AS Chemistry 9701: The Full Tri-iodomethane Reaction and Its Two Products

Pentan-2-one, $\text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3$ , is warmed with an excess of alkaline aqueous iodine (iodine dissolved in sodium hydroxide solution), the tri-iodomethane test.

(a) State the observation made, and give the systematic name of the compound responsible for it. [2]

(b) In this reaction, the three hydrogen atoms of the $\text{CH}_3$ group next to the carbonyl group are first replaced by iodine atoms; hydroxide ions then cleave the carbon-carbon bond between this trihalogenated carbon and the carbonyl carbon. Identify the other organic product of the reaction (the salt formed alongside tri-iodomethane), giving both its name and its formula. [3]

(c) Pentan-3-one, $\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_3$ , is treated with the same reagent under the same conditions. State, with a reason, whether a pale yellow precipitate is observed. [2]

Part (a): The observation

Pentan-2-one contains a methyl ketone group, $\text{CH}_3\text{CO}-$ (the $\text{CH}_3$ at one end of the chain is bonded directly to the carbonyl carbon). Warming it with alkaline aqueous iodine gives a pale yellow precipitate of tri-iodomethane, $\text{CHI}_3$ .

Part (b): Identifying the other product

The mechanism happens in two stages:

The three hydrogen atoms of the $\text{CH}_3$ group next to the carbonyl carbon are successively replaced by iodine atoms, giving the trihalogenated intermediate $\text{CI}_3\text{COCH}_2\text{CH}_2\text{CH}_3$ .
Hydroxide ions then attack this intermediate’s carbonyl carbon; the carbon-carbon bond between the $\text{CI}_3$ carbon and the carbonyl carbon breaks, releasing the carbanion $\text{CI}_3^-$ (which is then protonated by water to give the neutral $\text{CHI}_3$ precipitate) and leaving the remaining part of the molecule as a carboxylate ion.

Removing the $\text{CI}_3$ carbon from pentan-2-one ( $\text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3$ , 5 carbon atoms) leaves a 4-carbon carboxylate ion, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COO}^-$ , formed alongside a $\text{Na}^+$ counter-ion from the sodium hydroxide present. This is sodium butanoate, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COO}^-\text{Na}^+$ .

Part (c): Testing pentan-3-one

Pentan-3-one, $\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_3$ , has an ethyl group, $\text{CH}_2\text{CH}_3$ , attached to the carbonyl carbon on each side. There is no $\text{CH}_3$ group bonded directly to the carbonyl carbon. Since it contains neither the $\text{CH}_3\text{CO}-$ arrangement nor a $\text{CH}_3\text{CH(OH)}-$ arrangement, it does not react to form tri-iodomethane. No precipitate is observed, even though pentan-3-one is a ketone and would still give a positive result with 2,4-DNPH.

Final answers

(a) A pale yellow precipitate of tri-iodomethane, $\text{CHI}_3$ .
(b) Sodium butanoate, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COO}^-\text{Na}^+$ .
(c) No precipitate; pentan-3-one has no $\text{CH}_3$ group bonded directly to its carbonyl carbon (only ethyl groups on either side).