Ethene is hydrated with dilute $H X_{2} SO X_{4}$ to give A. A is heated over Cu at 573 K to give B. B reacts with $CH X_{3} MgBr$ followed by hydrolysis to give C. C is heated with conc. $H X_{2} SO X_{4}$ at 443 K to give D. Which of these compounds give a yellow precipitate with $I X_{2} / NaOH$ ?

Answer: A, B, C

A is ethanol, B is ethanal, C is propan-2-ol and D is propene. Ethanol, ethanal and propan-2-ol all contain the

CH X_{3} CH (OH) X^{-}

CH X_{3} CO X^{-}

unit (after oxidation by hypoiodite) and give iodoform. Propene has neither group and gives no precipitate.