Electrophilic Addition in Alkenes

⚡ Electrophilic Addition Reactions

• A reaction mechanism shows how a reaction occurs, step by step.

• Alkenes undergo addition reactions to form saturated compounds.

• The mechanism for this is called electrophilic addition.

🔬 Key Points:

✅ The C=C double bond is a region of high electron density due to the presence of π-electrons.
✅ Electrophiles are attracted to this high electron density.
✅ An electrophile is an electron pair acceptor and is often:

• A positively charged ion (e.g., H⁺)

• A molecule with a δ⁺ charge (e.g., HBr)

🧪 The Reaction Between But-2-ene and Hydrogen Bromide

• HBr adds to but-2-ene, forming a single addition product.

• The reaction occurs via electrophilic addition, forming a carbocation intermediate before the final product.

📌 Equation Example:
But-2-ene + HBr → 2-bromobutane

📌 Mechanism: (Curly arrows show electron movement!)

1. HBr is polar (H δ⁺—Br δ⁻).

2. π-electrons from the C=C bond attack the H δ⁺, breaking the double bond.

3. A carbocation forms, and Br⁻ is left over.

4. Br⁻ attacks the carbocation, forming the final product.

🔄 The Reaction Between Propene and Bromine (Br₂)

• Br₂ is non-polar, but electrons in C=C repel Br₂ electrons, inducing a temporary dipole (Br δ⁺—Br δ⁻).

• The reaction follows electrophilic addition like before.

📌 Equation Example:
Propene + Br₂ → 1,2-dibromopropane

📌 Mechanism Steps:

1. π-electrons repel Br₂ electrons, creating a temporary induced dipole.

2. π-electrons attack Br δ⁺, breaking the Br-Br bond.

3. A carbocation forms, and Br⁻ is left over.

4. Br⁻ attacks the carbocation, forming the final product.

💡 What happens with asymmetrical alkenes?

• When HX (e.g., HBr) adds to an asymmetrical alkene, two possible products form.

📌 Example: Propene + HBr

• Two possible carbocations:

  • Primary (less stable) → minor product (1-bromopropane)

  • Secondary (more stable) → major product (2-bromopropane)

🔬 Carbocation Stability

• Carbocations are classified by how many alkyl groups (R) are attached:

  • Primary (least stable) → 1 alkyl group

  • Secondary (more stable) → 2 alkyl groups

  • Tertiary (most stable) → 3 alkyl groups

✅ More alkyl groups = more electron donation = charge spread out = more stability
✅ Tertiary carbocations form faster and lead to the major product!

📜 Markovnikov’s Rule

• Markovnikov’s Rule (1870) states:

  • In the first step of the mechanism, the H⁺ (from the H-Br) prefers to attach to the carbon with the most hydrogen atoms.

  •    The more stable carbocation is favoured.