Michael addition: practice and mechanism
The malonate enolate is a soft, stabilised nucleophile and attacks the β-carbon of the enone (1,4-addition). The C=O group is kept.
Example
The stabilised malonate enolate is a soft nucleophile and attacks the Michael acceptor at the β carbon (1,4-addition). A new C–C bond forms and the carbonyl or nitrile group is kept.
Mechanism step by step
- 1
Ethoxide deprotonates diethyl malonate between the two ester groups: a stabilised enolate.
- 2
The enolate attacks the β-carbon of the enone (conjugate addition); a new enolate forms.
- 3
Protonation at the α-carbon gives the 1,5-dicarbonyl compound.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Stabilised enolates do not add to the C=O group (1,2) but conjugatively to the β-carbon (1,4). The product keeps the C=O and loses the C=C bond.
- The esters are kept: NaOEt neither hydrolyses nor decarboxylates them. That would need H₃O⁺ and heat afterwards.
Often confused with: 1,4-addition with cuprates (Gilman) · Robinson annulation
Frequently asked questions
Why is the CH₂ between two C=O groups so acidic?
The negative charge of the enolate spreads over both carbonyl oxygens. The pKa is about 11 to 13, so ethoxide is enough for complete deprotonation.
Which nucleophiles add 1,4?
Soft, stabilised ones: malonate and acetoacetate enolates, cuprates, amines, thiolates, enamines. Hard ones like Grignard or LiAlH₄ usually attack 1,2.