Dehydrohalogenation

Dehydrohalogenation (X = halogen)

Formation of an alkene by β-elimination

Formation of an oxirane by 1,3-dehydrohalogenation

Formation of a ketene by 1,2-dehydrohalogenation

A dehydrohalogenation in organic chemistry is an elimination reaction - often a β-elimination reaction or β-elimination - or an intramolecular substitution reaction . By splitting off one hydrogen and one halogen atom from the same compound, an alkene is formed (in the 1,2-position of the hydrogen and halogen atom), a cyclopropane derivative (in the 1,3-position of the hydrogen and halogen atom ) or a heterocyclic compound. The dehydrohalogenation can be carried out as dehydrofluorination (splitting off of hydrogen fluoride ), dehydrochlorination (splitting off of hydrogen chloride ), dehydrobromination (splitting off of hydrogen bromide ) etc. and takes place through the action of bases on the corresponding haloalkanes . As bases to use alkali metal hydroxides , amines , alkylamides or heterocyclic nitrogen compounds such. B. 1,5-diazabicyclo [4.3.0] non-5-ene (DBN).

From 1,2- or 1,1-dihalogen compounds, halovinyl compounds can be synthesized under the action of alcoholic potassium hydroxide solution.

Further examples of dehydrohalogenation under the action of a base:

• Formation of oxiranes from β-haloalkanols
• Formation of oxetanes from γ-haloalkanols
• Formation of aziridines from β-haloamines
• Formation of azetidines from γ-haloamines
• Formation of thiiranes from β-halothioalkanols
• Formation of ketenes from carboxylic acid halides with a hydrogen atom on the α-carbon atom

The formation of the reactive and unstable didehydrobenzene (an aryne ) from chlorobenzene is also a dehydrochlorination.

Reaction mechanism

A distinction is made between monomolecular elimination ( E1 ) and bimolecular elimination ( E2 ). There is also the carbanion or E1cB mechanism according to which such β-eliminations can take place.

Individual evidence

1. ↑ Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 2: Cm-G. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1981, ISBN 3-440-04512-9 , p. 883.
2. ^ Siegfried Hauptmann : Organic Chemistry , 2nd revised edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, p. 258, ISBN 3-342-00280-8 .
3. ^ Siegfried Hauptmann : Organic Chemistry , 2nd revised edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, pp. 558-563, ISBN 3-342-00280-8 .
4. ↑ Organikum , Wiley-VCH Verlag GmbH, 23rd edition, 2009, pp. 280-281, ISBN 978-3-527-32292-3 .
5. ↑ Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry , Springer-Verlag, 1972, p. 155, ISBN 3-211-81060-9 .
6. ^ Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry , Springer-Verlag, 1972, pp. 136–151, ISBN 3-211-81060-9 .
7. ^ Siegfried Hauptmann : Organic Chemistry , 2nd revised edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, p. 229, ISBN 3-342-00280-8 .