Question

The Claisen condensation converts 2 molecules of an ester into a ?-keto ester. The reaction starts with the ester in an alkoxide/alcohol solution and is worked up with acid to form the neutral ?-keto ester product.

Step 3. Draw the ester-containing intermediate produced from step 2 and draw the next reactant or reagent, if applicable. Add curved arrows.

Step 4. Draw the ester-containing intermediate produced from step 3 and draw the next reactant or reagent, if applicable. Add curved arrows.

Step 5. Draw the ester-containing intermediate produced from step 4 and draw the final reagent, H3O . Add curved arrows.

Answer 1

Concepts and reason

The Claisen condensation is a reaction between any two esters or an ester and a carbonyl in the presence of a strong base that leads to a new carbon-carbon bond formation in the product. Generally, in this type of condensation reaction, a carbanion is the intermediate. A carbanion is generated when a proton is abstracted from the ester by the base.

Fundamentals

• Curved arrow mechanism: Curved arrow mechanism explains how the reaction is proceeding and the clear transfer of electrons. In this curved arrow, two parts are there: (1) Head (2) Tail.

• Head indicates the destination of electron pair while the tail part indicates the excess electron pair on a molecule or a pi bond.

• Curved arrow always indicates the electron flow path in the reaction mechanism. That is, a curved arrow directed towards positive charge or electron deficient site from the electron cloud.

• General Claisen condensation reaction is depicted below:

Given reaction is drawn below:

Ethoxide is strong base, and it abstracts the acidic proton from substrate that leads to carbanion intermediate.

Produced carbanion intermediate reacts with one more ester to give condensed product.

Ethoxide elimination from oxyanion is depicted below:

Due to the basic nature of ethoxide ion, it abstracts the proton from the active methylene part of

the product.

On protonation to the carbanion intermediate:

Ans:

The complete curved arrow mechanism is shown below: