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!)
HBr is polar (H δ⁺—Br δ⁻).
π-electrons from the C=C bond attack the H δ⁺, breaking the double bond.
A carbocation forms, and Br⁻ is left over.
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.