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TitleHaloform Reaction
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Haloform reaction

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The haloform reaction is a chemical reaction where a haloform (CHX3, where X is a halogen) is

produced by the exhaustive halogenation of a methyl ketone (a molecule containing the R-CO-

CH3 group) in the presence of a base.

R may be H, alkyl or aryl. The reaction can be used to

produce CHCl3, CHBr3 or CHI3.


 1 Scope

 2 Mechanism

 3 Uses

 4 Iodoform test

 5 History

 6 References

[edit] Scope

Substrates that successfully undergo the haloform reaction are 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 ethanal, respectively. 1,3-Diketones such as

acetylacetone also give the haloform reaction. Beta-ketoacids such as acetoacetic acid will also

give the test upon heating. The halogen used may be chlorine, bromine, or iodine. Fluoroform

(CHF3) cannot be prepared from a methyl ketone by the haloform reaction due to the instability

of hypofluorite, but compounds of the type RCOCF3 do cleave with base to produce fluoroform

(CHF3); this is equivalent to the second and third steps in the process shown above.

[edit] Mechanism

In the first step, the halogen disproportionates in the presence of hydroxide to give the halide and


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X2 + OH−→ XO−+ X− + H
(X = Cl, Br, I) (Unbalanced)

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

depicted below should be hypohalite and the water≡H2O should be hypohalic acid≡HXO):

If a methyl ketone is present, it reacts with the hypohalite in a three-step process:

(1) R-CO-CH3 + 3 OX
→ R-CO-CX3 + 3 OH−

(2) R-CO-CX3 + OH− → RCOOH + −CX3
(3) RCOOH + −CX3 → RCOO− + CHX3

The detailed reaction mechanism is as follows:

Under basic conditions, the ketone undergoes keto-enol tautomerization. The

enolate undergoes electrophilic attack by the hypohalite (containing a halogen

with a formal +1 charge). When the α position has been exhaustively
halogenated, the molecule undergoes a nucleophilic acyl substitution by

hydroxide, with −CX3 being the leaving group stabilized by three electron-

withdrawing groups. The −CX3 anion abstracts a proton from either the

carboxylic acid formed or the solvent, and forms the haloform.

[edit] Uses

This reaction was traditionally used to determine the presence of a methyl

ketone, or a secondary alcohol oxidizable to a methyl ketone through the

iodoform test. Nowadays, spectroscopic techniques such as NMR and infrared

spectroscopy are preferred because they require small samples, may be non-

destructive (for NMR) and are easy and quick to perform.

It was formerly used to produce iodoform and bromoform and even chloroform

in industry.
[citation needed]

In organic chemistry, this reaction may be used to convert a terminal methyl

ketone into the appropriate carboxylic acid.

[edit] Iodoform test

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negative and positive iodoform test

When iodine and sodium hydroxide are used as the reagents, a positive reaction

gives iodoform. Iodoform (CHI3) is a pale-yellow substance. Due to its high

molar mass caused by the three iodine atoms, it is solid at room temperature (cf.

chloroform and bromoform). It is insoluble in water and has an antiseptic smell.

A visible precipitate of this compound will form from a sample only when either

a methyl ketone, ethanal, ethanol, or a methyl secondary alcohol is present.

[edit] History

The haloform reaction is one of the oldest organic reactions known.

In 1822,

Serullas reacted ethanol with iodine and sodium hydroxide in water to form

sodium formate and iodoform, called in the language of that time hydroiodide of

carbon. In 1831, Justus von Liebig reported the reaction of chloral with calcium

hydroxide to chloroform and calcium formate. The reaction was rediscovered by

Adolf Lieben in 1870. The iodoform test is also called the Lieben haloform

reaction. A review of the Haloform reaction with a history section was

published in 1934.

[edit] References

1. ^ Chakrabartty, in Trahanovsky, Oxidation in Organic Chemistry, pp 343-370,
Academic Press, New York, 1978

2. ^ László Kürti and Barbara Czakó (2005). Strategic Applications of Named
Reactions in Organic Synthesis. Amsterdam: Elsevier. ISBN 0-12-429785-4.

3. ^ Reynold C. Fuson and Benton A. Bull (1934). "The Haloform Reaction".
Chemical Reviews 15 (3): 275–309

Haloform Reaction

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This reaction has been used in qualitative analysis to indicate the presence of a methyl ketone. The
product iodoform is yellow and has a characteristic odour. The reaction has some synthetic utility in the
oxidative demethylation of methyl ketones if the other substituent on the carbonyl groups bears no
enolizable α-protons.

Mechanism of the Haloform Reaction

The reaction readily proceeds to completion because of the acidifying effect of the halogen substituents.

Chapter 18: Enols and Enolates

The Haloform reaction

Reaction type : Nucleophilic substitution

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Step 3:
Steps 1 and 2 repeat twice more yielding the
trihalogenated ketone.

Step 4:
The hydroxide now reacts as a nucleophile at the
electrophilic carbonyl carbon, with the C=O becoming a
C-O single bond and the oxygen is now anionic.

Step 5:
Reform the favourable C=O and displace a leaving group,
the trihalomethyl system which is stabilised by the 3
halogens. 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).

Haloform reaction

Compounds which have one of the following structural units, on heating with a halogen (

Cl2, Br2 or I2) in the presence of a base like NaOH or KOH give rise to a haloform

(chloroform, Iodoform).

As in Acetaldehyde, Acetone Ethanol, 2-Propanol


It is prepared by heating bleaching powder [Cl2 + Ca(OH)2 ] and ethanol. It is Chloroform

is formed as a colorless liquid. It is generally preserved in the presence of small amounts

of ethanol (negative catalyst) to prevent the formation of carbonyl chloride that is

phosgene which is highly poisonous.

Chloroform was used as an anesthetic during surgery; it is no longer used now because of

severe side effects like liver and cardiac toxicity. An anesthetic used at present is

Halothane. ( F3CCHBrCl).

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Net reaction using NaOH as the base


Note: 1. First step is oxidation of alcohol by the halogen.

Aldehydes and ketones with a methyl group next to the carbonyl group, will also give rise

to haloform.

The difference is the oxidation step will not be there.

Substitution of the three α-hydrogens by halogen atoms take place in the second step.
The hydrogens replaced by halogens are all from the same side of the carbonyl group, in

compounds like

CH3COCH3. Once hydrogen is replaced by halogen the other hydrogen atoms at that carbon

become more acidic causing further substitution at the same place.

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