Dear students. For the record….

  1. Triiodomethane (iodoform) reaction on methyl carbonyls e.g. ethanal. Does the carboxylate anion get further oxidised to CO2?
    (Yes, I know, the reaction equation below in the picture isn’t balanced. In organic chemistry we rarely worry about balancing equations, but where necessary we show the ratios of organic compounds to each other), so I’m not worried about the NaOH or I or even the H’s here!)

Iodoform reaction on ethananal gif

The syllabus states COOH and (COOH)2 can undergo oxidation using the mild oxidising reagent in Tollens ‘silver mirror’ test, and you know the iodoform is an oxidative cleavage reaction, i.e. Iodine in alkaline solution is an oxidising agent.

A question arises therefore as to whether the mild alkaline iodine solution can oxidise the carboxylate species (shown in red) produced via the iodoform test, just like the tollens reagent did for COOH? (Tollens reagent also employs alkaline conditions)

The (current) answer to that question is no.

Why? Non-scientific answer: I can not find a single reference to this reaction occurring.

Scientific guess reason: The oxidative strength of the I2(NaOH aq) will be different from that of the [Ag(NH3)2]+ used in Tollens reagent, so I’m presuming I2(NaOH aq) is not a strong enough oxidising agent to cause the anion to turn into CO2. I don’t know the reduction potential for I2 and Ag+ in alkaline conditions. I only know the values for 1M aq solution.

I can’t recall effervescence ever occurring before when I’ve done the iodoform test on ethanal.

Although not scientifically accurate, one way to think of it, i.e. a MIND TRICK, is to look at it this way:

Tollens oxidised HCOOH and (COOH)2, i.e. the acid form!
but after the iodoform test….
we end up with a carboxylate anion form and not the acid form.


I will post a comment about the oxidation of alkyl side chains in acidic medium here later…. Watch this space.

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