Dehydrohalogenation
In chemistry, dehydrohalogenation is an elimination reaction which removes a hydrogen halide from a substrate. The reaction is usually associated with the synthesis of alkenes, but it has wider applications.
Dehydrohalogenation from alkyl halides
Traditionally, alkyl
Base-promoted reactions to alkenes
When treated with a strong base many alkyl chlorides convert to corresponding alkene.[1] It is also called a β-elimination reaction and is a type of elimination reaction. Some prototypes are shown below:
Here
In general, the reaction of a haloalkane with potassium hydroxide can compete with an SN2 nucleophilic substitution reaction by OH− a strong, unhindered nucleophile. Alcohols are however generally minor products. Dehydrohalogenations often employ strong bases such as potassium tert-butoxide (K+ [CH3]3CO−).
Base-promoted reactions to alkynes
Upon treatment with strong base,
Thermal cracking
On an industrial scale, base-promoted dehydrohalogenations as described above are disfavored. The disposal of the alkali halide salt is problematic. Instead thermally-induced dehydrohalogenations are preferred. One example is provided by the production of vinyl chloride by heating 1,2-dichloroethane:[3]
- CH2Cl-CH2Cl → CH2=CHCl + HCl
The resulting HCl can be reused in oxychlorination reaction.
Thermally induced dehydrofluorinations are employed in the production of fluoroolefins and hydrofluoroolefins. One example is the preparation of 1,2,3,3,3-pentafluoropropene from 1,1,2,3,3,3-hexafluoropropane:
- CF2HCH(F)CF3 → CHF=C(F)CF3 + HF
Other dehydrohalogenations
Epoxides
- CH3CH(OH)CH2Cl + KOH → CH3CH(O)CH2 + H2O + KCl
Isocyanides
The
- KOH + CHCl3 → KCl + H2O + CCl2
Two successive base-mediated dehydrochlorination steps result in formation of the isocyanide.[5]
Coordination compounds
Dehydrohalogenation is not limited to organic chemistry. Some
For example, salts that contain acidic cations hydrogen bonded to halometallate anions will often undergo dehydrohalogenation reactions reversibly:[6]
- [B–H]+···[X–MLn]− ⇌ [B–MLn] + HX
where B is a basic ligand such as a pyridine, X is a halogen (typically chlorine or bromine), M is a metal such as cobalt, copper, zinc, palladium or platinum, and Ln are spectator ligands.
References
External links
- Dehydrohalogenation of Alkyl Halides Archived 2021-04-11 at the Wayback Machine