Phenols and ethers

MDCAT Chemistry · Chemistry · Alcohols and phenols. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All MDCAT Chemistry topics

The idea

In phenol, C₆H₅OH, the –OH group is joined directly to a benzene ring. When phenol loses H⁺, the negative charge of the phenoxide ion spreads into the ring. This stabilises the ion, so phenol is a much stronger acid than alcohols or water, though still weaker than carboxylic acids.

A lone pair on the oxygen also feeds electrons into the ring. The ring is activated, so phenol undergoes electrophilic substitution far more easily than benzene, at positions 2, 4 and 6.

Ethers, R–O–R′, have an oxygen atom between two carbon groups. They have no O–H bond, so their molecules cannot hydrogen-bond to each other and they boil at low temperatures.

Rules to remember

Acid strength: carboxylic acid > phenol > water > alcohol. K_a of phenol is about 1 × 10⁻¹⁰
Phenol reacts with NaOH to give sodium phenoxide and water, but it gives no CO₂ with Na₂CO₃ or NaHCO₃
Electron-withdrawing groups (–NO₂) make a phenol more acidic; electron-donating groups (–CH₃) make it less acidic
Bromine water at room temperature: white precipitate of 2,4,6-tribromophenol. Dilute HNO₃: 2- and 4-nitrophenol. Concentrated HNO₃ + H₂SO₄: 2,4,6-trinitrophenol (picric acid)
Phenol + zinc dust (heat) → benzene + ZnO. Phenol + methanal → Bakelite. Phenol + CH₃COCl → phenyl ethanoate + HCl
Phenol (carbolic acid): colourless crystals, melting point 41 °C, absorbs moisture from air, turns pink in air by oxidation. Catechol, resorcinol and quinol are benzene-1,2-, 1,3- and 1,4-diol
Ethers are named alkoxyalkanes, with the smaller group as alkoxy: CH₃OC₂H₅ is methoxyethane. Oxygen is sp³, so C–O–C is bent, about 110°. Williamson: RONa + R′X → ROR′ + NaX

Common mistake

Students say phenol releases CO₂ from sodium carbonate because it is an acid. Phenol is weaker than carbonic acid, so it reacts with NaOH only; giving CO₂ with a carbonate is the test for a carboxylic acid.

Worked example

A compound X dissolves in aqueous NaOH, gives no gas with NaHCO₃ solution, and gives a white precipitate with bromine water. Is X ethanol, phenol or ethanoic acid?

X reacts with NaOH, so it is acidic enough to form a salt. Ethanol is not, so X is not ethanol.
X gives no CO₂ with NaHCO₃, so it is weaker than carbonic acid. Ethanoic acid would give CO₂, so X is not ethanoic acid.
A white precipitate with bromine water at room temperature is 2,4,6-tribromophenol, which confirms an activated benzene ring with –OH.

Answer: X is phenol

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Keep going

← Alcohols Aldehydes and ketones →Take a MDCAT Chemistry test