Q.Give the equations of reactions for the preparation of phenol from cumene.

The key idea is electrophilic aromatic substitution via an intermediate hydroperoxide, followed by acid-catalyzed rearrangement (the cumene process).

Step 1: Cumene (isopropylbenzene) is oxidised with air to form cumene hydroperoxide.

Step 2: The hydroperoxide is treated with dilute acid, which causes a rearrangement (Hock rearrangement) to give phenol and acetone.

The equations are:

1. Oxidation:

   $\text{C}_6\text{H}_5\text{CH}(\text{CH}_3)_2 + \text{O}_2 \xrightarrow{\text{air, heat}} \text{C}_6\text{H}_5\text{C}(\text{OOH})(\text{CH}_3)_2$

2. Acid-catalysed cleavage: …

The cumene process (also called the Hock process) is one of the most important industrial routes to phenol. It’s a beautiful example of how a seemingly simple aromatic substitution can be engineered into a high-yield, atom-economical process. The trick is that we don’t directly substitute the benzene ring — instead, we first install a side chain that can be oxidised, then rearrange that oxidised side chain into a hydroxyl group on the ring.

Let’s walk through the chemistry step by step.

1. Start with cumene (isopropylbenzene)

   Cumene is benzene with an isopropyl group ($-CH(CH_3)_2$) attached. This alkyl group is the key — it’s going to be our handle for introducing oxygen.

2. First step: Autoxidation to cumene hydroperoxide

   Cumene is treated with oxygen (from air) at around 90–130°C, often with a radical initiator. The reaction proceeds via a free-radical chain mechanism:

   • A radical abstracts a hydrogen from the benzylic carbon (the carbon directly attached to the ring), forming a benzylic radical.
   • This radical reacts with $O_2$ to give a peroxy radical.
   • The peroxy radical abstracts another hydrogen from a cumene molecule, forming cumene hydroperoxide and regenerating the benzylic radical.

   The overall reaction is:

The product is cumene hydroperoxide — a molecule with an $-OOH$ group attached to the same carbon that was originally benzylic.

3. Second step: Acid-catalysed cleavage (the Hock rearrangement)

   This is the clever part. Cumene hydroperoxide is treated with dilute sulfuric acid (or another strong acid) at around 60–80°C. The acid protonates the $-OOH$ group, which then undergoes a rearrangement that is essentially an electrophilic aromatic substitution — but on the oxygenated side chain, not directly on the ring.

   The mechanism:

   • Protonation of the $-OH$ of the hydroperoxide makes it a better leaving group ($-OH_2^+$).
   • The $-OOH$ group then undergoes a 1,2-shift: the phenyl group (the benzene ring) migrates from the carbon to the adjacent oxygen, while water leaves.
   • This forms a carbocation intermediate that is resonance-stabilised by the oxygen.
   • Water attacks the carbocation, and after deprotonation, we get phenol and acetone.

   The overall reaction is:

   $C_6H_5-C(CH_3)_2-OOH \xrightarrow{H^+} C_6H_5OH + (CH_3)_2C=O$ …

Method: Cumene Hydroperoxide Process (Hock Process)

This is an industrial method for preparing phenol and acetone from cumene (isopropylbenzene). The key concept is that the tertiary benzylic C–H bond in cumene is highly reactive toward autoxidation.

Steps

1. Autoxidation of cumene Cumene is treated with oxygen (air) at elevated temperature (~100–130°C) to form cumene hydroperoxide.

2. Acid-catalyzed cleavage The hydroperoxide is treated with dilute sulfuric acid. It undergoes a rearrangement (Hock rearrangement) followed by hydrolysis to yield phenol and acetone.

Overall reaction …

Here are the common mistakes students make when writing the preparation of phenol from cumene (the cumene–phenol process), along with how to avoid each.

Mistake 1: Writing the wrong starting material or forgetting the air oxidation step

• What students do: They jump straight to the acid decomposition, writing something like:

  Cumene + $H_2SO_4$ → Phenol + Acetone

  (This is wrong — cumene does not react directly with acid to give phenol.)

• Why it’s wrong: The process has two distinct steps:

  1. Air oxidation of cumene (isopropylbenzene) to cumene hydroperoxide.
  2. Acid-catalysed cleavage of the hydroperoxide to phenol and acetone.

• How to avoid: Always write the two-step sequence clearly.

Correct Step 1 (oxidation):

Correct Step 2 (cleavage):

Mistake 2: Using the wrong reagent or catalyst in the second step

• What students do: They write $HNO_3$, $HCl$, or concentrated $H_2SO_4$ for the cleavage.

• Why it’s wrong: The cleavage requires dilute acid (usually dilute $H_2SO_4$). Concentrated acids can cause side reactions (sulfonation, oxidation of phenol).

• How to avoid: Memorise: dilute $H_2SO_4$ is the standard catalyst. Write it clearly as:

  $\xrightarrow[\text{(60–80°C)}]{\text{dil. } H_2SO_4}$

Mistake 3: Forgetting to mention by-product (acetone)

• What students do: They write only phenol as the product.

• Why it’s wrong: The reaction is atom-economical — one mole of cumene gives one mole of phenol and one mole of acetone. Leaving out acetone is incomplete.

• How to avoid: Always write both products in the second equation:

(Phenol + Acetone)

Mistake 4: Writing incorrect structural formulae

• What students do: They write cumene as $\ce{C6H5CH2CH2CH3}$ (n-propylbenzene) or forget the hydroperoxide group.

• Why it’s wrong: Cumene is isopropylbenzene — the side chain is $\ce{-CH(CH3)2}$, not $\ce{-CH2CH2CH3}$.

• How to avoid: Draw or write the structure carefully:

  • Cumene: $\ce{C6H5-CH(CH3)2}$ …