R3.4.11 Electrophilic addition reactions & Markovnikov's rule

Electrophilic Addition

Alkenes undergo electrophilic addition due to the high electron density in the C=C bond

the π bond breaks, allowing new atoms to bond, forming a larger molecule.

Electrophilic Addition: Mechanism Step 1

Electrophile approaches the C=C bond; heterolytic fission occurs

π electrons form a bond with electrophile, generating a carbocation intermediate.

Electrophilic Addition: Mechanism Step 2

Anion formed from fission uses a lone pair to bond with the carbocation

Product is a saturated molecule with added atoms.

Ethene + HBr Mechanism

HBr is polar

H bonds with one carbon forming a carbocation

Br⁻ attacks carbocation to form bromoethane.

Ethene + Br₂ Mechanism

Br₂ becomes polar near C=C; Br bonds to one carbon forming a carbocation

Br⁻ attacks carbocation forming 1,2-dibromoethane.

Ethene + ICl Mechanism

ICl is polar

I bonds to one carbon forming a carbocation

Cl⁻ bonds with carbocation forming 1-chloro-2-iodoethane.

Markovnikov's Rule

In electrophilic addition of HX to an asymmetrical alkene, the H adds to the carbon with more hydrogens.

The major product forms via the more stable carbocation intermediate.

Applies when predicting regioselectivity of unsymmetrical alkenes.

Product Distribution in Asymmetrical Alkenes

Alkenes like propene reacting with HBr can form two products.

According to Markovnikov's Rule, the major product has H on the more H-rich carbon.

Stability of the intermediate carbocation determines the major product.

Carbocation Stability and Major Product

More stable carbocations lead to the major product in addition reactions.

Stability order: tertiary > secondary > primary.

Electron-donating alkyl groups stabilize carbocations via inductive effect.

Heterolytic Fission in HX Addition

In electrophilic addition, the H-X bond breaks heterolytically forming X⁻

H⁺ adds to the alkene carbon forming a carbocation

X⁻ then bonds to the carbocation forming the final product.

Example: Propene + HBr

Two possible products

1-bromopropane (from primary carbocation)

2-bromopropane (from secondary carbocation)

2-bromopropane is the major product due to carbocation stability.

Positive Inductive Effect

Alkyl groups donate electron density stabilizing carbocations.

More alkyl groups mean greater carbocation stability.

Explains why tertiary carbocations are most stable.