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Introduction to the Reactions of Enols and Enolates

Haloform Reaction

If a methyl ketone is halogenated with chlorine, bromine, or iodine under basic conditions, a trihalomethyl ketone is obtained. However, due to the strong electron-withdrawing effect of three halogen atoms, the trihalomethyl group is a good leaving group. Halogenation of methyl ketones under basic conditions is, therefore, usually followed by a haloform reaction! Haloform reaction yields both the corresponding trihalomethane (haloform) and the carboxylic acid.

Haloform reaction.

Iodoform reaction

Iodoform reaction.

The iodoform reaction may be analytically used for the detection of methyl ketones. For this purpose, the sample is treated with hypoiodite as a source of iodine in basic solution. Iodoform, which is generated by the iodoform reactions then precipitates as a yellow, characteristically smelling solid substance. However, alcohols that contain an α methyl group, such as ethanol or isopropyl alcohol, may interfere with the detection of methyl ketones by iodoform reaction, as they can be oxidized to the corresponding methyl ketone by hypochlorite.

Experimental movie: Iodoform reaction.

Exercise 1

Exercise 2

Exercise 3

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