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CAIE A-Level Chemistry 17.1.6 Iodoform Test for Methyl Carbonyls

Practise identifying a CH3CO– group through alkaline iodine and predicting triiodomethane and carboxylate products.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • locate CH3CO– in an aldehyde or ketone before predicting a positive alkaline-I2 test
  • state that a pale-yellow or yellow precipitate of CHI3 forms in a positive reaction
  • draw the second carbon-containing product as the correct RCO2− ion after cleavage

17.1.6—Iodoform test for CH3CO- group question 1

[Maximum number: 1]

4P⃗,Q4 \vec{P}, Q and R all have the molecular formula C3H6O\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}. They are all structural isomers of each other.
P and Q each contain an oxygen atom bonded directly to a carbon atom that is sp2\mathrm{sp}^{2} hybridised. R contains an oxygen atom bonded directly to a carbon atom that is sp3\mathrm{sp}^{3} hybridised.

P and Q(C3H6O)\mathbf{Q}\left(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\right) both form an orange precipitate when reacted with 2,4-DNPH. Only Q produces a yellow precipitate when reacted with alkaline aqueous iodine.

Identify the yellow precipitate formed by the reaction of Q with alkaline aqueous iodine.

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