The cumene process is the main industrial way to make phenol. Cumene (isopropylbenzene) reacts with air (O2) to form cumene hydroperoxide. This intermediate is then treated with dilute acid (H+/H2O), which splits it into TWO products: phenol and acetone. Memory hook: "Cumene + air, then acid = Phenol AND Acetone." Acetone is the free by-product you must never forget.
Cumene reacts with air (O2) to give cumene hydroperoxide, which dilute acid (H+/H2O) splits into two products: phenol and acetone.
Your doubts, answered
What exactly is cumene?
Cumene is isopropylbenzene, C6H5-CH(CH3)2. It is a benzene ring joined to an isopropyl group. In the cumene process this molecule is the starting material. NEET often writes it as C6H5CH(CH3)2, so learn to read that formula as cumene at a glance.
What are the two steps of the cumene process?
Step 1: Cumene reacts with air (O2). This oxidises the special C-H bond and makes cumene hydroperoxide, C6H5-C(CH3)2-O-O-H. Step 2: This hydroperoxide is treated with dilute acid (H+/H2O). It breaks apart to give phenol and acetone. Two easy steps: air first, acid second.
What is the intermediate in the cumene process?
The intermediate is cumene hydroperoxide, C6H5-C(CH3)2-O-O-H. Notice the -O-O-H part (a peroxide) sits on the carbon that has the benzene ring and two CH3 groups. NEET 2019 asked for this exact structure, so memorise the -O-O-H (peroxide), not -O- (ether).
Why does the O2 attack that particular carbon?
The carbon holding the two methyl groups and the ring is a benzylic 3rd-degree (tertiary) position. The radical formed there is stable, so air attacks that C-H bond and not the ring or the methyl groups. This is why the peroxide forms on that central carbon.
What are the products of the cumene process?
Two products: phenol (C6H5OH) and acetone (CH3COCH3). Phenol is the main product we want. Acetone is a valuable by-product made for free. NEET loves to ask for the by-product, and the answer is acetone.
Is acetone the by-product or is water?
Acetone (CH3COCH3) is the by-product. Water is NOT counted as a product here. When a question says 'name the by-product of the cumene process,' write acetone. This one fact appears again and again in NEET.
How can I make cumene itself for this process?
Cumene is made by Friedel-Crafts alkylation: benzene + propene (or 1-chloropropane with anhydrous AlCl3). Watch out: a 1st-degree propyl cation rearranges by a hydride shift to the more stable 2nd-degree isopropyl cation, so you get cumene (isopropylbenzene), not n-propylbenzene. NEET 2018 tested this rearrangement.
⚠️ The NEET trap ✗ The intermediate is cumene peroxide written as C6H5-O-O-CH(CH3)2, an ether-like oxygen chain. ✓ The intermediate is cumene HYDROPEROXIDE, C6H5-C(CH3)2-O-O-H. The -O-O-H group sits on the central carbon that carries the ring and both methyl groups, not on the oxygen next to the ring. 🧠 Hydroperoxide ends in -O-O-H on carbon. If the structure has no O-O-H, or the O sits between ring and carbon, it is wrong (NEET 2019).
Real NEET questions
NEET 2019
The structure of intermediate A in the following reaction is: C6H5CH(CH3)2 + O2 -> A ; A --(H+/H2O)--> C6H5OH + CH3COCH3
A · C6H5-O-CH(CH3)2
B · C6H5-C(CH3)2-O-O-H ✓
C · C6H5-O-O-CH(CH3)2
D · C6H5-CH2-C(CH3)(OOH)H
Solution: Air (O2) oxidises the benzylic tertiary C-H of cumene to form cumene hydroperoxide, C6H5-C(CH3)2-O-O-H. This -O-O-H sits on the central carbon holding the ring and two CH3 groups. With H+/H2O it rearranges and hydrolyses to phenol and acetone. So A is option (B).
NEET 2018
Identify the major products P, Q, R: C6H6 + CH3CH2CH2Cl --(anhyd. AlCl3)--> P ; P --(i) O2 (ii) H3O+/heat--> Q + R
A · P: CH3CH(OH)CH3, Q: acetone, R: phenol
B · P: C6H5CH2CH2CH3, Q: benzaldehyde, R: benzoic acid
C · P: C6H5CH2CH2CH3, Q: benzaldehyde, R: CH3CH2OH
Solution: In Friedel-Crafts alkylation the 1st-degree propyl cation rearranges by a 1,2-hydride shift to the stable 2nd-degree isopropyl cation, giving cumene (P). Cumene + O2 gives cumene hydroperoxide, and H3O+/heat splits it into phenol (Q) and acetone (R). Answer (D).
NEET 2026
Match the transformation with the reagents. D. C6H5CH(CH3)2 -> C6H5OH (cumene to phenol) matches which reagent set? (II) (i) O2 ; (ii) H2O/H+
A · Cumene to phenol matches (II): (i) O2 (ii) H2O/H+ ✓
B · Cumene to phenol matches (I): oleum then NaOH heat then H+
C · Cumene to phenol matches (III): CH3OH/H+ then H2 catalyst
D · Cumene to phenol matches (IV): conc. H2SO4 heat then H+/H2O
Solution: The cumene process uses (i) O2 to form cumene hydroperoxide and (ii) H2O/H+ to hydrolyse it into phenol and acetone. That is reagent set (II). The oleum route (I) is the benzene-sulphonic-acid method for making phenol from benzene, a different path.
Solved Alcohols, Phenols And Ethers NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It makes most of the world's phenol and also gives acetone as a free by-product. Two useful chemicals from one cheap process makes it very economical, which is why NCERT says most phenol is made this way.
What is the difference between cumene and cumene hydroperoxide?
Cumene is C6H5CH(CH3)2 (isopropylbenzene), the starting material. Cumene hydroperoxide is C6H5C(CH3)2-O-O-H, the intermediate formed after air (O2) adds a -O-O-H group. Cumene has a C-H; the hydroperoxide has C-O-O-H.
Does the cumene process give only phenol?
No. It always gives two products: phenol and acetone. If a NEET question asks for the by-product, the answer is acetone, not water.
What reagents split cumene hydroperoxide into phenol and acetone?
Dilute acid and water (H+/H2O), often with heating. The acid triggers a rearrangement (aryl group migration) and then hydrolysis, giving phenol plus acetone.
Is the cumene process the same as making phenol from chlorobenzene?
No. Chlorobenzene, benzene sulphonic acid and diazonium salts are separate routes to phenol. The cumene process is unique because it also produces acetone. See the other-methods page for those alternative routes.
