Haloform reaction
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The haloform reaction is a chemical reaction where a haloform (CHX3) is produced by the multiple halogenation of a methyl ketone (a molecule containing the R-CO-CH3 group) in the presence of a base [1] . R may be H, alkyl or aryl. The reaction can be used to produce CHCl3, CHBr3 or CHI3.
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[edit] Uses
The reaction has three application area:
- In analytical chemistry to identify an acetyl group in a molecule with the iodoform test.
- In the chemical industry it is used for the production of iodoform and bromoform in former times even for chloroform.
- In organic chemistry it is a possible method to transform an acetyl group into a carboxyl group.
[edit] Reaction mechanism
The reaction mechanism for the reaction of a methyl ketone with chlorine is a three step process:
- (2) R-CO-CCl3 + OH− → RCOOH + −CCl3
an exhaustive chlorination replacing all the hydrogen atoms on the methyl group with chlorine followed by a nucleophilic acyl substitution of the hydroxide anion with the chloromethyl anion as the leaving group and finally proton transfer from the carboxylic acid to get to chloroform.
The graphical depiction of the reaction of a methyl ketone with a hypobromite:
where the first step displays a keto-enol tautomerism.
Haloforms can also be prepared directly from any secondary alcohol containing the CH3-CHOH- group (such as isopropyl alcohol), since this is oxidised under the reaction conditions to the corresponding methyl ketone, which can then undergo the haloform reaction.
With chlorine, it is possible to use sodium hypochlorite as a convenient source of both base and chlorine. Fluoroform (CHF3) may not be prepared from a methyl ketone by this reaction, but compounds of the type RCOCF3 do cleave with base to produce CHF3; this is equivalent to the second and third steps in the process shown above. The iodoform test is a laboratory test for the presence of a methyl ketone or the related alcohol, by formation of a pale yellow precipitate of iodoform.
[edit] History
The haloform reaction is one of the oldest organic reactions around [2]. In 1822 Serullas reacted ethanol with iodine and sodium hydroxide in water to sodium formate and iodoform, called in the language of that time hydroiodide of carbon. In 1831 Justus Liebig reported the reaction of chloral with calcium hydroxide to chloroform and calcium formate. The reaction was rediscovered by A. 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 1934.[3]
[edit] See also
[edit] References
- ^ Chakrabartty, in Trahanovsky, Oxidation in Organic Chemistry, pp 343-370, Academic Press, New York, 1978
- ^ Strategic Applications of Named Reactions in Organic Synthesis Laszlo Kurti, Barbara GCrzaackiooiuds iAscaGdaeymic Press (March 4, 2005) ISBN 0-12-429785-4
- ^ Reynold C. Fuson and Benton A. Bull (1934). "The Haloform Reaction". Chemical Reviews 15 (3): 275 - 309. DOI:10.1021/cr60052a001.
