Naming Alkanes

Introduction

With millions of organic compounds, chemists need a naming system in which every name describes exactly one structure and every structure has one agreed name. The system used worldwide is set by IUPAC . The good news is that it is built from a few simple parts. For straight-chain alkanes you need just two: a stem that counts the carbons and the ending -ane . Learn ten stems and you can name — and draw — ten alkanes instantly.

Core explanation

Two parts to every name. An organic name is built from pieces that each carry information:

- The stem tells you how many carbon atoms are in the longest chain. - The suffix tells you which homologous series the compound belongs to. For alkanes the suffix is -ane .

The stems. These are used for every homologous series, not just alkanes:

Carbons Stem Alkane Formula --- --- --- --- 1 meth- methane CH₄ 2 eth- ethane C₂H₆ 3 prop- propane C₃H₈ 4 but- butane C₄H₁₀ 5 pent- pentane C₅H₁₂ 6 hex- hexane C₆H₁₄ 7 hept- heptane C₇H₁₆ 8 oct- octane C₈H₁₈ 9 non- nonane C₉H₂₀ 10 dec- decane C₁₀H₂₂

Where the stems come from. The first four stems are historical: methane is linked to methanol ("wood spirit"), and butane to butyric acid, found in rancid butter. From five onwards the stems come from Greek or Latin number words, which you may know from shapes and words such as pentagon, hexagon, octopus and decade.

Using the same stems elsewhere. The stem stays the same when the series changes. Ethane, ethene and ethanol all have two carbons; only the ending changes to show the family. Learning the stems once therefore pays off across the whole of organic chemistry.

Name to formula. Read the stem to find n, then apply CₙH₂ₙ₊₂. Hexane: hex- = 6, so C₆H₁₄.

Formula to name. Count the carbons, pick the stem and add -ane. C₉H₂₀ has nine carbons, so it is nonane. Always check the hydrogen count fits the alkane formula first — C₉H₁₈ would not be an alkane.

Branched alkanes need extra parts in their names, such as "methyl" and position numbers; these build on the same stems and are covered later in this unit.

Step-by-step reasoning

To name a straight-chain hydrocarbon from its formula:

1. Check it fits CₙH₂ₙ₊₂; if so, it is an alkane. 2. Count the carbon atoms, n. 3. Choose the stem for that number. 4. Add the suffix -ane to the stem.

Visual explanation

Draw a name as two coloured blocks: a blue block for the stem ("prop") and a red block for the ending ("ane"). Beneath the blue block draw three carbon atoms in a chain; beneath the red block write "single bonds only". The name maps directly onto the structure.

Real-world analogy

Organic names work like postal addresses. The house number and street (the stem) say exactly where you are along the chain of houses, and the town (the suffix) says which family of streets you are in. Change either part and you reach a different place.

Real-world example

Petrol quality is described by an "octane rating". The name comes from octane, C₈H₁₈ — in fact a branched isomer of it — which burns smoothly in engines and is used as the reference fuel. Heptane, C₇H₁₆, which causes engine "knocking", is the other reference fuel.

Why?

Why do we need systematic names rather than common names? Common names such as "marsh gas" give no clue to structure and vary between languages. A systematic name such as methane tells any chemist, anywhere, exactly which molecule is meant.

Common misconception

"Butane has four hydrogen atoms because but- means four." The stem counts carbon atoms only. Butane has four carbons and ten hydrogens, C₄H₁₀.

Worked example

Question: Name the alkane C₇H₁₆ and give the formula of pentane.

Reasoning: C₇H₁₆: n = 7 and 2 × 7 + 2 = 16, so it is an alkane; seven carbons gives hept-, so heptane. Pentane: pent- = 5, hydrogens = 2 × 5 + 2 = 12.

Answer: Heptane; C₅H₁₂.

Quick check

1. What does the stem prop- tell you about a molecule? Answer: Its main chain contains three carbon atoms.

Exam focus

The first four stems — meth-, eth-, prop-, but- — are essential and cannot be worked out from number words, so memorise them. Spell names carefully: "methene" or "butaine" will not score. Use a mnemonic such as "Monkeys Eat Peeled Bananas" for the first four.

Advanced insight

IUPAC names extend well beyond ten: C₁₁H₂₄ is undecane, C₁₂H₂₆ dodecane and C₂₀H₄₂ icosane. The same rules also handle rings (cyclohexane), branches (2-methylpropane) and functional groups, all by adding prefixes, numbers and suffixes around the same stem.

Summary

Alkane names have a stem showing the number of carbon atoms and the ending -ane. The stems for one to ten carbons are meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non- and dec-. From a name, find n and use CₙH₂ₙ₊₂; from a formula, count the carbons and pick the stem. The same stems are used for every homologous series.

Practice questions

1. Name the alkane with the formula C₄H₁₀. Answer: Butane. 2. Give the molecular formula of octane. Answer: C₈H₁₈. 3. Name the alkane C₁₀H₂₂ and state what the suffix of its name tells you. Answer: Decane; the -ane ending shows it is an alkane, with single carbon–carbon bonds only. 4. A student names C₆H₁₂ as hexane. Explain the mistake. Answer: C₆H₁₂ does not fit CₙH₂ₙ₊₂ (hexane is C₆H₁₄), so it is not an alkane; it has two fewer hydrogens than hexane.