- Haloform reaction
The haloform reaction is a chemical reaction where a haloform (CHX3, where X is a
halogen ) is produced by the exhaustivehalogenation of a methylketone (a molecule containing the R-CO-CH3 group) in the presence of a base. [Chakrabartty, in Trahanovsky, "Oxidation in Organic Chemistry", pp 343-370, Academic Press, New York, 1978] R may be Hydrogen,alkyl oraryl . The reaction can be used to produce CHCl3, CHBr3 or CHI3.cope
Substrates which successfully undergo the haloform reaction are methyl ketones and secondary alcohols oxidizable to methyl ketones, such as isopropanol. The halogen used may be chlorine, bromine, and iodine.
Fluoroform (CHF3) cannot be prepared from a methyl ketone by the haloform reaction due to the instability ofhypofluorite , but compounds of the type RCOCF3 do cleave with base to producefluoroform (CHF3); this is equivalent to the second and third steps in the process shown above.Mechanism
In the first step, the halogen disproportionates in the presence of
hydroxide to give the halide and hypohalide:: X2 + OH−→ XO−+ X− + H+ (X = Cl, Br, I)
If a secondary alcohol is present, it is oxidized to a ketone by the hypohalide:
If a methyl ketone is present, it reacts with the hypohalide 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 hypohalide (containing a halide with a formal +1 charge). When the alpha position has been exhaustively halogenated, the molecule undergoes a
nucleophilic acyl substitution byhydroxide , with −CX3 being the leaving group stabilized by threeelectron withdrawing group s. The −CX3 anion abstracts a proton from either the carboxylic acid formed, or the solvent, and forms the haloform.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 andinfrared spectroscopy are preferred because they require small samples, may be non-destructive (for NMR) and are easy and quick to perform.Formerly, it was used to produce
iodoform andbromoform and evenchloroform industrially.Fact|date=September 2007In
organic chemistry , this reaction may be used to convert a terminal methyl ketone into the appropriate carboxylic acid.It also use for test priamry, secondary alochol.
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 highmolar mass due to the threeiodine atoms, it is solid at room temperature (c.f. chloroform and bromoform). It isinsoluble in water and has anantiseptic smell. A visible precipitate of this compound will form from a sample only when a methyl ketone is present.History
The haloform reaction is one of the oldest
organic reaction s around [cite book | title = Strategic Applications of Named Reactions in Organic Synthesis | author = László Kürti and Barbara Czakó | publisher = Elsevier | location = Amsterdam | year = 2005 | isbn = 0-12-429785-4] . In 1822 Serullas reactedethanol withiodine andsodium hydroxide in water tosodium formate and iodoform, called in the language of that time "hydroiodide of carbon". In 1831Justus Liebig reported the reaction ofchloral withcalcium hydroxide tochloroform and calcium formate. The reaction was rediscovered byAdolf Lieben in 1870. Theiodoform test is also called the Lieben haloform reaction. A review of the Haloform reaction with a history section was published 1934. [cite journal|title =The Haloform Reaction|author=Reynold C. Fuson and Benton A. Bull|journal=Chemical Reviews
volume= 15|issue= 3|year= 1934|pages= 275–309|doi= 10.1021/cr60052a001]References
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