Preparation of Alkanes: MDCAT Chemistry notes
Preparation of Alkanes for MDCAT: hydrogenation, Wurtz, Kolbe and decarboxylation methods, plus petroleum fractions, cracking, reforming and octane number.
Methods of preparing alkanes
| Method | Reaction |
|---|---|
| Hydrogenation of alkenes and alkynes (Sabatier–Senderens) | $\mathrm{CH_2{=}CH_2 + H_2 \xrightarrow{Ni,\ \Delta} CH_3CH_3}$ (Pt or Pd work at room temperature) |
| Reduction of alkyl halides | $\mathrm{RX + 2[H] \xrightarrow{Zn/HCl} RH + HX}$ |
| Wurtz reaction | $\mathrm{2RX + 2Na \xrightarrow{dry\ ether} R{-}R + 2NaX}$ |
| Decarboxylation (soda lime) | $\mathrm{RCOONa + NaOH \xrightarrow{CaO,\ \Delta} RH + Na_2CO_3}$ |
| Kolbe's electrolysis | $\mathrm{2RCOONa + 2H_2O \to R{-}R + 2CO_2 + 2NaOH + H_2}$ (R–R forms at the anode) |
| Reduction of aldehydes and ketones (Clemmensen) | $\mathrm{R_2C{=}O \xrightarrow{Zn(Hg)/HCl} R_2CH_2}$ |
- The Wurtz reaction and Kolbe's electrolysis double the carbon chain: $\mathrm{CH_3Br}$ gives ethane.
- Decarboxylation removes one carbon: sodium acetate gives methane.
Alkanes from petroleum
Crude petroleum is separated by fractional distillation. Lighter fractions have smaller molecules and lower boiling points. In order of increasing boiling point:
natural gas < gasoline (petrol) < kerosene < diesel oil < lubricating oil < heavy oil / residue
Cracking
Cracking breaks large, less useful molecules into smaller ones. Mainly C–C bonds are broken, giving smaller alkanes plus alkenes.
- Thermal cracking uses heat at high temperature and pressure only.
- Catalytic cracking uses a catalyst (e.g. silica–alumina) at about 500 °C and 2 atm. It gives better-quality gasoline, rich in branched and aromatic hydrocarbons with a higher octane number.
- Steam cracking is used mainly to produce alkenes.
Reforming
In the FSc definition, reforming is the conversion of straight-chain hydrocarbons into branched-chain (and cyclic or aromatic) ones. This raises the octane number.
Octane number
The octane number measures a fuel's resistance to knocking. The scale is fixed by two reference fuels: iso-octane (2,2,4-trimethylpentane) = 100 and n-heptane = 0. Branched and aromatic hydrocarbons have high octane numbers, while straight chains have low ones.
Related facts
- Destructive distillation of coal (heating out of contact with air) gives coke, coal tar, coal gas and ammoniacal liquor.
- Complete combustion of the saturated hydrocarbon methane: $\mathrm{CH_4 + 2O_2 \to CO_2 + 2H_2O}$. Ethene and ethyne are unsaturated. Controlled oxidation of methane (Cu, 400 °C, 200 atm) gives methanol, which is not combustion.
Common MDCAT traps
- Natural gas is the lowest-boiling fraction. When it is not among the options, choose gasoline.
- Reforming means straight chains become branched chains. Cracking means big molecules become small ones.
- n-Heptane is 0 on the octane scale, not 100.
- Catalytic cracking conditions are about 500 °C and 2 atm, a low pressure.
- Wurtz and Kolbe give symmetrical alkanes with twice the carbon count.
Quick revision
- Hydrogenation of alkenes over Ni gives alkanes.
- Sodium acetate + soda lime gives methane.
- Catalytic cracking gives higher-octane gasoline.
- Iso-octane is 100 and n-heptane is 0.
- Coal gives coke, coal tar and coal gas on destructive distillation.