Organic Reactions & Mechanisms (CHEM 220) · Module 7 of 9
Electrophilic Addition to Alkenes
How alkenes react with HX, halogens, and water — and why the regiochemistry and stereochemistry differ.
Markovnikov Addition via Carbocation
- In addition of HX, like HBr, to an alkene without peroxides, the alkene's pi electrons attack H⁺ first, forming the more stable carbocation, and then the halide ion attacks that carbocation.
- This is Markovnikov's rule: H adds to the carbon with more existing hydrogens, the halide to the more substituted carbon.
Why does HBr addition to an alkene, without peroxides, follow Markovnikov's rule?
Solution. The reaction proceeds through the more stable carbocation intermediate — formed by protonating the alkene in the way that generates the more substituted, more stable cation — so the halide ends up on the more substituted carbon.
Halogenation & the Halonium Ion
- Addition of X₂, like Br₂, to an alkene proceeds through a cyclic halonium ion intermediate, which a halide nucleophile then opens by attacking from the opposite face — resulting in anti addition.
Why does halogenation of an alkene result in anti addition rather than syn addition?
Solution. The reaction forms a cyclic halonium ion intermediate that blocks nucleophilic attack from the same face it formed on, forcing the halide nucleophile to attack from the opposite face, resulting in anti stereochemistry.
Acid-Catalyzed Hydration
- Acid-catalyzed hydration of an alkene follows a mechanism similar to Markovnikov HX addition: protonation forms the more stable carbocation, then water attacks, followed by loss of a proton to give the alcohol — also Markovnikov-selective.
What determines the regiochemistry (Markovnikov vs. anti-Markovnikov) of acid-catalyzed alkene hydration?
Solution. Like HX addition, it proceeds through the more stable carbocation intermediate, so the OH group ends up on the more substituted carbon — Markovnikov addition.
Stereochemistry of Addition Reactions
- Carbocation-intermediate mechanisms, like Markovnikov HX addition and hydration, generally give a mixture of stereochemical outcomes, since the planar carbocation can be attacked from either face.
- Halonium-ion mechanisms (halogenation) give a specific, predictable anti stereochemical outcome, since the nucleophile is forced to attack from the face opposite the halonium ion.
Why might halogenation give a more predictable stereochemical outcome than Markovnikov HBr addition to the same alkene?
Solution. Halogenation proceeds through a halonium ion that constrains nucleophilic attack to the opposite face (anti addition only), while HBr addition through a free carbocation can be attacked from either face, giving a mix of stereochemical outcomes.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What does Markovnikov's rule predict for HX addition to an alkene?
2. What intermediate forms during halogenation (X₂ addition) of an alkene?