Hydrohalogenation

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A hydrohalogenation reaction is the

hydrogen halides like hydrogen chloride or hydrogen bromide to alkenes to yield the corresponding haloalkanes.[1][2][3]

Hydrogen bromide addition to an alkene

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

anion
.

A simple example of a hydrochlorination is that of indene with hydrogen chloride gas (no solvent):[4]

hydrochlorination of indene

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.

Hydrohalogenation of alkynes

Anti-Markovnikov addition

In the presence of

free radical halogenation, in which the peroxide promotes formation of the bromine radical. However, this process is restricted to addition of HBr. Of the other hydrogen halides (HF, HCl, and HI), only HCl reacts similarly, and the process is too slow for synthetic use. (With HF and HI, the energy released in the halogen-carbon addition does not suffice to cleave another hydrogen-halogen bond. Consequently the chain cannot propagate.)[7][8]

The resulting 1-bromoalkanes are versatile

With

Michael acceptors the addition is also anti-Markovnikov because now a nucleophilic X reacts in a nucleophilic conjugate addition for example in the reaction of HCl with acrolein.[10]

Addition of HCl to acrolein
Addition of HCl to acrolein

Scope

Recent research has found that adding

alumina to H-Cl (or H-Br) in dichloromethane increases the rate of reaction making it an easy one to carry out.[citation needed
]

References

  1. .
  2. ^ R. A. Pacaud & C. F. H. Allen. "α-Hydroindone". Organic Syntheses; Collected Volumes, vol. 2, p. 336.
  3. OCLC 14214254
    .
  4. OCLC 866584251.{{cite book}}: CS1 maint: location missing publisher (link
    )
  5. .
  6. .
  7. ^ C. Moureu & R. Chaux (1941). "β-Chloropropionic acid". Organic Syntheses; Collected Volumes, vol. 1, p. 166.