Alkanes

Section 1 of 15Crude oil and fractional distillation

Crude oil

• Crude oil a mixture of mostly alkanes and the world’s main source of organic chemicals

Alkanes

• Alkanes saturated hydrocarbons with only single C–C bonds, made of hydrogen and carbon only, as straight or branched chains

Fractions

• Fractions the mixture is separated into fractions of similar chain length and boiling point, each with its own uses

Link

• Link the size of the molecule decides the fraction it enters, and it relates to the number of carbon and hydrogen atoms

Section 2 of 15The fractionating column

• Temperature gradient
  • the column is hot at the bottom and cooler at the top

1

Crude oil is heated and vaporises.

2

The vapours enter the column.

3

Hydrocarbons with high boiling points condense at the bottom.

4

Those with lower boiling points rise and condense at the top.

1Other compounds

• Other compounds crude oil contains small amounts of sulfur compounds, which burn to sulfur dioxide, a cause of acid rain

2Physical process

• Physical process no covalent bonds break, and only intermolecular forces are overcome as the oil is heated

3Syllabus note

• Syllabus note the fractions need not be memorised, but the principle and its purpose must be understood

Section 3 of 15Economics of cracking

Supply

• Supply how much of a fraction fractional distillation produces

Demand

• Demand how much customers want to buy

Imbalance

• Imbalance petrol, kerosene and diesel are in high demand but short supply, whereas fuel oil and bitumen are in low demand but high supply
  • (surplus fractions)

Solution

• Solution some surplus fractions are broken down into smaller, more useful compounds

Section 4 of 15Cracking

Cracking = an endothermic process that breaks surplus long-chain alkanes into smaller, more useful compounds

Mechanism

• Mechanism single C–C covalent bonds break in long-chain alkanes

Products

• Products a smaller alkane and one or more alkenes with lower M_r

Uses

• Uses low-M_r alkanes make good fuels, and alkenes are useful starting materials

Example

• Example decane cracks into octane and ethene

Section 5 of 15Thermal and catalytic cracking

	Thermal cracking	Catalytic cracking
Temperature	High, up to 1000 °C	Lower, about 450 °C
Pressure	High, up to 70 atm	Slight pressure
Catalyst	None	Zeolite
Products	Alkanes and a high percentage of alkenes	Motor fuels and aromatic hydrocarbons

Section 6 of 15Complete and incomplete combustion

Use

• Use alkanes are burnt on a large scale as fuels

Complete combustion

• Complete combustion excess oxygen oxidises all carbon and hydrogen to carbon dioxide and water, as in octane

Incomplete combustion

• Incomplete combustion limited oxygen oxidises all hydrogen to water but only some carbon, which becomes carbon monoxide

Soot

• Soot with very little oxygen, carbon forms

Car engines

• Car engines incomplete combustion often occurs because oxygen is limited

Section 7 of 15Pollutants: carbon monoxide and oxides of nitrogen

Exhaust pollutants

• Exhaust pollutants carbon monoxide, nitrogen oxides (NO_x), unburned hydrocarbons and sulfur dioxide,
  • which have serious environmental and health effects

Carbon monoxide

• 2C₈H₁₈ + 17O₂ → 16CO + 18H₂O
• It is toxic and odourless, causing dizziness, loss of consciousness and death.
• It binds to haemoglobin, reducing the transport of oxygen and removal of carbon dioxide.

Oxides of nitrogen

• Nitrogen is normally unreactive, but high temperature and pressure in an engine oxidise it: N₂ + O₂ → 2NO and N₂ + 2O₂ → 2NO₂.
• They dissolve and react with water and oxygen to form nitric acid, causing acid rain.
• Acid rain corrodes buildings, harms plant and aquatic life, and damages human health.

Section 8 of 15Pollutants: hydrocarbons and sulfur dioxide

Pollutants: hydrocarbons and sulfur dioxide

Unburned hydrocarbons

• Unburned hydrocarbons nitrogen oxides react with them in air to form photochemical smog, and they harm lungs, eyes and plants

Sulfur dioxide

• Sulfur dioxide sulfur impurities oxidise, S(s) + O₂(g) → SO₂(g), and SO₂ forms sulfuric acid in water, causing acid rain

Sulfur scrubbing (flue gas desulfurisation)

• Sulfur scrubbing (flue gas desulfurisation) a slurry of calcium oxide and calcium carbonate is sprayed into waste gases
• CaO(s) + 2H₂O(l) + SO₂(g) + ½O₂(g) → CaSO₄·2H₂O(s), forming gypsum via calcium sulfite.
• CaCO₃ + ½O₂ + SO₂ → CaSO₄ + CO₂.
• It removes 90–95% of the sulfur dioxide.

Section 9 of 15Catalytic converters

Design

• Design precious metals such as platinum are coated on a honeycomb for a large surface area

Reactions

• Oxidation of CO2CO + O₂ → 2CO₂.
• Reduction of NO2CO + 2NO → 2CO₂ + N₂.
• Oxidation of unburnt hydrocarbons, such as octane to CO₂ and H₂O.

Pollutant	Formation	Consequence	Catalytic removal
Carbon monoxide	Incomplete combustion in car engines	Toxic gas	Oxidation to CO2
Oxides of nitrogen	Oxidation of nitrogen in car engines	Acid rain	Reduction to N2
Unburned hydrocarbons	Unburned fuel	Smog with nitrogen oxides	Oxidation to CO2 and H2O
Sulfur dioxide	Oxidation of sulfur impurities	Acid rain	Oxidation to calcium sulfate (scrubbing)

• Carbon dioxide
  • it is not toxic, but it is a pollutant that causes global warming

Section 10 of 15Free radical substitution of methane

Reaction

• Reaction a hydrogen atom is substituted by a halogen under suitable conditions, in three stages

Initiation

• The Cl–Cl bond breaks using UV energy.
• Each atom takes one electron in homolytic fission: Cl–Cl → 2Cl•.

Section 11 of 15Propagation

Reactivity

• Reactivity halogen radicals are very reactive and attack unreactive alkanes

Step 1

• A C–H bond breaks homolytically to give an alkyl radical.
• CH₄ + Cl• → •CH₃ + HCl.

Step 2

• The alkyl radical attacks another chlorine molecule to give a halogenoalkane.
• •CH₃ + Cl₂ → CH₃Cl + Cl•.
• The chlorine radical is regenerated and repeats the cycle.

Section 12 of 15Termination

Definition

• Definition two radicals combine to form one molecule, ending the chain

Possible products

• •CH₃ + Cl• → CH₃Cl, •CH₃ + •CH₃ → CH₃CH₃, and Cl• + Cl• → Cl₂

Exam point

• Exam point do not give the equation that reforms the original halogen, because it is often ignored by mark schemes

Bromine

• Bromine free radical substitution with bromine follows similar steps

Section 13 of 15Further substitution

Limitation

• Limitation a mixture of products forms, so the reaction is unsuitable for preparing a specific halogenoalkane

Excess halogen

• Excess halogen all hydrogens are eventually substituted

Substitution	Propagation steps
Second	CH3Cl + Cl• → •CH2Cl + HCl; •CH2Cl + Cl2 → CH2Cl2 + Cl•
Third	CH2Cl2 + Cl• → •CHCl2 + HCl; •CHCl2 + Cl2 → CHCl3 + Cl•
Complete	CHCl3 + Cl• → •CCl3 + HCl; •CCl3 + Cl2 → CCl4 + Cl•

Section 14 of 15Key terms

Term	Meaning
Fractional distillation	Separation of crude oil into fractions by boiling point.
Cracking	Breaking long-chain alkanes into shorter alkanes and alkenes.
Incomplete combustion	Burning in limited oxygen, producing CO or soot.
Catalytic converter	A device with precious metal catalysts that removes pollutants from exhaust.
Flue gas desulfurisation	Removing sulfur dioxide from waste gases using calcium compounds.

Term	Meaning
Initiation	The step that forms radicals by homolytic fission.
Propagation	Steps in which a radical reacts and another radical is regenerated.
Termination	A step in which two radicals combine.
Alkyl radical	A radical formed by removing a hydrogen atom from an alkane.

Section 15 of 15Exam technique

1

Saying fractional distillation breaks covalent bonds — it only overcomes intermolecular forces.

2

Saying cracking is exothermic — it is endothermic.

3

Saying nitrogen oxides form from fuel nitrogen — they form from air nitrogen in hot engines.

4

Forgetting carbon dioxide as a pollutant — it causes climate change.

1

Giving a termination step that reforms the halogen — this is often ignored in marking.

2

Omitting the dot on a radical — the unpaired electron must be shown.

3

Saying UV light breaks the C–H bond first — it breaks the halogen–halogen bond.

4

Expecting a single product — further substitution gives a mixture.