydes and ketones is hybridized and electrophilic due to the polar nature of the bond, where oxygen carries a partial negative charge () and carbon carries a partial positive charge ().
In Step 1 of the mechanism, a nucleophile () attacks the electrophilic carbonyl carbon from a direction approximately perpendicular to the plane of the hybridized orbitals. This changes the hybridization of the carbon atom from to .
The attack results in the formation of a tetrahedral alkoxide intermediate where the negative charge resides on the oxygen atom.
In Step 2, the intermediate captures a proton () from the reaction medium or the reagent itself to produce a neutral addition product.
Reactivity trends: Aldehydes are generally more reactive than ketones toward nucleophilic addition due to: 1. Steric reasons (ketones have two bulky groups), and 2. Electronic reasons (two alkyl groups in ketones reduce the electrophilicity of the carbon more than one alkyl group in aldehydes via the effect).
πFormulae
π‘Examples
Problem 1:
Explain the formation of cyanohydrin from Ethanal () using the nucleophilic addition mechanism.
Solution:
- Generation of nucleophile: .
- Attack: The ion attacks the carbonyl carbon of to form the intermediate .
- Protonation: .
Explanation:
The cyanide ion () acts as a strong nucleophile. The base () is required to catalyze the reaction by generating from the weak acid .
Problem 2:
Arrange the following in increasing order of reactivity towards nucleophilic addition: , , .
Solution:
Explanation:
Formaldehyde () is most reactive as it has no alkyl groups to cause steric hindrance or reduce the positive charge on carbon via the effect. Acetone () is least reactive due to the presence of two electron-releasing methyl groups.