Mechanism Overview

(The red arrows are curved or curly arrows showing electron movement.)

Reaction involves two steps.

Step 1: (slow)

The alkene pi (p) 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.

Step 2:

The electron deficient carbocation reacts with an electron rich X^- ion.

Examples

Reactant. Click for larger model in new window.	Reagent(s) and conditions (yield: %)	Product (fraction formed: %) Click for larger model in new window.	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.