Phenol is hydroxybenzene (C6H5OH). The aromatic ring activates reactions at ortho and para positions. CBSE Class 12 and NEET test preparation, acidity and reactions of phenol. Expect one to two questions a year.
Core Concepts
Preparation
From cumene (isopropylbenzene) via hydroperoxide rearrangement — the main industrial route. From chlorobenzene with NaOH at 350°C and 300 atm. From benzenediazonium salt by warming with water.
Cumene process in detail:
Cumene is oxidised by air to cumene hydroperoxide, which is then cleaved by dilute acid to give phenol and acetone. The beauty of this process is that both products are commercially valuable — phenol for resins and acetone for solvents.
From diazonium salt:
This is the lab method. It starts from aniline: aniline → diazonium salt (NaNO2 + HCl at 0-5°C) → phenol (warming with water). The diazonium route is important because it connects amines to phenols.
Acidity of phenol
More acidic than alcohols because the phenoxide ion is stabilised by resonance with the ring. Less acidic than carboxylic acids. pKa about 10. Reacts with NaOH but not with NaHCO3.
After losing H+:
- Phenoxide ion: The negative charge delocalises into the benzene ring through 5 resonance structures. The charge is spread over the oxygen and the ring carbons.
- Ethoxide ion: The negative charge stays localised on one oxygen. No resonance stabilisation.
Greater stabilisation of the conjugate base → easier to lose H+ → stronger acid.
Effect of substituents on phenol acidity:
- Electron-withdrawing groups (EWG) like -NO2 increase acidity by stabilising the phenoxide ion further. p-Nitrophenol (pKa 7.2) is more acidic than phenol.
- Electron-donating groups (EDG) like -CH3 decrease acidity. p-Cresol (pKa 10.3) is less acidic than phenol.
- 2,4,6-trinitrophenol (picric acid, pKa 0.4) is as acidic as mineral acids — it reacts even with NaHCO3.
Electrophilic aromatic substitution
OH is an activator and ortho-para director. Phenol reacts faster than benzene in nitration, halogenation and sulphonation. Bromination gives 2,4,6-tribromophenol with excess Br2 water.
Why -OH activates the ring: The lone pair on oxygen donates electron density into the ring through resonance, making the ring more electron-rich and more attractive to electrophiles. This makes phenol about 1000 times more reactive than benzene towards electrophilic substitution.
Bromination:
With excess bromine water, all three available positions (2 ortho + 1 para) are brominated. The product (2,4,6-tribromophenol) is a white precipitate. This is a qualitative test for phenol.
Nitration: With dilute HNO3 at low temperature, gives a mixture of ortho and para nitrophenols. With concentrated HNO3 + H2SO4, gives 2,4,6-trinitrophenol (picric acid).
Reimer-Tiemann reaction
Phenol + CHCl3 + NaOH → salicylaldehyde. Adds a formyl group at the ortho position. Classic method for aromatic aldehyde synthesis.
The mechanism involves generation of dichlorocarbene (:CCl2) from CHCl3 in alkaline conditions, which attacks the electron-rich ring at the ortho position.
Kolbe’s reaction
Phenol + CO2 + NaOH → sodium salicylate. Acidification gives salicylic acid, the precursor to aspirin.
Reaction with FeCl3
Phenol gives a violet colour with neutral FeCl3. A quick qualitative test for phenols. The colour arises from formation of a coloured iron-phenol complex. Different phenols give different colours (some give blue, green, or red).
Williamson synthesis of ethers
Phenol can be converted to anisole (methoxybenzene) by treating sodium phenoxide with methyl iodide:
This is an SN2 reaction. Always use a primary alkyl halide with phenoxide to avoid elimination.
Worked Examples
In phenoxide, the negative charge delocalises into the benzene ring through resonance, stabilising the anion. Ethoxide has no such delocalisation, so it is less stable and ethanol is less acidic.
Salicylic acid + acetic anhydride → aspirin (acetylsalicylic acid) + acetic acid. The OH of salicylic acid is acetylated. Simple one-step synthesis.
p-Nitrophenol > phenol > p-cresol. The -NO2 group is electron-withdrawing (-I and -R effects), stabilising the phenoxide ion. The -CH3 group is electron-donating (+I effect), destabilising the phenoxide ion. More stable conjugate base = stronger acid.
Benzene needs a Lewis acid catalyst (FeBr3) for bromination because the ring is not electron-rich enough. Phenol’s -OH group donates electron density into the ring, making it reactive enough to attack Br2 directly without a catalyst.
Summary of Phenol Reactions
| Reaction | Reagent | Product | Type |
|---|---|---|---|
| Bromination | Br2/H2O | 2,4,6-tribromophenol | EAS |
| Nitration (dilute) | Dilute HNO3 | o- and p-nitrophenol | EAS |
| Reimer-Tiemann | CHCl3 + NaOH | Salicylaldehyde | EAS |
| Kolbe | CO2 + NaOH | Salicylic acid | EAS |
| FeCl3 test | Neutral FeCl3 | Violet colour | Qualitative |
| Ether formation | CH3I + NaOH | Anisole | SN2 |
| Acetylation | (CH3CO)2O | Aspirin (from salicylic acid) | Esterification |
Common Mistakes
Saying phenol is more acidic than carboxylic acids. It is less acidic.
Writing that phenol reacts with NaHCO3. It does not — phenol is too weak an acid.
Confusing Reimer-Tiemann and Kolbe. Reimer-Tiemann gives aldehyde; Kolbe gives carboxylic acid.
Saying phenol needs a Lewis acid catalyst for bromination. It does not — the activated ring reacts directly with bromine water.
Forgetting that picric acid (2,4,6-trinitrophenol) is strongly acidic. Three nitro groups make the phenoxide ion extremely stable, making picric acid comparable to mineral acids.
Exam Weightage and Revision
Phenol reactions carry 1-2 NEET questions per year and 3-5 marks in CBSE Class 12 boards. The most tested areas: acidity comparison, Reimer-Tiemann reaction, Kolbe reaction, and bromination. JEE asks about substituent effects on acidity.
| Question Type | NEET Frequency | Difficulty |
|---|---|---|
| Acidity comparison | Every year | Medium |
| Named reaction (Reimer-Tiemann/Kolbe) | Most years | Medium |
| Bromination product | Every 2 years | Easy |
| FeCl3 test | Occasional | Easy |
| Effect of substituents on acidity | JEE focus | Hard |
The single most common question: “Arrange phenol, ethanol, and acetic acid in order of acidity.” Answer: acetic acid > phenol > ethanol. Know the reasoning (resonance stabilisation of conjugate base).
Practice Questions
Q1. Why does phenol dissolve in NaOH but not in NaHCO3?
Phenol (pKa ~10) is a weaker acid than carbonic acid (pKa1 ~6.3). NaOH is a strong base that can deprotonate phenol: . But NaHCO3 is a weak base — it can only deprotonate acids stronger than carbonic acid. Since phenol is weaker than carbonic acid, NaHCO3 cannot deprotonate it. Carboxylic acids (pKa ~4-5) are stronger and do react with NaHCO3.
Q2. Write the product of Kolbe’s reaction. How is aspirin made from this product?
Kolbe’s reaction: sodium phenoxide + CO2 (125°C, 4-7 atm) → sodium salicylate. Acidification gives salicylic acid (2-hydroxybenzoic acid). To make aspirin: treat salicylic acid with acetic anhydride → acetylsalicylic acid (aspirin) + acetic acid. The phenolic -OH is acetylated while the -COOH remains free.
Q3. Arrange in decreasing order of acidity: phenol, 2,4-dinitrophenol, p-cresol.
2,4-Dinitrophenol > phenol > p-cresol. Two -NO2 groups (strong EWG) strongly stabilise the phenoxide ion through -I and -R effects. The -CH3 group (EDG, +I effect) destabilises the phenoxide. More stable anion = stronger acid.
FAQs
Why is phenol a solid at room temperature while ethanol is a liquid?
Phenol (MP 41°C) has stronger intermolecular forces than ethanol (MP -114°C). Phenol molecules form strong hydrogen bonds, and the flat aromatic rings allow efficient pi-stacking between molecules. This combination gives phenol higher melting and boiling points.
Is phenol toxic?
Yes. Phenol is corrosive to skin and toxic if ingested. Concentrated phenol causes severe chemical burns. Historically, it was used as carbolic acid for surgical antisepsis by Joseph Lister in the 1860s — revolutionary at the time, but replaced by milder antiseptics since.
What is the difference between phenol and alcohol?
Both have an -OH group, but in phenol the -OH is directly bonded to an aromatic ring, while in alcohols it is bonded to an aliphatic carbon. This structural difference causes major property differences: phenol is more acidic, undergoes electrophilic substitution on the ring, and gives a colour with FeCl3.
Memorise three reactions of phenol — Reimer-Tiemann, Kolbe, bromination. Covers most NEET questions on phenol.
Coupling Reaction of Phenol
Phenol can couple with diazonium salts to form brightly coloured azo dyes:
The product is p-hydroxyazobenzene — an azo dye. The -N=N- (azo) group acts as a chromophore, giving intense colour. Azo dyes are the largest class of synthetic dyes used in the textile industry.
This reaction works because the -OH group activates the ring enough for it to be attacked by the weakly electrophilic diazonium cation. The coupling happens preferentially at the para position.
Phenol vs Alcohol — Complete Comparison
| Property | Phenol (C6H5OH) | Ethanol (C2H5OH) |
|---|---|---|
| Acidity (pKa) | ~10 | ~16 |
| Reacts with NaOH | Yes | No |
| Reacts with NaHCO3 | No | No |
| Reacts with Na metal | Yes | Yes |
| FeCl3 test | Violet colour | No colour |
| Bromination | Without catalyst (Br2/H2O) | Needs catalyst or UV |
| Structure | -OH on aromatic ring | -OH on aliphatic carbon |
| Intermolecular forces | H-bonding + pi-stacking | H-bonding only |
| Melting point | 41°C (solid) | -114°C (liquid) |
The fundamental reason for all these differences: in phenol, the lone pair on oxygen interacts with the aromatic ring through resonance, making the O-H bond more polar and easier to break (more acidic), and making the ring more electron-rich (more reactive toward electrophiles).
Industrial Importance of Phenol
Phenol is one of the most important industrial chemicals, with global production exceeding 10 million tonnes per year. Uses include:
- Bakelite (phenol + formaldehyde): first fully synthetic plastic, used in electrical insulators
- Bisphenol A (2 phenol + acetone): raw material for polycarbonate plastics and epoxy resins
- Aspirin (via salicylic acid from Kolbe’s reaction): one of the world’s most-used drugs
- Antiseptic (dilute phenol solutions): Lister’s carbolic acid spray revolutionised surgery in the 1860s
- Nylon-6 (via caprolactam from cyclohexanone, which comes from phenol): one of the most common fibres
Phenol is the gateway to aromatic hydroxyl chemistry. The combination of acidity and ring activation makes it unusually reactive.