Halogen addition
Bromine addition converts alkenes into vicinal dibromoalkanes.
- Reagents
- Br2 in an organic solvent
- Conditions
- room temperature
- Reaction class
- electrophilic addition
- Equation
- RCH=CHR' + Br2 -> RCHBr-CHBrR'
Overview
Bromine addition converts alkenes into vicinal dibromoalkanes.
Transformation
- Equation
- RCH=CHR' + Br2 -> RCHBr-CHBrR'
- Reagents
- Br2 in an organic solvent
- Environment
- room temperature
- Reaction class
- electrophilic addition
- Mechanism
- electrophilic addition
- Evidence level
- textbook core
Reference procedure
Microscale reference: bromine addition test
Cyclohexene + bromine → brominated addition product
Materials and quantities
- Potassium bromate(V), 0.1 mol dm⁻³ — 10 drops
- Potassium bromide, 0.2 mol dm⁻³ — 20 drops
- Hydrochloric acid, 1 mol dm⁻³ — five drops
- Hexane — enough to half-fill the preparation well
- Cyclohexene — three drops
- Prepared bromine-in-hexane solution — three drops
Apparatus
- 24-well plastic plate
- Standard plastic pipettes used as micro separating funnels
- Plastic Petri dish
- Eye protection
Procedure
- Combine ten drops of bromate solution, 20 drops of bromide solution and five drops of hydrochloric acid in one well; leave for five minutes for bromine formation.
- Add hexane until the well is about half-full, draw the liquid into a plastic pipette, invert it and mix gently until bromine transfers into the upper hexane layer.
- Separate the lower aqueous layer and place the bromine-containing hexane layer in a clean well.
- Place three drops of bromine-in-hexane solution in a test well, add three drops of cyclohexene, and record the rapid loss of bromine colour.
Work-up and isolation
- Do not isolate or handle the small-scale addition product in this diagnostic method.
- Decontaminate remaining bromine solution using the disposal procedure specified by the source and local laboratory rules.
Critical controls
- Use a well-ventilated laboratory, eye protection and suitable gloves; bromine and hexane are hazardous and hexane is highly flammable.
- Keep liquids contained in the plastic pipette during extraction and invert gently so solvent is not expelled.
- Bromine decolourisation detects unsaturation; product analysis is listed separately.
Scope and limitations
- Scope
- Alkenes add bromine across the double bond to form vicinal dibromoalkanes.
- Limitations
- The displayed reagent and equation are specifically for bromination. Chlorine can undergo the analogous addition, but it requires its own reagent record; stereochemical outcomes depend on substrate and conditions.
Related reactions
- Hydrochlorination: Alkenes → Chloroalkanes
Addition of hydrogen chloride across an alkene can form a chloroalkane.
- Hydrobromination: Alkenes → Bromoalkanes
Addition of hydrogen bromide across an alkene can form a bromoalkane.
- Hydroiodination: Alkenes → Iodoalkanes
Addition of hydrogen iodide across an alkene can form an iodoalkane.
- Elimination: Chloroalkanes → Alkenes
Base-promoted elimination removes HCl to form an alkene.
- Elimination: Bromoalkanes → Alkenes
Base-promoted elimination removes HBr to form an alkene.
- Elimination: Iodoalkanes → Alkenes
Base-promoted elimination removes HI to form an alkene.
- Cracking: Alkanes → Alkenes
Thermal or catalytic cracking converts long-chain alkanes into shorter molecules including alkenes.
- Hydrogenation: Alkenes → Alkanes
Hydrogenation reduces alkenes to alkanes.
- Dihydroxylation: Alkenes → Diols
Cold, dilute manganate(VII) oxidises an alkene to a vicinal diol without the oxidative cleavage associated with stronger conditions.
- Addition polymerisation: Alkenes → Addition polymers
Alkenes can form addition polymers by chain-growth addition.
- Hydration: Alkenes → Alcohols
Catalytic hydration converts alkenes into alcohols.
- Dehydration: Alcohols → Alkenes
Dehydration converts alcohols into alkenes.
- Epoxidation: Alkenes → Epoxides
A peroxyacid transfers oxygen across the alkene in a concerted step, forming two C-O bonds while breaking the C=C pi bond. Both bonds form from one face, retaining the alkene substituents’ relative stereochemistry in the epoxide.
References
- Pearson Edexcel Level 3 Advanced GCE in Chemistry specificationPearson Education Limited · Pearson qualifications · 2024
Supports the textbook-core organic reaction routes used for displayed route and mechanism content.
- Testing for unsaturation with bromine on a microscaleRoyal Society of Chemistry · RSC Education · 2018
Microscale bromine-in-hexane preparation and three-drop alkene addition test.