Topic 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)

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.

Warm an appropriate alcohol with alkaline aqueous iodine. A yellow precipitate of tri-iodomethane, CHI₃, indicates an oxidisable CH₃CH(OH)R unit: oxidation first gives CH₃COR, which undergoes the iodoform reaction. Ethanol is included with R = H; propan-2-ol is positive, whereas propan-1-ol and a typical tertiary alcohol are negative.

Worked example

Combine 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?

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 aqueous medium the acidity order is phenol > water > ethanol. Phenoxide is stabilised by delocalisation of negative charge through the aromatic system; ethoxide has no corresponding delocalisation and its electron-donating ethyl group destabilises the negative charge relative to hydroxide. Compare the conjugate bases, not just the O-H bond polarity.

Phenol reacts with NaOH but does not normally release CO₂ from aqueous hydrogencarbonate, whereas a carboxylic acid does. Ethanol does not react appreciably with aqueous NaOH to form ethoxide, although dry ethanol reacts with sodium metal. Metal reaction and aqueous base reaction are different tests.