Difficulty: Intermediate–Advanced | Prerequisites: Chirality and R/S Configuration notes, Stereochemical Relationships notes (Docs 1 and 2 of this set), alkene structure and bonding.
This is where stereochemistry meets reaction chemistry. When a reagent adds across a double bond, the stereochemical outcome depends on the mechanism. Some reactions add both groups from the same face (syn addition), others from opposite faces (anti addition), and some produce a mixture. For each reaction, you need to know three things: what is the major product (regiochemistry), what is the spatial arrangement (stereochemistry), and whether the product is a single stereoisomer or a racemic mixture. These questions dominate the second half of the chapters 6/7 exam.
Bromination (Br2) gives anti addition through a bromonium ion intermediate. Halohydrin formation (Br2/H2O) gives anti addition with Markovnikov regiochemistry for the OH. Hydroboration-oxidation (BH3 then H2O2/NaOH) gives syn addition with anti-Markovnikov regiochemistry. Acid-catalysed hydration (H3O+) gives Markovnikov addition through a carbocation. Catalytic hydrogenation (H2/Pt) gives syn addition. Whether a product is racemic depends on whether the alkene face is equally accessible from both sides.
Regioselective
A reaction that preferentially forms one constitutional isomer over another. In simple terms, the reagent has a preference for which carbon it attaches to.
Stereoselective
A reaction that preferentially forms one stereoisomer over another. Think of it as the reaction favouring a particular 3D arrangement in the product.
Stereospecific
A reaction where the stereochemistry of the starting material dictates the stereochemistry of the product. Every stereospecific reaction is stereoselective, but not every stereoselective reaction is stereospecific.
Syn addition
Both new groups add to the same face of the double bond. Think of both groups arriving from the same side of the alkene.
Anti addition
The two new groups add to opposite faces of the double bond. One group comes from above, the other from below.
Markovnikov addition (Markovnikov's rule)
The hydrogen (or electrophilic part) adds to the less substituted carbon of the double bond, and the nucleophilic part adds to the more substituted carbon. In simple terms: hydrogen goes where there is already more hydrogen.
Anti-Markovnikov addition
The opposite of Markovnikov: the nucleophilic 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).
Halohydrin
A compound with a halogen and a hydroxyl group on adjacent carbons. Formed by reacting an alkene with Br2 (or Cl2) in water.
Racemic mixture
A 50:50 mixture of both enantiomers of a product. Formed when an achiral starting material reacts without any chiral influence, giving equal amounts of R and S product.
Reagent: Br2 (no solvent or in CCl4/CH2Cl2)
Mechanism: Electrophilic addition through a bromonium ion intermediate. Br2 approaches the alkene, forming a cyclic bromonium ion on one face. Bromide ion then attacks from the opposite face.
Regiochemistry: Not applicable (both atoms are Br, so there is no regiochemical question).
Stereochemistry: Anti addition. The two bromines end up on opposite faces of the former double bond.
Racemic? If the starting alkene is symmetric or achiral, the product is typically a racemic mixture because the bromonium ion can form equally on either face.
Reagent: Br2 in water
Mechanism: Same bromonium ion intermediate as halogenation, but water (the solvent, present in large excess) acts as the nucleophile instead of bromide.
Regiochemistry: Markovnikov-like. Water attacks the more substituted carbon of the bromonium ion (the carbon better able to stabilise partial positive charge), so OH ends up on the more substituted carbon and Br on the less substituted carbon.
Stereochemistry: Anti addition. Water attacks from the face opposite the bromine bridge.
Racemic? Yes, when the starting alkene is achiral, because the bromonium ion forms on either face with equal probability.
Reagent: Step 1: BH3 (or BH3·THF). Step 2: H2O2, NaOH.
Mechanism: BH3 adds across the double bond in a single concerted step (a four-membered transition state). No carbocation intermediate. The oxidation step replaces B with OH, with retention of configuration.
Regiochemistry: Anti-Markovnikov. The boron (and ultimately the OH) ends up on the less substituted carbon. This is because the boron is the larger atom and sterics direct it to the less hindered position.
Stereochemistry: Syn addition. Both H and OH are delivered to the same face of the double bond.
Racemic? If the alkene is symmetric about the double bond, both faces are equally accessible, giving a racemic mixture. If one face is blocked (e.g. by a ring substituent), a single stereoisomer may predominate.
Reagent: H3O+ (dilute acid, water as solvent)
Mechanism: Protonation of the double bond forms a carbocation. Water then attacks the carbocation.
Regiochemistry: Markovnikov. The proton adds to the less substituted carbon (forming the more stable carbocation), and water attacks the more substituted carbon.
Stereochemistry: Not stereospecific. The carbocation is planar (sp2), so the nucleophile can attack from either face. This gives a mixture of stereoisomers.
Racemic? Yes, if new stereocentres are created, because the flat carbocation is equally accessible from both sides.
Special note: If the alkene is an enol ether or a particularly stable alkene, there may be no reaction. Also watch for the possibility of carbocation rearrangements (hydride and methyl shifts).
Reagent: H2 gas with a metal catalyst (Pt, Pd, or Ni)
Mechanism: Both the alkene and H2 adsorb onto the metal surface. Both hydrogen atoms are delivered from the metal surface to the same face of the alkene.
Regiochemistry: Not applicable (both atoms added are hydrogen).
Stereochemistry: Syn addition. Both hydrogens come from the same face.
Racemic? Depends on the substrate. If the alkene is symmetric and both faces are equally accessible, the product is racemic. If one face is blocked (e.g. by a bulky group on a ring), one diastereomer may predominate.
A reaction can be regioselective without being stereoselective, and vice versa.
Regioselective but not stereoselective: Acid-catalysed hydration of propene. The OH adds preferentially to C2 (Markovnikov), so the reaction is regioselective. But the carbocation at C2 is flat, so water attacks from either face, giving a racemic mixture with no stereochemical preference.
Stereoselective but not regioselective: Bromination of a symmetric alkene such as cyclohexene. There is no regiochemical choice (both carbons of the double bond are equivalent), but the reaction is anti-stereospecific, giving trans-1,2-dibromocyclohexane exclusively.
Hydroboration-oxidation is widely used in synthesis when anti-Markovnikov, syn-selective alcohol formation is needed. For instance, in synthesising a natural product where a specific stereocentre must have a defined configuration, the predictable syn selectivity of this reaction gives chemists control that acid-catalysed hydration cannot.
Students often confuse regioselectivity with stereoselectivity. Regioselectivity is about which carbon gets which group. Stereoselectivity is about the 3D spatial arrangement. They are independent properties: a reaction can be one, both, or neither.
Students frequently forget that acid-catalysed hydration goes through a carbocation, which means (a) the product is Markovnikov, (b) stereochemistry is mixed because the carbocation is flat, and (c) rearrangements are possible.
A common error is assuming hydroboration-oxidation gives Markovnikov products. It gives anti-Markovnikov. The OH ends up on the less substituted carbon.
Students sometimes write that bromination is "syn" because both bromines add. Both bromines do add, but they add anti (from opposite faces) because of the bromonium ion intermediate.
⚠️ "Give the major product INCLUDING stereochemistry" is a high-frequency exam instruction. You must show wedge/dash notation and state whether the product is racemic.
⚠️ Hydroboration-oxidation vs acid-catalysed hydration is a favourite comparison. Know both the regiochemistry and the stereochemistry of each.
⚠️ The question "explain how a reaction can be regioselective but not stereoselective" (and vice versa) has appeared on past exams. Have a concrete example ready for each.
⚠️ Watch for "NR" (no reaction) traps. If the reagent cannot react with the substrate (e.g. H3O+ with a compound that is not an alkene), the answer is NR.
True or false: Hydroboration-oxidation gives Markovnikov addition of water across a double bond.
False. It gives anti-Markovnikov addition.
Fill in the blank: Bromination of an alkene proceeds through a ______ ion intermediate, resulting in ______ addition.
Bromonium; anti.
True or false: Acid-catalysed hydration of an alkene is stereospecific.
False. The carbocation intermediate is planar, so the nucleophile can attack from either face.
Fill in the blank: In catalytic hydrogenation, both hydrogens are delivered from the ______ face of the alkene.
Same (syn addition).
True or false: A reaction that is regioselective must also be stereoselective.
False. The two properties are independent.
Q: Cyclopentene reacts with Br2. What is the major product, including stereochemistry? Is it formed as a racemic mixture?
A: Trans-1,2-dibromocyclopentane. Bromination proceeds through a bromonium ion, giving anti addition: the two bromines are on opposite faces of the ring (trans). It is formed as a racemic mixture because the bromonium ion can form on either face of the symmetric alkene.
Q: 1-methylcyclopentene reacts with Br2/H2O. What is the major product, including regiochemistry and stereochemistry?
A: The product is a bromohydrin with OH on C1 (the more substituted carbon) and Br on C2 (the less substituted carbon), with anti stereochemistry. The bromonium ion forms first; water then attacks the more substituted carbon from the opposite face. The product is racemic.
Q: 1-methylcyclopentene reacts with BH3, followed by H2O2/NaOH. What is the major product?
A: 2-methylcyclopentanol, with the OH on C2 (the less substituted carbon), formed by syn addition. This is anti-Markovnikov regiochemistry. The product is racemic if both faces of the alkene are equally accessible.
Q: A trisubstituted alkene reacts with H3O+. What is the regiochemistry? What can you say about the stereochemistry?
A: Markovnikov regiochemistry: the proton adds to the less substituted carbon, forming a carbocation on the more substituted carbon, and water attacks there. The stereochemistry is mixed (not stereospecific) because the carbocation is sp2-hybridised and planar, allowing nucleophilic attack from either face. If a new stereocentre is formed, expect a racemic mixture.
Q: Briefly explain how a reaction could be regioselective but not stereoselective. Give an example.
A: A reaction is regioselective when it preferentially places groups on specific carbons, but not stereoselective when it shows no preference for the 3D arrangement of those groups. Example: acid-catalysed hydration of propene. OH goes preferentially to C2 (Markovnikov, regioselective), but the flat carbocation at C2 allows water to attack from either face, so there is no facial selectivity (not stereoselective).
Q: How could a reaction be stereoselective but not regioselective? Give an example.
A: A reaction is stereoselective when it controls the 3D arrangement of the product, but not regioselective when there is no constitutional choice to make. Example: bromination of cyclohexene. Both carbons of the double bond are equivalent, so there is no regiochemical question, but the reaction is stereospecifically anti, giving only the trans product.
The stereochemical outcomes of alkene additions feed directly into later chapters on substitution reactions (SN1, SN2). SN2 is stereospecific (inversion of configuration), while SN1 proceeds through a planar carbocation and gives racemisation, which mirrors the logic of anti addition vs carbocation-mediated addition covered here. Elimination reactions (E1, E2) are the reverse of additions and carry their own stereochemical requirements. Understanding how mechanism controls stereochemistry in additions prepares you for those chapters.
Alkene reactions, addition reactions, electrophilic addition, bromination, halogenation, halohydrin formation, bromohydrin, hydroboration-oxidation, syn addition, anti addition, anti-Markovnikov, Markovnikov's rule, acid-catalysed hydration, catalytic hydrogenation, H2 Pt, Br2, Br2 H2O, BH3, H2O2 NaOH, H3O+, bromonium ion, carbocation, regioselective, stereoselective, stereospecific, racemic mixture, stereochemistry of reactions, CHEM 237, organic chemistry, UW, University of Washington, chapters 6 and 7