8.2 Hydrohalogenation

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HYDROHALOGENATION
Less Substituted
More Substituted
Two Groups Added
H
X (Cl, Br, or I)
Reagents Added HX (HCl, HBr, or HI)
Regioselectivity Markovnikov
Stereospecificity None
Other Characteristics Intermediate is a carbocation making rearrangements possible.

Hydrohalogenation

Hydrohalogenation results in the Markovnikov addition of a hydrogen (less substituted side) and a halogen (Cl, Br, or I--more substituted side) across an alkene forming an alkyl halide.  There is no stereospecificity associated with this reaction, but the intermediate is a carbocation so rearrangements are possible.  The reagent is either HCl, HBr, or HI.

hydrohalogenation of alkenes

Hydrohalogenation Mechanism

Step 1: The pi electrons of the alkene attack a hydrogen of HBr resulting in the formation of a carbocation and a bromide ion.  The carbocation is a high energy intermediate making this the rate determining step (slow step) of the reaction.

 

Step 2: When there is no favorable rearrangement the bromide ion carries out nucleophilic attack on the carbocation forming an alkyl halide.  The carbocation has a trigonal planar geometry and the bromide ion can attack from either side explaining the lack of stereospecificity for this reaction.  When attack occurs at a chiral center as in this example both the R and S stereoisomers are produced.

Hydrohalogenation of Alkenes Mechanism