Summary — What This Chapter Covers
- Alcohols react with acyl chlorides at room temperature to give esters.
- Phenol is made from phenylamine through a diazonium salt, which is then warmed with water.
- Phenol reacts with NaOH(aq), Na, diazonium salts, dilute HNO3 and bromine water.
- Acidity: phenol > water > ethanol, explained by how well the negative charge on the anion is stabilised.
- The –OH group activates the ring and directs to the 2-, 4- and 6-positions; other phenols such as naphthol react in the same way.
1. Alcohols
32.1
Alcohols with acyl chlorides
An alcohol reacts with an acyl chloride at room temperature to form an ester and steamy fumes of HCl.
Making ethyl ethanoate
CH3COCl + CH3CH2OH → CH3COOCH2CH3 + HCl
Compared with esterification using a carboxylic acid (heat, concentrated H2SO4 catalyst, reversible), the acyl chloride route is fast, needs no catalyst or heating, and goes to completion.
Practice Question
Name the ester formed from propanoyl chloride and methanol, and write the equation.
2. Phenol
32.2
Making phenol
- React phenylamine with HNO2 (made in situ from NaNO2 and dilute HCl) below 10 °C to form benzenediazonium chloride:
C6H5NH2 + HNO2 + HCl → C6H5N2+Cl− + 2H2O
- Warm the diazonium salt solution with water:
C6H5N2+Cl− + H2O → C6H5OH + N2 + HCl
The first step is kept below 10 °C because the diazonium salt decomposes at higher temperatures. In the second step you warm it on purpose to make that decomposition happen in water.
Reactions of phenol
| Reagent and conditions | Product and equation |
|---|
| NaOH(aq) | Sodium phenoxide: C6H5OH + NaOH → C6H5O−Na+ + H2O |
| Na(s) | Sodium phenoxide and hydrogen: 2C6H5OH + 2Na → 2C6H5O−Na+ + H2 |
| Diazonium salt, in NaOH(aq), below 10 °C | An azo compound (yellow-orange dye): C6H5N2+ + C6H5OH → C6H5N=NC6H4OH + H+ (coupling at the 4-position) |
| Dilute HNO3(aq), room temperature | Mixture of 2-nitrophenol and 4-nitrophenol: C6H5OH + HNO3 → HOC6H4NO2 + H2O |
| Br2(aq), room temperature | Bromine water decolourised, white precipitate of 2,4,6-tribromophenol: C6H5OH + 3Br2 → C6H2Br3OH + 3HBr |
Phenol does not react with carbonates to give CO2: it is too weak an acid. This distinguishes it from carboxylic acids.
Acidity of phenol, water and ethanol
Phenol is a weak acid: C6H5OH ⇌ C6H5O− + H+. The more stable the anion formed, the more the equilibrium lies to the right, and the stronger the acid.
| Compound | Approx. pKa | Anion and its stability |
|---|
| Phenol | 10.0 | Phenoxide: a lone pair (p orbital) on O overlaps with the ring's π system, so the negative charge is delocalised into the ring. The charge density on O is lower, so it attracts H+ less. |
| Water | 14.0 | Hydroxide: charge stays on O, with no delocalisation and no electron-donating group. |
| Ethanol |
about 16 |
electron-donating
+
|
Order of acid strength
phenol > water > ethanol
Memorise