chemistry

Haloform reaction

Haloform reaction is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Haloform reaction rather than just read about it. In short: In chemistry, the haloform reaction (also referred to as the Lieben haloform reaction) is a chemical reaction in which a haloform (CHX3, where X is a halogen) is produced by the exhaustive halogenation of an acetyl group (R−C(=O)CH3, where R can be either a hydrogen atom, an alkyl or an aryl group), in the presence of a base. The reaction can be used to transform acetyl groups into carboxyl groups (R−C(=O)OH) or to…

Key takeaways

Haloform reaction belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Haloform reaction to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Haloform reaction from memory before moving on to harder problems.

Reference excerpt

In chemistry, the haloform reaction (also referred to as the Lieben haloform reaction) is a chemical reaction in which a haloform (CHX3, where X is a halogen) is produced by the exhaustive halogenation of an acetyl group (R−C(=O)CH3, where R can be either a hydrogen atom, an alkyl or an aryl group), in the presence of a base. The reaction can be used to transform acetyl groups into carboxyl groups (R−C(=O)OH) or to produce chloroform (CHCl3), bromoform (CHBr3), or iodoform (CHI3). Note that fluoroform (CHF3) can't be prepared in this way.

Mechanism In the first step, the halogen dis-proportionates in the presence of hydroxide to give the halide and hypohalite.

Br 2 + 2 OH − ⟶ Br − + BrO − + H 2 O {\displaystyle {\ce {Br2 + 2 OH- -> Br- + BrO- + H2O}}}

If a secondary alcohol is present, it is oxidized to a ketone by the hypohalite:

If a methyl ketone is present, it reacts with the hypohalite in a three-step process: 1. Under basic conditions, the ketone undergoes keto-enol tautomerisation. The enolate undergoes electrophilic attack by the hypohalite (containing a halogen with a formal +1 charge).

2. When the α(alpha) position has been exhaustively halogenated, the molecule reacts with hydroxide, with −CX3 being the leaving group stabilized by three electron-withdrawing groups. In the third step the −CX3 anion abstracts a proton from either the solvent or the carboxylic acid formed in the previous step, and forms the haloform. At least in some cases (chloral hydrate) the reaction may stop and the intermediate product isolated if conditions are acidic and hypohalite is used.

Scope 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. 1,3-Diketones such as acetylacetone also undergo this reaction. β-ketoacids such as acetoacetic acid will also give the test upon heating. Acetyl chloride and acetamide do not undergo this reaction. The halogen used may be chlorine, bromine, iodine or sodium hypochlorite. Fluoroform (CHF3) cannot be prepared by this method as it would require the presence of the highly unstable hypofluorite ion. However ketones with the structure RCOCF3 do cleave upon treatment with base to produce fluoroform; this is equivalent to the second and third steps in the process shown above.

Applications

Laboratory scale

This reaction forms the basis of the iodoform test which was commonly used in history as a chemical test to determine the presence of a methyl ketone, or a secondary alcohol oxidizable to a methyl ketone. When iodine and sodium hydroxide are used as the reagents a positive reaction gives iodoform, which is a solid at room temperature and tends to precipitate out of solution causing a distinctive cloudiness. In organic chemistry, this reaction may be used to convert a terminal methyl ketone into the analogous carboxylic acid.

Industrially It was formerly used to produce iodoform, bromoform, and even chloroform industrially. A variant of this reaction is used to manufacture deuterated chloroform, in reaction of hexachloroacetone with heavy water catalysed by base:

O=C(CCl3)2 + D2O → 2 CDCl3 + CO2 Further variant uses decomposition of calcium trichloroacetate in heavy water:

Ca(CCl3CO2)2 + D2O → 2 CDCl3 + CaCO3 + CO2

As a by-product of water chlorination Water chlorination can result in the formation of haloforms if the water contains suitable reactive impurities (e.g. humic acid). Haloforms, specifically chloroform, is suspected to be carcinogenic and for causing the weak association between consumption of chlorinated water and cancer.

History The haloform reaction is one of the oldest organic reactions known. In 1822, Georges-Simon Serullas added potassium metal to a solution of iodine in ethanol and water to form potassium formate and iodoform, called in the language of that time hydroiodide of carbon. In 1832, Justus von Liebig reported the reaction of chloral with calcium hydroxide to form chloroform and calcium formate. The reaction was rediscovered by Adolf Lieben in 1870. The iodoform test is also called the Lieben iodoform reaction. A review of the haloform reaction with a history section was published in 1934.

References

Illustrations

Haloform reaction: Negative and positive iodoform test — Negative and positive iodoform test

Worked examples

Example 1 — a first encounter with Haloform reaction

Start with the simplest possible case. Write down what Haloform reaction claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Haloform reaction before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Haloform reaction ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Haloform reaction

In research: Haloform reaction appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Haloform reaction in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Haloform reaction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Carbon-heteroatom bond forming reactions, Halogenation reactions, Organic redox reactions, so understanding it makes those chapters shorter.
In everyday life: Look for Haloform reaction outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Haloform reaction in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Haloform reaction means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Haloform reaction out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Haloform reaction in simple terms?

Why does Haloform reaction matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Haloform reaction?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Haloform reaction.