Addition Reactions of Alkenes

Full Notes

Reactivity of Alkenes

A carbon–carbon double bond is made up of a sigma (σ) and pi (π) bond. The π-bond is formed from the sideways overlap of two p-shaped orbitals and is weaker (lower bond enthalpy) than the σ-bond.

Because the pi-bond is slightly weaker than the sigma bond, the double bond can break in reactions and open up, allowing other atoms or groups to bond to the carbons. When this happens, an addition reaction occurs.

This makes alkenes more reactive than alkanes and electrophiles can attack the electron-rich π-bond.

An electrophile is an electron pair acceptor, often carrying a positive charge or δ⁺ region (e.g. H⁺, Brδ⁺).

Addition Reactions of Alkenes

Alkenes react by electrophilic addition reactions and there are several examples you need to know.

Addition of H₂ (Hydrogenation)

Alkenes react with hydrogen in hydrogenation reactions to form alkanes.

• Reagents: H₂ gas
• Catalyst: Ni or Pt
• Conditions: 150–200 °C

Example: CH₂=CH₂ + H₂ → CH₃–CH₃

Addition of Halogen – X₂ (Halogenation)

Alkenes react with halogens in halogenation reactions to form dihalogenoalkanes.

• Reagent: Br₂ or Cl₂
• Conditions: Room temperature
• Forms dihalogenoalkane

Example: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

This reaction is also used as a test for unsaturation. Bromine water turns from orange to colourless when added to an alkene.

Addition of Hydrogen Halide – HX

Alkenes react with hydrogen halides to form halogenoalkanes.

• Reagent: HX (e.g. HBr)
• Conditions: Room temperature
• Forms halogenoalkanes

Example: CH₂=CHCH₃ + HBr → CH₃CHBrCH₃ (major)

Addition of Steam – Hydration

Alkenes react with steam, H₂O(g), to form alcohols.

• Reagents: H₂O(g), catalyst: H₃PO₄
• Conditions: 300 °C, 60 atm
• Forms alcohol

Example: CH₂=CH₂ + H₂O → CH₃CH₂OH

Mechanism of Electrophilic Addition

When alkenes react with electrophiles, the reaction follows a standard electrophilic addition mechanism.

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).
• 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.

For ExampleElectrophilic addition mechanism for Bromine + Ethene

1. Br₂ molecule approaches C=C
   (polarised by electron density)
2. Double bond breaks, Br⁺ forms bond
   Carbocation intermediate forms
3. Br⁻ ion attacks carbocation
   Forms CH₂Br–CH₂Br

Major and Minor Products

When adding HX 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

More stable carbocation = major product. This explains Markovnikov’s rule (Major product will be the one where H from HX bonds to carbon in C=C that is bonded to the most hydrogens) .

Example Propene + HBr

Secondary carbocation → 2-bromopropane (major)
Primary carbocation → 1-bromopropane (minor)

Summary

• Alkenes are reactive due to their weak π-bond, which breaks easily in reactions.
• They undergo electrophilic addition reactions with H₂, halogens, hydrogen halides, and steam to form different products.
• Electrophilic addition involves carbocation intermediates and nucleophilic attack.
• Markovnikov’s rule predicts the major product based on carbocation stability.