Defining Markovnikov's Rule
Markovnikov's rule states that when a protic acid (HX) is added to an unsymmetrical alkene, the acidic hydrogen atom attaches to the carbon atom of the double bond that already has the greater number of hydrogen atoms. The halide (X) group attaches to the carbon atom that has fewer hydrogen atoms.
Section 2: The Principle of Carbocation Stability
The underlying principle behind Markovnikov's rule is the stability of the carbocation intermediate formed during the reaction. The addition of the hydrogen ion creates a carbocation, and the reaction proceeds through the most stable intermediate possible. Carbocation stability follows the order: tertiary (most stable) > secondary > primary (least stable).
Section 3: A Practical Example
Consider the reaction of propene (CH₃-CH=CH₂) with hydrogen bromide (HBr). The hydrogen from HBr can add to either the first or second carbon of the double bond. If it adds to the first carbon (CH₂), it forms a more stable secondary carbocation. If it adds to the second carbon (CH), it forms a less stable primary carbocation. Therefore, the reaction favors the formation of the secondary carbocation, and the bromide ion attacks this carbon, resulting in 2-bromopropane as the major product.
Section 4: Importance in Organic Synthesis
Markovnikov's rule is fundamentally important in organic chemistry because it allows chemists to predict the regioselectivity of electrophilic addition reactions. By understanding this rule, chemists can control the outcome of reactions to synthesize specific desired molecules, which is a cornerstone of pharmaceutical and materials science.