Electrophilic Addition Reactions to Alkenes
Alkenes are electron-rich due to the π-bond, making them susceptible to attack by electrophiles (electron-deficient species). Electrophilic addition is one of the most important reaction classes in organic chemistry, underpinning many drug synthesis routes and metabolic transformations. The SAPC exam frequently tests regioselectivity (Markovnikov vs. anti-Markovnikov) and stereoselectivity (syn vs. anti addition).
General Mechanism
Step 1: The π-electrons of the alkene attack the electrophile (E⁺), forming a cyclic bromonium ion (halogenation) or a carbocation (other additions). This is the rate-determining step.
Step 2: The nucleophile attacks the more substituted carbon of the intermediate (carbocation), opening the ring in the case of halonium ions.
Addition of Halogens (Halogenation)
**Br₂ and Cl₂ add across the C=C bond in anti fashion.
Mechanism:
- Br₂ approaches the π-bond → bromonium ion forms (3-membered ring with Br⁺)
- Br⁻ attacks from the back side (anti to the bromonium) — anti addition results
Stereochemical Outcome:
- In cycloalkenes, anti addition gives trans products (one axial, one equatorial in chair cyclohexane)
- Anti addition is diagnostic for bromonium ion intermediate — you can prove the mechanism by observing stereochemistry
Test for Alkenes: Br₂ in CCl₄ (brown/red colour) decolourizes as Br₂ adds — used as a qualitative test for unsaturation.
Addition of Hydrogen Halides (HX)
Regioselectivity — Markovnikov’s Rule: The hydrogen adds to the less substituted carbon, and the halogen adds to the more substituted carbon.
Rationale: The intermediate is a carbocation — more substituted carbocations are more stable, so the electrophile adds to form the most stable carbocation possible.
Carbocation Rearrangements: If a more stable carbocation can form via hydride or methyl shift, rearrangement occurs and the major product reflects the rearranged carbocation.
Anti-Markovnikov Addition: Occurs in the presence of peroxides (radical mechanism). Hydrogen adds to the more substituted carbon (opposite of Markovnikov’s rule). Only works with HBr; HBr peroxides do not apply to HCl or HI.
Addition of Water (Hydration)
Acid-catalyzed hydration: H₂O adds across the double bond following Markovnikov’s rule (H to less substituted carbon, OH to more substituted).
Mechanism: H⁺ from acid protonates the alkene → carbocation → water attacks → deprotonation to give alcohol.
Oxymercuration-Reduction (Markovnikov, no rearrangement): Hg(OAc)₂ + H₂O adds OH Markovnikov without carbocation rearrangement. The subsequent NaBH₄ reduction replaces Hg with H. This is the laboratory method for Markovnikov addition without side reactions.
Hydroboration-Oxidation (Anti-Markovnikov): BH₃ (borane) adds syn and anti-Markovnikov; H₂O₂/NaOH oxidation replaces B with OH → syn addition, anti-Markovnikov. Stereospecific: adds OH syn (both to same face).
Addition of Sulfuric Acid (H₂SO₄)
Concentrated H₂SO₄ adds to alkenes Markovnikov-wise to give alkyl hydrogen sulfates. The product is water-soluble and can be hydrolyzed back to the alcohol. This reaction is the basis for separating alkenes from hydrocarbon mixtures.
Oxidation Reactions
Syn Addition of Dihalogens (Br₂, Cl₂): Anti addition across C=C, giving vicinal dihalides. Cyclohexene + Br₂ → trans-1,2-dibromocyclohexane.
Epoxidation (Peracid): Epoxides form by syn addition of oxygen — peracids (RCOOOH) transfer oxygen to the π-bond. This is syn addition (both C-O bonds form on the same face).
Dihydroxylation — Syn Addition of Two OH Groups:
| Reagent | Stereochemistry | Product |
|---|---|---|
| OsO₄ / NaHSO₃ or K₂OsO₄.H₂O | Syn addition | Cis-diol (both OH same side) |
| Cold KMnO₄ | Syn addition | Cis-diol |
| Ozonolysis (reductive workup Zn/H₂O or DMS) | Cleaves C=C | Aldehydes/ketones |
| Ozonolysis (oxidative workup H₂O₂) | Cleaves C=C | Carboxylic acids |
Ozonolysis
Ozonolysis cleaves the C=C bond:
- Disubstituted alkene carbon → ketone (C=O with two carbon substituents)
- Monosubstituted alkene carbon → aldehyde (C=O with one H)
- Unsubstituted (=CH₂) → formaldehyde (H₂C=O)
Reductive workup (Zn/CH₃COOH or DMS) gives aldehydes/ketones. Oxidative workup (H₂O₂) gives carboxylic acids.
Polymerization
Alkenes undergo addition polymerization: thousands of monomer units (styrene, ethylene, propylene) link via repeated addition across C=C bonds, catalysed by Ziegler-Natta catalysts or free radical initiators. Understanding polymerization is important for pharmaceutical packaging materials and biodegradable polymer drugs.
Addition to Cyclohexene
Anti Addition (Br₂, Cl₂): Results in trans relationship between substituents — one axial, one equatorial on adjacent carbons.
Syn Addition (OsO₄, epoxidation): Both substituents end up on the same face — both axial or both equatorial.
SAPC Examination Tips
- Markovnikov = “more stable carbocation intermediate” — always explain regioselectivity via the intermediate, not just the rule
- Anti = back-side attack (Br₂/Cl₂) vs. Syn = same-face addition (OsO₄, hydroboration-oxidation) — distinguish clearly
- Ozonolysis products — count substituents on each carbon of the original double bond: 2 substituents = ketone, 1 = aldehyde, 0 = formaldehyde
- Carbocation rearrangements — always check if a 1,2-hydride or 1,2-methyl shift could give a more stable carbocation
- Peracid epoxidation always adds syn — if you need to know which face the oxygen adds to, check for stereoisomeric products
- Practical application — hydration of alkenes is the reverse of dehydration (elimination); Le Chatelier’s principle applies — excess water drives hydration forward
Reaction Summary Table
| Reaction | Reagent | Regioselectivity | Stereochemistry | Intermediate |
|---|---|---|---|---|
| Halogenation | Br₂, Cl₂ | — | Anti | Bromonium/chloronium ion |
| HX addition | HBr, HCl | Markovnikov | Anti (via carbocation) | Carbocation |
| HX + peroxides | HBr (peroxide) | Anti-Markovnikov | — | Radical |
| Hydration | H₂O/H₂SO₄ | Markovnikov | — | Carbocation |
| Oxymercuration | Hg(OAc)₂/H₂O then NaBH₄ | Markovnikov, no rearrangement | — | Mercurinium ion |
| Hydroboration | BH₃ then H₂O₂/NaOH | Anti-Markovnikov | Syn | — |
| Dihydroxylation | OsO₄ or cold KMnO₄ | — | Syn | — |
| Epoxidation | RCOOOH | — | Syn | — |