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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

1. Attack of Electrophile (Slow/RDS): The $\pi$-electrons attack the electrophile, forming a Carbocation intermediate.
2. 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).

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