How Alkenes React with Electrophiles- Mechanism
What Electrophilic Addition Actually Is
Alkenes are electron-rich hydrocarbons. That double bond? It's loaded with π-electrons sitting there, waiting to be attacked. Electrophiles—species that love electrons—can't resist them.
When an electrophile meets an alkene, it steals electron density. The π-bond breaks. New bonds form. This is electrophilic addition, and it's the core reaction type you need to understand for alkenes.
No catalysts needed for this to happen. The alkene is reactive enough on its own. That's the bitter truth about unsaturated hydrocarbons—they're thermodynamically unstable compared to their saturated counterparts.
The Mechanism: Two Steps, No Magic
Skip the fancy animations. Here's what actually happens:
Step 1: The Electrophile Attacks
The electrophile approaches the π-bond and accepts electron density. The π-electrons form a bond with the electrophile. The alkene carbons each lose one electron from the π-bond, leaving one carbon with a positive charge.
You've just created a carbocation intermediate. This is the rate-determining step. Slower than step 2, and it controls your reaction kinetics.
Step 2: The Nucleophile Strikes
The nucleophile—an anion or negatively charged species—attacks the carbocation. It forms the second new bond. The carbocation disappears. Product forms.
That's it. Addition complete. The nucleophile always attacks the more substituted carbon in the intermediate. This isn't a rule—it's just physics. The more substituted carbocation is more stable, so it forms preferentially.
Why the Carbocation Matters
The carbocation intermediate determines everything: regioselectivity, stereochemistry, and whether your reaction gives you the product you want.
Carbocations rearrange. They shift hydrides or alkyl groups to form more stable intermediates. A secondary carbocation next to a tertiary? It'll rearrange. Your product will reflect the more stable structure, not your original alkene.
This is where students mess up. They draw the mechanism as if the carbocation can't move. It can. It does. Plan for rearrangements when you're predicting products.
Common Electrophilic Addition Reactions
Halogenation (Br₂, Cl₂)
Bromine or chlorine adds across the double bond. The electrophile is the halogen itself—specifically, the partially positive end of the halogen molecule.
Mechanism involves a bromonium ion intermediate, not a carbocation. The halide attacks from the opposite side of the bond formation. This gives you anti-addition—the two substituents end up on opposite faces of the molecule.
Anti-addition means trans products from cis alkenes. Syn addition doesn't happen here. Remember that for your exams.
Hydrohalogenation (HBr, HCl, HI)
HX adds across the double bond. Hydrogen is the electrophile. The halide is the nucleophile.
Markovnikov's Rule applies: the hydrogen adds to the carbon with more hydrogens already attached. The halide ends up on the more substituted carbon.
This happens because the major carbocation intermediate forms on the more substituted carbon. The nucleophile then attacks that carbon.
Peroxides flip the regioselectivity with HBr. Anti-Markovnikov addition occurs. The mechanism changes—it goes through a free radical intermediate instead. This is an exception you need to memorize.
Hydration (H₂O with Acid Catalysis)
Water adds to an alkene under acidic conditions. Same mechanism as hydrohalogenation, but water is the nucleophile instead of a halide.
Markovnikov addition applies. The OH ends up on the more substituted carbon. This is how you make alcohols from alkenes industrially.
Osmium tetroxide (OsO₄) gives syn addition if you need the opposite stereochemistry. That reaction is expensive and toxic, but it delivers.
Hydroboration-Oxidation
BH₃ adds to the alkene, then oxidation replaces the boron with OH. Net result: anti-Markovnikov, syn addition.
The borane adds with the boron going to the less hindered carbon. Oxidation replaces B with OH in the same stereochemical position. This is the cleanest way to get anti-Markovnikov alcohols.
Ozonolysis
Ozone cleaves the double bond completely. Each carbon becomes a carbonyl—aldehyde or ketone depending on substitution.
This reaction tells you exactly where the double bond was in your original molecule. It's analytical chemistry's best friend. Work backwards from the carbonyl products to identify your alkene structure.
Comparing the Reactions
| Reaction | Electrophile | Regioselectivity | Stereochemistry | Intermediate |
|---|---|---|---|---|
| Halogenation (Br₂/Cl₂) | Halogen | No regioselectivity issue | Anti | Bromonium/Chloronium ion |
| Hydrohalogenation | H⁺ | Markovnikov | No stereochemistry | Carbocation |
| Acid-catalyzed Hydration | H⁺ | Markovnikov | No stereochemistry | Carbocation |
| Hydroboration-Oxidation | BH₃ | Anti-Markovnikov | Syn | No stable intermediate |
| Ozonolysis | O₃ | Cleaves bond | N/A | None |
How to Predict Products: A Practical Approach
Stop guessing. Follow this checklist every time:
- Identify the alkene structure. Count carbons, identify substitution pattern. Is it terminal? Disubstituted? Trisubstituted?
- Identify the reagent. What electrophile is attacking? H⁺? Br₂? BH₃? This determines everything.
- Determine regioselectivity. Markovnikov or anti-Markovnikov? Check for exceptions like peroxide conditions with HBr.
- Determine stereochemistry. Syn or anti? Some reactions don't care about stereochemistry. Some demand specific outcomes.
- Check for rearrangements. Will the carbocation shift before the nucleophile attacks? If a more stable carbocation can form, assume it does.
Work through the mechanism step by step. Draw the intermediate. Then draw the product from that intermediate. Don't skip steps.
What About Stereochemistry?
Some additions give you stereoisomers. Some don't. Know the difference.
Reactions with carbocation intermediates: No stereocontrol. The planar carbocation can be attacked from either face equally. You get a racemic mixture if the product is chiral.
Reactions with cyclic intermediates (bromonium ion): Anti addition. Trans products from cis alkenes. Cis products from trans alkenes. Stereochemistry is predictable.
Syn addition reactions: Both substituents add to the same face. Hydroboration-oxidation and OsO₄ do this. Cis alkenes give meso products or enantiomeric pairs. Trans alkenes give racemic mixtures.
The Bottom Line
Electrophilic addition to alkenes follows predictable patterns. The mechanism is always the same—electrophile attacks, carbocation forms, nucleophile attacks. Variations come from the nature of the electrophile and whether stable intermediates form.
Your job is to identify which electrophile you're working with, predict which carbocation intermediate will form, and draw the product. That's it. No shortcuts, no tricks. Just apply the mechanism.