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Iodoform anomaly?

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Iodoform anomaly?

Post  sangbmt on Fri Jan 07, 2011 1:49 am

Familiar with the iodoform test? Then read on...

I have just tried the iodoform test using reagent grade methanol and iodine dissolved in 1moldm-3 NaOH. When the aqueous alkaline iodine was added to the methanol a substantial quantity of precipitate was formed. If my understaning of the reaction is correct this shouldn't happen!

According to my undergrad organic chemistry textbook, the mechanism involves the following steps:

(1) the oxidation of the alcohol to a carbonyl group
(2) hydroxide ion deprotonates the carbon adjacent to the cabonyl group to form an enolate ion
(3) enolate attacks the I2 so that one iodine atom is added to the carbon (the one that lost the H in step 2)
(4) Repeat steps 2-3- further addition of iodine atoms until CI3 is formed which is a better leaving group than OH
(5) Nucleophilic attack of hydroxide ion at carbonyl carbon (which has been happening throughout)
(6) Negative charge on oxygen refroms c=o and C-C bond broken to eliminate CI3- leaving an acid
(7) The CI3- carbanion deprotonates the acid to form triiodomethane and an alkanoate ion

Step 2-7 seem to rule out the possibility of methanol participating in this reaction since there is no carbon adjacent to the carbonyl group. If it works for methanol, where does the triiodomethane come from? Can anyone help???


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