Additions to alkenes accompanied by 1,2-hydride shifts

by James

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.

Examples:

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.

Notes: 

 

{ 3 comments… read them below or add one }

Raj

hey James I think you made a mistake with the description reaction, a third R group was added to the product

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james

Thank you Raj.

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Chris Munson

Will the predominant product be the hydride shift mechanism? How much more energetically favorable is this product?
Thanks!

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