Crude Oil, Hydrocarbons & Polymers
Organic chemistry studies carbon-based compounds, beginning with crude oil: a finite mixture of hydrocarbons formed over millions of years from ancient plankton. Fractional distillation separates crude oil by boiling point into useful fractions (such as petrol, kerosene, and diesel), while cracking converts long-chain alkanes into shorter alkanes and alkenes.
- Hydrocarbon
- A compound containing only hydrogen and carbon atoms.
- Alkane
- A saturated hydrocarbon with the general formula CnH2n+2.
- Alkene
- An unsaturated hydrocarbon containing a carbon-carbon double bond, with the general formula CnH2n.
Curly-Arrow Organic Mechanisms (Nucleophilic Substitution & Addition)
3 Golden Rules for Curly Arrows:
1. Start at electrons: Every curly arrow MUST start at a lone pair of electrons or the middle of a covalent bond (never start an arrow on a δ+ or positive charge).
2. Show dipoles: Always label δ+ and δ− on polar bonds like C–Br or H–Br.
3. Balance charges: If a neutral molecule loses a leaving ion (:Br⁻), the intermediate carbocation must carry a formal + charge.
Key Concepts & Exam Definitions (5)
- Hydrocarbon
- A compound containing only hydrogen and carbon atoms.
- Alkane
- A saturated hydrocarbon with the general formula CnH2n+2.
- Alkene
- An unsaturated hydrocarbon containing a carbon-carbon double bond, with the general formula CnH2n.
- Cracking
- The thermal decomposition of long-chain hydrocarbons into shorter, more useful alkanes and alkenes.
- Polymerisation
- A chemical reaction where many small molecules (monomers) join together to form a large molecule (polymer).
Quick-Fire Flashcards (5)
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What is the general formula for an alkane?
How do boiling points change as the hydrocarbon chain length increases?
What is the test for an alkene?
Name the first four alkanes.
Why is cracking used?
Exam-Style Questions & Mark Schemes (3)
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- Q1
Describe how fractional distillation separates crude oil.
Click “Check Mark Scheme” to compare your answer, Crude oil is heated and vaporised. The column has a temperature gradient (hot at bottom, cooler at top). Vapours rise and condense at their specific boiling points, separating into fractions.
- Q2
Write the balanced chemical equation for the complete combustion of methane (CH4).
Click “Check Mark Scheme” to compare your answer, CH4 + 2O2 -> CO2 + 2H2O
- Q3
Explain how poly(ethene) is formed.
Click “Check Mark Scheme” to compare your answer, Ethene monomers undergo addition polymerisation, where their double bonds break and they join together to form a long chain polymer called poly(ethene).
Common Exam Questions (3)
What is crude oil?
Crude oil is a finite, non-renewable fossil fuel formed from the remains of ancient marine organisms buried in mud over millions of years. Chemically, it is a complex mixture of hydrocarbons, mostly homologous alkanes.
What is fractional distillation?
Fractional distillation separates crude oil into fractions with similar chain lengths and boiling points: vaporised crude oil enters a fractionating column that is hot at the bottom and cool at the top, so hydrocarbons condense at different heights according to their boiling points.
What are alkanes and alkenes?
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂ containing only single covalent C-C bonds. Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ containing at least one carbon-carbon double bond (C=C).
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