Organic Chemistry: Haloform Reaction

[maverick280857]

Hi

What are the necessary and sufficient conditions for a compound to give a positive Haloform Test (Iodoform test for instance)?

Is it necessary to have a CH_{3}CO- group or is it necessary to have a XCH_{2}- group where X is some group? Is the occurrence of a CH_{3}CO- group a sufficient condition?

I would be very grateful if someone could help me with this...

Thanks and cheers
Vivek

 

[chem_tr]

Haloform test is ideally positive for methyl ketones; the C=O group attached to a methyl group increases the overall acidity of methyl protons due to tautomerization to enol form. So the X you wrote should be a strong electron withdrawing substituent, or at least a group with a highly electronegative atom.

However, you should take into account that the reactant is alkali iodine, or hypoiodite; so any reactant capable of reducing hypoiodite would consume it, causing a positive error. It would be wise to include a bit excess of hypoiodite.

Try to find the mechanism responsible for iodoform formation, the mechanism probably goes from an enol intermediate.

Good luck.

 

[maverick280857]

Thanks chem_tr...

I am referring to the reaction of a given compound with KOH/I2 which we call the Iodoform Test. The mechanism as I know it involves first the iodination of the Me group attached to the carbonyl carbon to a CI3 group followed by "base removal of H+ ions" with a I- nucleophile successively attacking the intermediate to finally form iodoform (precipitate).

So it now does appear that having a MeCO- group is a necessary condition...

Thanks and cheers
vivek