1. Alkanes
14.1
Making alkanes
- Hydrogenation of an alkene: H2(g) with a Pt or Ni catalyst and heat. CH2=CH2 + H2 → CH3CH3
- Cracking of a longer-chain alkane: heat with Al2O3 catalyst. Products are a shorter alkane and at least one alkene, e.g. C10H22 → C8H18 + C2H4.
Crude oil contains more heavy fractions than are needed and too little petrol and naphtha. Cracking converts the heavy fractions into more useful lower-Mr alkanes (fuels such as petrol) and alkenes (feedstock for polymers and other chemicals).
Why alkanes are unreactive
C–C and C–H bonds are strong (C–H about 410 kJ mol−1), and carbon and hydrogen have very similar electronegativities, so C–H bonds are almost non-polar. There is no δ+ or δ− site to attract a nucleophile or an electrophile, so polar reagents such as acids, alkalis and oxidising agents do not react with alkanes.
Combustion
Complete combustion (excess oxygen) gives CO2 and H2O: C3H8 + 5O2 → 3CO2 + 4H2O. Incomplete combustion (limited oxygen) gives CO and/or C (soot): C3H8 + 3½O2 → 3CO + 4H2O.
Free-radical substitution
Ethane reacts with Cl2 (or Br2) in ultraviolet light: C2H6 + Cl2 → C2H5Cl + HCl.
| Step | Equations |
|---|
| Initiation (UV splits Cl–Cl homolytically) | Cl2 → 2Cl• |
| Propagation (radical used and regenerated) | Cl• + C2H6 → C2H5• + HCl
C2H5• + Cl2 → C2H5Cl + Cl• |
| Termination (two radicals combine) | Cl• + Cl• → Cl2
C2H5• + Cl• → C2H5Cl
C2H5• + C2H5• → C4H10 |
The product is a mixture: further substitution gives C2H4Cl2 and beyond, and termination gives butane. A large excess of ethane favours monosubstitution.
Engine pollutants and catalytic removal
| Pollutant | Origin | Effect |
|---|
| CO | incomplete combustion | toxic: binds to haemoglobin and reduces oxygen transport |
| NO, NO2 (NOx) | N2 and O2 from air react at the high temperature in the engine | acid rain; photochemical smog; respiratory irritant |
| unburnt hydrocarbons | fuel not fully burnt | photochemical smog with NOx (ground-level ozone); some are carcinogenic |
A catalytic converter (Pt, Pd, Rh on a ceramic honeycomb) removes them: 2CO + 2NO → 2CO2 + N2; hydrocarbons and CO are oxidised to CO2 and H2O.
Worked Example
Dodecane, C12H26, is cracked to give octane and ethene only. Write the equation.
Practice Question
Write the two propagation steps for the reaction of methane with bromine in UV light.
2. Alkenes