Hydrogenation
Hydrogenation reduces alkenes to alkanes.
- Reagents
- H2
- Conditions
- nickel catalyst, heat
- Reaction class
- addition
- Equation
- RCH=CHR' + H2 -> RCH2-CH2R'
Overview
Hydrogenation reduces alkenes to alkanes.
Transformation
- Equation
- RCH=CHR' + H2 -> RCH2-CH2R'
- Reagents
- H2
- Environment
- nickel catalyst, heat
- Reaction class
- addition
- Mechanism
- catalytic addition
- Evidence level
- textbook core
Scope and limitations
- Scope
- Alkenes add hydrogen across the C=C bond to form alkanes.
- Limitations
- This is a heterogeneous catalytic addition route; surface-catalyst details are outside the organic route summary.
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.
- Free radical chlorination: Alkanes → Chloroalkanes
Photochemical chlorination can convert alkanes into chloroalkanes.
- Free radical bromination: Alkanes → Bromoalkanes
Photochemical bromination can convert alkanes into bromoalkanes.
- Cracking: Alkanes → Alkenes
Thermal or catalytic cracking converts long-chain alkanes into shorter molecules including alkenes.
- Halogen addition: Alkenes → Dihalogenoalkanes
Bromine addition converts alkenes into vicinal dibromoalkanes.
- 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.
- Complete combustion: Alkanes → Carbon dioxide
Alkanes burn completely in excess oxygen to form carbon dioxide and water.
- 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.