Acetoacetic ester synthesis: practice and mechanism
Like the malonic ester synthesis, but with acetoacetic ester: alkylation, hydrolysis and decarboxylation give a methyl ketone. R–X becomes R–CH₂–CO–CH₃.
Example
As in the malonic ester synthesis the enolate is alkylated, hydrolysed and decarboxylated. With acetoacetic ester a methyl ketone remains: the haloalkane R–X becomes R–CH₂–CO–CH₃.
Mechanism step by step
- 1
Ethoxide deprotonates ethyl acetoacetate at the CH₂ between the keto and ester groups.
- 2
The enolate is alkylated by the haloalkane via SN2.
- 3
Heating in acid hydrolyses the esters to the β-keto acid or malonic acid, which loses CO₂ through a six-membered transition state (decarboxylation).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- The β-keto acid loses CO₂ on heating. What remains is the methyl ketone without a COOH group.
Often confused with: Malonic ester synthesis · Carboxylic acid from Grignard and CO₂ · SN2 with cyanide (chain extension)
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.
Malonic or acetoacetic ester?
Malonic ester → carboxylic acid (R–CH₂–COOH). Acetoacetic ester → methyl ketone (R–CH₂–CO–CH₃).