Q67E - Organic Chemistry (Mcmurry)

Question

Acid-catalyzed dehydration of 3-hydroxy-3-phenylcyclohexanone leads to an unsaturated ketone. What possible structures are there for the product? At what position in the IR spectrum would you expect each to absorb? If the actual product has an absorption at $\mathbf{1670}{\mathbf{cm}}^{-1}$, what is its structure?

Step-by-Step Solution

Answer

The Acid-catalyzed dehydration of 3-hydroxy-3-phenylcyclohexanone is shown below.

The compound B is an $\text{α,β}$unsaturated ketone, and an additional conjugation occurs with the phenyl ring, which lowers the IR absorption value below ${\text{1685\hspace{0.17em}cm}}^{\text{-1}}$. Thus, compound B has an IR absorption at ${\text{1670\hspace{0.17em}cm}}^{\text{-1}}$.

Step-by-Step Solution Step 1: Acid-catalyzed dehydration

The acid-catalyzed dehydration of tertiary alcohols proceeds via E1 elimination mechanism, removing water from a compound. The product formed is an alkene, and the major product is that which has conjugation.

Step 2: The acid-catalyzed dehydration of 3-hydroxy-3-phenylcyclohexanone

The acid-catalyzed dehydration of 3-hydroxy-3-phenylcyclohexanone is the formation of alkene bonds. Two compounds, A and B, are possible, and the reaction is shown below.

                Acid-catalyzed dehydration of 3-hydroxy-3-phenylcyclohexanone

Step 3: The structure of the product that has absorption at 1670   cm - 1

Compound A is a cyclic and non-conjugated enone. The infrared absorption of the carbonyl group occurs at ${\text{1715\hspace{0.17em}cm}}^{\text{-1}}$.

Compound B is an $\text{α,β}$ unsaturated ketone, and there an additional conjugation occurs with the phenyl ring, which lowers the IR absorption value below ${\text{1685\hspace{0.17em}cm}}^{\text{-1}}$ . This is because the actual IR absorption takes place at ${\text{1670\hspace{0.17em}cm}}^{\text{-1}}$. Thus, compound B is correct.