Electrophilic Addition Reactions
Mechanisms, Regioselectivity (Markovnikov vs Anti-Markovnikov), and Stereochemistry.
By chemca Team • Updated Jan 2026
Unsaturated hydrocarbons (Alkenes and Alkynes) are electron-rich due to the presence of $\pi$-bonds. They undergo addition reactions initiated by the attack of an Electrophile ($E^+$).
1. General Mechanism
Step-by-Step
- Attack of Electrophile (Slow/RDS): The $\pi$-electrons attack the electrophile, forming a Carbocation intermediate.
- Attack of Nucleophile (Fast): The nucleophile attacks the carbocation to form the final addition product.
$$>C=C< + E-Nu \xrightarrow{\text{Slow}} >C^+-C(E)< \xrightarrow{Nu^-, \text{Fast}} >C(Nu)-C(E)<$$
2. Addition of Hydrogen Halides (HX)
Markovnikov's Rule
The electrophile ($H^+$) adds to the carbon atom of the double bond that has more hydrogen atoms (to form the more stable carbocation).
$$CH_3-CH=CH_2 + HBr \rightarrow \underset{2^\circ \text{ Carbocation (Stable)}}{CH_3-CH^+-CH_3} \xrightarrow{Br^-} \underset{\text{2-Bromopropane}}{CH_3-CH(Br)-CH_3}$$
Rearrangement Alert: Since a Carbocation is formed, Rearrangement (Hydride/Methyl shift) occurs to form a more stable carbocation ($3^\circ > 2^\circ$) before the nucleophile attacks.
Peroxide Effect (Anti-Markovnikov)
Specifically for HBr in the presence of Peroxide ($R_2O_2$). Mechanism involves Free Radicals, not Carbocations.
$$CH_3-CH=CH_2 + HBr \xrightarrow{\text{Peroxide}} CH_3-CH_2-CH_2-Br$$
3. Addition of Halogens ($X_2$)
Cyclic Halonium Ion Mechanism
Reaction with $Br_2$ or $Cl_2$ in inert solvent ($CCl_4$).
- No Carbocation: A three-membered Cyclic Halonium Ion is formed. No rearrangement.
- Stereochemistry: Anti-Addition. The nucleophile attacks from the side opposite to the halogen bridge.
- Test for Unsaturation: Decolorization of Reddish-Brown Bromine solution.
$$\text{cis-2-Butene} + Br_2 \rightarrow \text{Racemic (dl) 2,3-Dibromobutane}$$
$$\text{trans-2-Butene} + Br_2 \rightarrow \text{Meso 2,3-Dibromobutane}$$
4. Addition of Water (Hydration)
Three Methods compared
| Method | Reagents | Regioselectivity | Intermediate | Rearrangement |
|---|---|---|---|---|
| Acid Catalyzed | $H_2O / H^+$ | Markovnikov | Carbocation | Yes |
| Oxymercuration-Demercuration (OMD) | 1. $Hg(OAc)_2, H_2O$ 2. $NaBH_4$ |
Markovnikov | Cyclic Mercurinium Ion | No |
| Hydroboration-Oxidation (HBO) | 1. $B_2H_6, THF$ 2. $H_2O_2, OH^-$ |
Anti-Markovnikov | Four-center transition state | No (Syn-Addition) |
Example: 3,3-Dimethyl-1-butene
- Acid Hydration: 2,3-Dimethylbutan-2-ol (Rearranged $3^\circ$ Alcohol).
- OMD: 3,3-Dimethylbutan-2-ol (Markovnikov, No rearrangement).
- HBO: 3,3-Dimethylbutan-1-ol (Anti-Markovnikov).
Knowledge Check
Test your understanding of Addition Reactions
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