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Alkanes

From Crude Oil (a mixture of Hydrocarbons). Cracking Pass Alkane over HEATED Al 2 O 3 catalyst. Fractional Distillation. Alkanes. Uses Non Renewable source. Reactivity No Reaction with acid or alkalis No Reaction with Br 2 ?. Fractional Distillation. Separation by Boiling Point

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Alkanes

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  1. From Crude Oil (a mixture of Hydrocarbons) Cracking Pass Alkane over HEATED Al2O3 catalyst Fractional Distillation Alkanes Uses Non Renewable source Reactivity No Reaction with acid or alkalis No Reaction with Br2?

  2. Fractional Distillation • Separation by Boiling Point • Each fraction has a different number of Carbons (C1-C4) (C5-C7) (C8-C10)…(C30-C40) • What varies as the molecule size changes? Home

  3. Uses Fuels What do we do when we use them as fuels? Feedstock for alkenes

  4. Practical One Place a small amount of Hexane (C6H14) Octane (C8H18) Paraffin (C20H42) Onto a heat proof mat in turn and ignite with a Bunsen Burner How easy to they ignite? How sooty is the flame? What products are formed in a good supply of Oxygen? What products are formed in a poor supply of Oxygen? Why do the flames get more sooty?

  5. Combustion Ease of combustion decreases as more C in alkane Flame gets more sooty as more C in alkane CO2 and H2O formed ONLY if oxygen supply is good CO and C (soot) formed if poor air supply (Called INCOMPLETE combustion) As alkanes have more C in molecule more moles of O2 per mole of alkane are required for complete combustion WRITE EQUATIONS

  6. Ease of combustion • Vapour Burns • Less Vapour as molecule increases in size at same temperature • Stronger VDW HOME

  7. Isomerisation and ReformingWeakens VDW so more vapourBetter ignition • Straight to Branched • Straight to cyclic

  8. Practical 2 • Reactions of alkanes • Observations of Bromine reaction • Br2 decolourised AND an acidic gas produced • Reaction only occurs when UV light is present

  9. Bond Breakage X-X  X. + X. X-X  X+ + X-

  10. Free radical substitution chlorination of methane i.e. homolytic breaking of covalent bonds Overall reaction equation CH4 + Cl2 CH3Cl + HCl Conditions ultra violet light excess methane to reduce further substitution

  11. Free radical substitution mechanism Cl + Cl CH4+ Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl CH3+ Cl CH3 + CH3 ultra-violet Cl2 initiation step twopropagation steps termination step CH3Cl minor termination step Already leads to multiple products CH3CH3

  12. Further free radical substitutions Overall reaction equations CH3Cl + Cl2 CH2Cl2 + HCl CH2Cl2 + Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4 + HCl Conditions ultra-violet light excess chlorine

  13. Further propagation steps and termination steps CH3Cl + Cl CH2Cl + HCl CH2Cl + Cl2 CH2Cl2+ Cl CH2Cl + Cl CH3 + CH3 twopropagation steps termination step CH2Cl2 termination step CH3CH3

  14. Cracking • Is the product a bigger or smaller molecule than the paraffin? • Is the product more or less flammable than the paraffin? • What happens when Br2 is added to the product? Smaller as it is a gas (so weaker VDW so lower bp) More flammable (as it is a gas already) Br2 decolourised without UV light (it is an alkene) HOME

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