Haloform reaction
The haloform reaction (also called the unicorn reaction ) is an organic chemical reaction. Methyl ketones are split into carboxylic acids by the action of hypohalites or halogens ( chlorine , bromine or iodine ) in an alkaline solution with loss of the methyl group. A trihalomethane is formed from the split off carbon atom.
The Haloform reaction was previously used analytically to detect methyl ketones or alcohols with a CH 3 –CHOH group. When iodine is used as a halogen, iodoform is formed , which has a characteristic odor, making it easy to detect. This analysis method is also called the iodoform sample .
Reaction mechanism
The reaction proceeds under very mild conditions with mostly very good yields and tolerates the presence of further functional groups in the educt. The following shows the assumed reaction mechanism using bromine as the halogen.
First step
First, the ketone 1 used is deprotonated on its methyl group by a base (here hydroxide ion ), whereby an enolate 2 is formed which, due to the keto-enol tautomerism, is in equilibrium with a ketone deprotonated in the α-position. The enolate 2 can now react with a halogen molecule (here a bromine molecule as an example), so that a ketone 3 which is simply brominated in the α position and a bromide ion are formed. The bromide ion is irrelevant for the further reaction. The repetition of this reaction step is favored by the electron-withdrawing bromine substituent, as a result of which the ketone 4 , triple brominated in the α-position , is obtained quantitatively .
Second step
The ketone 4 is attacked nucleophilically by a hydroxide ion . This creates (formally) an alcoholate 5 , which breaks down into a carboxylic acid 6 and a tribromomethane anion 7 through rearrangement of its electrons . The anion 7 stabilizes its charge through the −I effect of the halogen atoms and is therefore a good leaving group . It is quickly protonated to bromoform 8 in an aqueous medium .
The haloform reaction can also be carried out with alcohols that contain a CH 3 -CHOH group, the alcohol first being dehydrated to the ketone . Also, ethanol reacts in this manner, as an intermediate acetaldehyde is formed.
Use of trihalomethanes
Trihalomethanes find z. B. Use in the production of irritant gases or disinfectants. Probably the best known trihalomethane is chloroform . In the past it was widely used as an anesthetic, but today it is mainly used as a solvent.
Individual evidence
- ^ FA Carey, RJ Sundberg: Organic Chemistry , Wiley-VCH Verlag 2004, ISBN 3-527-29217-9 .
- ^ Louis F. Fieser and Mary Fieser: Organic Chemistry , 2nd Edition, Verlag Chemie, Weinheim 1982, ISBN 3-527-25075-1 , pp. 405-406.
- ^ A b Rainer Beckert, Hans-Joachim Knölker, Egon Fanghänel , Peter Metz, Wolf D. Habicher and Klaus Schwetlick: Organikum , 23rd edition, Wiley-VCH Verlag, 2009, ISBN 978-3-527-32292-3 , p 454-455.
- ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 605.
- ^ Organic-chemie.ch: Haloform reaction
- ^ A b K. Peter C. Vollhardt, Neil E. Schore: Organische Chemie , Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2005, 4th edition, H. Butenschön, ISBN 3-527-31380-X , p 923