Electrophilic Alkene Addition Mechanism

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Page ID: 1137

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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

Reagent(s) and conditions (yield: %)	Product (fraction formed: %)	Comments.
HBr acetic acid (90%)		Regioselective: Markovnikov
HBr acetic acid (90%)		Note the bromine attached to the C-2 position from the more stable secondary carbocation
HBr acetic acid (90%)		Regioselective: Markovnikov
HBr acetic acid (90%)		Note the bromine attached to the C-2 position from the more stable tertiary carbocation
HCl (100%)		Regioselective: Markovnikov
HCl (100%)		Note the chlorine attached to the C-1 position from the more stable tertiary carbocation
KI H₃PO₄, 80ºC (90%)
KI H₃PO₄, 80ºC (90%)
HCl	(40%) + (60%)	Rearrangement occurs (hydride migration)
HCl	+	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)
HCl	+	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)