Difficulty: Intermediate | Prerequisites: CIP priority rules, R/S configuration, carbocation stability, Markovnikov's rule basics
Alkene addition reactions are the bread and butter of CHEM 237. The pi bond of an alkene acts as a nucleophile, attacking an electrophile, and the result is a new single bond with substituents added across the former double bond. What makes these reactions interesting (and exam-worthy) is stereochemistry: depending on the mechanism, the two new groups may end up on the same face of the molecule (syn addition) or on opposite faces (anti addition). This set of notes covers four reaction types tested on Exam 4, with a focus on predicting the major product and its stereochemistry.
Br₂/H₂O gives anti addition of Br and OH (bromohydrin) via a bromonium ion. BH₃ followed by H₂O₂/NaOH gives syn addition of H and OH with anti-Markovnikov regiochemistry. HBr adds across the double bond following Markovnikov's rule through a carbocation intermediate. H⁺/H₂O (acid-catalysed hydration) gives the Markovnikov alcohol, also via a carbocation.
Syn addition
Both new substituents add to the same face of the double bond. Think of it as both groups arriving from the same side.
Anti addition
The two new substituents add to opposite faces of the double bond. One group goes "up" while the other goes "down" relative to the plane of the alkene.
Markovnikov's rule
In the addition of HX across an alkene, the hydrogen attaches to the carbon with more hydrogens already on it, and the X group goes to the more substituted carbon. In simple terms, "the rich get richer." The underlying reason is carbocation stability: the more substituted carbocation is more stable.
Anti-Markovnikov addition
The opposite regiochemistry: the new group ends up on the less substituted carbon. Hydroboration-oxidation is the classic example.
Bromonium ion
A three-membered ring intermediate formed when Br₂ reacts with an alkene. The bromine bridges both carbons of the former double bond, blocking one face and forcing the nucleophile to attack from the opposite side (anti addition).
Carbocation
A positively charged carbon intermediate. Stability order: tertiary > secondary > primary > methyl. Carbocation intermediates allow for racemisation at the cation centre because the nucleophile can attack from either face of the planar sp² carbon.
Bromohydrin
The product of Br₂/H₂O addition to an alkene: one carbon gets Br, the adjacent carbon gets OH, in an anti arrangement.
Hydroboration-oxidation
A two-step sequence: (1) BH₃ adds across the double bond in a syn, anti-Markovnikov fashion; (2) H₂O₂/NaOH oxidises the C–B bond to a C–OH bond with retention of configuration. The net result is syn addition of water with anti-Markovnikov regiochemistry.
Racemic mixture (racemate)
A 50:50 mixture of both enantiomers, formed when a new stereocentre is created with no facial selectivity. Shows no net optical rotation.
Reagent and conditions
Br₂ in water (H₂O acts as the nucleophilic solvent).
Mechanism summary
Br₂ approaches the alkene. The pi electrons attack one bromine, forming a bromonium ion (a three-membered ring with Br bridging the two carbons). The other bromine leaves as Br⁻.
Water (present in large excess) attacks the more substituted carbon of the bromonium ion from the opposite face (anti attack, backside).
Result: anti addition of Br and OH across the double bond.
Regiochemistry
The OH ends up on the more substituted carbon (the carbon better able to stabilise partial positive charge in the bromonium ion).
Br ends up on the less substituted carbon.
Stereochemistry
Anti addition. The Br and OH are on opposite faces.
If a new stereocentre is formed, consider whether a racemic mixture results (it does when the starting alkene is symmetric about the double bond) or whether a single pair of enantiomers/diastereomers forms.
What it does
Converts an alkene to an alcohol with anti-Markovnikov regiochemistry and syn stereochemistry.
Step 1: Hydroboration
BH₃ adds across the double bond in a single concerted step (no intermediate).
The boron attaches to the less substituted carbon; the hydrogen goes to the more substituted carbon.
Because it is concerted, both additions happen from the same face: syn addition.
Step 2: Oxidation
H₂O₂ in NaOH replaces the C–B bond with a C–OH bond.
This step proceeds with retention of configuration (the OH ends up on the same face where the B was).
Net result
Anti-Markovnikov alcohol (OH on the less substituted carbon).
Syn addition overall (H and OH on the same face).
If the starting alkene is symmetric (e.g. cyclohexene), the product is a racemic mixture of enantiomers.
Real-world note
Hydroboration-oxidation is widely used in synthesis because it gives the opposite regiochemistry to acid-catalysed hydration, giving chemists control over where the OH ends up.
Reagent
HBr (no peroxides).
Mechanism
The pi electrons of the alkene attack H⁺, forming a carbocation on the more substituted carbon (Markovnikov).
Br⁻ then attacks the carbocation.
Regiochemistry
Markovnikov: Br ends up on the more substituted carbon.
Stereochemistry
The carbocation intermediate is sp² (planar), so Br⁻ can attack from either face.
If a new stereocentre is created, expect a racemic mixture (or use a squiggle bond to indicate mixed stereochemistry).
With peroxides (ROOR)
Peroxides switch the mechanism to a radical chain, giving anti-Markovnikov addition of HBr. This was not directly tested on this exam, but it is worth knowing the distinction.
Reagent
Dilute acid (H⁺ or H₃O⁺) in water.
Mechanism
H⁺ protonates the alkene to form a carbocation on the more substituted carbon (Markovnikov).
Water attacks the carbocation, then loss of a proton gives the alcohol.
Regiochemistry
Markovnikov: OH ends up on the more substituted carbon.
Stereochemistry
The carbocation is planar (sp²), so water attacks from both faces.
If the product has a new stereocentre, expect a racemic mixture.
Comparison with hydroboration-oxidation
Both reactions convert an alkene to an alcohol, but they give opposite regiochemistry.
Acid-catalysed hydration: Markovnikov alcohol.
Hydroboration-oxidation: anti-Markovnikov alcohol.
This complementary pair is a favourite exam comparison.
Students often confuse the regiochemistry of hydroboration-oxidation with acid-catalysed hydration. Both make alcohols, but BH₃/H₂O₂ gives anti-Markovnikov and H⁺/H₂O gives Markovnikov. Keep them straight by remembering that in hydroboration, the boron (and later the OH) goes to the less hindered carbon.
Students often forget that bromohydrin formation is anti addition. The bromonium ion blocks one face, so the nucleophile must come from the other side.
Students often draw a single enantiomer when a racemic mixture should be shown. Any time a carbocation intermediate is involved, the nucleophile can attack from either face, so both enantiomers form equally.
The exam hint about using a squiggle bond (~) is worth remembering. When a reaction creates a stereocentre with no facial selectivity, a squiggle bond says "both configurations form here."
⚠️ Always show stereochemistry where appropriate. Flat, ambiguous drawings lose marks.
⚠️ For a symmetric starting alkene (like cyclohexene), syn addition still gives a racemic mixture of enantiomers, because the reagent can approach from either face of the ring.
⚠️ When the alkene has a substituent that makes one face more accessible, consider whether the product is a single diastereomer or a mixture.
True or False: Hydroboration-oxidation gives Markovnikov regiochemistry.
Fill in the blank: Br₂/H₂O addition proceeds through a ___ ion intermediate.
True or False: A carbocation intermediate is sp² hybridised and planar.
Fill in the blank: Syn addition means both groups add to the ___ face of the alkene.
True or False: Acid-catalysed hydration and hydroboration-oxidation give the same product.
Answers: 1. False (anti-Markovnikov). 2. Bromonium. 3. True. 4. Same. 5. False (same functional group but different regiochemistry; the OH goes to different carbons).
Q: Methylcyclohexene reacts with Br₂ in H₂O. What is the major product, and what is its stereochemistry?
A: A bromohydrin with OH on the more substituted carbon (the one that bore the methyl group's side of the double bond) and Br on the less substituted carbon. The Br and OH are anti to each other (trans) because of the bromonium ion mechanism.
Q: Cyclohexene is treated with BH₃, followed by H₂O₂/NaOH. What product forms and what is its stereochemistry?
A: Cyclohexanol. The OH and H add syn (cis) across the double bond. Because cyclohexene is symmetric, the reaction gives a racemic mixture of the two enantiomeric trans products... no, rather: the syn addition means the H and OH are cis on the ring. A racemic mixture of enantiomers forms because BH₃ can approach from either face.
Q: An alkene with a large tert-butyl group reacts with HBr. Where does the Br end up, and is the product a single enantiomer or a mixture?
A: The Br goes to the more substituted carbon (Markovnikov). The carbocation intermediate is planar, so Br⁻ attacks from both faces, giving a racemic mixture if a new stereocentre is created.
Q: You want to prepare a secondary alcohol from a terminal alkene. Which reagent set gives you the alcohol on the terminal (less substituted) carbon?
A: Hydroboration-oxidation (BH₃, then H₂O₂/NaOH). This is the anti-Markovnikov method. Acid-catalysed hydration would place the OH on the internal (more substituted) carbon instead.
These addition reactions connect to the stereochemistry fundamentals (CIP priority, R/S assignment) covered in the companion notes, because every product with a new stereocentre needs its configuration assigned. They also connect forward to CHEM 238's treatment of alkynes, where similar additions occur but the starting point is a triple bond rather than a double bond. The concept of syn vs anti addition reappears in epoxide ring-opening and catalytic hydrogenation.
alkene addition, electrophilic addition, Markovnikov, anti-Markovnikov, syn addition, anti addition, bromohydrin, bromonium ion, hydroboration, oxidation, BH3, H2O2, NaOH, HBr, acid-catalysed hydration, carbocation, racemic mixture, stereochemistry of addition, regioselectivity, stereoselectivity, CHEM 237, organic chemistry reactions, University of Washington