Nearly every alkene reaction is the same two-step move — π bond attacks an electrophile, a cation forms, a nucleophile traps it — differing only in the intermediate.
The one alkene, three destinations
1. The π Bond Is the Nucleophile — It Attacks the Electrophile
The π electrons form a new σ bond to an electrophile, leaving the other carbon a cation that a nucleophile then captures.
2. Adding HX Runs Through the More Stable Carbocation — That Is Markovnikov's Rule
The π bond grabs the proton of HX to give the more stable carbocation (3° > 2° > 1°), so X lands on the more substituted carbon.
Markovnikov addition of HBr to propene.
Watch for rearrangements: a free carbocation can undergo a 1,2-hydride or alkyl shift to upgrade a 2° cation to 3°.
3. Acid-Catalyzed Hydration Adds H–OH the Same Markovnikov Way
Water plus catalytic acid runs the same mechanism with water as nucleophile — OH on the more substituted carbon, and it can rearrange too.
Acid-catalyzed hydration puts OH on the more substituted carbon.
4. Halogenation Goes Through a Bridged Halonium Ion — Forcing Anti Addition
With Br2 or Cl2 the halogen bridges both carbons as a cyclic halonium ion, blocking one face so X– must attack from the back — giving anti vicinal dihalides with no rearrangement.
Anti addition of Br₂ gives 1,2-dibromopropane.
5. Halohydrins Form When Water Opens the Halonium at the More Substituted Carbon
Run halogenation in water and water opens the halonium from the back face (anti) at the more substituted carbon, giving a halohydrin — OH on the more substituted carbon, X on the neighbor.
6. The Syn Additions (Hydroboration, Hydrogenation) Break This Pattern on Purpose
Hydroboration–oxidation is concerted, so it is syn, never rearranges, and puts OH on the less substituted carbon (anti-Markovnikov); hydrogenation (H2, Pd) adds two H syn to reduce the alkene.
Hydrogenation: a concerted syn delivery of two H atoms.
7. Summary
Open carbocation (HX, hydration): Markovnikov · non-stereospecific · can rearrange · Bridged halonium (Br2, halohydrins): anti · no rearrangement · nucleophile opens at more substituted C · Concerted (hydroboration, hydrogenation): syn · no rearrangement · hydroboration is anti-Markovnikov.
Ask "what is the intermediate?" first, and the regiochemistry and stereochemistry follow.
Quiz yourself
Tap a question to reveal the answer — free, no login.
2-Bromopropane (CC(C)Br). Protonating the terminal CH2 gives a secondary carbocation, which is more stable than the primary one; bromide then traps that cation. This is Markovnikov's rule — Br ends up on the more substituted carbon.
Because the intermediate is a bridged bromonium ion, not an open carbocation. The bromine bridges both carbons and blocks one face, so the incoming Br– is forced to attack from the opposite face — anti addition. The same bridging is why halogenation never rearranges.
Water opens the bromonium ion at the more substituted carbon (more partial positive charge), so OH lands there and Br on the less substituted carbon — a bromohydrin (CC(O)CBr) — added anti.
Only the open-carbocation pathways — HX addition and acid-catalyzed hydration — can undergo 1,2-hydride or alkyl shifts. Halogenation, halohydrin formation, hydroboration, and hydrogenation all avoid a free carbocation, so they don't rearrange. A product whose skeleton implies a shifted, more-stable cation is the tell.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.