Give the mechanism of Markownikoffs rule as applied to unsaturated hydrocarbons.
Propene (unsymmetrical alkene) reacts with hydrogen iodide (unsymmetrical reagent) to form 2-iodopropane. Addition takes place according to Markownikoffs rule.
Mechanism: The reaction involves electrophilic addition. It is initiated by the attack of an electrophile (H+) on propene molecule. Two types of intermediate carbocations are formed.
The carbocation which is mainly formed depends on upon its relative stability. Since secondary carbocation (I) is more stable than the primary carbocation (II), therefore, it would be formed faster and than in preference to carbocation (II). The carbocation (I) then takes up the iodide ion of the HI to form isopropyl iodide as the main product.
On the basis of above explanation. Markownikoffs rule may also be stated as The electrophilic addition to an unsymmetrical alkene always occurs through the formation of a more stable carbocation intermediate.
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of the HI to form isopropyl iodide as the main product. 
