Alcohols, Phenols and Ethers • Topic 2 of 3

Phenols

A phenol has the –OH group attached directly to an sp2 carbon of an aromatic ring. The simplest is phenol itself, C6H5OH (carbolic acid). Because the oxygen lone pair conjugates with the ring, phenols behave very differently from alcohols: they are more acidic and their ring is strongly activated toward electrophiles.

Preparation

From haloarenes (Dow process). Chlorobenzene + NaOH at 623 K and 320 atm, then acidification, gives phenol. From benzenesulphonic acid. Fusion with NaOH (alkali fusion) then acidification gives phenol. From cumene (industrial). Cumene (isopropylbenzene) is air-oxidised to cumene hydroperoxide, which on treatment with dilute acid gives phenol and acetone together. From diazonium salts. Benzenediazonium chloride warmed with water (or dilute acid) gives phenol with evolution of N2.

Acidic character

Phenol (pKa ≈ 10) is far more acidic than ethanol (pKa ≈ 16) because its conjugate base, the phenoxide ion, is stabilised by resonance: the negative charge is delocalised onto the ortho and para ring carbons. In an alkoxide the charge stays localised on oxygen. Electron-withdrawing groups (–NO2, especially at o/p) increase acidity (picric acid is strongly acidic); electron-releasing groups (–CH3, –OCH3) decrease it. Phenol is still weaker than carboxylic acids.

Electrophilic aromatic substitution

The –OH group is strongly activating and o/p-directing. Nitration with dilute HNO3 gives o- and p-nitrophenol; conc. HNO3 gives 2,4,6-trinitrophenol (picric acid). Halogenation with bromine water gives 2,4,6-tribromophenol (white precipitate) instantly, while in CS2 at low temperature mono-substitution dominates. Kolbe's reaction: sodium phenoxide + CO2 under pressure, then H+, gives salicylic acid (2-hydroxybenzoic acid). Reimer–Tiemann reaction: phenol + CHCl3 + NaOH gives salicylaldehyde (2-hydroxybenzaldehyde) via a dichlorocarbene intermediate.

Oxidation and detection

Phenol is readily oxidised; with chromic acid (Na2Cr2O7/H2SO4) it gives the conjugated diketone benzoquinone. Phenols give a characteristic violet/blue colour with neutral FeCl3 (forming a coloured iron–phenoxide complex), a standard distinction test from alcohols, which give no colour.

Resonance stabilisation of the phenoxide ion delocalising the negative charge to ortho and para positionsO⁻OO⁻⁻↔↔oxygen-localisedortho carbanionpara carbanionNegative charge is shared over O and the o-/p-ring carbons → phenoxide is resonance-stabilised, so phenol is more acidic than alcohols.
Solved Examples
1
Worked Example
Explain why phenol is more acidic than ethanol.
Solution
  1. Acidity depends on stability of the conjugate base after losing H+.
  2. Phenoxide ion delocalises the negative charge into the ring (resonance onto o- and p-carbons).
  3. Ethoxide keeps the charge localised on oxygen and the alkyl group pushes electron density onto it, destabilising it.
  4. The more stable, resonance-delocalised phenoxide makes phenol the stronger acid (pKa ~10 vs ~16).

Answer: Resonance stabilisation of phenoxide (absent in ethoxide) makes phenol much more acidic than ethanol.

2
Worked Example
Arrange in increasing acidity: phenol, p-nitrophenol, p-cresol (4-methylphenol).
Solution
  1. –NO2 is electron-withdrawing and at para it stabilises phenoxide by resonance → raises acidity.
  2. –CH3 is electron-releasing → destabilises phenoxide → lowers acidity.
  3. Phenol is the reference (no substituent effect).

Answer: p-cresol < phenol < p-nitrophenol.

3
Worked Example
Name the product and reagents of Kolbe's reaction on phenol.
Solution
  1. Treat phenol with NaOH to form sodium phenoxide (more nucleophilic ring).
  2. React with CO2 at about 400 K and 4–7 atm pressure.
  3. Acidify the product.
  4. The CO2 adds ortho to the –OH giving 2-hydroxybenzoic acid.

Answer: Salicylic acid (2-hydroxybenzoic acid); reagents: NaOH, then CO2 under pressure, then H+.

4
Worked Example
What is observed when phenol is treated with bromine water, and why is it so easy?
Solution
  1. The –OH group is strongly activating and o/p-directing.
  2. In aqueous medium the highly reactive phenol undergoes multiple substitution.
  3. Bromine substitutes at the 2-, 4- and 6-positions.
  4. A white precipitate of 2,4,6-tribromophenol forms immediately, even without a catalyst.

Answer: A white precipitate of 2,4,6-tribromophenol forms instantly because –OH strongly activates the ring.

5
Worked Example
Identify reagents X in the Reimer–Tiemann reaction and the product, and name the reactive intermediate.
Solution
  1. Phenol is treated with CHCl3 and aqueous NaOH at about 340 K.
  2. NaOH + CHCl3 generate dichlorocarbene :CCl2, the electrophilic intermediate.
  3. It attacks ortho to –OH; hydrolysis of the –CHCl2 group then gives –CHO.

Answer: X = CHCl3 + NaOH; intermediate = dichlorocarbene (:CCl2); product = salicylaldehyde (2-hydroxybenzaldehyde).

6
Worked Example
How would you chemically distinguish phenol from ethanol in two test tubes?
Solution
  1. Add a few drops of neutral FeCl3 solution to each sample.
  2. Phenol forms an iron–phenoxide complex giving a violet/blue colour.
  3. Ethanol gives no colour change.
  4. (Alternatively, phenol gives a white precipitate with bromine water; ethanol does not.)

Answer: Neutral FeCl3 gives a violet colour with phenol but no colour with ethanol.

Key Points

  • Phenol has –OH on an sp2 aromatic carbon; prepared from chlorobenzene (Dow), benzenesulphonic acid (alkali fusion), cumene (+ acetone), or diazonium salt + warm water.
  • Phenol (pKa ~10) is more acidic than alcohols because the phenoxide ion is resonance-stabilised; still weaker than carboxylic acids.
  • Electron-withdrawing groups (–NO2) at o/p raise acidity; electron-releasing groups (–CH3, –OCH3) lower it.
  • –OH activates the ring (o/p-directing): bromine water → 2,4,6-tribromophenol; HNO3 → nitrophenols / picric acid; Kolbe → salicylic acid; Reimer–Tiemann (via :CCl2) → salicylaldehyde.
  • Phenol oxidises to benzoquinone; neutral FeCl3 gives a violet colour, the standard test distinguishing phenols from alcohols.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Phenol is more acidic than ethanol because:
Explanation: The negative charge of phenoxide is delocalised into the ring; ethoxide cannot do this, so phenol is the stronger acid.
Q2.The industrial cumene process gives phenol together with:
Explanation: Cumene hydroperoxide decomposes in dilute acid to give phenol and acetone (propanone).
Q3.Reaction of phenol with CHCl3 and NaOH (Reimer–Tiemann) gives:
Explanation: Dichlorocarbene attacks ortho and hydrolysis gives the –CHO group, forming salicylaldehyde.
Q4.Which is the strongest acid?
Explanation: The electron-withdrawing –NO2 at para stabilises phenoxide most, making p-nitrophenol the strongest acid here.
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