Markovnikov's Addition: Rule & Mechanism | Chemca

Organic Chemistry

Markovnikov's Addition: Rule & Mechanism

By Chemca Editorial Team • Last Updated: January 2026 • 9 min read

Markovnikov's Rule governs the outcome of the electrophilic addition of asymmetrical reagents (like $HX, H_2O, H_2SO_4$) to asymmetrical alkenes. The rule predicts which constitutional isomer will be the major product based on the stability of the carbocation intermediate.

1. The Rule

Statement

"In the addition of a protic acid $HX$ to an asymmetric alkene, the acidic hydrogen atom ($H^+$) attaches to the carbon atom with more hydrogen atoms (less substituted), and the negative part ($X^-$) attaches to the carbon atom with fewer hydrogen atoms (more substituted)."

"The rich get richer (in Hydrogen)."

2. General Reaction

Addition of Hydrogen Bromide to Propene.

$$ \underbrace{CH_3-CH=CH_2}_{\text{Propene}} + HBr \rightarrow \underbrace{CH_3-CH(Br)-CH_3}_{\text{2-Bromopropane (Major)}} + \underbrace{CH_3-CH_2-CH_2Br}_{\text{1-Bromopropane (Minor)}} $$

3. Detailed Mechanism

The selectivity is driven by the stability of the Carbocation Intermediate.

Step 1: Protonation (Rate Determining Step)

The pi-bond attacks the electrophilic hydrogen ($H^+$). Two possible carbocations can form:

Path A: H adds to C1 $\rightarrow$ Forms $CH_3-\overset{+}{C}H-CH_3$ ($2^\circ$ Carbocation, More Stable).
Path B: H adds to C2 $\rightarrow$ Forms $CH_3-CH_2-\overset{+}{C}H_2$ ($1^\circ$ Carbocation, Less Stable).

Since the secondary carbocation is more stable (due to hyperconjugation and inductive effect), it forms faster.

Step 2: Nucleophilic Attack

The bromide ion ($Br^-$) attacks the more stable secondary carbocation rapidly.

$$ CH_3-\overset{+}{C}H-CH_3 + Br^- \rightarrow CH_3-CH(Br)-CH_3 $$

4. Carbocation Rearrangements

Crucial Exception

Markovnikov's rule in its simple phrasing ("H goes to C with more H") fails if the carbocation rearranges.
Correct Rule: The reaction proceeds via the most stable carbocation intermediate.

Example: 3,3-Dimethyl-1-butene + HCl

1. Protonation gives $2^\circ$ carbocation.
2. 1,2-Methyl Shift occurs to form a more stable $3^\circ$ carbocation.
3. $Cl^-$ attacks the $3^\circ$ carbon.
Product: 2-Chloro-2,3-dimethylbutane (Rearranged product).

5. Anti-Markovnikov Addition (Peroxide Effect)

Also known as the Kharasch Effect.

In the presence of Organic Peroxides ($R-O-O-R$), the addition of HBr (only HBr, not HCl or HI) to unsymmetrical alkenes follows the Anti-Markovnikov rule.

$$ R-CH=CH_2 + HBr \xrightarrow{\text{Peroxide}} R-CH_2-CH_2Br $$
Mechanism: Free Radical Addition. The stability of the Carbon Radical dictates the product ($3^\circ > 2^\circ > 1^\circ$).

6. Summary of Reagents

Reagent	Condition	Rule
HCl, HI, $H_2SO_4$	Any	Markovnikov
HBr	No Peroxide	Markovnikov
HBr	With Peroxide	Anti-Markovnikov

Markovnikov Quiz

Test your concepts on Addition reactions. 10 MCQs with explanations.

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