Alkanes and alkenes

Introduction to Alkanes and Alkenes

Welcome to one of the most important chapters in organic chemistry! In this section, we are going to explore two families of hydrocarbons: the alkanes and the alkenes. A hydrocarbon is simply a compound made of only carbon and hydrogen atoms.

Understanding these molecules is the key to understanding how we get energy from fuels and how we create modern materials like plastics. Don't worry if the names sound a bit like a foreign language at first—once you spot the patterns, you'll find they are actually very logical!

The Alkanes: Saturated Hydrocarbons

Alkanes are the simplest family of hydrocarbons. They are often used as fuels (like the methane in your kitchen stove or the butane in a camping heater).

1. What does "Saturated" mean?

Alkanes are called saturated hydrocarbons. This means that all the bonds between the carbon atoms are single covalent bonds \( (C-C) \). Because they only have single bonds, the carbon atoms are "full"—they are bonded to the maximum possible number of hydrogen atoms.

2. The First Four Alkanes

You need to know the names and formulas for the first four alkanes. A great way to remember the order is the mnemonic: Monkeys Eat Peeled Bananas.

• Methane: \(CH_4\)
• Ethane: \(C_2H_6\)
• Propane: \(C_3H_8\)
• Butane: \(C_4H_{10}\)

3. The General Formula

Notice the pattern? For every \(n\) carbon atoms, there are \(2n + 2\) hydrogen atoms. The general formula for alkanes is \(C_nH_{2n+2}\).

Quick Takeaway: Alkanes have single bonds only and are saturated.

The Alkenes: Unsaturated Hydrocarbons

Alkenes are similar to alkanes, but they have a "hidden power"—a double bond between two carbon atoms \( (C=C) \).

1. What does "Unsaturated" mean?

Alkenes are unsaturated. This is because the \(C=C\) double bond could potentially break open to bond with more atoms. They are not "full" of hydrogen yet.

2. The First Three Alkenes

Note: There is no such thing as "methene" because you need at least two carbons to form a double bond!

• Ethene: \(C_2H_4\)
• Propene: \(C_3H_6\)
• Butene: \(C_4H_8\)

3. The General Formula

The general formula for alkenes is simpler: \(C_nH_{2n}\). They have exactly twice as many hydrogens as carbons.

Quick Takeaway: Alkenes have at least one double bond \( (C=C) \) and are unsaturated.

Distinguishing Alkanes and Alkenes

If you were given two clear liquids, one an alkane and one an alkene, they would look identical. How do we tell them apart? We use bromine water.

The Bromine Water Test

Bromine water is an orange-brown solution. When you add it to a hydrocarbon and shake it, here is what happens:

• Alkanes: No reaction happens. The solution stays orange.
• Alkenes: The double bond reacts with the bromine. This is an addition reaction. The bromine atoms add across the double bond, and the solution turns from orange to colourless (it is "decolourised").

Addition Reaction Example

When ethene reacts with bromine, it forms 1,2-dibromoethane:

\(C_2H_4 + Br_2 \rightarrow C_2H_4Br_2\)

Common Mistake: Don't say the bromine water turns "clear." "Clear" just means you can see through it (like glass). You must use the word colourless (like water).

Complete Combustion

Both alkanes and alkenes can be burned as fuels. When there is plenty of oxygen available, this is called complete combustion.

The Rule for Combustion

During complete combustion, the carbon and hydrogen atoms in the fuel react with oxygen to produce only two things: carbon dioxide and water.

The word equation:
\(Hydrocarbon + Oxygen \rightarrow Carbon\ Dioxide + Water\)

Example: Complete Combustion of Methane

\(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)

Example: Complete Combustion of Ethene

\(C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O\)

Note: If there isn't enough oxygen, incomplete combustion occurs, producing toxic carbon monoxide or soot (carbon). You can review more about this in Topic 8: Fuels.

Summary Table for Revision

Use this table to quickly check your knowledge before the exam:

What's next? Now that you understand the basics of these hydrocarbons, you are ready to see how alkenes can be joined together to make long chains in the Polymers chapter, or how they relate to Alcohols and Carboxylic Acids.