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Haloform Reaction: Definition & Examples

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson the haloform reaction will be introduced and defined, in addition to the mechanism being outlined and the importance of the products discussed.

What is the Haloform Reaction?

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.

For example, many of the FDA-approved drugs a lot of us rely on contain one or more fluorine atoms. The placement of a fluorine on a small molecule can have very profound effects on its biological activity inside the body. Countless examples of molecules isolated from marine organisms have been reported in which a molecule that has very potent antibiotic activity contains one or more bromine atoms. Furthermore, halogenated organic solvents are very common in the lab setting and find wide use in various applications.

In this lesson, we are going to learn about a common organic transformation known as the haloform reaction.

Definition and General Reaction Form

The haloform reaction involves some very simple ingredients: we need a methyl ketone, a halogen in molecular form (i.e. Cl2, Br2, or I2), and a base (usually some sort of metal hydroxide i.e. NaOH). Notice in the reaction below, the 'R' group represents any general organic side chain and could be virtually anything. The 'X' below (teal) represents any of the four halogens.

Generic haloform reaction
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In the reaction above, the general products of any haloform reaction are a carboxylate salt (deprotonated form of a carboxylic acid) and a haloform (some sort of CHX3 where 'X' can be any halogen). An important thing to realize is that the carbon portion of the haloform always comes from the methyl ketone substrate (labeled in blue).

Haloform Reaction Mechanism

Now that we've seen what the general form of a haloform reaction looks like, let's take a more detailed look at the different steps involved in the mechanism. Remember that a mechanism is basically a detailed 'story' of how the chemical reaction occurs and uses curved arrows to show the flow of electrons in the context of bond-making and bond-breaking events.

Step 1

In the first step of the reaction, the hydroxide (-OH) base deprotonates one of the hydrogens from the methyl ketone, generating an enolate intermediate (which is a carbon adjacent to a carbonyl with a formal -1 charge).

Deprotonation of the methyl ketone to form an enolate
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Step 2

Once the enolate is formed, it then acts as a nucleophile (which is something with a negative charge) to attack a molecule of the molecular halogen to produce the halogenated ketone and a halogen ion.

Nucleophilic attack of the enolate on the halogen
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Step 3

Repeat steps 1 and 2 TWO more times to form trihalogenated ketone.

Method of forming trihalogenated ketone
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Step 4

In this step, the hydroxide anion acts as a nucleophile and attacks the carbonyl carbon forming an alkoxide intermediate.

Nucleophilic attack of hydroxide on trihalogenated ketone
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Step 5

In step 5, the carbon-oxygen bond reforms, followed by ejection of the CX3 anion and formation of a carboxylic acid. Normally, we wouldn't think of a carbon atom being a good leaving group (which is something that gets ejected from the substrate), but in this case the presence of the three halogen atoms greatly stabilizes the anion.

Formation of carboxylic acid and haloform anion
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