Difficulty: Intermediate | Prerequisites: Familiarity with alkene structure, Lewis acids/bases, carbocation stability, and basic stereochemistry (R/S configuration, wedge-dash notation).
Electrophilic addition is the central reaction class for alkenes in organic chemistry. The pi bond in an alkene is electron-rich and reacts with electrophiles, breaking the double bond and forming two new sigma bonds. This lecture reviews the three distinct mechanisms by which addition happens, each producing different stereochemical and regiochemical outcomes.
If you understand these three mechanisms, you can predict the products of nearly every alkene addition reaction in the course. The mechanisms also set up the oxidation and reduction reactions covered in the companion notes.
Alkene addition reactions follow one of three mechanisms. Mechanism I (carbocation intermediate) gives syn + anti products with Markovnikov regiochemistry. Mechanism II (three-membered ring intermediate) gives exclusively anti products. Mechanism III (concerted, simultaneous addition) gives exclusively syn products. Knowing which mechanism applies tells you both the stereochemistry and the regiochemistry of the product.
Syn addition
Addition of two groups to the same face of the double bond. Think of it as both new bonds forming on the same side of the flat alkene.
Anti addition
Addition of two groups to opposite faces of the double bond. In simple terms, one group attaches from above and the other from below.
Markovnikov addition (Markovnikov regiochemistry)
The electrophilic atom or proton adds to the less substituted carbon of the alkene, placing the positive charge (or partial positive charge) on the more substituted, more stable carbon. In simple terms, "the rich get richer" : the hydrogen goes to the carbon that already has more hydrogens.
Anti-Markovnikov addition
The electrophilic atom adds to the more substituted carbon, placing the new substituent on the less substituted carbon. Hydroboration is the classic example.
Carbocation
A carbon atom bearing a formal positive charge. Stability order: 3° > 2° > 1° > methyl. Carbocations are the key intermediate in Mechanism I.
Three-membered ring intermediate (bridged ion)
A cyclic intermediate formed when an electrophile (such as Br⁺) bridges across both carbons of the former double bond. This locks the geometry and forces the nucleophile to attack from the opposite face, giving anti addition. Examples include bromonium ions and iodonium ions.
Electrophile
An electron-poor species that accepts electron density from the alkene's pi bond. In simple terms, it is the "attacker" that initiates the reaction.
Nucleophile
An electron-rich species that donates a pair of electrons to the electrophilic intermediate in the second step of the addition.
Regioselectivity
Which carbon of the double bond each group ends up on. Determined by carbocation stability (Mechanism I) or by partial charge matching and steric effects (Mechanisms II and III).
Stereoselectivity
Whether the two groups add to the same face (syn) or opposite faces (anti) of the double bond.
H⁺ (from HX, H₃O⁺, or ROH₂⁺) adds to the alkene first, forming a carbocation on the more substituted carbon (Markovnikov).
The nucleophile (X⁻, H₂O, ROH) then attacks the planar carbocation.
Because the carbocation is sp² and flat, the nucleophile can attack from either face.
Stereoselectivity: syn + anti products (mixture of both).
Regioselectivity: Markovnikov (more stable carbocation forms).
Reactions that use this mechanism: hydrohalogenation (HCl, HBr, HI), acid-catalysed hydration (H₂O + H⁺), acid-catalysed addition of alcohols (ROH + H⁺).
The electrophile (e.g. Br₂, Cl₂, or X₂ with H₂O/ROH) adds to form a cyclic bridged ion (bromonium, chloronium, or iodonium ion).
The ring blocks one face of the molecule. The nucleophile must open the ring from the opposite side.
Stereoselectivity: exclusively anti addition.
Regioselectivity: the nucleophile (X⁻, H₂O, or ROH) attacks the carbon that best stabilises the partial positive charge (the more substituted carbon in halohydrin formation).
Reactions that use this mechanism: halogenation (Br₂, Cl₂), halohydrin formation (X₂ + H₂O or HOX), haloether formation (X₂ + ROH or ROX).
Two groups that are connected to each other add to the alkene at the same time, from the same face.
No intermediate; the reaction passes through a single cyclic transition state.
Stereoselectivity: exclusively syn addition.
Regioselectivity: matching of δ⁺/δ⁻ or size-based.
Reactions that use this mechanism: hydroboration (BH₃, then H₂O₂/NaOH), catalytic hydrogenation (H₂ + Pd, Pt, Ni), dihydroxylation (OsO₄ or KMnO₄), epoxidation (mCPBA).
Reaction | Electrophile | Reagent (2nd step) | Stereoselectivity | Regioselectivity | Mechanism |
|---|---|---|---|---|---|
Hydrohalogenation | HX (HCl, HBr, HI) | X⁻ | syn + anti | Markovnikov | I |
Hydration | H₃O⁺ | H₂O | syn + anti | Markovnikov | I |
Alcohol addition | ROH₂⁺ | ROH | syn + anti | Markovnikov | I |
Halogenation | X₂ (Br₂, Cl₂) | X⁻ | anti | n/a (symmetric) | II |
Halohydrin | X₂ + H₂O or HOX | H₂O | anti | OH to more substituted C | II |
Haloether | X₂ + ROH or ROX | ROH | anti | OR to more substituted C | II |
Hydroboration | BH₃ or B₂H₆ | H₂O₂, NaOH | syn | anti-Markovnikov | III |
Students often think Markovnikov's rule is about "hydrogen goes to the carbon with more hydrogens." The underlying principle is carbocation stability: the proton adds so that the more stable carbocation forms. The hydrogen-counting shortcut works in most cases but fails with rearrangements or unusual substituents.
Students frequently assume all addition reactions give anti products because halogenation (Br₂) is often the first reaction taught. Mechanism I reactions (HBr, H₂O/H⁺) give a mixture of syn and anti, not exclusively anti.
Confusing anti-Markovnikov with anti addition. These are two different concepts: anti-Markovnikov describes which carbon the groups attach to (regiochemistry), while anti addition describes which face they add from (stereochemistry). Hydroboration is anti-Markovnikov AND syn addition.
In halohydrin formation, students sometimes place the OH on the wrong carbon. The nucleophile (OH) goes to the more substituted carbon because that carbon bears more of the partial positive charge in the bridged intermediate.
⚠️ Expect to be given an alkene and a reagent and asked to draw the product with correct stereochemistry. You must know which mechanism applies to determine syn vs. anti.
⚠️ Regiochemistry questions often test whether you can distinguish Markovnikov from anti-Markovnikov. Hydroboration (anti-Markovnikov, syn) vs. acid-catalysed hydration (Markovnikov, syn + anti) is a classic comparison.
⚠️ Drawing enantiomers and diastereomers from addition reactions is heavily tested. Mechanism II reactions give enantiomeric pairs (anti addition from either face of the alkene). Mechanism I reactions can give diastereomers because both syn and anti products form.
⚠️ The summary table of all addition reactions (electrophile, intermediate, second-step reagent, stereoselectivity, regioselectivity, mechanism) is essentially the exam cheat sheet. Know it cold.
True or false: HBr addition to an alkene gives exclusively anti products. (False, it gives a mixture of syn and anti via Mechanism I.)
Fill in the blank: In Mechanism II, the nucleophile attacks from the ______ face relative to the electrophile. (Opposite.)
True or false: Hydroboration gives Markovnikov products. (False, it gives anti-Markovnikov products.)
Fill in the blank: Mechanism III produces exclusively ______ addition products. (Syn.)
True or false: In halohydrin formation, the OH group ends up on the less substituted carbon. (False, it goes to the more substituted carbon.)
Q: An unsymmetrical alkene reacts with HBr. What is the regiochemistry and stereochemistry of the product?
A: Markovnikov regiochemistry (Br goes to the more substituted carbon). A mixture of syn and anti addition products forms (Mechanism I, carbocation intermediate is planar).
Q: What reagents would you use to convert an alkene to a bromohydrin? What is the stereochemistry?
A: Br₂ + H₂O (or NBS + H₂O). The product is anti addition, with OH on the more substituted carbon and Br on the less substituted carbon (Mechanism II).
Q: How does hydroboration-oxidation differ from acid-catalysed hydration in terms of regiochemistry and stereochemistry?
A: Hydroboration-oxidation (BH₃, then H₂O₂/NaOH) gives anti-Markovnikov, syn addition. Acid-catalysed hydration (H₂O/H⁺) gives Markovnikov, syn + anti addition. Both place an OH on the alkene, but on different carbons and with different stereochemical outcomes.
Q: Br₂ is added to a cis-disubstituted alkene. How many stereoisomeric products form, and what is their relationship?
A: Two products form. They are enantiomers of each other (both are anti addition products, one from top-face and one from bottom-face attack on the bromonium ion).
Q: Classify each reaction by mechanism type: (a) HCl + alkene, (b) Br₂ + alkene, (c) BH₃ + alkene.
A: (a) Mechanism I (carbocation). (b) Mechanism II (bromonium ion). (c) Mechanism III (concerted syn addition).
This material connects directly to the oxidation reactions of alkenes (dihydroxylation, ozonolysis, epoxidation), which all follow Mechanism III and give syn addition. Understanding why Mechanism III is always syn is the bridge to predicting products for OsO₄, KMnO₄, mCPBA, and O₃.
The stereochemistry principles here also apply to ring-opening reactions of epoxides, where acid-catalysed opening is analogous to Mechanism II (anti, nucleophile to more substituted carbon) and mirrors halohydrin formation.
Carbocation stability from this lecture recurs in elimination reactions (E1) and SN1 substitution.
Related Terms / Search Tags
electrophilic addition, alkene addition reactions, syn addition, anti addition, Markovnikov rule, anti-Markovnikov, carbocation intermediate, bromonium ion, three-membered ring intermediate, halonium ion, regioselectivity, stereoselectivity, hydrohalogenation, hydration, halogenation, halohydrin formation, hydroboration, concerted addition, CHEM 210, organic chemistry II, University of Michigan