R3.4.11 Electrophilic addition reactions & Markovnikov's rule

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12 Terms

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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.

<p>Alkenes undergo electrophilic addition due to the high electron density in the C=C bond</p><p>the π bond breaks, allowing new atoms to bond, forming a larger molecule.</p>
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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.

<p>Electrophile approaches the C=C bond; heterolytic fission occurs</p><p>π electrons form a bond with electrophile, generating a carbocation intermediate.</p>
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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.

<p>Anion formed from fission uses a lone pair to bond with the carbocation</p><p>Product is a saturated molecule with added atoms.</p>
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Ethene + HBr Mechanism

HBr is polar

H bonds with one carbon forming a carbocation

Br⁻ attacks carbocation to form bromoethane.

<p>HBr is polar</p><p>H bonds with one carbon forming a carbocation</p><p>Br⁻ attacks carbocation to form bromoethane.</p>
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Ethene + Br₂ Mechanism

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

Br⁻ attacks carbocation forming 1,2-dibromoethane.

<p>Br₂ becomes polar near C=C; Br bonds to one carbon forming a carbocation</p><p>Br⁻ attacks carbocation forming 1,2-dibromoethane.</p>
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Ethene + ICl Mechanism

ICl is polar

I bonds to one carbon forming a carbocation

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

<p>ICl is polar</p><p>I bonds to one carbon forming a carbocation</p><p>Cl⁻ bonds with carbocation forming 1-chloro-2-iodoethane.</p>
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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.

<p>In electrophilic addition of HX to an asymmetrical alkene, the H adds to the carbon with more hydrogens. </p><p>The major product forms via the more stable carbocation intermediate. </p><p>Applies when predicting regioselectivity of unsymmetrical alkenes.</p>
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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.

<p>Alkenes like propene reacting with HBr can form two products. </p><p>According to Markovnikov's Rule, the major product has H on the more H-rich carbon. </p><p>Stability of the intermediate carbocation determines the major product.</p>
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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.

<p>More stable carbocations lead to the major product in addition reactions. </p><p>Stability order: tertiary &gt; secondary &gt; primary. </p><p>Electron-donating alkyl groups stabilize carbocations via inductive effect.</p>
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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.

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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.

<p>Two possible products</p><p>1-bromopropane (from primary carbocation)</p><p>2-bromopropane (from secondary carbocation)</p><p>2-bromopropane is the major product due to carbocation stability.</p>
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Positive Inductive Effect

Alkyl groups donate electron density stabilizing carbocations.

More alkyl groups mean greater carbocation stability.

Explains why tertiary carbocations are most stable.

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