organic chem 3.3.2 alkanes

aqa chemistry

Crude oil

A complex mixture of hydrocarbons, mainly alkanes.

Fractional distillation

To separate crude oil into fractions based on boiling points.

Boiling points of hydrocarbons

Due to differences in molecular size and strength of van der Waals forces.

Short-chain hydrocarbons

Condense at the top (lower boiling point) of the fractionating column.

Uses of fractions from crude oil

Gasoline for car fuel, bitumen for roads.

Cracking

The breaking of long-chain alkanes into shorter alkanes and alkenes.

Economic importance of cracking

Short-chain hydrocarbons and alkenes are more useful and in higher demand.

Conditions for thermal cracking

High pressure (~70 atm), high temperature (~1000°C).

Products of thermal cracking

Mostly alkenes.

Conditions for catalytic cracking

Slight pressure, ~450°C, zeolite catalyst.

Products of catalytic cracking

Branched alkanes, motor fuels, aromatic hydrocarbons.

Products of complete combustion of alkanes

CO₂ and H₂O.

Products of incomplete combustion

CO, carbon (soot), and H₂O.

Pollutants from internal combustion engines

NOₓ, CO, unburned hydrocarbons.

Catalytic converter

Converts harmful gases into less harmful products (e.g. CO + NO → CO₂ + N₂).

Acid rain from fuel combustion

SO₂ from sulfur in fuels.

Removal of SO₂ from flue gases

Using CaO or CaCO₃ in a neutralisation reaction.

Conditions for chlorination of methane

UV light.

Mechanism of chlorination of methane

Free radical substitution.

Initiation step in chlorination

Cl₂ → 2Cl• (under UV light).

Propagation steps in chlorination

Radicals react with molecules to produce new radicals.

Example of a propagation step

Cl• + CH₄ → HCl + CH₃•.

Termination step

Two radicals combine to form a stable product (e.g. Cl• + CH₃• → CH₃Cl).