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|Two Groups Added|| || |
|Reagents Added||HX (HCl, HBr, or HI)|
|Other Characteristics||Intermediate is a carbocation making rearrangements possible.|
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.
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.