Difficulty: Intermediate to Advanced | Prerequisites: Chapter 11 (alkene structure, pi bonds as nucleophiles), Chapter 7 (reaction mechanisms, carbocations).
This is the heart of alkene chemistry: the reactions that break the pi bond and add new atoms across the double bond. If Chapter 11 taught you what alkenes are and how they form, Chapter 12 teaches you what they do. Every reaction here follows the same basic logic (electrophile attacks the nucleophilic pi bond), but the details of mechanism, stereochemistry, and regiochemistry differ for each reagent. You need to keep those details straight, because exams will test them heavily. This first part covers the foundational reactions: catalytic hydrogenation, H-X addition, acid-catalysed hydration, halogenation (Br₂/Cl₂), and halonium ion chemistry with various nucleophiles.
Alkene addition reactions replace the weak pi bond with two new sigma bonds, which is why they are thermodynamically favourable (ΔH is negative). The nucleophilic pi bond attacks an electrophile, and then a nucleophile completes the addition. Whether the product is syn or anti, Markovnikov or anti-Markovnikov, and racemic or stereospecific depends entirely on which reagent and mechanism you are using.
Electrophilic addition
The general reaction type for alkenes: an electrophile is attracted to the electron-rich pi bond, initiating addition across the double bond. Think of it as the pi bond "reaching out" to grab something electron-poor.
Markovnikov's rule
In the addition of H-X to an alkene, the hydrogen adds to the less substituted carbon (the one with more H atoms already) and the X group adds to the more substituted carbon. The underlying reason: this pathway forms the more stable carbocation intermediate.
Anti-Markovnikov addition
The opposite regiochemistry: the hydrogen ends up on the more substituted carbon and the other group on the less substituted carbon. Seen in hydroboration-oxidation and radical bromination (covered in Part 2).
Syn addition
Both new groups add to the same face of the double bond. Example: catalytic hydrogenation, where both hydrogen atoms are delivered from the metal surface to one face.
Anti addition
The two new groups add to opposite faces of the double bond. Example: bromine addition through a bromonium ion intermediate, where the nucleophile attacks from the back side.
Catalytic hydrogenation
Addition of H₂ across a double bond using a metal catalyst (Pd/C, PtO₂, or Raney Ni). Concerted syn addition. Reduces the alkene to an alkane.
Carbocation rearrangement
A hydride or alkyl shift that converts a less stable carbocation to a more stable one. Relevant in H-X addition and acid-catalysed hydration whenever a secondary carbocation could rearrange to a tertiary one.
Halonium ion (bromonium/chloronium ion)
A three-membered ring intermediate formed when Br₂ or Cl₂ reacts with an alkene. The halogen bridges across both carbons, preventing rotation. The nucleophile then opens the ring from the opposite face, giving anti addition.
Vicinal dihalide
A molecule with two halogen atoms on adjacent (vicinal) carbons. The product of halogen (X₂) addition to an alkene.
Regioselectivity
Preference for bond formation at one position over another. Markovnikov's rule is a statement of regioselectivity.
Stereoselectivity / stereospecificity
Stereoselectivity means one stereochemical outcome is favoured over another. Stereospecificity means the mechanism dictates exactly one stereochemical outcome (e.g. anti addition through a bromonium ion is stereospecific).
The alkene's pi bond is nucleophilic (electron-rich)
An electrophile (X-Y) is attracted to the pi electrons
The pi bond breaks, and two new sigma bonds form (one to X, one to Y)
This is exothermic: you are replacing one weak pi bond with two stronger sigma bonds (ΔH < 0)
Whether addition is syn or anti, and where each group ends up (regiochemistry), depends on the specific mechanism
A mixture of stereoisomers results unless a chiral source is present (nearby chiral centre or chiral catalyst/reagent)
Reagents: H₂ with a metal catalyst (Pd/C, PtO₂, or Raney Ni)
Common solvents: MeOH, EtOH, EtOAc (ethyl acetate)
Mechanism: concerted syn addition. Both H atoms are delivered from the catalyst surface to the same face of the alkene.
The catalyst activates H₂ (H-H bond would not break easily on its own)
The alkene coordinates to the metal surface
Both H atoms transfer to the same face of the alkene (syn)
The alkane dissociates from the catalyst
Stereospecific: always syn addition. If the alkene is cis or trans, the relative stereochemistry of the product is determined by which face the catalyst approaches.
The actual catalytic cycle involves oxidative addition, migratory insertion, and reductive elimination at the metal centre
Produces an alkane from an alkene (reduction)
Planar alkene means the catalyst can approach from the top or bottom face equally, giving a racemic product from an achiral, planar alkene
Reagents: HBr, HCl, HI
Mechanism: stepwise, through a carbocation intermediate
Step 1: The pi bond attacks the proton of H-X. The proton adds to one carbon, forming a carbocation on the adjacent carbon.
Step 2: The halide ion (nucleophile) attacks the carbocation from either face.
Regiochemistry: Markovnikov. The H adds to the less substituted carbon; the X adds to the more substituted carbon. This is because the more substituted carbocation is more stable.
Stereochemistry: both syn and anti addition products form (the carbocation is planar and sp2, so the nucleophile can attack from either face). Racemic mixture from an achiral substrate.
Carbocation rearrangements are possible. If the initial carbocation is secondary and a hydride or methyl shift would produce a tertiary carbocation, it will rearrange.
Overall exothermic
Reagents: H₂O with a strong acid catalyst (H₂SO₄, H₃PO₄)
Mechanism: stepwise, through a carbocation intermediate (same logic as H-X addition)
Step 1: Protonation of the alkene by the acid to form the more stable carbocation (Markovnikov)
Step 2: Water (nucleophile) attacks the carbocation
Step 3: Deprotonation to give the alcohol product
Regiochemistry: Markovnikov. The OH ends up on the more substituted carbon.
Stereochemistry: both syn and anti (carbocation is planar, water attacks from either face)
Carbocation rearrangement is possible
This reaction is reversible: the reverse is dehydration of an alcohol (E1, with acid and heat). Le Chatelier's principle: excess water drives hydration forward; excess acid and heat drives dehydration.
Note: isomerisation can also occur under these conditions (the carbocation may lose a different proton than expected)
Reagents: Br₂ or Cl₂ (only these two, not F₂ or I₂ under standard conditions)
Solvent: typically CCl₄ or DCM (CH₂Cl₂)
Product: vicinal dihalide (two halogens on adjacent carbons)
Mechanism: stepwise through a halonium ion (bromonium or chloronium ion)
Step 1: The pi bond attacks Br₂. One Br bonds to both carbons, forming a three-membered bridged ring (bromonium ion). The other Br leaves as Br⁻.
Step 2: Br⁻ attacks the more substituted carbon of the bromonium ion from the opposite face (backside attack, similar to SN2 logic).
Stereochemistry: anti addition, stereospecific. The two Br atoms end up on opposite faces.
Cis alkene gives one pair of enantiomers
Trans alkene gives a different pair (diastereomers relative to the cis product)
The bromonium ion intermediate prevents free rotation, which is why anti addition is enforced
Stereochemistry must be the same within a given 3-membered ring (filled octets on the bridging halogen)
Halohydrin formation
Reagents: Br₂ or Cl₂ in H₂O (water is the solvent and the nucleophile)
Mechanism: same halonium ion intermediate, but water opens the ring instead of the halide
The halonium ion forms as before
H₂O attacks the more substituted carbon (Markovnikov-like: water goes to the more substituted side)
Deprotonation gives the halohydrin (OH and X on adjacent carbons)
Anti addition (nucleophile attacks opposite face from the bridging halogen)
Stereospecific and regioselective
Markovnikov's rule applies to where the nucleophile (water) attacks
Useful for epoxide synthesis: treat the halohydrin with base to do an intramolecular SN2, forming a three-membered epoxide ring
Haloethers
Reagents: Br₂ in MeOH (or another alcohol solvent)
Same halonium ion mechanism; the alcohol opens the ring instead of water
Same regiochemistry and stereochemistry as halohydrin formation
Other reagents that open halonium ions
BrCl, BrCN, ICl, RSCl can all generate unsymmetrical halonium-type intermediates
The more electronegative halogen tends to bridge, and the nucleophile attacks the more substituted carbon
Thiol nucleophiles (HSCN, thiols) behave similarly
Reaction | Reagents | Mechanism | Regiochemistry | Stereochemistry | Rearrangement? |
|---|---|---|---|---|---|
Hydrogenation | H₂, Pd/C (or PtO₂, Raney Ni) | Concerted | N/A (both H) | Syn | No |
H-X addition | HBr, HCl, or HI | Stepwise, carbocation | Markovnikov | Syn + anti (racemic) | Yes |
Hydration | H₂O, H₂SO₄ | Stepwise, carbocation | Markovnikov (OH on more sub.) | Syn + anti (racemic) | Yes |
Halogenation (X₂) | Br₂ or Cl₂ | Stepwise, halonium ion | Anti-Markovnikov-like (both same group) | Anti | No |
Halohydrin | Br₂/Cl₂ in H₂O | Stepwise, halonium ion | Markovnikov (OH on more sub. C) | Anti | No |
Catalytic hydrogenation is used industrially to convert liquid vegetable oils into solid fats (margarine production) and to reduce unsaturated compounds in pharmaceutical synthesis. Halohydrin chemistry is a key step in the industrial synthesis of propylene oxide, one of the most important chemical intermediates for making polyurethane foams, solvents, and antifreeze.
Students often assume that all addition reactions are syn. They are not. Hydrogenation is syn, but halogenation through a halonium ion is anti. You must know the mechanism to predict the stereochemistry.
Students confuse where the nucleophile goes with where the electrophile goes. In Markovnikov addition, the H (from H-X) goes to the less substituted carbon, and the X goes to the more substituted carbon. The nucleophile ends up on the carbon that was the carbocation.
Students forget that hydration is reversible. The same carbocation intermediate that forms the alcohol can also lose a proton to re-form the alkene (dehydration). Conditions matter.
Thinking that Br₂ addition gives a mixture of syn and anti products. It does not. The bromonium ion enforces anti addition exclusively.
⚠️ For every addition reaction, you should be able to state: the reagents, the mechanism type (concerted vs stepwise), the regiochemistry (Markovnikov or anti-Markovnikov), the stereochemistry (syn, anti, or both), and whether rearrangement is possible. The reagent summary table above is your exam checklist.
⚠️ Drawing the bromonium ion intermediate correctly is essential. The three-membered ring, the positive charge on the bridging halogen, and the anti attack of the nucleophile are frequently tested.
⚠️ Predict the product, including stereochemistry, when given a cis or trans alkene with Br₂. Cis and trans starting materials give different diastereomeric products.
⚠️ Know the difference between halohydrin formation (X₂ in water) and simple halogenation (X₂ in an inert solvent). The solvent determines the nucleophile.
True or false: Catalytic hydrogenation gives anti addition of H₂. (False. It is syn addition.)
Fill in the blank: In Markovnikov addition of HBr to an alkene, the Br ends up on the ______ substituted carbon. (More)
True or false: Acid-catalysed hydration is irreversible. (False. The reverse is dehydration.)
Fill in the blank: Br₂ addition to an alkene proceeds through a ______ ion intermediate. (Bromonium)
True or false: Halohydrin formation gives syn addition. (False. It gives anti addition, because the nucleophile opens the halonium ion from the opposite face.)
Q: Draw the product of HBr addition to 1-methylcyclohexene. Show regiochemistry and explain.
A: The product is 1-bromo-1-methylcyclohexane. Protonation at the less substituted alkene carbon gives a tertiary carbocation at the more substituted position. Br⁻ then attacks the tertiary carbocation. This is Markovnikov addition.
Q: What product forms when cis-2-butene reacts with Br₂ in CCl₄? Include stereochemistry.
A: Anti addition gives a vicinal dibromide. Starting from cis-2-butene, anti addition of two Br atoms across the double bond gives the (2R,3R) and (2S,3S) pair of enantiomers (a racemic mixture of the anti-addition products).
Q: An alkene is treated with Br₂ in water. The product is a bromohydrin, not a vicinal dibromide. Why?
A: Water is present in large excess as the solvent. After the bromonium ion forms, water (a better nucleophile by concentration) attacks the more substituted carbon faster than the Br⁻ counterion does. The result is a bromohydrin (Br and OH on adjacent carbons) rather than a dibromide.
Q: Why does acid-catalysed hydration of 3-methyl-1-butene sometimes give 2-methyl-2-butanol instead of the expected 3-methyl-2-butanol?
A: Carbocation rearrangement. Protonation of the double bond initially gives a secondary carbocation at C2. A hydride shift converts this to a more stable tertiary carbocation at C2 of the rearranged skeleton, and water then attacks the tertiary centre.
Q: You want to reduce an alkene to an alkane with syn stereochemistry. What reagent and conditions do you use?
A: H₂ with a metal catalyst (Pd/C, PtO₂, or Raney Ni) in an appropriate solvent (MeOH, EtOH, or EtOAc). Catalytic hydrogenation delivers both H atoms to the same face.
These reactions build directly on the nucleophilic character of the pi bond from Chapter 11. The carbocation intermediates in H-X addition and hydration revisit the same stability principles (tertiary > secondary > primary) from Chapters 7 and 9. Halohydrin chemistry connects forward to epoxide synthesis and ring-opening reactions. The stereochemical reasoning (syn vs anti, racemic mixtures) is the same framework you will use in every reaction chapter going forward.
Electrophilic addition, alkene addition reactions, Markovnikov's rule, Markovnikov addition, anti-Markovnikov, syn addition, anti addition, catalytic hydrogenation, Pd/C, PtO₂, Raney nickel, H-X addition, hydrohalogenation, HBr addition, HCl addition, acid-catalysed hydration, hydration of alkenes, halogenation, bromination, chlorination, bromonium ion, chloronium ion, halonium ion, vicinal dihalide, halohydrin, bromohydrin, haloether, carbocation rearrangement, stereoselectivity, stereospecificity, regioselectivity, racemic mixture