11. Phenols
🎯 Learning Outcomes
By the end of this chapter, students should be able to:
- Describe the nomenclature of phenols briefly.
- Show the preparation of phenol from chlorobenzene, diazonium salt and benzenesulphonic acid.
- State the physical properties of phenol.
- Describe the acidic nature of phenol (comparison with alcohol and water).
- Explain the chemical properties of phenol — with NH₃, Zn, Na, benzenediazonium chloride, phthalic anhydride, acylation, Kolbe’s reaction, Reimer–Tiemann’s reaction, and electrophilic substitution (nitration, sulphonation, bromination, Friedel–Crafts alkylation).
- Describe the tests of phenol (FeCl₃ test, aq. bromine test & Liebermann’s test).
- State important uses of phenol.
1 Introduction & Nomenclature▶
Phenols are a class of organic compounds with the general formula Ar–OH, where Ar– is an aryl group (most commonly C₆H₅–). The –OH group is attached directly to the aromatic ring.
When an –OH group is instead attached to a side-chain alkyl group of benzene, the compound is an aromatic alcohol, not a phenol — this distinction is a common exam trap.
Other examples of phenols
Substituted phenols are named as o-, m-, p- derivatives, or by their common/trivial names shown in brackets below.
2 Preparation of Phenol▶
1. From chlorobenzene (Dow process)
Phenol can be prepared by nucleophilic substitution of chlorobenzene with aqueous NaOH at 360 °C and 300 atm pressure. First used industrially by Dow Chemical, hence the Dow process.
2. From diazonium salt
Benzenediazonium chloride is converted to phenol simply by boiling with water.
3. From benzenesulphonic acid
Benzenesulphonic acid gives sodium phenoxide on fusion with NaOH; acidic hydrolysis of the phenoxide then yields phenol.
3 Physical Properties▶
- Simplest phenols are colourless liquids or low-melting solids.
- They have a characteristic phenolic odour.
- Quite high boiling points, due to intermolecular hydrogen bonding.
- Phenol is somewhat soluble in water (solubility ≈ 9 g/100 mL) because it can hydrogen-bond with water.
4 Acidic Nature of Phenol▶
a. Ionisation and action on litmus
Phenol ionises in aqueous solution into phenoxide ion and hydronium ion. It therefore turns blue litmus red.
b. Reaction with active metals
Phenol reacts with active metals such as sodium or potassium, forming a phenoxide (phenolate) salt with liberation of hydrogen gas.
c. Reaction with caustic alkalis (NaOH, KOH)
Phenol dissolves in strong alkali solutions to form a phenolate salt and water.
d. Reaction with carbonates and bicarbonates
Phenol ionises in aqueous solution as:
Phenol itself is a resonance hybrid of the following five structures:
Similarly, the phenoxide ion is a resonance hybrid of these five structures:
The phenoxide ion is more stable than phenol because none of its resonance structures involve charge separation (the negative charge is simply delocalised over O and the ring). Phenol’s resonance structures, by contrast, involve separating + and – charges, which is energetically costly. So the ionisation equilibrium of phenol lies considerably to the right, producing more H₃O⁺ — phenol is more acidic than an alcohol.
Alcohols and alkoxide ions have no such resonance stabilisation, so their equilibrium lies well to the left (alcohol favoured):
Comparison of acidic strength with water
Phenol is more acidic than water: on ionisation phenol forms the resonance-stabilised phenoxide ion, whereas water forms a hydroxide ion that is not resonance-stabilised.
| Order of acidity (strongest → weakest) |
|---|
| Carboxylic acids > Phenol > Water > Alcohols |
It appears that this quiz is not set up correctly.5 Chemical Reactions of Phenol▶
1. Action with ammonia
Under drastic conditions — high temperature, high pressure, and ZnCl₂ catalyst — phenol reacts with ammonia to give aniline.
2. Action with zinc
Phenol gives benzene when heated with zinc dust (removal of the –OH group).
3. Action with benzenediazonium chloride (coupling reaction)
Phenol undergoes a coupling reaction with benzenediazonium chloride to form p-hydroxyazobenzene, a useful dye.
4. Acylation reaction
Phenol reacts with acid chlorides (alkanoyl chlorides) or acid anhydrides (alkanoic anhydrides) in the presence of sodium ethanoate (sodium acetate) to form a phenyl ester.
5. Kolbe’s reaction (carboxylation)
When sodium phenoxide is heated with CO₂ at 135 °C under 4–7 atm pressure, sodium salicylate forms as the major product; acidification then gives salicylic acid. Introducing a –COOH group into a molecule is called carboxylation.
6. Reimer–Tiemann’s reaction
Heating chloroform with phenol in the presence of aqueous NaOH/KOH at 60–70 °C, followed by acidification, gives a mixture of o-hydroxybenzaldehyde (salicylaldehyde, major) and p-hydroxybenzaldehyde (minor).
7. Action with phthalic anhydride
Phenol and phthalic anhydride condense in the presence of conc. H₂SO₄ to form phenolphthalein, a famous acid–base indicator.
6 Electrophilic Substitution▶
- –OH is an activating group in electrophilic aromatic substitution, so these reactions occur faster in phenol than in benzene.
- Products are ortho- and para-substituted, since –OH is an o-, p-directing group.
I. Halogenation
Phenol reacts with aqueous bromine to give 2,4,6-tribromophenol, a white precipitate — this reaction is used as a test for phenol.
II. Nitration
Phenol reacts with concentrated nitric acid to form 2,4,6-trinitrophenol (picric acid) — used as an explosive, in medicine, and as a yellow dye.
Mononitrophenol can be obtained at low temperature using dilute nitric acid.
III. Sulphonation
Phenol on sulphonation gives different products depending on temperature — mainly the ortho isomer at low temperature and the para isomer at high temperature.
IV. Friedel–Crafts alkylation
It appears that this quiz is not set up correctly.7 Laboratory Tests for Phenol▶
1. FeCl₃ (ferric chloride) test
Phenol reacts with FeCl₃ solution to give a green, blue, violet, or red coloured, water-soluble complex.
2. Aqueous bromine test
Covered above under Halogenation — phenol gives a white precipitate of 2,4,6-tribromophenol with aqueous bromine.
3. Liebermann’s test
Phenols give a red/brown colouration on warming with sodium nitrite dissolved in conc. H₂SO₄. This colouration changes to an intense blue or green on addition of NaOH.
8 Uses of Phenol▶
- Production of Bakelite — a thermosetting plastic.
- Manufacture of dyes, explosives (e.g. picric acid), etc.
- Manufacture of drugs (e.g. aspirin, phenacetin, salicylic acid).
- Preparation of phenolphthalein, an indicator.
- As a disinfectant and germicide.
- Manufacture of cyclohexanol, used as a solvent for rubber.
9 Solved Problems▶
Solution: This is an esterification reaction, just like that between an aliphatic alcohol and a carboxylic acid — here it produces an aromatic ester.
(Always write IUPAC names and side products.)
It appears that this quiz is not set up correctly.📌 Chapter Summary
- Phenol = Ar–OH, with –OH directly on the aromatic ring (not the side chain, which would make it an aromatic alcohol instead).
- Preparation: Dow process (chlorobenzene + NaOH, 360 °C/300 atm), from diazonium salt (+ H₂O), or from benzenesulphonic acid (via NaOH fusion then acidification).
- Acidic nature: phenol > water > alcohol in acidity, because the phenoxide ion is resonance-stabilised with no charge separation; reacts with Na/K and NaOH/KOH, but not with carbonates/bicarbonates.
- Key reactions: with NH₃ (→ aniline), Zn (→ benzene), benzenediazonium chloride (→ azo dye), acylation (→ esters), Kolbe’s reaction (→ salicylic acid), Reimer–Tiemann (→ salicylaldehyde), phthalic anhydride (→ phenolphthalein).
- Electrophilic substitution: –OH is o-, p-directing and ring-activating — bromination, nitration, sulphonation, and Friedel–Crafts all occur faster than in benzene.
- Tests: FeCl₃ (coloured complex), aq. bromine (white ppt.), Liebermann’s test (red/brown → blue/green with NaOH).
⚡ Don’t confuse
Phenol vs. aromatic alcohol: in phenol, –OH is on the ring itself (C₆H₅OH); in an aromatic alcohol like benzyl alcohol, –OH is on a side-chain carbon (C₆H₅CH₂OH) — the two behave very differently, and only phenol shows the acidic/resonance chemistry of this chapter.
Sodium phenoxide vs. sodium salicylate vs. salicylic acid: these are the three successive stages of Kolbe’s reaction (ONa → ONa+COONa → OH+COOH) — track which –COOH/–COONa is present at each step.