Electrophilic addition is one of the most important reaction mechanisms in IB Chemistry. It explains how alkenes—molecules with C=C double bonds—react with electrophiles to form saturated organic products. Because alkenes appear repeatedly in exam questions, understanding electrophilic addition is crucial for predicting products, drawing mechanisms, and explaining regioselectivity.
What Is Electrophilic Addition?
Electrophilic addition is a reaction where an electrophile attacks the electron-rich C=C double bond of an alkene, leading to the addition of two atoms or groups across the double bond.
Learn why ionic and covalent bonds form under different conditions and how electronegativity, energy and structure determine bonding type.
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carbocation intermediate
2. Nucleophilic attack
A nucleophile (often the negative part of the electrophile) attacks the carbocation. A new bond forms, completing the addition reaction.
The C=C double bond becomes a C–C single bond, and two new atoms/groups attach.
Example: Addition of Hydrogen Halides (HX) to Alkenes
This is the most common IB example.
Step 1: Protonation
H⁺ from HX attacks the double bond → forms carbocation.
Step 2: Nucleophilic Attack
X⁻ attacks the carbocation → forms an alkyl halide.
Product:
Alkene + HX → Halogenoalkane
Markovnikov’s Rule
When an unsymmetrical alkene reacts with HX, two possible carbocations can form. One is more stable.
Markovnikov's rule: “The hydrogen adds to the carbon with more hydrogens already, and the halogen adds to the carbon with fewer hydrogens.”
Why? Because the reaction proceeds through the most stable carbocation, and more substituted carbocations (secondary or tertiary) are more stable.
This governs the major product in many IB reactions.
Example: Addition of Bromine (Br₂) to Alkenes
This reaction is used as a test for unsaturation.
Step 1: Electrophilic attack
The electron-rich double bond polarizes Br₂. One Br becomes partially positive and acts as the electrophile.
Step 2: Formation of a bromonium ion
Three-membered ring forms temporarily.
Step 3: Nucleophilic attack
Br⁻ attacks the more positive carbon.
Product:
A dibromoalkane
The reaction:
Decolorizes bromine water
Is an important qualitative test in IB labs
Carbocation Stability
Carbocation stability controls which product forms in electrophilic addition.
Stability order: Tertiary > Secondary > Primary
Why?
Alkyl groups donate electron density
They stabilize the positive charge
Therefore, electrophilic addition usually forms:
The most stable intermediate
The major product consistent with Markovnikov’s rule
Electrophiles in IB Chemistry
Common electrophiles include:
H⁺ (from acids like HCl, HBr)
Br₂ and Cl₂
HBr or HCl
H₂SO₄ (in hydration reactions)
Any positively charged or electron-deficient species can act as an electrophile.
Real-World Importance
Electrophilic addition is central to:
Polymer formation
Industrial alkene reactions
Synthetic chemistry
Drug design pathways
Many plastic materials originate from electrophilic addition mechanisms.
Common IB Misunderstandings
“The nucleophile attacks first.”
Incorrect—the electrophile always attacks first.
“Only charged species can be electrophiles.”
False. Br₂ is neutral but becomes an electrophile when polarized.
“All products are symmetric.”
Not true—regioselectivity matters, especially with unsymmetrical alkenes.
FAQs
Why does the reaction form a carbocation?
Because the π electrons attack the electrophile, temporarily leaving one carbon electron-deficient.
Why does bromine water lose color?
The alkene reacts with Br₂, removing it from solution.
Do all alkenes undergo electrophilic addition?
Yes—electrophilic addition is the characteristic reaction of alkenes.
Conclusion
Electrophilic addition is a fundamental organic reaction where an electrophile attacks the π bond of an alkene, forming a carbocation and then a saturated product. It explains how alkenes react with halogens, hydrogen halides, and sulfuric acid, and it underpins major concepts like Markovnikov’s rule and carbocation stability. Mastery of electrophilic addition is essential for IB Chemistry exams.
