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Phenols

A phenol has –OH joined straight to a benzene ring carbon. It is made from chlorobenzene (NaOH, 623 K, 300 atm), benzenesulphonic acid, diazonium salts or cumene. Phenol is more acidic than alcohols and water because the phenoxide ion spreads its negative charge over the ring; electron-pulling groups like –NO₂ (at ortho/para) raise acidity, electron-pushing groups like –CH₃ lower it. The –OH group activates the ring at ortho and para positions: bromine water gives 2,4,6-tribromophenol, dilute HNO₃ gives o- and p-nitrophenol, NaOH + CO₂ gives salicylic acid (Kolbe) and CHCl₃ + NaOH gives salicylaldehyde (Reimer–Tiemann).

🎬 Step-by-step story

  1. Meet phenol. The –OH is fixed straight onto a carbon of the benzene ring. In alcohols it sat on an ordinary carbon. This one change makes phenol acidic.
  2. Making phenol: take chlorobenzene, heat it with NaOH under high pressure. Cl leaves, O⁻Na⁺ joins; then dilute acid turns it into phenol.
  3. Acidity: the H of –OH leaves as H⁺. The negative charge left behind does not stay on O – it spreads to the ortho and para carbons (yellow). Spread-out charge = stable ion.
  4. Compare acids: taller bar = stronger acid. Ethanol < water < phenol < nitrophenols < picric acid. –NO₂ pulls the charge further and helps.
  5. Because –OH pushes electrons into the ring, ortho and para positions are very active. Bromine water replaces all three of those H atoms: 2,4,6-tribromophenol.
  6. Free play: pick a reagent, guess which ring positions change, then press Show to check.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

If phenol has an –OH, why is it not called an alcohol?

In alcohols –OH sits on an sp³ carbon. In phenol it sits on a ring (sp²) carbon and its lone pair mixes with the ring, so its properties are different – for example, it is acidic enough to react with NaOH.

Why can't we just boil chlorobenzene with NaOH like a haloalkane?

The C–Cl bond on a ring has partial double-bond character and is very strong, so it needs 623 K and 300 atm to break.

Phenoxide has the charge on O too – so why is it more stable than ethoxide?

In phenoxide the charge can move onto three ring carbons (ortho, ortho, para). Sharing the charge over many atoms lowers its energy. In ethoxide it is stuck on one O.

Why does a –NO₂ group at meta help less than at para?

At ortho or para the –NO₂ can take the ring's negative charge by resonance. At meta it can only pull weakly through bonds, so the effect is smaller.

Why does phenol give tribromophenol in water, but only mono-bromophenol in CS₂?

Water helps phenol form some phenoxide ion, which is even more electron-rich, and it helps split Br₂. In non-polar CS₂ at low temperature the ring is less active, so only one Br goes in.

Why does the Kolbe reaction put –COOH at ortho?

The phenoxide ion is very electron-rich at ortho and para. CO₂, a weak electrophile, attacks mainly the ortho carbon, giving salicylic acid.

What is a phenol? Structure and naming

A phenol has an –OH group joined directly to a carbon of a benzene ring (an sp² carbon). The simplest one, C₆H₅OH, is itself called phenol (IUPAC: benzenol). Methyl phenols are called cresols; dihydroxy benzenes are catechol (1,2), resorcinol (1,3) and hydroquinone (1,4).

Because the ring carbon is sp² and O's lone pair is shared with the ring, the C–O bond in phenol is a little shorter and stronger than in methanol.

Preparation of phenol

1. From haloarenes (Dow process)

Chlorobenzene + NaOH at 623 K and 300 atm → sodium phenoxide; then dilute HCl → phenol. High temperature and pressure are needed because the C–Cl bond on a ring is very strong.

2. From benzenesulphonic acid

Benzene + oleum → benzenesulphonic acid; fuse with molten NaOH → sodium phenoxide; acidify → phenol.

3. From diazonium salts

Aniline + NaNO₂ + HCl at 273–278 K → benzenediazonium chloride. Warm it with water: N₂ gas bubbles off and phenol forms.

4. From cumene (industry)

Cumene (isopropylbenzene) + air → cumene hydroperoxide; with dilute acid it splits into phenol + propanone (acetone). Two useful products from one process – that is why most phenol is made this way.

Physical properties

Phenol is a colourless solid (it turns pink in air on standing), with a boiling point higher than that of arenes and haloarenes of similar mass because of hydrogen bonding. It dissolves a little in water (H-bonds with water), and it burns skin, so it is handled carefully.

Acidity of phenols

Phenol turns blue litmus red only weakly, but it does react with NaOH to form sodium phenoxide – alcohols do not. It does not react with NaHCO₃ (carboxylic acids do). So acid strength: alcohols < water < phenol < carboxylic acids.

Why is phenol more acidic than ethanol?

  1. The ring pulls electrons from O (sp² carbon is more electron-hungry), so O–H is easier to break.
  2. After H⁺ leaves, the phenoxide ion's negative charge spreads to the ortho and para carbons through resonance. A charge shared over many atoms is more stable. In ethoxide the charge is stuck on O, and the ethyl group even pushes more electrons onto it.

Effect of groups on the ring

Smaller pKa = stronger acid. Phenol pKa ≈ 10, ethanol ≈ 15.9.

Reactions of phenols

A. Reactions of the O–H

With Na: 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂. With NaOH: sodium phenoxide + water. Esterification: phenol + acid chloride or anhydride → ester; e.g. salicylic acid + ethanoic anhydride → aspirin.

B. Electrophilic substitution on the ring (ortho/para)

C. Other reactions

Heating with zinc dust removes O → benzene. Oxidation with Na₂Cr₂O₇/H₂SO₄ → benzoquinone.

Key formulas and definitions

Worked examples

1. Why does phenol dissolve in NaOH but not in NaHCO₃?

Step 1: NaOH is a strong base, so even a weak acid like phenol (pKa 10) gives up H⁺ to it → sodium phenoxide dissolves. Step 2: NaHCO₃ is a weak base; its conjugate acid H₂CO₃ (pKa ≈ 6.4) is stronger than phenol, so the reaction does not go forward. Only acids stronger than H₂CO₃, like carboxylic acids, release CO₂.

2. Arrange in increasing acidity: phenol, p-nitrophenol, p-cresol, ethanol.

Ethanol (no ring, charge stuck on O) is weakest. p-Cresol has –CH₃ pushing electrons → weaker than phenol. p-Nitrophenol has –NO₂ pulling charge → stronger. Order: ethanol < p-cresol < phenol < p-nitrophenol.

3. Phenol has pKa = 10. Find its Ka.

pKa = −log Ka, so Ka = 10^(−pKa) = 10^(−10) = 1 × 10⁻¹⁰.

4. Ethanol has pKa 15.9 and phenol 10.0. How many times stronger an acid is phenol?

Ratio of Ka = 10^(15.9 − 10.0) = 10^5.9 ≈ 7.9 × 10⁵. Phenol is about 8 lakh times stronger.

5. What mass of 2,4,6-tribromophenol (M = 331 g/mol) forms from 9.4 g of phenol (M = 94 g/mol) with excess bromine water?

Moles of phenol = 9.4 / 94 = 0.1 mol. 1 mol phenol → 1 mol tribromophenol. Mass = 0.1 × 331 = 33.1 g.

6. How many grams of Br₂ (M = 160 g/mol) are used up in Example 5?

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr. 0.1 mol phenol needs 0.3 mol Br₂ = 0.3 × 160 = 48 g.

7. The cumene process gives phenol and propanone together. If 60 kg of cumene (M = 120 g/mol) reacts fully, what masses of phenol (94) and propanone (58) form?

Moles of cumene = 60 000 / 120 = 500 mol. Each mole gives 1 mol phenol and 1 mol propanone. Phenol = 500 × 94 = 47 kg; propanone = 500 × 58 = 29 kg.

8. o-Nitrophenol boils lower than p-nitrophenol. Why, and how is this used?

In o-nitrophenol the –OH and –NO₂ are side by side and form an H-bond inside one molecule (intramolecular), so molecules do not hold each other. In p-nitrophenol H-bonds join different molecules (intermolecular). So the ortho isomer is more volatile and is removed by steam distillation, leaving the para isomer.

Common mistakes

Practice quiz

1. Phenol reacts with bromine water to give:
2. Which is the strongest acid?
3. The Kolbe reaction gives:
4. The cumene process gives phenol and:
5. The –OH group in phenol directs incoming groups to:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

Why is phenol more acidic than alcohol?

After losing H⁺, phenol forms a phenoxide ion whose negative charge spreads over the ring by resonance, making it stable. The alkoxide ion from an alcohol keeps the charge on one O.

What is the test for phenol?

It gives a white precipitate with bromine water, and a violet colour with neutral FeCl₃ solution.

How is phenol made in industry?

Mostly by the cumene process: cumene is oxidised by air and split with acid into phenol and acetone.

Where this is taught

PolandLiceum ogólnokształcące, klasa IIAlcohols and phenols
PolandLiceum ogólnokształcące, klasa IIIAlcohols and phenols
CBSE (India)Class 12Alcohols, Phenols and Ethers
Russia10 классOxygen-containing compounds
Russia10 классOxygen-containing compounds

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