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
Alkanes are named using IUPAC (International Union of Pure and Applied Chemistry) systematic nomenclature. The name has two parts: a prefix indicating the number of carbons in the longest chain, and the suffix '-ane' indicating it is an alkane (all single bonds). The prefixes for 1 to 8 carbons are: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8). These prefixes are universal across organic chemistry, they also appear in alcohols (methanol, ethanol) and acids (methanoic acid).
Drawing structural formulas: methane has a central carbon with four H atoms radiating out. Ethane is H₃C–CH₃, two carbons each with three H atoms and bonded to each other. Propane is H₃C–CH₂–CH₃, three carbons in a chain. Butane adds one more CH₂ unit. For each additional carbon in the chain, two more H atoms are added to satisfy the tetravalency of carbon, this is where the '+2' in CₙH₂ₙ₊₂ comes from: the two end carbon atoms each carry one extra H compared with the interior CH₂ units.
Heptane (C₇H₁₆): 7 carbons in a chain. Formula check: C₇H₂×7+2 = C₇H₁₆. Heptane is a key component of petrol, its combustion in car engines is used as a reference point (0 on the octane scale) to measure how resistant petrol is to premature ignition (knock).
Ampol's fuel specifications for Australian petrol are defined by the chain length and branching of alkane components, isooctane (a branched C₈H₁₈ alkane) rates 100 on the octane scale; heptane (straight-chain C₇H₁₆) rates 0. The '95 RON' printed on premium petrol pumps at Australian service stations means the fuel behaves like a 95:5 mixture of isooctane and heptane.
Chemists describe the same alkane in three different ways, and each one tells you something different. The molecular formula gives only the count of each kind of atom in one molecule. For propane this is C₃H₈, telling you there are 3 carbons and 8 hydrogens, but nothing about how they are joined. The displayed formula (also called the full structural formula) shows every single atom and every single bond drawn out, so you can see exactly which atom is bonded to which. For propane the displayed formula shows a central carbon joined to two end carbons, with H atoms filling every remaining bond. This is the most detailed picture, but it takes the most space to draw.
The condensed formula sits in between: it groups the atoms together without drawing the bonds, so it is quick to write while still showing the order of the carbons in the chain. Propane condenses to CH₃CH₂CH₃, and butane to CH₃CH₂CH₂CH₃. Reading left to right you can count the carbons in the chain and see that each end carbon is a CH₃ group while each interior carbon is a CH₂ group. Use the molecular formula when you only need the atom counts, the displayed formula when you need to show structure clearly, and the condensed formula when you want a fast, compact way to write the structure.
Butane (C₄H₁₀) written three ways. Molecular formula: C₄H₁₀ (4 carbons, 10 hydrogens). Condensed formula: CH₃CH₂CH₂CH₃, two CH₃ end groups with two CH₂ groups between them. Displayed formula: a chain of four carbons drawn out with a single bond between each pair of carbons and an H atom shown on every spare carbon bond. All three describe the same molecule.
When chemists and medicine manufacturers label compounds, they often use displayed (structural) formulas rather than just atom counts. A structural formula communicates exactly how the atoms are joined, so a researcher in any country can read it and build the same molecule without confusion.
Sometimes two different molecules share exactly the same molecular formula yet are not the same substance. These are called structural isomers: molecules with the same molecular formula but a different arrangement of atoms. The clearest example is C₄H₁₀. One arrangement is butane, where all four carbons are joined in a single straight chain (CH₃CH₂CH₂CH₃). The other arrangement is 2-methylpropane (also called isobutane), where three carbons form a chain and the fourth carbon branches off the middle carbon. Both molecules contain 4 carbons and 10 hydrogens, so both have the molecular formula C₄H₁₀, but the atoms are connected in different ways.
Because the atoms are arranged differently, structural isomers can have different physical properties. Straight-chain butane boils at about −1 °C, while branched 2-methylpropane boils at about −12 °C, the branched molecule has a slightly lower boiling point because its shape lets the molecules pack together less closely. As alkanes get larger they have more possible isomers: pentane (C₅H₁₂) has three structural isomers. In Year 9 you only need to recognise straight-chain versus branched arrangements; the detailed rules for naming branched chains come later.
Butane and isobutane both have the molecular formula C₄H₁₀. Butane is a straight chain of 4 carbons and boils at about −1 °C. Isobutane (2-methylpropane) has a branched arrangement and boils at about −12 °C. Same atoms, different arrangement, different boiling point.
The branched C₈H₁₈ isomer isooctane is prized in petrol because it burns smoothly and resists knocking, giving high-octane fuel. Its straight-chain isomer, octane, burns far less smoothly. Two molecules with the same molecular formula behave very differently at the petrol pump just because of how their atoms are arranged.
Structural isomers have the same formula but a different of atoms. For example, butane is a chain, while its isomer 2-methylpropane is . Because the atoms are arranged differently, the two isomers can have different physical properties such as point.
At the start of this lesson, you saw that chemists in different countries need to refer to the exact same molecule without confusion. IUPAC nomenclature solves this: a prefix tells you how many carbons are in the chain, and the suffix '-ane' tells you it is an alkane with all single bonds. You also learned three ways to represent an alkane, molecular, displayed, and condensed formulas, and that structural isomers share a molecular formula but arrange their atoms differently.
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, and write its molecular and condensed formulas? 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.