Hydrohalogenation

A hydrohalogenation reaction is the electrophilic addition of hydrogen halides like hydrogen chloride or hydrogen bromide to alkenes to yield the corresponding haloalkanes.^[1]^[2]^[3]

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 hydrogen halide (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 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 (Ad_E3) or stepwise by first protonating the alkyne to form a vinyl cation, followed by attack of HX/X⁻ to give the product (Ad_E2) (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.

^ Solomons, T.W. Graham; Fryhle, Craig B. (2003), Organic Chemistry (8th ed.), Wiley, ISBN 0-471-41799-8
^ Smith, Janice G. (2007), Organic Chemistry (2nd ed.), McGraw-Hill, ISBN 978-0-07-332749-5
^ 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 (20): 7433–7434, doi:10.1021/ja00176a075.
^ R. A. Pacaud & C. F. H. Allen. "α-Hydroindone". Organic Syntheses; Collected Volumes, vol. 2, p. 336.
^ Lowry, Thomas H. (1987). Mechanism and theory in organic chemistry. Richardson, Kathleen Schueller. (3rd ed.). New York: Harper & Row. ISBN 0-06-044084-8. OCLC 14214254.
^ Vollhardt, K. Peter C. (January 2014). Organic chemistry : structure and function. Schore, Neil Eric, 1948- (Seventh ed.). New York, NY. ISBN 978-1-4641-2027-5. OCLC 866584251.{{cite book}}: CS1 maint: location missing publisher (link)

[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 978-0-07-332749-5

[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 (20): 7433–7434, doi:10.1021/ja00176a075.

[4] R. A. Pacaud & C. F. H. Allen. "α-Hydroindone". Organic Syntheses; Collected Volumes, vol. 2, p. 336.

[5] Lowry, Thomas H. (1987). Mechanism and theory in organic chemistry. Richardson, Kathleen Schueller. (3rd ed.). New York: Harper & Row. ISBN 0-06-044084-8. OCLC 14214254.

[6] Vollhardt, K. Peter C. (January 2014). Organic chemistry : structure and function. Schore, Neil Eric, 1948- (Seventh ed.). New York, NY. ISBN 978-1-4641-2027-5. OCLC 866584251.{{cite book}}: CS1 maint: location missing publisher (link)

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