Lesson 21 of 34

Use a structural pattern for iodoform, and delocalisation for phenol

An OH group alone does not guarantee a positive iodoform test or phenol-like acidity.

A-Level 9476 (2026-2027)

I can infer the iodoform-active alcohol unit.Syllabus
Syllabus 9476, 11.6(c). Infer the iodoform-active alcohol unit.
I can describe phenol chemistry.Syllabus
Syllabus 9476, 11.6(d). Describe phenol chemistry.
I can explain relative aqueous acidity of water, phenol and ethanol.Syllabus
Syllabus 9476, 11.6(e). Explain relative aqueous acidity of water, phenol and ethanol.

Use a structural pattern for iodoform, and delocalisation for phenol

An OH group alone does not guarantee a positive iodoform test or phenol-like acidity.

Make a guess. You are not marked.

Which is the stronger acid, phenol or ethanol?

Ethanol, as its O-H bond is more polar
Ethanol, as the ring pushes electrons onto O
Phenol, as the ring stabilises its anion

Show the answer

Phenol, as the ring stabilises its anion

The negative charge of the phenoxide ion spreads into the benzene ring, which stabilises it. The ethoxide ion has no such delocalisation.

Warm the alcohol with alkaline aqueous iodine. A yellow precipitate of tri-iodomethane, CHI₃, shows that the alcohol has a CH₃CH(OH)R unit. The alcohol is first oxidised to CH₃COR, which then gives the iodoform reaction. Ethanol counts, with R = H. Propan-2-ol gives a positive test. Propan-1-ol and a typical tertiary alcohol do not.

Worked exampleCombine oxidation and the iodoform pattern

An alcohol is oxidised to a ketone and gives yellow CHI₃ with warm alkaline iodine. What local structure is supported?

Try it first. Then show the worked stepsHide the worked steps

Formation of a ketone suggests a secondary alcohol.
The iodoform reaction requires a methyl group next to the OH-bearing carbon.
Combine the clues rather than using either alone.

Answer

CH₃CH(OH)R with R a carbon group. The evidence identifies a local unit, not the complete carbon skeleton.

Phenol reactions
Reagent	Observation/product	Reason
Aqueous NaOH	C₆H₅OH + OH^- → C₆H₅O^- + H₂O.	Phenol is acidic enough to form phenoxide with hydroxide.
Sodium	2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂.	Replacement of the O-H hydrogen produces hydrogen gas.
Dilute HNO₃	Mixture of 2-nitrophenol and 4-nitrophenol.	OH activates the ring and directs substitution to 2/4 positions.
Aqueous Br₂	Decolourisation and a white precipitate of 2,4,6-tribromophenol.	Strong ring activation permits multiple substitution without an AlBr₃ catalyst.

In water, the order of acid strength is phenol > water > ethanol. In the phenoxide ion, the negative charge spreads into the benzene ring, which stabilises it. The ethoxide ion cannot spread its charge. Its ethyl group also pushes electrons onto the oxygen, which makes the ion less stable than hydroxide. Compare how stable the anions are, not just how polar the O-H bond is.

Phenol reacts with NaOH, but it does not release CO₂ from aqueous hydrogencarbonate. A carboxylic acid does both. Ethanol hardly reacts with aqueous NaOH, although dry ethanol does react with sodium metal. Reacting with a metal and reacting with an aqueous base are different tests.