Haloform reaction

The haloform reaction (also Einhorn reaction ) is an organic chemical reaction. In this case, methyl ketones are cleaved by action of a hypohalite or halogens (chlorine, bromine or iodine) in an alkaline solution with loss of one carbon atom of the carboxylic acid. But by far the most important product in this reaction is the resulting trihalomethanes.

The haloform reaction is also used to detect methyl ketones or alcohols having one CH3 -CHOH- group. When using iodine as the halogen produced iodoform, having a characteristic odor, which makes it easy to detect. This analysis method is also called Iodoformprobe.

Reaction mechanism

The reaction proceeds under very mild conditions, usually with very good yields and tolerates the presence of other functional groups in the starting material. Hereinafter the assumed reaction mechanism using bromine is present as halogen.

First step

First the ketone 1 with a base ( hydroxide ion here ) is deprotonated at its methyl group to form a 2 enolate is produced which is due to keto -enol tautomerism a deprotonated α -position to the ketone is in equilibrium. The enolate 2 can now with a halogen molecule (here, a bromine molecule example) react, so that in α -position easily brominated ketone 3 and a bromide ion is formed. The bromide ion is irrelevant for the further reaction. Repeating this reaction steps, so you can get into α -position trisubstituted brominated ketone 4.

Second Step

The ketone 4 is attacked by a nucleophilic hydroxide ion. This results in a 5 alcoholate, which cleaves by rearrangement of its electrons in a carboxylic acid 6 and tribromomethane anion 7. The tribromomethane anion 7 stabilizes its charge by the - I effect of the halogen atoms. In aqueous (or better as in this acidic ) environment is rapidly protonated to bromoform 8.

The haloform reaction is also carried out with alcohols containing a CH 3 -CHOH- group, wherein the alcohol is first dehydrated to the ketone. Ethanol also reacts in this manner, as an intermediate product, acetaldehyde is formed.

Use of trihalomethanes

Trihalomethanes find application, for example in the production of irritant gases or disinfectants. The most famous trihalomethane is chloroform. Earlier, it was often used as an anesthetic, but today it is mainly used as a solvent.