What Is a Hydrocarbon?

Introduction

The gas in a kitchen hob, the petrol in a car, the wax in a candle and the plastic bag that carried your shopping all have something in common: they are made from, or are themselves, hydrocarbons . These are the simplest family of organic compounds, and they are the starting point for almost everything else in organic chemistry. Understanding what a hydrocarbon is — and what it is not — gives you the vocabulary for the rest of this unit.

Core explanation

Definition. A hydrocarbon is a compound that contains carbon and hydrogen atoms only . The word itself tells you this: hydro- for hydrogen and -carbon for carbon. If a molecule contains even one atom of any other element — oxygen, nitrogen, chlorine and so on — it is not a hydrocarbon.

Checking a formula. The quickest test is to read the molecular formula:

Formula Name Hydrocarbon? --- --- --- CH₄ methane yes C₃H₈ propane yes C₂H₄ ethene yes C₆H₆ benzene yes C₂H₅OH ethanol no — contains oxygen CH₃Cl chloromethane no — contains chlorine CO₂ carbon dioxide no — no hydrogen, and it contains oxygen

The carbon skeleton. In every hydrocarbon the carbon atoms join to one another to form a backbone — a straight chain, a branched chain or a ring. This is catenation at work. Each carbon atom forms four covalent bonds in total. Any bonds not used to link carbon atoms together are used to bond to hydrogen atoms, and each hydrogen atom forms exactly one bond. The bonds are covalent because carbon and hydrogen share electrons rather than transferring them.

Why so many? Because carbon chains can be any length, can branch, can close into rings and can contain single, double or triple carbon–carbon bonds, there are millions of possible hydrocarbons. Chemists sort them into families. The two you will meet first are the alkanes , which contain only single bonds, and the alkenes , which contain at least one carbon–carbon double bond.

Where hydrocarbons come from. Most hydrocarbons we use come from fossil fuels . Natural gas is mainly methane. Crude oil is a complex mixture of hundreds of different hydrocarbons. Some hydrocarbons are also made by living things: plants release small amounts of isoprene, and cattle and wetlands release methane.

What they are used for. Hydrocarbons are used overwhelmingly as fuels , because they release a lot of energy when they burn. The rest are used as feedstock — raw materials for making plastics, solvents, medicines and many other products.

Step-by-step reasoning

To decide whether a substance is a hydrocarbon:

1. Write out or look up its molecular formula. 2. List every element symbol in the formula. 3. Check that carbon is present and hydrogen is present. 4. Check that no other symbol appears. If only C and H appear, it is a hydrocarbon.

Visual explanation

Imagine a ball-and-stick model of propane, C₃H₈. Three black balls (carbon) sit in a zigzag row joined by sticks. Eight smaller white balls (hydrogen) stick out around them so that every black ball has four sticks. There are no balls of any other colour — that is what makes it a hydrocarbon.

Real-world analogy

Think of a building set that contains only two kinds of piece: large connector blocks with four studs (carbon) and small caps with one stud (hydrogen). You can build an enormous variety of shapes — long rods, branched trees, rings — but every model uses only those two kinds of piece.

Real-world example

Liquefied petroleum gas (LPG), used for camping stoves and some vehicles, is a mixture of propane and butane. Both are hydrocarbons that are gases at room temperature but turn into liquids when squeezed under moderate pressure, so a lot of fuel can be stored in a small cylinder.

Why?

Why do hydrocarbons make such good fuels? When they burn, the carbon and hydrogen atoms end up in carbon dioxide and water. The bonds formed in these products are much stronger overall than the bonds broken in the fuel and oxygen, so a large amount of energy is released as heat.

Common misconception

"Any compound containing carbon and hydrogen is a hydrocarbon." Ethanol, C₂H₅OH, and glucose, C₆H₁₂O₆, contain carbon and hydrogen, but they also contain oxygen, so they are not hydrocarbons. The key word is only .

Worked example

Question: Which of these are hydrocarbons: C₄H₁₀, CH₃COOH, C₈H₁₈, C₂H₅Br?

Reasoning: C₄H₁₀ contains only C and H. CH₃COOH contains oxygen. C₈H₁₈ contains only C and H. C₂H₅Br contains bromine.

Answer: C₄H₁₀ (butane) and C₈H₁₈ (octane) are hydrocarbons; the other two are not.

Quick check

1. Is carbon dioxide, CO₂, a hydrocarbon? Explain. Answer: No. It contains no hydrogen, and it contains oxygen as well as carbon.

Exam focus

When defining a hydrocarbon, always include the word only : "a compound containing hydrogen and carbon only". Examiners do not give the mark for "a compound containing hydrogen and carbon", because that also describes ethanol and sugars.

Advanced insight

Hydrocarbons are non-polar or only very slightly polar, because carbon and hydrogen have similar electronegativities. This explains why they do not dissolve in water, why oil and water separate into layers, and why hydrocarbon solvents such as hexane are good at dissolving grease and other non-polar substances.

Summary

A hydrocarbon is a compound of carbon and hydrogen only. Carbon atoms form a skeleton of chains or rings, each carbon making four bonds and each hydrogen one. Most hydrocarbons come from crude oil and natural gas. They are used mainly as fuels and as feedstock for plastics and other chemicals. Any extra element in the formula means the compound is not a hydrocarbon.

Practice questions

1. Define the term hydrocarbon. Answer: A compound that contains carbon and hydrogen atoms only. 2. Explain why glucose, C₆H₁₂O₆, is not a hydrocarbon. Answer: It contains oxygen atoms as well as carbon and hydrogen, so it is not made of carbon and hydrogen only. 3. How many bonds does each carbon atom form in a hydrocarbon, and how many does each hydrogen atom form? Answer: Each carbon atom forms four covalent bonds; each hydrogen atom forms one. 4. Name the two main fossil-fuel sources of hydrocarbons and give the main use of hydrocarbons. Answer: Crude oil and natural gas; hydrocarbons are used mainly as fuels.