Step 1: Understanding the reaction.
In the preparation of phenol from cumene, cumene is first oxidized to cumene hydroperoxide, which then decomposes to produce phenol and acetone (propanone). Acetone is the valuable byproduct obtained in this process.
Step 2: Analyzing the options.
(A) Propanal: Incorrect. Propanal is not a byproduct in the preparation of phenol from cumene.
(B) Propan - 1 - ol: Incorrect. Propan - 1 - ol is not formed during the process.
(C) Propan - 2 - ol: Incorrect. Propan - 2 - ol is not a product of this reaction.
(D) Propanone: Correct. Propanone (acetone) is the valuable byproduct obtained during the preparation of phenol from cumene.
Step 3: Conclusion.
The correct answer is (D) Propanone, as it is the valuable byproduct obtained in the preparation of phenol from cumene.