18P - Organic Chemistry (Mcmurry)

Question

Unlike most β-diketones, the following β-diketone has no detectable enol content and is about as acidic as acetone. Explain.

 

Step-by-Step Solution

Answer

The given β-diketone is a bicyclic ketone. Removing the proton from the bridgehead carbon will form a double bond on the bridgehead. This will form a highly strained enol. Hence, the given β-diketone cannot exist in the enol form.

But removing a proton from the methylene group adjacent to the carbonyl groups forms an enol very easily. Hence, enolisation like this in acetone can also occur; therefore, the given β-diketone is as acidic as the acetone. 

 

Step 1: Solution of the problem

The enol of a β-diketone is highly strained as the bridgehead carbon accepts the proton. Again there is a double bond in the ring junction. Therefore, it cannot easily forms. The enolisation can take place on the opposite side of the carbon. Hence, the β-diketone is as acidic as the acetone.

 

 

Step 2: Explanation of the solution

The given β-diketone is a bicyclic ketone. Removing the proton from the bridgehead carbon will form a double bond on the bridgehead. This will form a highly strained enol. Hence, the given β-diketone cannot exist in the enol form.

But removing a proton from the methylene group adjacent to the carbonyl groups forms an enol very easily. Hence, enolisation like this in acetone can also occur; therefore, the given β-diketone is as acidic as the acetone. 

 

Step 3: Conclusion

Thus, the given β-diketone has no detectable enol content and is as acidic as acetone is explained.