Organic Chemistry · Alkenes: Reactions and Synthesis
Halogenation of Alkenes: Addition of X2
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In 30 seconds
Halogenation is the addition of a halogen molecule X2 across the carbon–carbon double bond of an alkene. The π bond breaks and two new carbon–halogen σ bonds form, so the product is a vicinal dihalide A molecule with halogen atoms on adjacent carbons Full entry → — a molecule with two halogen atoms on adjacent carbons:
alkene + X2 ⟶ vicinal dihalide
For cyclohexene, the balanced transformation is:
C6H10 + Br2 ⟶ C6H10Br2
Bromine and chlorine react cleanly with most alkenes at room temperature and need no catalyst. Fluorine is far too reactive to control — it attacks the solvent and even the product — while iodine does not add under ordinary conditions because the addition is thermodynamically unfavorable. The reaction also serves as the classic test for unsaturation: a red-brown solution of bromine in dichloromethane turns colorless as it reacts with a carbon–carbon double bond.
Why this matters
Vicinal dihalides are useful synthetic intermediates, and the addition itself is a workhorse of laboratory and industrial chemistry. Ethylene reacts with chlorine on an industrial scale to give 1,2-dichloroethane, which is pyrolyzed to vinyl chloride, the monomer of PVC. On the exam side, halogenation is the first reaction you meet that is stereospecific The reactant's geometry fully determines the product's stereochemistry Full entry →: the geometry of the alkene determines the stereochemistry of the product, and the mechanism (a cyclic halonium ion A positively charged three-membered ring containing a halogen (e.g., bromonium ion) Full entry →, not a free carbocation) explains why no rearrangements occur. Understanding this reaction sets up the rest of the chapter's additions.
The college version
Core Concepts
The reaction and its scope
An alkene reacts with one mole of X2 (X = Cl or Br) to give the 1,2-dihalide. The reaction works for essentially all alkenes and needs no acid, base, or catalyst — the π electrons are nucleophilic enough to attack the halogen molecule directly. Common solvents are dichloromethane (CH2Cl2), carbon tetrachloride, or hexane: inert, nonpolar liquids that cannot compete with the alkene or intercept the intermediate. Bromine is used most often: it is easy to handle and its red color gives a built-in endpoint for monitoring.
Mechanism: the cyclic halonium ion
The mechanism has two steps. First, the alkene's π electrons attack one bromine atom of Br2, pushing the other bromine out as bromide ion (Br-). The positive bromine that remains is held in a three-membered ring by the two alkene carbons — a bromonium ion:
alkene + Br2 ⟶ bromonium ion + Br-
Second, the bromide ion attacks one of the ring carbons from the back side, opening the ring and forming the second carbon–bromine bond. Because the intermediate is a bridged, positively charged ring rather than a flat carbocation, it cannot rearrange by hydride or alkyl shifts — a key difference from HX addition reactions.
Stereochemistry: anti addition
The back-side attack of the bromide ion forces the two bromines onto opposite faces of the alkene: this is anti addition Both new groups attach from opposite faces of the π bond Full entry →. The consequence is dramatic for cyclic alkenes. Cyclohexene, for example, gives only trans-1,2-dibromocyclohexane, never the cis isomer. For acyclic alkenes, anti addition means the stereochemical outcome depends on the geometry of the starting alkene: a (Z)-alkene and an (E)-alkene with the same substituents give different products (see Worked Example 2). This is why halogenation is called stereospecific — the stereochemistry of the reactant dictates the stereochemistry of the product.
Reactivity trends and practical conditions
Reactivity follows F2 ≫ Cl2 > Br2 ≫ I2: fluorine reacts explosively and is never used for simple halogenation, while iodine reacts so poorly that loss of I2 is favored at equilibrium. The solvent matters too: in a nucleophilic solvent such as water or an alcohol, the solvent intercepts the halonium ion and a halohydrin or haloether forms instead (the next topic).
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Anti addition (halogenation) | Syn addition (hydrogenation, hydroboration) | Halogenation adds from opposite faces; syn reactions add from the same face |
| Markovnikov's rule | Halogenation regiochemistry | Markovnikov's rule applies to HX additions; X2 addition has no H, but the halonium ion still opens preferentially at the more substituted carbon |
| Halonium ion | Carbocation | A bridged cyclic cation cannot rearrange; a free carbocation can — this is why halogenation shows no rearrangements |
| Vicinal dihalide | Geminal dihalide | Vicinal = halogens on adjacent carbons; geminal = both halogens on the same carbon |
| Bromine "test" color change | Proof of a specific product | Decolorization only shows a C=C reacted; it does not identify the product structure |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a double bond is two friends holding both hands. A bromine molecule is a pair of acrobats sharing one rope: the double bond grabs one acrobat, pushing the other away, and the grabbed acrobat now sits in a ring between the two friends. The pushed-away acrobat returns and attaches from underneath — so the two acrobats end up on opposite sides. That is anti addition: the halogens land on opposite faces of the alkene.
Worked example
Example 1: Cyclohexene plus bromine
Predict the product and explain the mechanism when cyclohexene reacts with Br2 in dichloromethane.
Reasoning. The π electrons of cyclohexene attack one bromine of Br2, expelling Br- and forming a bromonium ion bridged across the two alkene carbons. The bromide ion then attacks one of those carbons from the back side, opening the ring. Because the attack is back-side, the two bromines end up on opposite faces of the former double bond.
Answer. The product is trans-1,2-dibromocyclohexane (SMILES: Br[C@H]1C@@HCCCC1, trans relationship). In the chair conformation the two C–Br bonds point in opposite directions, and the diaxial arrangement minimizes dipole–dipole repulsion.
Example 2: (Z)- and (E)-2-butene give different products
Show why (Z)-2-butene and (E)-2-butene give different stereoisomers of 2,3-dibromobutane.
Reasoning. In (Z)-2-butene (SMILES: C/C=C\C) the methyl groups are on the same side of the double bond; in (E)-2-butene (SMILES: C/C=C/C) they are on opposite sides. Anti addition of Br2 places the two bromines on opposite faces of the alkene. When the same alkene has two stereocenters forming, the relationship between them is set by the alkene geometry.
Answer. (Z)-2-butene gives meso-2,3-dibromobutane — a single compound with an internal mirror plane. (E)-2-butene gives a racemic mixture of (2R,3R)- and (2S,3S)-2,3-dibromobutane (SMILES: CC(Br)C(Br)C). The two starting alkenes therefore give products that are not identical, which is the signature of a stereospecific reaction.
Example 3: Stoichiometry with dimensional analysis
How many grams of Br2 are needed to react completely with 0.250 mol of cyclohexene?
Formula first. The reaction is 1:1, so moles of bromine equal moles of alkene. The mass comes from:
m = n × M
where m is mass in grams, n is amount in moles, and M is molar mass in g/mol.
Substitution. The molar mass of Br2 is M = 2 × 79.90 g/mol = 159.80 g/mol. Since 0.250 mol of alkene requires 0.250 mol of Br2:
m = 0.250 mol × 159.80 gmol = 39.95 g
Answer. About 40.0 g of bromine is required.
Key takeaways
- Br2 and Cl2 add to alkenes without a catalyst, giving vicinal (1,2) dihalides; F2 is too reactive and I2 too unreactive.
- The mechanism goes through a cyclic halonium ion (e.g., bromonium ion), not a free carbocation — so no rearrangements occur.
- Addition is anti (the two halogens attach from opposite faces); cyclohexene gives trans-1,2-dibromocyclohexane.
- (Z)-2-butene gives meso-2,3-dibromobutane; (E)-2-butene gives a racemic mixture — a stereospecific outcome.
- One mole of Br2 reacts per mole of alkene (1:1 stoichiometry) — useful for stoichiometry calculations.
- The bromine color test: red-brown Br2 decolorizes in the presence of a C=C bond.
- In nucleophilic solvents (water, alcohols), the solvent competes with Br- and halohydrins or haloethers form instead.
- General lab-safety principle: bromine and chlorine are corrosive, volatile, and toxic; handle them only in a fume hood with appropriate gloves and eye protection.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What intermediate forms when an alkene reacts with Br2, and what does it explain?
Show answer
A cyclic bromonium ion (a positively charged three-membered ring holding the bromine between the two alkene carbons). Its bridged structure forces back-side attack (anti addition) and prevents rearrangements.
Is halogenation a syn or anti addition? What product does cyclohexene give?
Show answer
Anti addition. Cyclohexene gives trans-1,2-dibromocyclohexane.
Why does halogenation never show carbocation rearrangements, while HX addition sometimes does?
Show answer
The halonium ion is a bridged cation; the positive charge is spread over the ring, and no free carbocation exists long enough to undergo hydride or alkyl shifts.
What product does (Z)-2-butene give with Br2, and what does (E)-2-butene give?
Show answer
(Z)-2-butene gives meso-2,3-dibromobutane; (E)-2-butene gives a racemic mixture of (2R,3R)- and (2S,3S)-2,3-dibromobutane.
Why is F2 unsuitable for alkene halogenation, and why is I2 usually unreactive?
Show answer
Fluorine is so reactive that it attacks the solvent and product uncontrollably; the addition of iodine to an alkene is thermodynamically unfavorable, so the reverse reaction dominates.
How many grams of Br2 react with 0.500 mol of an alkene?
Show answer
0.500 mol × 159.80 g/mol = 79.90 g.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- vicinal dihalide
- A molecule with halogen atoms on adjacent carbons
- halonium ion
- A positively charged three-membered ring containing a halogen (e.g., bromonium ion)
- anti addition
- Both new groups attach from opposite faces of the π bond
- stereospecific
- The reactant's geometry fully determines the product's stereochemistry
- electrophile
- An electron-poor species that accepts electron density
Sources & references
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