Alkene Reactions- Mechanisms and Examples
What Makes Alkenes React
Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. That double bond is the reason alkenes are chemically reactive. The pi bond (the second bond in the C=C) is weaker than a sigma bond, making it the preferred site for attack by reagents.
Unlike alkanes, which only burn or crack, alkenes participate in a wide range of organic reactions. If you're studying organic chemistry, mastering alkene reactions isn't optional—it's the foundation for understanding almost everything that comes next.
The Major Reaction Types
Electrophilic Addition
This is the most important reaction class for alkenes. Electrophiles (electron-deficient species) attack the electron-rich pi bond. Here's how the major addition reactions work.
Hydrogenation
Hydrogen adds across the double bond in the presence of a metal catalyst like Pt, Pd, or Ni. The pi bond breaks, and hydrogen atoms attach to each carbon.
Example: Ethene + H₂ → Ethane
Halogenation
Halogens like Br₂ and Cl₂ add across the double bond. The result is a vicinal dihalide—two halogen atoms on adjacent carbons.
Example: Propene + Br₂ → 1,2-dibromopropane
This reaction also serves as a simple test for alkenes. Bromine is reddish-brown; when it reacts with an alkene, the color disappears.
Hydrohalogenation
Hydrogen halides (HCl, HBr, HI) add to alkenes. The hydrogen attaches to one carbon, and the halogen attaches to the other.
Example: Ethene + HCl → Chloroethane
Markovnikov's Rule
When the alkene is unsymmetrical, the hydrogen adds to the carbon that already has more hydrogens. The halogen goes to the more substituted carbon.
Think of it this way: the carbocation intermediate forms on the more stable carbon—the one with more alkyl groups attached. The halide then attacks that carbocation.
Example: Propene + HBr → 2-bromopropane (not 1-bromopropane)
Hydration (Addition of Water)
Water adds to alkenes in the presence of acid catalyst (usually H₂SO₄). This follows Markovnikov's rule.
Example: Ethene + H₂O → Ethanol
Without acid, water won't react with alkenes. The catalyst is essential.
Oxidation Reactions
Alkenes oxidize differently depending on the oxidizing agent used.
Combustion
Complete combustion produces CO₂ and H₂O. Alkenes burn with a sooty flame—more soot than alkanes because of the higher carbon content.
Syn Hydroxylation
Using cold, dilute KMnO₄ (or OsO₄) adds hydroxyl groups to both carbons of the double bond. This produces a diol (vicinal diol).
Example: Cyclohexene + KMnO₄ → Cyclohexane-1,2-diol
Oxidative Cleavage
Strong oxidizing agents (hot KMnO₄ or O₃) break the double bond entirely. The carbons become carbonyl groups (aldehydes, ketones, or carboxylic acids depending on substitution).
Example: Ethene + Hot KMnO₄ → CO₂ + H₂O
Example: 2-butene + O₃ → Acetaldehyde (from each end)
Polymerization
Alkenes (particularly ethene and propene) polymerize under pressure with a catalyst to form polyethylene and polypropylene. This is addition polymerization—monomers simply add together without losing any atoms.
The double bond opens up, and thousands of monomer units link into long chains. That's why these plastics are called addition polymers.
Comparison of Addition Reactions
| Reaction | Reagents | Product Type | Regioselectivity |
|---|---|---|---|
| Hydrogenation | H₂, Pt/Pd/Ni | Alkane | Not applicable |
| Halogenation | Br₂ or Cl₂ | Dihalide | Anti addition |
| Hydrohalogenation | HCl, HBr, HI | Alkyl halide | Markovnikov |
| Hydration | H₂O, H₂SO₄ | Alcohol | Markovnikov |
| Syn Hydroxylation | Cold KMnO₄ or OsO₄ | Diol | Syn addition |
| Oxidative Cleavage | Hot KMnO₄ or O₃ | Carbonyl compounds | Breaks C=C bond |
How to Predict Alkene Reaction Products
Follow this step-by-step approach:
- Identify the functional group. You have an alkene—look for the C=C bond.
- Identify the reagent. Is it an electrophile? An oxidant? This tells you the reaction type.
- Check for symmetry. If the alkene is symmetrical, there's only one possible product. If not, apply Markovnikov's rule for addition reactions.
- Count carbons in the product. Addition reactions keep the carbon count the same. Cleavage reactions reduce it.
- Consider stereochemistry if relevant. Halogenation gives anti products. Syn hydroxylation gives syn products.
Common Reaction Patterns to Memorize
- Addition of H₂ → Alkane (saturated)
- Addition of X₂ → Vicinal dihalide
- Addition of HX → Alkyl halide (Markovnikov)
- Addition of H₂O → Alcohol (Markovnikov)
- Cold dilute KMnO₄ → Diol
- Hot KMnO₄ or O₃ → Cleavage products
Worked Example
Question: What product forms when 2-methyl-2-butene reacts with HBr?
Solution:
2-methyl-2-butene is an unsymmetrical alkene. The double bond is between a carbon with two methyl groups and a carbon with one methyl group.
According to Markovnikov's rule, hydrogen adds to the carbon with more hydrogens (the less substituted end), and bromine goes to the more substituted carbon where the carbocation is more stable.
Product: 2-bromo-2-methylbutane
The bromine ends up attached to the carbon that held two methyl groups to begin with.
Quick Reference Summary
- Alkenes react mainly through electrophilic addition
- Symmetrical alkenes give one product; unsymmetrical alkenes follow Markovnikov's rule
- Oxidation level increases: combustion → cleavage products
- Halogenation is anti; hydroxylation is syn
- Polymerization opens the double bond to form chains
These are the reactions you'll encounter most often in exams and in understanding reaction mechanisms. Practice predicting products until it becomes automatic.