Course: Organic Chemistry I | Source: Studocu, University of Minnesota Twin Cities
Difficulty: Introductory
Prerequisites: Basic understanding of atomic structure, chemical bonding, and the concept of hydrocarbons. If you are unfamiliar with covalent bonding or molecular formulas, review those first.
Big Picture
Alkanes are the simplest family of organic molecules and the starting point for nearly every organic chemistry course. They contain only carbon and hydrogen connected by single bonds, which makes them the baseline against which all other functional groups are compared. Understanding alkanes, their naming, their physical trends, and their (limited) reactivity gives you the vocabulary and logic you will use for the rest of the course. If you can confidently describe how and why alkane properties change with chain length, you are ready for alkenes, alkynes, and beyond.
Alkanes are hydrocarbons built entirely from carbon–carbon and carbon–hydrogen single bonds, following the general formula CₙH₂ₙ₊₂. They are low in reactivity (their main reaction is combustion), and their physical state at room temperature scales predictably with chain length: short chains are gases, mid-length chains are liquids, long chains are solids. Most of the fossil fuels you encounter daily, from natural gas to petrol, are mixtures of alkanes.
Alkane
A hydrocarbon containing only single covalent bonds between carbon atoms and hydrogen atoms. General formula: CₙH₂ₙ₊₂. In simple terms, these are the most basic organic molecules, chains of carbon with as much hydrogen as they can hold.
Hydrocarbon
An organic compound made up exclusively of carbon and hydrogen atoms. Think of it as the broadest family name for molecules that contain nothing but C and H.
Paraffin
An older, informal name for alkanes, from the Latin "parum affinis" (little affinity), referring to their low chemical reactivity. You will still see this word on wax candles and in petroleum industry literature.
Saturated hydrocarbon
A hydrocarbon in which every carbon–carbon bond is a single bond, meaning the molecule holds the maximum possible number of hydrogen atoms. In simple terms, there are no double or triple bonds, so the molecule is "full up" with hydrogen.
Homologous series
A sequence of compounds that share the same general formula and differ from one neighbour to the next by a constant unit (for alkanes, that unit is CH₂). Think of it as a family where each member is one carbon longer than the last.
Fractional distillation
A separation technique that sorts a mixture of liquids by their boiling points. In the petroleum industry, crude oil enters a fractionating column and the different hydrocarbon fractions (gases, petrol, kerosene, diesel, etc.) are collected at different heights.
Combustion
A chemical reaction in which a substance reacts with oxygen and releases energy as heat and light. For alkanes, complete combustion produces carbon dioxide and water. In simple terms, this is what happens when you burn natural gas on a hob.
Higher alkanes
Alkanes with four or more carbon atoms in the chain. The term is informal but appears frequently in introductory texts to distinguish the longer-chain members from methane, ethane, and propane.
Alkanes are named by the number of carbon atoms in the chain: methane (C₁), ethane (C₂), propane (C₃), butane (C₄), pentane (C₅), and so on.
Each member differs from the next by one CH₂ unit, forming a homologous series.
Alkanes with four or more carbons are sometimes called "higher alkanes."
C₁–C₄ (methane through butane): gases at room temperature.
C₅–C₂₀ (pentane through eicosane, roughly): liquids at room temperature.
Beyond C₂₀: waxy solids at room temperature.
The trend is straightforward: as chain length increases, London dispersion forces strengthen, so boiling points rise and the molecules become harder to pull apart.
All alkanes are colourless, odourless, and tasteless in their pure form.
Alkanes are notably unreactive. The C–H and C–C single bonds are strong and non-polar (or only very weakly polar), so they resist attack by most reagents.
They are unreactive towards acids, bases, and most oxidising agents under normal conditions.
The main reaction you need to know at this level is combustion: alkanes burn in oxygen to produce CO₂ and H₂O, releasing energy.
Complete combustion (excess O₂): CₙH₂ₙ₊₂ + excess O₂ → CO₂ + H₂O
Incomplete combustion (limited O₂): produces CO or soot (carbon) alongside water.
Crude oil is a complex mixture of hydrocarbons, mostly alkanes of varying chain lengths.
Fractional distillation heats crude oil in a tall column. Different fractions condense at different heights according to their boiling points.
Shorter chains (lower boiling points) rise higher; longer chains condense lower down.
This process yields fractions such as petroleum gas, petrol (gasoline), kerosene, diesel, and heavy fuel oils.
Fuels: Methane is the main component of natural gas (heating, cooking, electricity generation). Propane is used in portable gas cylinders and rural heating. Octane and other mid-chain alkanes form the bulk of petrol.
Chemical feedstocks: Alkanes serve as raw materials for producing plastics, solvents, synthetic rubber, and a wide range of industrial chemicals.
Combustion of alkanes releases CO₂ (a greenhouse gas) and, when incomplete, carbon monoxide and particulates.
Extraction and transport of crude oil carry risks of oil spills and habitat damage.
These concerns are a recurring exam and essay topic in introductory chemistry courses.
General formula for alkanes
CₙH₂ₙ₊₂
Where n = number of carbon atoms. This is the single most important formula in this topic. Every alkane, from methane (n = 1, CH₄) to any arbitrarily long chain, follows it.
Complete combustion (balanced general form)
CₙH₂ₙ₊₂ + ((3n + 1) / 2) O₂ → n CO₂ + (n + 1) H₂O
For methane specifically: CH₄ + 2 O₂ → CO₂ + 2 H₂O
First ten straight-chain alkanes (reference table)
Name | Formula | Carbons | State at 25 °C |
|---|---|---|---|
Methane | CH₄ | 1 | Gas |
Ethane | C₂H₆ | 2 | Gas |
Propane | C₃H₈ | 3 | Gas |
Butane | C₄H₁₀ | 4 | Gas |
Pentane | C₅H₁₂ | 5 | Liquid |
Hexane | C₆H₁₄ | 6 | Liquid |
Heptane | C₇H₁₆ | 7 | Liquid |
Octane | C₈H₁₈ | 8 | Liquid |
Nonane | C₉H₂₀ | 9 | Liquid |
Decane | C₁₀H₂₂ | 10 | Liquid |
Natural gas piped into homes for heating and cooking is mostly methane, the simplest alkane. The petrol in a car engine is a blend of C₅–C₁₂ alkanes, with octane ratings referring to how smoothly the mixture burns. Propane cylinders fuel barbecues and caravans. In the chemical industry, cracking longer alkanes into shorter ones is the basis of plastics manufacturing: polyethylene, for instance, starts as ethylene derived from ethane.
"Alkanes are completely non-reactive." They are unreactive towards most reagents under mild conditions, but they burn readily in oxygen and undergo halogenation under UV light. "Low reactivity" is more accurate than "no reactivity."
"All hydrocarbons are alkanes." Alkanes are one family of hydrocarbons. Alkenes (double bonds), alkynes (triple bonds), and aromatic compounds (ring structures with delocalised electrons) are all hydrocarbons too.
"Longer chain = higher reactivity." Chain length changes physical properties (boiling point, viscosity) but does not meaningfully change an alkane's chemical reactivity. The bonds are the same regardless of chain length.
"Fractional distillation is a chemical reaction." It is a physical separation technique. No bonds are broken or formed; the mixture is simply sorted by boiling point.
⚠️ Be able to write the general formula CₙH₂ₙ₊₂ and use it to predict the molecular formula of any straight-chain alkane given n.
⚠️ Know the physical-state trend: C₁–C₄ gases, C₅–C₂₀ liquids, above C₂₀ solids. Exams often ask you to predict state from carbon count.
⚠️ Be prepared to write and balance a combustion equation for a named alkane (methane and propane are favourites).
⚠️ Understand that alkanes are saturated, meaning every bond is a single bond. This distinguishes them from alkenes and alkynes.
⚠️ Fractional distillation is a physical process, not a chemical one. Do not confuse it with cracking, which breaks C–C bonds and is a chemical process.
True or false: Alkanes contain at least one carbon–carbon double bond. False. Alkanes contain only single bonds.
Fill in the blank: The general formula for alkanes is ____. CₙH₂ₙ₊₂
True or false: Pentane (C₅H₁₂) is a gas at room temperature. False. Pentane is a liquid. Gases stop at C₄ (butane).
Fill in the blank: Alkanes are separated from crude oil by a physical process called ____. Fractional distillation
True or false: Combustion of methane produces carbon dioxide and water. True.
Q: What is the molecular formula of an alkane with 7 carbon atoms?
A: C₇H₁₆. Apply CₙH₂ₙ₊₂ with n = 7: (2 × 7) + 2 = 16 hydrogen atoms.
Q: Write a balanced equation for the complete combustion of propane (C₃H₈).
A: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
Q: Explain why alkanes are described as "saturated" hydrocarbons.
A: Every carbon–carbon bond in an alkane is a single bond, so each carbon carries the maximum number of hydrogen atoms possible. There are no double or triple bonds that could accept additional atoms.
Q: A student claims that fractional distillation breaks the bonds in crude oil. Is this correct? Explain.
A: No. Fractional distillation is a physical separation. It sorts hydrocarbons by boiling point without breaking any chemical bonds. The process that breaks C–C bonds is cracking, which is a different, chemical process.
Q: Why do boiling points increase as alkane chain length increases?
A: Longer chains have more electrons and a larger surface area, which increases London dispersion forces between molecules. Stronger intermolecular forces mean more energy is needed to separate the molecules into the gas phase, so the boiling point rises.
This topic connects directly to alkenes and alkynes, which you will study next. The difference is the bond type: alkenes have at least one C=C double bond, alkynes at least one C≡C triple bond. Recognising that alkanes are the "saturated" baseline makes unsaturated compounds easier to understand.
The combustion reactions covered here reappear in thermochemistry and enthalpy, where you will calculate the energy released per mole of fuel burned.
Fractional distillation and cracking tie into industrial chemistry and polymer science, since short-chain alkenes produced by cracking are the monomers for common plastics like polyethylene and polypropylene.
alkanes, paraffins, saturated hydrocarbons, CₙH₂ₙ₊₂, homologous series, methane, ethane, propane, butane, pentane, octane, decane, fractional distillation, crude oil, petroleum, combustion of alkanes, complete combustion, incomplete combustion, London dispersion forces, van der Waals forces, boiling point trend, organic chemistry nomenclature, IUPAC naming, hydrocarbon classification, chemical feedstock, natural gas, gasoline components, cracking