Electrophilic Addition

Full Notes

Addition reactions of alkenes have been outlined in more detail here.
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Mechanism of Electrophilic Addition

Alkenes react with electrophiles in electrophilic addition reactions.

The high electron density within a carbon-carbon double bond attracts electrophiles and the reaction mechanism follows three basic steps:

Step 1: Electrophile Attraction

• The C=C bond has high electron density and attracts electrophiles (electron pair acceptors).
• Electrophiles include H⁺, Br₂, and H₂SO₄ (see below).

Step 2: Formation of Carbocation Intermediate

• The double bond breaks as one carbon gains a new bond with the electrophile.
• The other carbon becomes positively charged (carbocation).

Step 3: Forming final product

• A negative species (e.g., Br⁻, HSO₄⁻) bonds to the carbocation, forming the final product.

There are several addition reactions of alkenes you need to know:

2. Addition Reactions of Alkenes

Reaction with Hydrogen Bromide (HBr)

Equation: C₂H₄ + HBr → C₂H₅Br

Mechanism:

• HBr is polar (Hδ⁺—Brδ⁻), and H⁺ acts as the electrophile.
• H⁺ bonds to one carbon of the C=C bond, forming a carbocation.
• Br⁻ attacks the carbocation, forming a halogenoalkane.

Reaction with Bromine (Br₂) – Test for Unsaturation

Equation: C₂H₄ + Br₂ → C₂H₄Br₂

Mechanism:

• Br₂ is non-polar but gets polarised when approaching the high electron density of the alkene.
• One Br atom bonds to a carbon, forming a carbocation.
• The Br⁻ ion attacks the carbocation, forming a dihalogenoalkane.

Test for Alkenes:
Bromine water (Br₂(aq)) turns from orange to colourless if an alkene is present.

Reaction with Sulfuric Acid (H₂SO₄)

Equation: C₂H₄ + H₂SO₄ → C₂H₅OSO₃H

Mechanism:

• H⁺ from H₂SO₄ acts as an electrophile, bonding to the alkene.
• Carbocation forms and reacts with HSO₄⁻, forming an alkyl hydrogen sulfate.
• On hydrolysis with water, an alcohol is formed.
• C₂H₅OSO₃H + H₂O → C₂H₅OH + H₂SO₄

Major and Minor Products in Unsymmetrical Alkenes

When adding an electrophile to an unsymmetrical alkene, two possible products can form.

The two possible products won’t be formed in equal amounts. The product formed most is called the major product and the one formed the least is the minor product.

We can predict the major product based on the carbocation intermediate formed in the reaction – the major product forms from the most stable carbocation.

Alkyl (carbon) groups bonded to the positively charged carbon in the intermediate stabilise the positive charge by ‘giving’ electron density to the positively charged carbon. This is called the positive inductive effect.

The more alkyl groups there are bonded to the positively charged carbon, the more stable the carbocation is and the more likely it is to form.

Carbocation stability order: Tertiary > Secondary > Primary

Example Propene + HBr

Major Product: 2-Bromopropane (Formed from secondary Carbocation)
Minor Product: 1-Bromopropane (Formed from primary Carbocation)

Summary

• Alkenes undergo electrophilic addition because the C=C has high electron density.
• HBr adds to give halogenoalkanes
• Br₂ adds to give dihalogenoalkanes and decolourises bromine water
• H₂SO₄ gives alkyl hydrogen sulphates which hydrolyse to alcohols.
• Mechanism steps: electrophile attack → carbocation formation → anion addition.
• In unsymmetrical alkenes, the major product comes from the more stable carbocation (tertiary > secondary > primary) due to the positive inductive effect.