Petroleum and green chemistry

What you'll learn

Crude oil and natural gas are the foundation of the modern chemical industry and of the economies of several Caribbean nations, especially Trinidad and Tobago and, increasingly, Guyana. They provide fuels for transport and electricity and the feedstocks for plastics, fertilisers and countless other products. At the same time, their extraction and use have serious environmental consequences, which is why green chemistry — designing chemical products and processes that reduce waste and hazards — is increasingly important. This guide covers the composition of crude oil, fractional distillation, cracking (thermal and catalytic), reforming and isomerisation, octane numbers and fuel quality, the uses of petroleum fractions, the environmental impacts of the petroleum industry, alternative fuels such as biofuels, and the twelve principles of green chemistry, including atom economy.

CAPE questions ask you to explain how fractional distillation and cracking work, write cracking equations, discuss fuel quality and pollution, and apply green chemistry principles to evaluate processes.

Key terms and definitions

Crude oil (petroleum) — A complex mixture of hydrocarbons, mainly alkanes, cycloalkanes and arenes, formed from the remains of ancient marine organisms.

Fraction — A mixture of hydrocarbons with similar boiling points, separated by fractional distillation.

Fractional distillation — Separating a mixture into fractions according to boiling point, using a fractionating column with a temperature gradient.

Cracking — Breaking long-chain hydrocarbons into shorter, more useful molecules, including alkenes.

Catalytic cracking — Cracking using a zeolite catalyst at moderate temperatures.

Reforming — Converting straight-chain alkanes into branched, cyclic or aromatic hydrocarbons to improve petrol quality.

Octane number — A measure of a fuel's resistance to knocking (auto-ignition) in an engine.

Knocking — Premature ignition of the fuel–air mixture in an engine, reducing efficiency and causing damage.

Biofuel — A fuel produced from recently living organisms, such as ethanol from sugar cane or biodiesel from plant oils.

Green chemistry — The design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances.

Atom economy — The percentage of the mass of reactants that ends up in the desired product.

Core concepts

Composition of crude oil

Crude oil is a mixture of hundreds of hydrocarbons, from small gaseous molecules to large, viscous ones. It also contains small amounts of sulfur, nitrogen and oxygen compounds and metals. Trinidad's famous Pitch Lake is a natural deposit of asphalt — the heaviest fraction — which seeps to the surface.

Fractional distillation

1. Crude oil is heated to about 350–400 °C in a furnace, vaporising most of it.
2. The vapour enters the bottom of a fractionating column, which is hot at the bottom and cooler at the top.
3. As vapours rise, they cool; each fraction condenses at the level where the temperature falls below its boiling point and is collected on trays.
4. Short-chain molecules with low boiling points rise to the top; long-chain molecules condense near the bottom; the heaviest residue does not vaporise.

Separation works because boiling point increases with chain length (stronger London forces).

Cracking

There is more demand for short-chain fractions such as petrol and for alkenes than crude oil naturally supplies, while there is a surplus of long-chain fractions. Cracking solves this.

Thermal cracking: high temperatures (up to about 900 °C) and high pressures; produces a high proportion of alkenes by a free-radical mechanism.

Catalytic cracking: a zeolite catalyst (aluminosilicates) at about 450–500 °C and moderate pressure; produces branched alkanes and aromatics useful for high-quality petrol.

Example: C₁₄H₃₀ → C₈H₁₈ + 3C₂H₄ (tetradecane → octane + ethene).

Cracking equations must balance carbon and hydrogen; they typically produce a shorter alkane and at least one alkene.

Reforming and isomerisation

Isomerisation converts straight-chain alkanes into branched isomers, e.g. octane into 2,2,4-trimethylpentane. Reforming converts straight-chain alkanes into cycloalkanes and arenes, often using a platinum catalyst, producing hydrogen as a by-product.

Both increase the octane number of petrol because branched, cyclic and aromatic hydrocarbons burn more smoothly and resist knocking.

Fuel quality and the end of leaded petrol

Tetraethyl lead was once added to petrol to increase the octane number, but lead is toxic and damages catalytic converters, so it has been phased out worldwide. Modern petrol uses branched and aromatic hydrocarbons and oxygenates such as ethanol to achieve high octane numbers.

Environmental impacts of the petroleum industry

• Combustion products: carbon dioxide (enhanced greenhouse effect and climate change), carbon monoxide (toxic), unburnt hydrocarbons and particulates (smog and respiratory illness), nitrogen oxides and sulfur dioxide (acid rain).
• Oil spills from tankers, pipelines and platforms damage coral reefs, mangroves, beaches, fisheries and tourism — major concerns for Caribbean economies.
• Gas flaring wastes energy and emits greenhouse gases.
• Refinery emissions and effluents pollute air and water.

Reducing impacts: catalytic converters, removing sulfur from fuels (hydrodesulfurisation), carbon capture, strict regulation of offshore drilling, oil spill response plans, and moving towards renewable energy.

Alternative fuels

• Bioethanol from fermentation of sugar cane — used widely in Brazil and possible in the Caribbean sugar industry. It is renewable and can be considered close to carbon-neutral, but uses land that could grow food.
• Biodiesel from vegetable oils or waste cooking oil.
• Hydrogen for fuel cells — produces only water, but is currently mostly made from natural gas.
• Solar, wind and geothermal energy for electricity — the Caribbean has great solar potential, and islands such as Saint Lucia, Dominica and Montserrat have explored geothermal resources.

The principles of green chemistry

Green chemistry aims to make chemistry safer and more sustainable. Key principles include:

1. Prevent waste rather than treating it afterwards.
2. Maximise atom economy.
3. Use less hazardous syntheses.
4. Design safer chemicals.
5. Use safer solvents (water or supercritical CO₂ instead of toxic organic solvents).
6. Design for energy efficiency — work at ambient temperature and pressure where possible.
7. Use renewable feedstocks such as plant materials.
8. Reduce derivatives (avoid unnecessary steps).
9. Use catalysts rather than stoichiometric reagents.
10. Design for degradation — products should break down harmlessly.
11. Real-time analysis to prevent pollution.
12. Inherently safer chemistry to prevent accidents.

Atom economy = (Mᵣ of desired product ÷ total Mᵣ of all products) × 100. Addition reactions have 100% atom economy; substitution and elimination reactions produce by-products and have lower atom economy.

Worked examples

Example 1: Cracking equation

Complete the equation for cracking C₁₂H₂₆ to give octane and one other product.

C₁₂H₂₆ → C₈H₁₈ + C₄H₈ (butene). Check: C 12 = 8 + 4; H 26 = 18 + 8. ✓

Example 2: Atom economy

Calculate the atom economy for making ethanol by (a) hydration of ethene, (b) fermentation of glucose. (Mᵣ: C₂H₅OH = 46, CO₂ = 44)

(a) C₂H₄ + H₂O → C₂H₅OH: only one product, atom economy = 100%. (b) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂: atom economy = (2 × 46) ÷ (2 × 46 + 2 × 44) × 100 = 92 ÷ 180 × 100 = 51.1%.

Example 3: Explaining fractional distillation

Explain why petrol collects higher up the fractionating column than diesel. (2 marks)

Petrol molecules are shorter chains with weaker London forces and lower boiling points (1), so they remain as vapour until they reach the cooler upper parts of the column before condensing (1).

Example 4: Evaluating a fuel

Evaluate the use of bioethanol from sugar cane as a fuel in the Caribbean.

Bioethanol is renewable and uses an existing agricultural crop, could reduce fuel imports and support the sugar industry, and the CO₂ released was recently absorbed by the plants. However, growing cane uses land, water and fertilisers, fermentation and distillation require energy, and there may be competition with food production. It could be part of a mixed energy strategy alongside solar and other renewables.

Example 5: Atom economy of a cracking reaction

Calculate the atom economy for producing octane in the reaction C₁₀H₂₂ → C₈H₁₈ + C₂H₄. (Mᵣ: C₈H₁₈ = 114, C₂H₄ = 28)

Atom economy = 114 ÷ (114 + 28) × 100 = 80.3%. In practice, the ethene is not wasted — it is a valuable feedstock for making poly(ethene) and ethanol — so the overall process is more efficient than this figure suggests when all products are used.

Example 6: Explaining octane numbers

Explain why 2,2,4-trimethylpentane has a much higher octane number than heptane.

Branched alkanes such as 2,2,4-trimethylpentane burn more smoothly and are less likely to auto-ignite under compression, so they resist knocking. Straight-chain alkanes such as heptane ignite too early in the engine cycle, causing knocking. Isomerisation and reforming at refineries therefore convert straight chains into branched and cyclic molecules to improve petrol quality.

Example 7: Applying green chemistry principles

A company currently makes a product using a toxic organic solvent, a stoichiometric metal reagent and a reaction at 200 °C. Suggest three green chemistry improvements.

1. Replace the toxic solvent with water or supercritical carbon dioxide (safer solvents).
2. Use a catalyst instead of the stoichiometric reagent, reducing waste.
3. Find a catalyst that allows the reaction to run at a lower temperature, improving energy efficiency.

Further improvements could include using a renewable feedstock and designing the product to biodegrade after use.

Example 8: Desulfurisation of fuels

Why is sulfur removed from fuels at refineries, and how?

Sulfur compounds in fuels burn to form sulfur dioxide, which causes acid rain and respiratory problems, and also poisons the catalysts in catalytic converters. In hydrodesulfurisation, the fuel is reacted with hydrogen over a catalyst, converting sulfur compounds into hydrogen sulfide, which is removed and converted into sulfur that can be sold.

Fossil fuels and alternatives compared

Common mistakes and how to avoid them

• Saying the column is hottest at the top — it is hottest at the bottom.
• Unbalanced cracking equations.
• Confusing cracking with fractional distillation — cracking is a chemical change; distillation is physical separation.
• Saying biofuels are completely carbon-neutral — energy for farming and processing produces emissions.
• Calculating atom economy using reactants only — use the products.

How this links to your Internal Assessment

SBAs in this area could compare the energy released by different fuels (such as ethanol, kerosene and biodiesel) by calorimetry, investigate the cracking of liquid paraffin over a heated porcelain catalyst and test the gaseous product for unsaturation with bromine water, or make biodiesel from waste cooking oil and assess its properties. Strong investigations include risk assessments for flammable liquids, controlled variables, and evaluation using green chemistry principles.

Exam technique for Petroleum and green chemistry

• Explain fractional distillation using boiling points, temperature gradient and condensation.
• Write and check cracking equations for carbon and hydrogen balance.
• Link fuel quality to octane number and molecular structure.
• In green chemistry questions, name the principles and apply them specifically to the process given.

Quick revision summary

• Crude oil: mixture of hydrocarbons separated by fractional distillation (hot at bottom, cool at top).
• Fractions: refinery gases, petrol, naphtha, kerosene, diesel, fuel oil, lubricants, bitumen.
• Cracking (thermal or catalytic with zeolites) makes shorter alkanes and alkenes to meet demand.
• Reforming and isomerisation raise octane numbers.
• Environmental impacts: CO₂, CO, NOₓ, SO₂, particulates, oil spills, gas flaring.
• Alternatives: bioethanol, biodiesel, hydrogen, solar, wind, geothermal.
• Green chemistry: prevent waste, high atom economy, safer solvents, catalysts, renewable feedstocks, energy efficiency.