Haloform reaction
A base-catalyzed reaction producing haloforms from acetyl groups.
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) in the presence of a base. The reaction can be used to transform acetyl groups into carboxyl groups or to produce chloroform, bromoform, or iodoform. Fluoroform cannot be prepared in this way.
- type
- Chemical reaction
- key_contributors
- Georges-Simon Serullas, Justus von Liebig, Adolf Lieben
- field
- Organic chemistry
- known_for
- Production of haloforms, iodoform test for methyl ketones
- substrates
- Methyl ketones, secondary alcohols oxidizable to methyl ketones, ethanol, acetaldehyde, 1,3-diketones, β-ketoacids
Lore & Background
The haloform reaction is one of the oldest organic reactions known.
Reader's Guide
The haloform reaction has significant historical and practical importance in organic chemistry. It forms the basis of the iodoform test, which was commonly used 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, a positive reaction gives iodoform, a solid at room temperature that precipitates out of solution, causing a distinctive cloudiness. The reaction can also be used to convert a terminal methyl ketone into the analogous carboxylic acid. Industrially, it was formerly used to produce iodoform, bromoform, and chloroform. A variant is used to manufacture deuterated chloroform via reaction of hexachloroacetone with heavy water or decomposition of calcium trichloroacetate in heavy water. Water chlorination can result in the formation of haloforms if water contains suitable reactive impurities such as humic acid; chloroform is suspected to be carcinogenic and is associated with a weak link between chlorinated water consumption and cancer. Substrates are broadly limited to methyl ketones and secondary alcohols oxidizable to methyl ketones, with ethanol and acetaldehyde being the only primary alcohol and aldehyde to undergo the reaction. Acetyl chloride and acetamide do not undergo this reaction.
Did You Know?
- Fluoroform cannot be prepared by the haloform reaction because it would require the highly unstable hypofluorite ion.
- Water chlorination can produce haloforms like chloroform from impurities such as humic acid, and chloroform is suspected to be carcinogenic.
- A variant of the reaction uses hexachloroacetone with heavy water to manufacture deuterated chloroform.
More in Organic Chemistry And Reaction Mechanisms 1-22
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