23E - Organic Chemistry (Mcmurry)

Question

The two optically beta-keto acids below were decarboxylated using the conditions typically used for the acetoacetate synthesis. Will the ketone products also be optically active? Provide the complete mechanism to explain your answer.

Step-by-Step Solution

Answer

Step 1: Subpart (a)

The product is a racemic mixture. Hence it is optically inactive.

Step 2: Mechanism of the reaction

Step 3: Explanation

Elimination of carbon dioxide of the keto acid forms its enol form. The enol is planar and achiral. The tautomerization between the ketone and enol forms the enantiomers in equimolar proportions. Hence, the product becomes racemic and optically inactive.

 

Step 4: Conclusion

The complete reaction is

Step 5: Subpart (b)

The product is optically active.

Step 6: Mechanism of the reaction

Step 7: Explanation

Elimination of carbon dioxide of the keto acid forms its enol form. There is no chiral carbon in the reaction. The tautomerization between the ketone and enol forms the enantiomers in different proportions. Hence, the product becomes optically active.

Step 8: Conclusion

The complete reaction is