Hydrohalogenation
A hydrohalogenation reaction is the
If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents, an observation known as
A simple example of a hydrochlorination is that of indene with hydrogen chloride gas (no solvent):[4]
Alkynes also undergo hydrohalogenation reactions. Depending on the exact substrate, alkyne hydrohalogenation can proceed though a concerted protonation/nucleophilic attack (AdE3) or stepwise by first protonating the alkyne to form a vinyl cation, followed by attack of HX/X− to give the product (AdE2) (see electrophile for arrow pushing).[5] As in the case of alkenes, the regioselectivity is determined by the relative ability of the carbon atoms to stabilize positive charge (either a partial charge in the case of a concerted transition state or a full formal charge for a discrete vinyl cation). Depending on reaction conditions, the main product could be this initially formed alkenyl halide, or the product of twice hydrohalogenation to form a dihaloalkane. In most cases, the main regioisomer formed is the gem-dihaloalkane.[6] This regioselectivity is rationalized by the resonance stabilization of a neighboring carbocation by a lone pair on the initially installed halogen. Depending on relative rates of the two steps, it may be difficult to stop at the first stage, and often, mixtures of the mono and bis hydrohalogenation products are obtained.
Anti-Markovnikov addition
In the presence of
The resulting 1-bromoalkanes are versatile
With
Scope
Recent research has found that adding
References
- ISBN 0-471-41799-8
- ISBN 978-0-07-332749-5
- .
- ^ R. A. Pacaud & C. F. H. Allen. "α-Hydroindone". Organic Syntheses; Collected Volumes, vol. 2, p. 336.
- OCLC 14214254.
- )
- ISBN 0-471-60180-2
- ISBN 978-0-471-26418-7.
- ISBN 978-3527306732.
- ^ C. Moureu & R. Chaux (1941). "β-Chloropropionic acid". Organic Syntheses; Collected Volumes, vol. 1, p. 166.