Hydrohalogenation

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A hydrohalogenation reaction is the electrophilic addition of hydrohalic acids 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 Markovnikov's rule. This is due to the abstraction of a hydrogen atom by the alkene from the acid (HX) to form the most stable carbocation (relative stability: 3°>2°>1°>methyl), as well as generating a halogen anion.

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

hydrochlorination of indene

Anti-Markovnikov addition[edit]

In the presence of peroxides, HBr adds to a given alkene in an anti-Markovnikov addition fashion.[5] This regiochemistry follows from the reaction mechanism, which favors formation of the most stable carbon radical intermediate (relative stability: 3° > 2° > 1°> methyl). The mechanism for this reaction is similar to a chain reaction such as free radical halogenation in which the peroxide promotes the formation of the bromide radical. Therefore, in the presence of peroxides, HBr adds so that the bromine atom is added to the carbon bearing the most numerous hydrogen substituents and hydrogen atoms will add to carbons bearing fewest hydrogen substituents. However, this process is restricted to addition of HBr.

Other hydrogen halide (HF, HCl, HI) do not behaves in the manner described above.[6] The resulting 1-bromoalkanes are versatile alkylating agents. By reaction with dimethyl amine, they are precursors to fatty tertiary amines. By reaction with tertiary amines, long-chain alkyl bromides such as 1-bromododecane, give quaternary ammonium salts, which are used as phase transfer catalysts.[7]

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.[8]

Addition of HCl to acrolein

Scope[edit]

Recent research has found that adding silica gel or 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[edit]

  1. ^ Solomons, T.W. Graham; Fryhle, Craig B. (2003), Organic Chemistry (8th ed.), Wiley, ISBN 0-471-41799-8
  2. ^ Smith, Janice G. (2007), Organic Chemistry (2nd ed.), McGraw-Hill, ISBN 0-07-332749-2
  3. ^ P.J. Kropp; K.A. Dans; S.D. Crawford; M.W. Tubergen; K.D. Kepler; S.L Craig; V.P. Wilson (1990), "Surface-mediated reactions. 1. Hydrohalogenation of alkenes and alkynes", J. Am. Chem. Soc., 112 (112): 7433–7434, doi:10.1021/ja00176a075.
  4. ^ R. A. Pacaud and C. F. H. Allen. "α-Hydroindone". Organic Syntheses.; Collective Volume, 2, p. 336
  5. ^ March Jerry; (1885). Advanced Organic Chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wi===qqqqqwq1884ey & Sons, inc. ISBN 0-471-85472-7
  6. ^ The hydrogen-fluorine bond is simply too strong and therefore no fluorine radicals can be generated in the propagation step. Hydrogen chloride reacts too slowly, again reflecting the strength of the hydrogen-chlorine bond. Due to the weakness of the carbon-iodine bond necessary to complete the first step of the propagation phase, insufficient heat is released to proceed through the reaction successfully.
  7. ^ Dagani, M. J.; Barda, H. J.; Benya, T. J.; Sanders, D. C. (2012), "Bromine Compounds", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a04_405
  8. ^ C. Moureu and R. Chaux (1941). "β-Chloropropionic acid". Organic Syntheses.; Collective Volume, 1, p. 166