4.1.3 Reactions of Alkenes

⚡ The Reactivity of Alkenes

🧪 Why are alkenes more reactive than alkanes?

• Alkenes contain a C=C double bond, made up of:

  • σ-bond (sigma bond)

  • π-bond (pi bond)

• π-electrons are found above and below the plane of the σ-bond.

Since π-electrons are more exposed, they break easily, allowing alkenes to undergo addition reactions ✅ 

👉 The π-bond is weaker than the σ-bond, so it breaks first, allowing addition reactions to occur!

🔄 Addition Reactions of Alkenes

🌟 Alkenes react easily with small molecules, breaking the π-bond and forming new bonds!

📌 Common addition reactions:

✅ Hydrogenation (H₂, Ni catalyst)
✅ Halogenation (Cl₂, Br₂)
✅ Hydrogen halides (HCl, HBr)
✅ Hydration (H₂O, acid catalyst)

🔥 Hydrogenation of Alkenes

👨‍🔬 What happens?

• Alkenes react with hydrogen gas (H₂)

• Requires a nickel catalyst (Ni) at 423 K

• Converts alkenes → alkanes

📌 Example:

🛠 Propene + Hydrogen → Propane

🧪 Halogenation of Alkenes

⚡ Reaction with halogens (Cl₂, Br₂) → dihaloalkanes

📌 Example:

🧐  Bromine water test:

• Orange bromine water + alkene  → colourless (C=C bond present ✅ so unsaturated)

• No change if alkane (C-C only ❌ so unsaturated)

📌 Example Reaction:

Propene + Br₂ → 1,2-dibromopropane

this is an electrophilic addition reaction

💨 Addition of Hydrogen Halides (HCl, HBr)

💥 Reaction:

• Alkenes react with gaseous hydrogen halides at room temperature

• Forms haloalkanes (C–X bonds)

📌 Example Reaction:

Propene + HCl → 1-chloropropane / 2-chloropropane

📌 Key Concept:

🛠 Unsymmetrical alkenes → two possible products!

Click here to see the mechanism for this electrophilic addition reaction

💧 Hydration of Alkenes (H₂O Addition)

🚰 Formation of Alcohols

• Alkenes + steam (H₂O, g) → alcohols

• Requires phosphoric acid catalyst (H₃PO₄)

📌 Example:

Propene + H₂O → propan-1-ol / propan-2-ol

📌 Study Tip:

💡 Sulfuric acid (H₂SO₄) can also be used as the acid catalyst!

🎯 Summary of Reactions