Log in or sign up to add this lesson to a Custom Course. Log in or Sign up. When most people hear the term halogen in the context of chemistry, their minds almost immediately jump to thinking about substances that are very toxic and dangerous. Recall that when we refer to halogens, we're talking about either fluorine, chlorine, bromine, or iodine. Although there are certainly many halogenated chemicals we should always avoid, sometimes the presence of a halogen, especially on an organic molecule, can be very useful.

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Summary When methyl ketones are treated with the halogen in basic solution, polyhalogenaton followed by cleavage of the methyl group occurs.

The products are the carboxylate and trihalomethane, otherwise known as haloform. The halogenations get faster since the halogen stablises the enolate negative charge and makes it easier to form. Then a nucleophilic acyl substitution by hydroxide displaces the anion CX 3 as a leaving group that rapidly protonates.

This reaction is often performed using iodine and as a chemical test for identifying methyl ketones. Iodoform is yellow and precipitates under the reaction conditions. A yellow precipitate indicates a positive result in the iodoform test centre tube. Step 1: First, an acid-base reaction. Step 2: The nucleophilic enolate reacts with the iodine giving the halogenated ketone and an iodide ion.

Step 3: Steps 1 and 2 repeat twice more yielding the trihalogenated ketone. This gives the carboxylic acid. Step 6: An acid-base reaction. The trihalomethyl anion is protonated by the carboxylic acid, giving the carboxylate and the haloform trihalomethane. Ian Hunt , Department of Chemistry.


Haloform Reaction Mechanism

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Haloform reaction

The haloform reaction is a chemical reaction where a haloform CHX 3 , where X is a halogen is produced by the exhaustive halogenation of a methyl ketone RCOCH 3 , where R can be either a hydrogen atom, an alkyl or an aryl group , in the presence of a base. In the first step, the halogen disproportionates in the presence of hydroxide to give the halide and hypohalite example with bromine, but reaction is the same in case of chlorine and iodine; one should only substitute Br for Cl or I :. Under basic conditions, the ketone undergoes keto-enol tautomerization. At least in some cases chloral hydrate the reaction may stop and the intermediate product isolated if conditions are acidic and hypohalite is used. Substrates are broadly limited to methyl ketones and secondary alcohols oxidizable to methyl ketones, such as isopropanol. The only primary alcohol and aldehyde to undergo this reaction are ethanol and acetaldehyde , respectively.


Haloform Reaction: Definition & Examples


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