Q68E - Organic Chemistry (Mcmurry)

Question

The following reactions are unlikely to provide the indicated product in high yield. What is wrong with each?

Step-by-Step Solution

Answer

(a) In this reaction, the shown product will give other products by itself and the final products will be a mixture of condensation products.

(b) In this reaction, the given reactants are monoketone and α,β-unsaturated ketone. The monoketone will convert into an enamine and then react with the α,β-unsaturated. Therefore the final product will be a mixture of diketones.

(c) In this reaction, the given diketone can participate in internal aldol condensation reaction and will give a stable six membered ring. 

Answer of subpart (a): Solution of the problem

In this reaction, the shown product will give other products by itself and the final products will be a mixture of condensation products.

Explanation of the solution

Both the acetaldehyde and acetone contain α-hydrogen atoms. Therefore in the reaction medium two types of condensation can be happened – intramolecular and intermolecular. Hence, the final product will be a mixture of different products.

Answer of subpart (b): Solution of the problem

In this reaction, the given reactants are monoketone and α,β-unsaturated ketone. The monoketone will convert into an enamine and then react with the α,β-unsaturated. Therefore the final product will be a mixture of diketones.

Explanation of the solution

The given monoketone will not take place readily and the percentage of the product will be low. For a good percentage of yields, the monoketone will convert into an enamine and then participate in Michael addition. Now, the methyl cyclohexanone has two α-hydrogen atoms. Therefore the removal of these two hydrogen atoms will give two different products. Therefore the final product will be a mixture of diketones.

Answer of subpart (c): Solution of the problem

In this reaction, the given diketone can participate in internal aldol condensation reaction and will give a stable six membered ring. 

Explanation of the solution

If the base can remove the hydrogen atom from (a)-α carbon, then the diketone will form methylcyclohexanone (a stable six membered ketone).

If the base can remove the hydrogen atom from (b)-α carbon, then the diketone will form methylcyclobutanone (a relatively unstable four membered ketone).

 

As a six membered ring is more stable and has less strain than that of a four membered ring, for this the six membered ketone will be a major product.