8.2 Hydrohalogenation
PRE-HEALTH PREP
| HYDROHALOGENATION | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| 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 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.
