18.24:
Benzene to Phenol via Cumene: Hock Process
The Hock process converts benzene to phenol via cumene and cumene hydroperoxide.
The first step involves the Friedel–Crafts alkylation of benzene with propene to generate cumene.
In this step, propene reacts with phosphoric acid to form the isopropyl cation that adds to the benzene ring to form cumene.
After this, cumene undergoes a radical chain reaction where a radical initiator abstracts the benzylic hydrogen atom, providing a tertiary benzylic radical.
In the chain propagation step that follows, the benzylic radical reacts with an oxygen diradical to generate the cumene hydroperoxide radical. A subsequent reaction with another cumene molecule forms cumene hydroperoxide while simultaneously regenerating the benzylic radical.
Next, cumene hydroperoxide is protonated at the terminal oxygen to give the oxonium ion.
Hydrolytic rearrangement of the ion involving migration of the phenyl ring to oxygen, along with the simultaneous loss of water produces a carbocation.
The subsequent addition of water, a rearrangement through proton shift followed by deprotonation generates the phenol, along with the co-product—acetone.
18.24:
Benzene to Phenol via Cumene: Hock Process
The synthesis of phenol from benzene via cumene and cumene hydroperoxide is called the Hock process. First, a Friedel–Crafts alkylation reaction of benzene with propene gives cumene. Then cumene forms cumene hydroperoxide via a radical chain reaction. In the chain initiation step, the benzylic hydrogen is abstracted to give a benzylic radical. In the chain propagation step, the benzylic radical reacts with an oxygen diradical to form a cumene hydroperoxide radical. The cumene hydroperoxide radical reacts with another cumene molecule to yield cumene hydroperoxide, regenerating the benzylic radical. In the subsequent protonation step, cumene hydroperoxide gives an oxonium ion. Further, a hydrolytic rearrangement migrates the phenyl group to oxygen (with the loss of water) to yield the carbocation. Finally, the addition of water and a subsequent rearrangement involving a proton shift gives phenol as the product.