Naming and Representing Simple Alkanes
In 1979, IUPAC standardised organic naming so a Japanese and a German chemist could write C₂H₄ and both know they meant ethene, the molecule produced at 750 million tonnes per year globally.
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Q1 · If scientists around the world need to refer to the same molecule, how do you think they ensure they all mean exactly the same thing, even when speaking different languages?
Q2 · Why do you think it is so important for scientists to have one agreed, universal system for naming chemical compounds rather than each country using its own names?
● Know
- The six IUPAC prefixes for C1–C6 alkanes
- The difference between molecular, displayed, and condensed structural formulas
- How to convert between name and formula for simple alkanes
● Understand
- Why IUPAC naming is necessary for scientific communication
- How each formula type conveys different information about molecular structure
- What structural isomers are and why they have the same formula
● Can do
- Name alkanes from their formula
- Write molecular, displayed, and condensed formulas for C1–C6 alkanes
- Identify the correct IUPAC name from a displayed structure
Add a few drops of bromine water to a test tube of ethene gas and watch it instantly turn from orange-brown to colourless, the bromine atoms have broken open the double bond and attached themselves to the two carbon atoms, something that simply cannot happen with an alkane. Alkenes are hydrocarbons that contain at least one carbon–carbon double bond. The general formula is $C_nH_{2n}$ (compared to $C_nH_{2n+2}$ for alkanes). Having fewer hydrogen atoms than the maximum possible makes alkenes unsaturatedthe double bond can react with additional atoms. The simplest alkene is ethene (ethylene, C₂H₄); then propene (C₃H₆), butene (C₄H₈). Named with the '-ene' suffix: eth-ene, prop-ene, but-ene. The double bond restricts rotation around that bond, the four atoms directly attached to the double-bonded carbons are locked in a flat (planar) arrangement.
The double bond makes alkenes significantly more reactive than alkanes. In an alkane, all four bonds of each carbon are fully used; there is no easy site for reaction. In an alkene, the second bond of the double bond is weaker and less stable, it can break open to allow two new atoms to attach to the two carbons. This is called an addition reaction, and it is the key reaction type for alkenes. This reactivity is exactly why ethene is the building block of polyethylene, millions of molecules undergo addition reactions, chaining together.
Formula check for propene (n=3): CₙH₂ₙ = C₃H₆. Structural formula: CH₂=CH–CH₃. The double bond is between C1 and C2; C3 carries 3 H atoms. Propene is the monomer for polypropylene, used in car bumper bars, food containers, and rope fibres manufactured in Australian factories.
Qenos's Altona petrochemical complex in Melbourne produces ethene and propene from cracked petroleum. These two alkenes are the feedstock for Australia's entire domestic polyethylene and polypropylene production, packaging film, pipes, car parts, and food containers manufactured by companies across NSW and Victoria.
The double bond in alkenes undergoes addition reactions where the double bond opens and two new atoms or groups attach to the two carbon atoms. Key addition reactions: (1) Hydrogenationalkene + H₂ (with nickel catalyst) → alkane. This converts unsaturated fats (in margarine production). (2) Brominationalkene + Br₂ (bromine water) → dibromoalkane. The orange-brown bromine water is immediately decolourised, this is the standard test for unsaturation. (3) Hydrationalkene + H₂O → alcohol (with phosphoric acid catalyst). Ethene + H₂O → ethanol (industrial ethanol production).
The bromine water test is the most important practical test in this unit. Mix bromine water (orange-brown) with an unknown hydrocarbon. If the colour disappears: alkene present (the Br₂ adds across the double bond, forming a colourless dibromoalkane). If the colour persists: alkane present (alkanes don't react with bromine water under normal conditions, they have no double bond). This test distinguishes saturated from unsaturated hydrocarbons in seconds.
Ethene + bromine: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br (1,2-dibromoethane). The orange bromine water turns colourless. Ethane + bromine water: no reaction in the dark, the solution stays orange-brown. Same test, opposite result because one compound has a double bond and the other does not.
Industrial hydrogenation of vegetable oils (containing unsaturated C=C bonds) at Goodman Fielder's Australian food processing plants converts liquid sunflower oil into solid or semi-solid margarine. Adding H₂ across the C=C bonds raises the melting point, the same addition reaction used in the bromine test, just with hydrogen instead of bromine.
Alkynes contain at least one carbon–carbon triple bond. General formula: $C_nH_{2n-2}$. The simplest is ethyne (acetylene, C₂H₂, HC≡CH). Alkynes are even more unsaturated than alkenes, they have even fewer H atoms relative to carbons. The triple bond makes alkynes highly reactive and capable of two successive addition reactions. Polymerisationwhen alkene monomers chain together through addition reactions, is the most industrially important application of alkene chemistry. In the addition polymerisation of ethene, n molecules of CH₂=CH₂ undergo the double bond opening reaction repeatedly to form (–CH₂–CH₂–)ₙ: polyethylene.
The scale is extraordinary: the world produces 200 million tonnes of polyethylene per year from ethene. In Australia, Qenos's Altona plant and overseas-sourced polymer granules are converted into packaging film, pipes, and containers at hundreds of manufacturing sites. Meanwhile, ethyne (acetylene) burned in oxygen produces a flame at 3500 °C, hotter than any other common fuel gas. This oxyacetylene torch is the standard tool for cutting and welding steel at Australian construction and engineering sites, with 50,000+ registered welders using it across the country.
Addition polymerisation of ethene: n × CH₂=CH₂ → (–CH₂–CH₂–)ₙ. For grocery store plastic bags, n ≈ 10,000–100,000, the chain contains 10,000 to 100,000 ethene units. The double bonds in every monomer unit have all reacted, leaving only single bonds in the polymer, it is fully saturated and inert.
BOC's cylinder gas network supplies oxyacetylene (ethyne + O₂) to thousands of Australian fabrication workshops and construction sites. The 3500 °C flame achieves the only temperatures hot enough to cut through 100 mm structural steel plate in a single pass, essential for demolition of NSW bridges, shipbuilding at Austal's Henderson facility, and offshore oil platform fabrication.
Alkynes contain at least one carbon–carbon bond. The simplest alkyne is , also known as acetylene. In addition polymerisation, many small molecules called join into a long chain. When ethene polymerises, the C=C double bond in each monomer and links to its neighbours. This forms the long-chain polymer .
At the start of this lesson, you heard that ethylene, just two carbon atoms with a double bond, is the world's most-produced organic chemical (over 200 million tonnes a year) and the building block of polyethylene, the plastic that wraps almost every product you buy. One structural feature creates an entirely new class of chemistry.
Now that you've worked through the lesson, can you use IUPAC naming rules to correctly name a simple alkane you haven't seen before? How does having a standard naming system help chemists in different countries communicate precisely about molecules?
Q1. State the IUPAC names and molecular formulas for the alkanes with 1, 2, 3, 4, 5, and 6 carbon atoms.
Q2. Draw the displayed formula for propane (C₃H₈). Then write its condensed formula. Explain what additional information the displayed formula provides compared to the molecular formula.
Q3. Butane (C₄H₁₀) has a structural isomer called 2-methylpropane, also with formula C₄H₁₀. Explain what a structural isomer is and describe the structural difference between these two molecules.