Cracking
Thermal or catalytic cracking converts long-chain alkanes into shorter molecules including alkenes.
- Reagents
- zeolite catalyst
- Conditions
- high temperature
- Reaction class
- thermal cracking
- Equation
- long-chain alkane -> shorter alkane + alkene
Overview
Thermal or catalytic cracking converts long-chain alkanes into shorter molecules including alkenes.
Transformation
- Equation
- long-chain alkane -> shorter alkane + alkene
- Reagents
- zeolite catalyst
- Environment
- high temperature
- Reaction class
- thermal cracking
- Mechanism
- thermal cracking
- Evidence level
- textbook core
Reference procedure
Microscale reference: catalytic cracking
Liquid paraffin → shorter-chain hydrocarbons including an alkene
Materials and quantities
- Liquid paraffin — five or six drops on mineral wool; about 0.5 cm of the tube occupied
- Aluminium oxide — one microspatula
- Bromine water, 0.01 mol dm⁻³ — half-fill the receiving well; about 3 cm³ available per group
- Mineral wool — a small plug
- Ethanol or industrial denatured alcohol — microburner fuel
Apparatus
- Two microscale well plates with ported lids
- 10 cm³ gas syringe
- Straight narrow-bore glass tube and two L-shaped glass tubes
- Short lengths of silicone tubing
- Dropping pipette
- Ethanol microburner
- Eye protection
Procedure
- Connect the gas syringe to the inlet well and half-fill the receiving well with bromine water before joining the two wells through the glass reaction tube.
- Place mineral wool two to three centimetres into the inlet end of the straight tube, add five or six drops of liquid paraffin, and place one microspatula of aluminium oxide at the opposite end.
- Assemble the tube horizontally with the paraffin nearest the gas-syringe end and the catalyst nearest the bromine-water end.
- Heat the aluminium oxide strongly first while depressing the syringe very slowly, then flick the heat over the paraffin so its vapour passes across the hot catalyst.
- Continue the slow gas flow until the bromine-water colour no longer changes or the paraffin is exhausted, then stop heating and allow all glassware to cool before dismantling.
Critical controls
- Wear eye protection throughout setup, heating and dismantling; the narrow glass tubes can break while tubing is fitted.
- Keep the ethanol microburner upright and closed except as instructed; its vapour is highly flammable.
- Maintain a slow positive gas flow to reduce suck-back of bromine water into the hot tube.
- The product mixture shows unsaturation by bromine-water decolourisation.
Scope and limitations
- Scope
- Long-chain alkanes can be cracked to produce smaller alkanes and alkenes.
- Limitations
- Cracking gives mixtures; this route records the alkene-producing branch rather than product selectivity.
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.
- Hydrogenation: Alkenes → Alkanes
Hydrogenation reduces alkenes to alkanes.
- 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.
- Cracking hydrocarbons on a microscaleRoyal Society of Chemistry and Nuffield Foundation · RSC Education
Provides a microscale catalyst-tube method for cracking liquid paraffin and testing the gaseous product with bromine water.