Organic chemistry study guide · Reaction Mechanism MCAT Tutor
Alkene addition: Markovnikov, anti addition, and reagent-specific outcomes
Alkenes react by adding reagents across the π bond. Regiochemistry (which carbon gets H vs X) and stereochemistry (syn vs anti) depend on the mechanism — carbocation vs cyclic halonium vs concerted hydroboration. Markovnikov addition places H on the less substituted carbon and the electronegative group on the more substituted carbon for many electrophilic additions.
Written for undergraduate orgo, pre-med, and MCAT organic chemistry review — United States exam-style framing.
Short lesson
Markovnikov addition (HX, acid-catalyzed hydration)
Proton adds to give the more stable carbocation (more substituted center). Nucleophile (X⁻ or H₂O) attacks the more substituted carbon.
With HBr and peroxides, anti-Markovnikov radical addition can occur — a classic “read the conditions” exam point.
Acid-catalyzed hydration gives Markovnikov alcohol; rearrangements are possible if a better carbocation forms.
Anti addition: halogens and similar
Br₂ and Cl₂ add through a bromonium/chloronium ion; backside attack gives trans-1,2-dihalide (anti addition).
Same anti stereochemistry theme appears in epoxide opening under basic conditions and some osmylation sequences.
Syn vs anti summary
Hydroboration–oxidation: syn addition, anti-Markovnikov alcohol (H and OH end up on same face; OH on less substituted carbon).
Oxymercuration–demercuration: Markovnikov alcohol, no carbocation rearrangement — compare to direct hydration.
Catalytic hydrogenation: syn addition of H₂ (both H add from same side of the π bond).
Common traps on exams
Markovnikov without checking for peroxides
HBr + ROOR switches to radical anti-Markovnikov. The reagent line matters as much as the alkene.
Drawing syn dihalide from Br₂
Halogen addition is anti through the halonium intermediate — trans-1,2-dibromide on a simple alkene.
Confusing hydroboration with acid hydration
Hydroboration gives anti-Markovnikov and syn stereochemistry; H₃O⁺ hydration is Markovnikov and can rearrange.
Forgetting stereochemistry on achiral alkenes
Even when no chirality exists yet, exams expect correct cis/trans or syn/anti descriptors for dihalides and diols.
Practice-style prompts
Try these on paper, then check product and mechanism in OrgoMech.
- Propene + HBr — draw the major Markovnikov product. Repeat with HBr + peroxide.
- Cyclohexene + Br₂ — show the anti addition product stereochemistry.
- 1-methylcyclohexene + BH₃ then H₂O₂, OH⁻ — where does the OH install and what is the relative stereochemistry of H and OH?
- Which reagent gives Markovnikov alcohol without carbocation rearrangement: H₃O⁺ or oxymercuration?
- Predict the product of trans-2-butene + Br₂ in CH₂Cl₂.
Student questions
Common questions undergraduates ask about this topic before exams.
- What is Markovnikov addition?
- In electrophilic addition of HX or acid-catalyzed hydration, H adds to the less substituted carbon and the electronegative group adds to the more substituted carbon, following carbocation stability.
- When is anti-Markovnikov addition seen with HBr?
- With peroxides (ROOR), HBr adds via a radical chain: Br ends up on the less substituted carbon. Without peroxides, Markovnikov HBr addition is typical.
- Is bromine addition syn or anti?
- Anti addition through a bromonium ion — trans-1,2-dibromide on a simple alkene.
- How does hydroboration differ from acid hydration?
- Hydroboration–oxidation gives anti-Markovnikov alcohol with syn placement of H and OH. Acid-catalyzed hydration is Markovnikov and can rearrange via carbocations.
- Why use oxymercuration instead of H₃O⁺ hydration?
- Oxymercuration–demercuration gives Markovnikov alcohol without carbocation rearrangement — useful when rearrangement would change the product skeleton.
- Does catalytic hydrogenation show stereochemistry?
- Yes — syn addition of H₂ across the π bond from the same face (both hydrogens delivered to the same side of the former double bond).
- What alkene addition topics show up most on first exams?
- HX (with and without peroxides), halogen addition stereochemistry, and hydroboration versus hydration comparisons are high frequency in U.S. orgo 1.
Further reading (external)
Open textbooks and reference sites for the same topics covered above. We link out to established science education publishers — not affiliate pages.
- Alkene addition reactions (LibreTexts) — LibreTexts
- OpenStax — Reactions of alkenes — OpenStax
- PubChem — NIH NLM
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Drill this topic in OrgoMech
Predict the major product for your structure, then order the curved-arrow mechanism — the same skills tested on homework, finals, and the MCAT orgo section. Predict the major product, then order the curved arrows. Organic chemistry practice for reactions, mechanisms, and the days left before your final.