The Alkanes: Saturated Hydrocarbons

Introduction

The alkanes are the simplest homologous series of hydrocarbons and the main ingredients of natural gas, petrol, diesel and candle wax. Every carbon atom in an alkane is joined to its neighbours by single bonds only, and every spare bond holds a hydrogen atom. This makes the alkanes "full up" with hydrogen — chemists say they are saturated . Their simple structure explains both why they are such useful fuels and why they are otherwise rather unreactive.

Core explanation

General formula. All alkanes fit the general formula CₙH₂ₙ₊₂ . For every carbon there are two hydrogens, plus one extra hydrogen at each end of the chain. So an alkane with 3 carbons has 2 × 3 + 2 = 8 hydrogens: C₃H₈.

The first four alkanes:

n Name Molecular formula Structural formula --- --- --- --- 1 methane CH₄ CH₄ 2 ethane C₂H₆ CH₃CH₃ 3 propane C₃H₈ CH₃CH₂CH₃ 4 butane C₄H₁₀ CH₃CH₂CH₂CH₃

Bonding. Each carbon forms four single covalent bonds. In methane all four go to hydrogen atoms. In longer alkanes, each carbon bonds to one or two neighbouring carbons and to hydrogen atoms for the rest. The bonds around each carbon point to the corners of a tetrahedron with angles of about 109.5°, so a real alkane chain is a zigzag, not a straight line.

What saturated means. Because every carbon already has four single bonds, there is no way to add more atoms to the molecule without first breaking a bond. The molecule holds the maximum possible number of hydrogen atoms for its number of carbons. That is the meaning of saturated.

Reactivity. C–C and C–H bonds are strong and almost non-polar, so alkanes do not react with acids, alkalis or most other reagents at room temperature. Their two important reactions are:

- Combustion. Alkanes burn in oxygen, releasing a lot of energy. With plenty of oxygen the products are carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O. This is why they are used as fuels. - Substitution with halogens. In ultraviolet light, a halogen such as chlorine can replace hydrogen atoms one at a time, for example CH₄ + Cl₂ → CH₃Cl + HCl. This is slow without UV light.

Sources. Methane is the main component of natural gas; larger alkanes are separated from crude oil.

Step-by-step reasoning

To draw the displayed formula of an alkane:

1. Use the name to find the number of carbons (prop- = 3). 2. Draw the carbons in a row joined by single bonds. 3. Give each carbon four bonds in total. 4. Put a hydrogen on every bond that is not linking two carbons. 5. Count the hydrogens and check the total against CₙH₂ₙ₊₂.

Visual explanation

A displayed formula of ethane shows two C symbols side by side joined by a single line, with three H symbols attached to each C by lines — above, below and to the outside. The 3D model with SIM-ORG-001 shows the same molecule as two tetrahedra joined at one corner.

Real-world analogy

A saturated alkane is like a car park in which every space is taken. No more cars (hydrogen atoms) can fit in unless one leaves first. An unsaturated molecule, by contrast, has spaces still free.

Real-world example

Natural gas piped into homes for cooking and heating is typically 70–90% methane, with smaller amounts of ethane, propane and butane. A harmless but strong-smelling sulfur compound is added because methane itself has no smell, so that leaks can be detected quickly.

Why?

Why is the general formula CₙH₂ₙ₊₂? Each carbon in the middle of a chain uses two bonds to link to its neighbours and two for hydrogens. The two end carbons each have one extra bond free, which takes one extra hydrogen each — hence the "+2".

Common misconception

"Alkanes are straight molecules." Displayed formulae are drawn flat for convenience, but the bond angles are about 109.5°, so real chains zigzag in three dimensions and can twist freely about each single bond.

Worked example

Question: An alkane has 12 hydrogen atoms. Find its formula and name.

Reasoning: 2n + 2 = 12, so 2n = 10 and n = 5. An alkane with five carbons is pentane.

Answer: C₅H₁₂, pentane.

Quick check

1. Why are alkanes described as saturated? Answer: They contain only single carbon–carbon bonds, so no more hydrogen atoms can be added.

Exam focus

Learn the first four alkane names and formulae, and be able to draw their displayed formulae with every bond shown. When asked why alkanes are saturated, mention "single bonds only" and "no more atoms can be added". A missing hydrogen on a displayed formula loses the mark.

Advanced insight

Alkanes are sometimes called paraffins, from Latin words meaning "little affinity", reflecting their low reactivity. Their substitution reaction with chlorine goes by a free-radical chain mechanism started by UV light breaking Cl–Cl bonds. Because substitution can continue, it gives a mixture of products such as CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄.

Summary

Alkanes are saturated hydrocarbons with only single bonds and general formula CₙH₂ₙ₊₂. The first four are methane, ethane, propane and butane. Each carbon is tetrahedral, so chains zigzag. Alkanes are unreactive apart from combustion, which makes them excellent fuels, and substitution with halogens in UV light.

Practice questions

1. State the general formula of the alkanes. Answer: CₙH₂ₙ₊₂. 2. Write the molecular formula of the alkane with 10 carbon atoms. Answer: C₁₀H₂₂. 3. Write a balanced equation for the complete combustion of ethane. Answer: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. 4. Explain why alkanes are used as fuels but are otherwise described as unreactive. Answer: They release a large amount of energy when they burn, but their C–C and C–H bonds are strong and non-polar, so they react with few other substances.