Difficulty: Introductory | Prerequisites: General chemistry (Lewis structures, bonding, electronegativity)
Big Picture
Alkanes are the simplest organic molecules, built entirely from carbon and hydrogen with only single bonds. This chapter is the foundation for everything that follows in organic chemistry: the naming conventions (IUPAC nomenclature) you learn here carry through every functional group for the rest of the course, and the ideas about molecular shape and intermolecular forces come back constantly. You should already be comfortable drawing Lewis structures and understand basic ideas about electronegativity and polarity from general chemistry. If those feel shaky, revisit them before diving in.
Alkanes are saturated hydrocarbons containing only C–H and C–C single bonds. Their names follow a systematic set of IUPAC rules based on the longest carbon chain and any branches. They are non-polar, held together by weak London dispersion forces, and their boiling points rise with molecular size. Constitutional isomers share the same molecular formula but differ in how atoms are connected.
Hydrocarbon
A compound made up exclusively of carbon and hydrogen atoms. In simple terms, strip away every other element and you are left with a hydrocarbon.
Alkane
A saturated hydrocarbon containing only C–C and C–H single bonds, with the maximum possible number of hydrogen atoms. Think of it as: the most "filled up" a carbon skeleton can be with hydrogens.
Saturated
Describes a molecule that contains no double or triple bonds between carbon atoms. Every carbon–carbon bond is a single bond. Think of it as: all bonding capacity is used up on single bonds and hydrogens.
Unsaturated
Describes a molecule that contains at least one carbon–carbon double or triple bond. The molecule could accept more hydrogen atoms if those multiple bonds were broken.
Alkyl group
A fragment of an alkane that has lost one hydrogen atom, leaving a free bond to attach to something else. Named by replacing the "-ane" ending with "-yl" (methane becomes methyl, ethane becomes ethyl). Think of it as: the branch or substituent hanging off a main chain.
Constitutional isomers (structural isomers)
Molecules that share the same molecular formula but differ in how the atoms are connected to one another. They are genuinely different compounds with different physical properties. In simple terms, same ingredients, different wiring diagram.
Dispersion forces (London forces)
Weak, temporary attractive forces between non-polar molecules caused by fleeting, uneven distributions of electron density (instantaneous dipoles). These are the only intermolecular forces available to alkanes. Think of it as: a brief, random wobble in the electron cloud of one molecule nudges the electrons of its neighbour, creating a momentary attraction.
Combustion
The exothermic reaction of a hydrocarbon with oxygen (O₂), producing carbon dioxide (CO₂) and water (H₂O). This is why alkanes are useful as fuels.
Alkanes consist only of carbon and hydrogen, with all single bonds (no double or triple bonds).
They can be acyclic (open-chain, either linear or branched) or cyclic (forming a ring).
General formula for acyclic alkanes: CₙH₂ₙ₊₂
General formula for cyclic alkanes: CₙH₂ₙ (two fewer hydrogens because the ring closes off two bonding positions)
Physical properties of alkanes:
Non-polar, so they do not dissolve in water
Colourless
High energy content, which is why they burn well as fuels
The first four names (methane, ethane, propane, butane) are historical. From five carbons onward, the Greek or Latin number prefix tells you the chain length, followed by the suffix "-ane."
Alkane | Carbons | Boiling Point (°C) |
|---|---|---|
Methane | 1 | −164 |
Ethane | 2 | −89 |
Propane | 3 | −42 |
Butane | 4 | 1 |
Pentane | 5 | 36 |
Hexane | 6 | 69 |
Heptane | 7 | 99 |
Octane | 8 | 126 |
Nonane | 9 | 151 |
Decane | 10 | 174 |
You need to memorise these names, their carbon counts and the boiling-point trend. The bold prefixes (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-) are number prefixes that recur throughout organic chemistry.
Alkanes are non-polar, so they lack dipole–dipole interactions and hydrogen bonding.
The only intermolecular forces holding alkane molecules together are dispersion forces: temporary, induced dipoles from random fluctuations in electron density.
Greater surface area means more contact between molecules, which means stronger dispersion forces and a higher boiling point.
This is why boiling point climbs steadily as chain length increases.
Branching reduces surface area. For example, neopentane (C₅H₁₂, highly branched and compact) boils lower than n-pentane (C₅H₁₂, linear), even though they share the same molecular formula.
Other intermolecular forces exist in other molecule families (dipole–dipole interactions, hydrogen bonding), but alkanes rely solely on dispersion forces.
The combustion of an alkane is an exothermic reaction:
2\,C_8H_{18} + 25\,O_2 \rightarrow 16\,CO_2 + 18\,H_2OThis is the principle behind petrol engines, gas stoves and heating systems. Alkanes release a large amount of energy when burned.
Constitutional isomers have the same molecular formula but different connectivity between atoms.
They are distinct compounds with distinct physical properties (different boiling points, melting points, densities).
Simple rotations of a molecule do not produce a new constitutional isomer. You must break and reform bonds to convert one constitutional isomer into another.
Systematic approach to drawing all constitutional isomers of a given formula:
Start with the longest possible unbranched chain.
Shorten the chain by one carbon and attach it as a branch. Try every unique position.
Shorten further and add multiple branches.
Check for duplicates. Molecules that look different on paper but have the same connectivity are the same compound.
Example: C₄H₁₀ has 2 constitutional isomers (n-butane and isobutane). C₅H₁₂ has 3. The number grows rapidly with carbon count.
Students often think that rotating a molecule on paper creates a new constitutional isomer. It does not. If the connectivity is the same, it is the same molecule regardless of how you have drawn it.
Students sometimes assume that "saturated" means "full of carbons." It means full of hydrogens: every carbon–carbon bond is single, so the maximum number of C–H bonds are present.
A common error is confusing intermolecular forces with intramolecular bonds. Dispersion forces act between separate alkane molecules. The C–C and C–H bonds within a single molecule are covalent bonds, which are much stronger.
Some students believe branching increases boiling point because "more complexity means more interactions." The opposite is true: branching makes the molecule more compact, reducing surface area and weakening dispersion forces.
⚠️ You will almost certainly need to name alkanes using IUPAC rules and draw structures from IUPAC names. Practise both directions.
⚠️ Expect questions asking you to draw all constitutional isomers of a given formula. Use the systematic approach (longest chain first, then shorten and branch) and always check for duplicates.
⚠️ Understanding dispersion forces and how molecular size and shape affect boiling point is a classic exam topic. Be ready to explain why a branched isomer boils lower than its straight-chain counterpart.
⚠️ Memorise the names and carbon counts for methane through decane. These prefixes are used throughout the entire course.
True or false: An alkane with the formula C₆H₁₄ could be cyclic. (False, cyclic alkanes follow CₙH₂ₙ.)
Fill in the blank: The only intermolecular forces between alkane molecules are ______ forces. (Dispersion / London)
True or false: Butane and isobutane are constitutional isomers. (True)
Fill in the blank: The general formula for an acyclic alkane is ______. (CₙH₂ₙ₊₂)
True or false: A branched alkane has a higher boiling point than its straight-chain isomer with the same molecular formula. (False)
Q: What is the IUPAC name for an unbranched alkane with 7 carbon atoms?
A: Heptane.
Q: How many constitutional isomers does C₅H₁₂ have? Name or draw them.
A: Three. n-Pentane (straight chain), isopentane (2-methylbutane), and neopentane (2,2-dimethylpropane).
Q: Explain why n-pentane (bp 36 °C) boils at a higher temperature than neopentane (bp 9.5 °C), even though both have the formula C₅H₁₂.
A: n-Pentane is a longer, more extended molecule with greater surface area. This allows more contact between neighbouring molecules, leading to stronger London dispersion forces. More energy is needed to pull the molecules apart, so the boiling point is higher. Neopentane is compact and roughly spherical, so it has less surface area and weaker dispersion forces.
Q: Write the balanced equation for the complete combustion of propane (C₃H₈).
A: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
Q: A molecule has the formula C₆H₁₂. Could this be an acyclic alkane? Explain.
A: No. An acyclic alkane with 6 carbons would follow CₙH₂ₙ₊₂ and have the formula C₆H₁₄. C₆H₁₂ fits the formula CₙH₂ₙ, which corresponds to a cycloalkane (or a molecule with one degree of unsaturation, such as one double bond).
The naming conventions you learn here (parent chain, substituent prefixes, numbering rules) carry directly into every later chapter on functional groups, from alkenes to alcohols to carboxylic acids. The idea that molecular shape affects physical properties through intermolecular forces reappears when you study polarity, solubility and reaction mechanisms. Constitutional isomers are the first type of isomerism you encounter; stereoisomers (covered at the end of this chapter and in depth later) add another layer.
alkane, saturated hydrocarbon, CₙH₂ₙ₊₂, IUPAC nomenclature, constitutional isomers, structural isomers, London dispersion forces, van der Waals forces, boiling point trend, methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane, hydrocarbon, combustion, non-polar, molecular formula, acyclic, cyclic, organic chemistry chapter 2, CHEM 25500, Purdue