DAT Review · Organic Chemistry
Alkene Reactions: Master Reaction Table
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Scope: All major alkene addition reactions including regiochemistry (Markovnikov vs. anti-Markovnikov) and stereochemistry (syn vs. anti addition). Expect 4–6 questions. This is the most reaction-dense topic in DAT organic chemistry. You must know every reaction in the table below, including reagents, products, regiochemistry, and stereochemistry.
The college version
Core Review
The Master Alkene Reaction Table
| Reaction | Reagents | Regiochemistry | Stereochemistry | Product | Notes |
|---|---|---|---|---|---|
| Hydrohalogenation | HX (HCl, HBr, HI) | Markovnikov | None (carbocation intermediate) | Alkyl halide | Rearrangements possible. HBr/peroxides = anti-Markovnikov (radical) |
| Acid-catalyzed hydration | H₂O, H₂SO₄ (cat.) | Markovnikov | None (carbocation) | Alcohol | Rearrangements possible. Reversible. |
| Hydroboration-Oxidation | 1. BH₃·THF 2. H₂O₂, NaOH | Anti-Markovnikov | SYN addition (H and OH add to same face) | Alcohol | NO rearrangements. Concerted. |
| Oxymercuration-Demercuration | 1. Hg(OAc)₂, H₂O 2. NaBH₄ | Markovnikov | Anti addition (H₂O and Hg add anti) | Alcohol (after NaBH₄ replaces Hg with H) | NO rearrangements. Excellent lab method. |
| Halogenation | Br₂ or Cl₂ in CCl₄ | N/A (both carbons get X) | ANTI addition | Vicinal dihalide | Bromonium ion intermediate (3-membered ring). |
| Halohydrin formation | X₂, H₂O | OH on MORE substituted carbon | ANTI addition | Halohydrin (X and OH on adjacent carbons) | Water attacks the bromonium ion at the more substituted carbon. |
| Hydrogenation | H₂, Pt/Pd/Ni | N/A (both carbons get H) | SYN addition | Alkane | Heterogeneous catalysis. H₂ adds from the less hindered face. |
| Epoxidation | mCPBA (or RCO₃H) | N/A | SYN addition (O adds to same face) | Epoxide | Concerted. Stereochemistry of alkene is retained in epoxide. |
| Ozonolysis | 1. O₃ 2. Zn/H₂O (or Me₂S) | N/A (double bond cleaved) | N/A | Carbonyls (aldehydes/ketones) | Each alkene C becomes a carbonyl C. Oxidative workup (H₂O₂) oxidizes aldehydes to acids. |
| syn-Dihydroxylation | OsO₄ or cold KMnO₄ | N/A | SYN addition (both OH on same face) | Vicinal diol (cis) | OsO₄ is catalytic with NMO co-oxidant. |
| Carbene addition | CH₂I₂, Zn(Cu) | N/A | SYN addition | Cyclopropane | Simmons-Smith reaction. |
Markovnikov's Rule Explained
When HX adds to an unsymmetrical alkene, the H adds to the LESS substituted carbon (the one with more H's already), and X adds to the MORE substituted carbon. This forms the MORE stable carbocation intermediate.
Why: The transition state is carbocation-like. A more substituted carbocation is more stable (3° > 2° > 1°). The pathway through the more stable carbocation has a lower activation energy.
Anti-Markovnikov Products (Three Ways)
- HBr + peroxides: Radical mechanism. Br• adds to less substituted C (forms more stable radical).
- Hydroboration-oxidation: Concerted syn addition. Boron adds to less substituted C (steric and electronic preference), then oxidation replaces B with OH with retention.
- (Not covered in DAT scope: transition metal catalyzed routes).
Stereochemistry Deep Dive
Syn addition: Both new groups add to the SAME face of the alkene.
- Hydrogenation (H₂/Pt): Both H's add from the less hindered face.
- Hydroboration-oxidation: B and H add from same side; oxidation retains configuration.
- Epoxidation: O adds syn (cis-alkene → cis-epoxide).
- syn-Dihydroxylation: OsO₄ adds both OH's from the same face.
Anti addition: New groups add to OPPOSITE faces.
- Halogenation (Br₂): Bromonium ion opens from the backside (anti).
- Halohydrin formation: Water attacks from the opposite face of the bromonium ion.
Carbocation Rearrangements
Occur in: HX addition, acid-catalyzed hydration (any reaction with a carbocation intermediate).
Do NOT occur in: Hydroboration-oxidation, oxymercuration-demercuration, hydrogenation, epoxidation, halogenation (bromonium ion, not free carbocation).
A 1,2-hydride shift (H moves with its bonding electrons) or 1,2-alkyl shift converts a less stable carbocation to a more stable one. ALWAYS scan the substrate for potential rearrangements.
Ozonolysis — Structural Determination Tool
Ozonolysis cleaves the C=C bond. Each alkene carbon becomes a carbonyl group:
- RCH= → RCHO (aldehyde)
- R₂C= → R₂CO (ketone)
This is a powerful tool for determining alkene structure from the carbonyl fragments. DAT questions often give the ozonolysis products and ask for the original alkene.
Common Traps
- Using Markovnikov for hydroboration-oxidation: This is anti-Markovnikov! OH ends up on the LESS substituted carbon.
- Assuming rearrangements with all acid-catalyzed reactions: Oxymercuration-demercuration is acid-catalyzed but goes through a mercurinium ion, NOT a free carbocation — no rearrangements.
- Mixing up syn vs. anti: Hydrogenation = syn. Halogenation = anti. Hydroboration = syn. These are high-frequency DAT points.
- Forgetting stereochemistry of the starting alkene: cis-alkene + syn addition = cis product relationship. trans-alkene + anti addition = identity of product type depends on specific reaction.
Eli explains
The same idea, in plain words
Explain it like I’m 10
An alkene is like a double door. Different "teams" of reagents approach differently. Markovnikov's rule: the H+ (small, fast) goes through the less crowded door. When it's a "syn" reaction, both reagents come from the same hallway. "Anti" means they come from opposite sides. Hydroboration is the rebel team — it does the opposite of what Markovnikov says. Ozonolysis is like taking a saw to the double bond and seeing what pieces you get.
Key takeaways
- Markovnikov addition (HX, H₂O/H⁺, Hg(OAc)₂/H₂O): H goes to less substituted C.
- Anti-Markovnikov addition (BH₃ then H₂O₂/NaOH, HBr/peroxides): H goes to more substituted C.
- No rearrangements with hydroboration-oxidation or oxymercuration-demercuration.
- Anti addition with Br₂ and halohydrin formation; syn addition with hydrogenation and hydroboration.
- Ozonolysis serves as a structural proof: identify the alkene from its carbonyl fragments.
- 1-methylcyclohexene + HBr → ? Answer: 1-bromo-1-methylcyclohexane (Markovnikov). H adds to C-2 (less substituted, forming the more stable 3° carbocation at C-1), then Br⁻ attacks. No rearrangement because the carbocation is already 3°.
- 1-methylcyclohexene + 1. BH₃·THF 2. H₂O₂, NaOH → ? Answer: trans-2-methylcyclohexanol (anti-Markovnikov, syn addition). Boron adds to the LESS substituted C-2, H adds to C-1 from the same face. Oxidation replaces B with OH with retention → the OH and the added H are trans to each other (both added syn to the same face).
- cis-2-butene + Br₂ in CCl₄ → ? Answer: (2R,3S)-2,3-dibromobutane (meso) — anti addition to cis-alkene gives the meso compound. The bromonium ion opens from the backside at each carbon, producing anti stereochemistry. Starting from cis gives the meso product (achiral despite two stereocenters).
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Predict products for all major alkene addition reactions
- Apply Markovnikov's rule and identify when anti-Markovnikov products form
- Determine stereochemical outcomes: syn vs. anti addition
- Select appropriate reagents for targeted transformations
- Recognize when rearrangements occur and when they don't
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