Additions to alkenes accompanied by 1,2-hydride shifts
Description: When secondary (or primary) carbocations are formed, adjacent to a more substituted carbon, hydrogen atoms can shift. This leads to formation of a more stable carbocation.
Notes: Note how the blue hydrogen has shifted positions and the “X” [which could be Br, Cl, I, or OH] is attached to where the H used to be.
Notes: Note that the first two examples show rearrangement to give mroe stable tertiary carbocations, whereas the last example shows a rearrangement to give a resonance stabilized benzylic carbocation.
Mechanism: The first step of the reaction is formation of a carbocation. Here, the formation of a carbocation from addition of HBr to an alkene is shown (Step 1, arrows A and B). Since we have a secondary carbocation adjacent to a tertiary carbon, shift of a hydrogen to the secondary carbocation will results in a (more stable) tertiary carbocation (Step 2, arrow C). The tertiary carbocation is then trapped, in this case, by Br(–) (Step 3, arrows D). The rearrangement step goes through a transition state such as the one pictured.