Electrophilic Alkene Addition Mechanism
- Page ID
- 1137
Mechanism Overview
The electrophilic Alkene Adddition reaction involves two sequential steps. The red arrows are curved or curly arrows showing electron movement.
Step 1 (slow)
The alkene pi (π) electrons react with a source of electron deficient H. The most stable carbocation is formed. Occasionally the first-formed carbocation undergoes a rearrangement to a more stable carbocation. Formation of the carbocation is the rate limiting (slowest) step in the mechanism.
Step 2 (fast)
The electron deficient carbocation reacts with an electron rich X- ion.
Examples
Reactant | Reagent(s) and conditions (yield: %) |
Product (fraction formed: %) |
Comments. |
---|---|---|---|
HBr acetic acid (90%) |
Regioselective: Markovnikov | ||
Note the bromine attached to the C-2 position from the more stable secondary carbocation |
|||
HBr acetic acid (90%) |
Regioselective: Markovnikov | ||
Note the bromine attached to the C-2 position from the more stable tertiary carbocation | |||
HCl (100%) |
Regioselective: Markovnikov | ||
Note the chlorine attached to the C-1 position from the more stable tertiary carbocation | |||
KI H3PO4, 80ºC (90%) |
|||
HCl |
(40%) + (60%) | Rearrangement occurs (hydride migration) |
|
+ |
The first-formed carbocation is secondary, some of this carbocation reacts with Cl- but a hydride migration converts this into a more stable tertiary carbocation from which the majority of the product is derived. | ||
HCl |
(17%) (83%) |
Rearrangement occurs (methyd [CH3 with 2 electrons] migration) |
|
+ |
The first-formed carbocation is secondary, a methide migration converts this into the more stable tertiary carbocation. A small amount of the secondary carbocation reacts with Cl- before rearranging, but the majority rearranges before reacting. |
Contributors
- David Woodcock (Okanagan College, Associate Professor emeritus)