1. Petroleum Fractional Distillation
Alkanes are saturated hydrocarbons with the general formula \(\text{C}_n\text{H}_{2n+2}\). Crude oil is a complex mixture of branched and unbranched alkanes separated by fractional distillation:
Operation of the Fractionating Tower
- Crude oil is vaporised in a furnace at approximately \(350^\circ\text{C}\) and pumped into the bottom of the fractionating column.
- The column has a temperature gradient: hot at the bottom and cooler at the top.
- Vapour rises up through bubble caps. As fractions reach a tray where the temperature is just below their boiling point, they condense into liquid and are drawn off.
- Smaller molecules with lower boiling points rise to the top; larger molecules with higher boiling points condense near the base.
2. Industrial Cracking of Hydrocarbons
The economic demand for shorter-chain fuels and reactive alkenes exceeds their natural proportion in crude oil. Cracking breaks long-chain alkanes into shorter, more valuable alkanes and alkenes:
Thermal Cracking
- Conditions: High temperature (\(700-1200\text{ K}\)) and high pressure (up to \(70\text{ atm}\)).
- Mechanism: Homolytic C-C bond fission forming free radicals.
- Major Products: High percentage of alkenes (e.g. ethene, propene) used as feedstocks for addition polymerisation.
Catalytic Cracking
- Conditions: Moderate temperature (\(720\text{ K}\)), slight pressure (\(>1\text{ atm}\)), and a zeolite catalyst (aluminosilicate with honeycombed acidic active sites).
- Mechanism: Carbocation intermediate route.
- Major Products: Branched alkanes, cycloalkanes, and aromatic hydrocarbons with high octane numbers for motor fuels.
3. Combustion and Environmental Pollutants
- Complete Combustion: Excess oxygen produces \(\text{CO}_2\) and \(\text{H}_2\text{O}\): \[ \text{C}_8\text{H}_{18} + 12.5\text{O}_2 \rightarrow 8\text{CO}_2 + 9\text{H}_2\text{O} \]
- Incomplete Combustion: Limited oxygen produces toxic carbon monoxide (\(\text{CO}\)) and carbon particulates (soot). \(\text{CO}\) binds irreversibly to haemoglobin in red blood cells, preventing oxygen transport.
- Internal Combustion Engine Pollutants: High cylinder temperatures cause atmospheric nitrogen and oxygen to react, producing \(\text{NO}\) and \(\text{NO}_2\) (nitrogen oxides contribute to acid rain and photochemical smog).
- Catalytic Converters: Exhaust gases pass through a honeycomb coated with platinum, palladium, and rhodium: \[ 2\text{NO}(g) + 2\text{CO}(g) \rightarrow \text{N}_2(g) + 2\text{CO}_2(g) \]
4. Free-Radical Substitution Mechanism
A chemical species containing an unpaired valence electron, represented with a dot (\(\text{Cl}^\bullet\)). Highly reactive.
Alkanes react with halogens in the presence of ultraviolet (UV) light via a 3-stage chain reaction:
Stage 1: Initiation
UV light provides the energy for homolytic fission of the halogen-halogen bond. Each halogen atom retains one electron from the shared pair:
\[ \text{Cl}_2 \xrightarrow{UV} 2\text{Cl}^\bullet \]
Stage 2: Propagation (Chain Reaction)
Radicals attack molecules to form new radicals in a continuous cycle:
\[ \text{CH}_4 + \text{Cl}^\bullet \rightarrow \text{CH}_3^\bullet + \text{HCl} \]
\[ \text{CH}_3^\bullet + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet \]
Stage 3: Termination
Two free radicals collide and combine their unpaired electrons, removing radicals from the system:
- \(\text{CH}_3^\bullet + \text{Cl}^\bullet \rightarrow \text{CH}_3\text{Cl}\)
- \(\text{CH}_3^\bullet + \text{CH}_3^\bullet \rightarrow \text{C}_2\text{H}_6\) (Produces trace amounts of ethane!)
- \(\text{Cl}^\bullet + \text{Cl}^\bullet \rightarrow \text{Cl}_2\)
Limiting Polysubstitution: Excess methane is used to ensure a high yield of chloromethane and minimize further substitution to \(\text{CH}_2\text{Cl}_2, \text{CHCl}_3\), and \(\text{CCl}_4\).
5. Practice Questions
Initiation: \[ \text{Br}_2 \xrightarrow{UV} 2\text{Br}^\bullet \] (1 mark)
Propagation Step 1: \[ \text{C}_2\text{H}_6 + \text{Br}^\bullet \rightarrow \text{C}_2\text{H}_5^\bullet + \text{HBr} \] (1 mark)
Propagation Step 2: \[ \text{C}_2\text{H}_5^\bullet + \text{Br}_2 \rightarrow \text{C}_2\text{H}_5\text{Br} + \text{Br}^\bullet \] (1 mark)