Markovnikov S Rule — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Markovnikov's Rule Markovnikov's rule states that in the electrophilic addition of an unsymmetrical reagent (such as H-X or H-OH) to an unsymmetrical alkene, the hydrogen atom (or the electrophilic part) adds to the carbon atom of the double bond that already has a greater number of hydrogen atoms, leading to the formation of the more stable carbocation intermediate. Typically, these are solution-phase reactions that do not exhibit dramatic color changes, distinct smells, or precipitate formation. The reactions are generally exothermic, which may lead to a slight increase in temperature. When a gaseous hydrogen halide (like HCl) is bubbled through an alkene, the gas will dissolve and react. The addition reactions following Markovnikov's rule are generally exothermic, as a weaker pi bond is broken and two stronger sigma bonds are formed, leading to a net release of energy. Step 1: Protonation of the alkene's pi bond by the electrophile (H+) to form the most stable carbocation intermediate. The hydrogen adds to the less substituted carbon of the double bond, and the carbocation forms on the more substituted carbon. Step 2: Nucleophilic attack by the halide ion (X-) or water (H2O) on the positively charged carbon of the carbocation. Step 3: (For hydration only) Deprotonation of the protonated alcohol intermediate by a base (e.g., water) to yield the neutral alcohol product and regenerate the acid catalyst. Carbocation Rearrangements: Always check for possible 1,2-hydride or 1,2-alkyl shifts if a more stable carbocation (e.g., secondary to tertiary) can be formed, leading to rearranged products. Anti-Markovnikov Addition: Remember the peroxide effect for HBr, where in the presence of peroxides (ROOR), HBr adds anti-Markovnikov via a radical mechanism. This is a crucial exception. Symmetrical Alkenes: Markovnikov's rule is irrelevant for symmetrical alkenes because only one product is possible regardless of which carbon the hydrogen adds to. Stereochemistry: If a new chiral center is formed, remember that a racemic mixture is typically obtained due to the planar carbocation intermediate. Reagent Specificity: Do not confuse acid-catalyzed hydration (Markovnikov) with hydroboration-oxidation (anti-Markovnikov hydration).