Animated Solution for Chemistry - Organic Chemistry: The major product U in the following reactions is :

Welcome to a beautiful journey through one of the most industrially significant reaction sequences in organic chemistry! We are presented with a two-step synthesis. In the first step, a benzene ring is reacted with propene in the presence of an acid catalyst ($H^{+}$) under high pressure and heat to form an intermediate compound T.

Following this, compound T is subjected to oxidation using molecular oxygen ($O_2$) and a radical initiator to yield the final major product U. Our goal is to identify the exact structure of U.

The Friedel-Crafts Alkylation

Let's break down the first reaction. We have an alkene (propene) and an acidic medium. The first thing that happens is the protonation of the alkene. The pi electrons of propene attack the $H^{+}$ ion. According to Markovnikov's rule, the proton adds to the terminal carbon to generate the most stable carbocation possible.

This yields the secondary isopropyl cation, which is stabilized by hyperconjugation and the inductive effect of two methyl groups.

Now, this electrophilic carbocation encounters the electron-rich benzene ring. Through a classic Electrophilic Aromatic Substitution (EAS), specifically a Friedel-Crafts alkylation, the isopropyl group attaches to the benzene ring.

The resulting molecule, isopropylbenzene, is universally known in the chemical industry as Cumene. Therefore, our intermediate T is Cumene.

Radical Autoxidation

Moving on to the second step, Cumene is treated with $O_2$ and a radical initiator. This setup screams radical autoxidation. To predict the product, we must find the most reactive hydrogen atom in the Cumene molecule.

Look closely at the benzylic carbon—the carbon atom directly bonded to the benzene ring. It holds a single hydrogen atom. If a radical abstracts this specific hydrogen, it leaves behind a cumyl radical. This radical is exceptionally stable because its unpaired electron is delocalized over the entire benzene ring via resonance.

Because the benzylic $C-H$ bond is the weakest and most reactive, the oxygen molecule ($O_2$) readily inserts itself directly into this bond via a radical chain mechanism.

The insertion of the $O_2$ molecule forms a hydroperoxide group ($-O-O-H$) at the benzylic position.

This molecule is Cumene hydroperoxide. Comparing our derived structure with the given options, it perfectly matches option (B).

As a fascinating side note, this exact two-step sequence is the first half of the famous Hock Process. Industrially, Cumene hydroperoxide is subsequently treated with dilute acid to undergo a spectacular rearrangement, ultimately yielding Phenol and Acetone—two massively important chemicals in global manufacturing. Understanding this pathway is a must for any serious chemistry student!