Question

Following is an equation for hydroperoxidation of cumene. Propose a radical chain mechanism for this reaction. Assume that initiation is by an unspecified radical, \(\mathrm{R}\).

Short answer

Question: Describe the proposed radical chain mechanism for the hydroperoxidation of cumene, including the initiation, propagation, and termination steps. Answer: The proposed radical chain mechanism for the hydroperoxidation of cumene involves one initiation step, in which an unspecified radical reacts with cumene to produce a cumene radical. There are two propagation steps, where the cumene radical reacts with molecular oxygen to form a peroxyl radical and the peroxyl radical reacts with cumene to produce hydroperoxide, regenerating the cumene radical. Finally, the termination steps include the reaction of two cumene radicals to form a dimer, the reaction between a cumene radical and a peroxyl radical to form a stable product, and the reaction between two peroxyl radicals to form a stable product.

Step-by-step solution

Identify the initiation step

For this reaction, we are given that initiation is by an unspecified radical, R. We will represent cumene as \(\mathrm{C}\) and the hydroperoxide as \(\mathrm{H}\). The initiation step will involve the reaction of R with cumene (C) to produce a cumene radical. 1. \(\mathrm{R + C \rightarrow C\cdot}\)

Propagation steps

Now, we need to identify the propagation steps, consisting of a series of alternating reactions that maintain the radical species. In the hydroperoxidation of cumene, there are two main propagation steps: 2. The cumene radical (\(\mathrm{C\cdot}\)) reacts with a molecule of molecular oxygen (\(\mathrm{O_2}\)) to produce the peroxyl radical (\(\mathrm{C-O\cdot}\)): \(\mathrm{C\cdot + O_2 \rightarrow C-O\cdot}\) 3. The peroxyl radical (\(\mathrm{C-O\cdot}\)) reacts with another molecule of cumene (C) to produce the hydroperoxide (H) and regenerate the cumene radical (\(\mathrm{C\cdot}\)): \(\mathrm{C-O\cdot + C \rightarrow H + C\cdot}\) By these two steps, the radicals transform and propagate through the reaction.

Termination steps

Lastly, we have to identify the termination steps for the chain reaction. Termination occurs when two radical species react with each other to form a stable product, ending the chain. There are several possible termination steps, and we will mention three: 4. Two cumene radicals (\(\mathrm{C\cdot}\)) react to form a dimer: \(\mathrm{C\cdot + C\cdot \rightarrow C-C}\) 5. A cumene radical (\(\mathrm{C\cdot}\)) reacts with the peroxyl radical (\(\mathrm{C-O\cdot}\)) to form a stable product: \(\mathrm{C\cdot + C-O\cdot \rightarrow \mathrm{Product}}\) 6. Two peroxyl radicals (\(\mathrm{C-O\cdot}\)) react to form a stable product: \(\mathrm{C-O\cdot + C-O\cdot \rightarrow \mathrm{Product}}\) These termination steps end the chain process. In summary, the proposed radical chain mechanism for the hydroperoxidation of cumene involves one initiation step, two propagation steps, and the three termination steps listed above.