Markovnikov's Rule

Carbocation Stability: The stability of a carbocation increases with the number of alkyl groups attached to the positively charged carbon ($3^{\circ} > 2^{\circ} > 1^{\circ} > \text{methyl}$).

Inductive Effect: Alkyl groups are electron-donating; they push electron density toward the positive carbon, dispersing the charge and lowering the potential energy of the intermediate.

Hammond's Postulate: In an endothermic step (like carbocation formation), the transition state resembles the intermediate. Therefore, the more stable carbocation has a lower activation energy barrier and forms faster.

Hyperconjugation: Stability is also enhanced by the overlap of $\sigma$-bonds (from adjacent $C-H$ or $C-C$ bonds) with the empty $p$-orbital of the carbocation.

Step 1: Electrophilic Attack: The $\pi$-electrons of the double bond attack the electrophile (usually $H^{+}$). The hydrogen must bond to one of the two vinylic carbons.

Step 2: Carbocation Formation: Identify which carbon, if left with a positive charge, would be more substituted. The hydrogen bonds to the less substituted carbon to create the more substituted carbocation.

Step 3: Nucleophilic Attack: The remaining nucleophile (e.g., $Br^{-}$, $Cl^{-}$, or $H_{2}O$) attacks the carbocation to form the final sigma bond.

Decision Criteria: Always compare the substitution levels of the two carbons involved in the double bond before drawing the intermediate.

Identify Symmetry: First, check if the alkene is symmetric (e.g., but-2-ene). If it is, Markovnikov's rule does not apply because both carbons are equivalent, and only one product type is formed.

The 'Rich Get Richer' Mnemonic: Remember that the carbon that already has more hydrogens 'gets' the new hydrogen atom.

Watch for Rearrangements: Always check if the formed carbocation can become more stable via a hydride shift or methyl shift before the nucleophile attacks.

Reagent Awareness: If you see 'peroxides' ($ROOR$) with $HBr$, the rule is reversed (Anti-Markovnikov). However, this reversal typically only works for $HBr$, not $HCl$ or $HI$.

Ignoring the Intermediate: Students often try to memorize the final product without drawing the carbocation. Drawing the intermediate is the only way to ensure accuracy, especially when rearrangements are possible.

Applying to Non-Protic Additions: The rule is specifically for the addition of $HX$. For reagents like $Br_{2}$ in non-polar solvents, the mechanism involves a cyclic bromonium ion, not a simple carbocation, though the 'more substituted' logic still often applies in water.

Confusing Substitution Levels: Ensure you count the number of non-hydrogen groups attached to the double-bonded carbons correctly. A carbon bonded to two other carbons is secondary ($2^{\circ}$).