Welcome to Organic Chemistry!
In this chapter, we are going to explore Organic Chemistry. Don't let the name scare you! "Organic" simply refers to compounds that are based on the element carbon. We will specifically look at crude oil—how we find it, how we separate it, and how we turn it into the fuels and plastics we use every day. Think of crude oil as "buried sunshine" from millions of years ago that we’ve learned to use to power our modern world.
1. Crude Oil, Hydrocarbons, and Alkanes
Crude oil is a finite resource. This means it is "non-renewable"—once we use it all up, it’s gone! It is found in rocks and was formed over millions of years from the remains of ancient biomass (mostly tiny sea creatures called plankton) that were buried in mud.
What is a Hydrocarbon?
Most of the compounds in crude oil are hydrocarbons. As the name suggests, these are molecules made up of hydrogen and carbon atoms ONLY.
Common Mistake: Some students think hydrocarbons can contain oxygen. Remember: Hydro- (Hydrogen) and -Carbon (Carbon). That’s it!
The Alkanes
Most hydrocarbons in crude oil belong to a "family" called alkanes. They are called a homologous series because they all have the same general formula and similar properties.
The general formula for alkanes is: \( C_nH_{2n+2} \)
This formula is like a recipe. If you know the number of carbons (\( n \)), you can find the number of hydrogens by doubling that number and adding 2.
The First Four Alkanes
You need to remember the names and structures of the first four alkanes. Here is a handy mnemonic to help you remember the order:
Monkeys Eat Peeled Bananas
- Methane: \( CH_4 \)
- Ethane: \( C_2H_6 \)
- Propane: \( C_3H_8 \)
- Butane: \( C_4H_{10} \)
Quick Review: Alkanes are "saturated" molecules. This means all the carbon-carbon bonds are single bonds, so the carbon atoms are "full up" and cannot bond with any more atoms.
Key Takeaway: Crude oil is a mixture of hydrocarbons, mostly alkanes, formed from ancient plankton.
2. Fractional Distillation
Crude oil straight from the ground isn't very useful because it’s a mixture of too many different compounds. We need to separate it into groups of similar-sized molecules called fractions. We do this using fractional distillation.
How it works: Step-by-Step
- The crude oil is heated until most of it turns into a gas (vapour).
- The vapour enters a fractionating column, which is very hot at the bottom and gets cooler towards the top.
- The gases rise up the column.
- When a gas reaches a level that is at its boiling point, it condenses (turns back into a liquid) and is collected.
- Small molecules have low boiling points and come out at the top. Large molecules have high boiling points and come out at the bottom.
Why do we do this?
The fractions we get are used as fuels and feedstock.
Fuels: Petrol, diesel, kerosene, heavy fuel oil, and liquefied petroleum gases (LPG).
Feedstock: These are the raw materials used by the chemical industry to make solvents, lubricants, polymers (plastics), and detergents.
Did you know? The reason we can make so many different things from carbon is because carbon atoms can bond together to form long chains and rings!
Key Takeaway: Fractional distillation separates crude oil based on boiling points. Smaller molecules are at the top; larger molecules are at the bottom.
3. Properties of Hydrocarbons
As the size of the hydrocarbon molecule changes, its physical properties change too. This determines what we use the fuel for.
The Three Big Trends
As the molecules get larger (more carbon atoms):
- Boiling Point Increases: They need more heat to turn into a gas.
- Viscosity Increases: They become thicker and flow less easily (like honey vs. water).
- Flammability Decreases: They are harder to ignite (burn).
Combustion (Burning)
When we burn hydrocarbons in plenty of air, it is called complete combustion. The carbon and hydrogen in the fuel are oxidised (they react with oxygen).
The products of complete combustion are always carbon dioxide and water.
The general word equation:
Hydrocarbon + Oxygen \(\rightarrow\) Carbon Dioxide + Water (+ Energy)
Quick Review Box:
Short chains = low boiling point, runny, very flammable (make great fuels!).
Long chains = high boiling point, thick, not very flammable.
Key Takeaway: Small hydrocarbons make the best fuels because they are very flammable and easy to turn into gas.
4. Cracking and Alkenes
Fractional distillation often leaves us with too many "long-chain" hydrocarbons and not enough "short-chain" ones (like petrol). We fix this using a process called cracking.
What is Cracking?
Cracking is the process of breaking down large, less useful hydrocarbons into smaller, more useful ones. It is a thermal decomposition reaction (breaking down using heat).
Methods of Cracking:
- Catalytic Cracking: Heat the hydrocarbons to vaporise them and pass them over a hot catalyst.
- Steam Cracking: Mix the hydrocarbon vapours with steam and heat them to a very high temperature.
The Products of Cracking
Cracking always produces a mixture of:
1. An alkane (used for fuels).
2. Another type of hydrocarbon called an alkene.
What are Alkenes?
Alkenes are more reactive than alkanes. They are used to make polymers (plastics) and are the starting materials for many other chemicals.
How to test for Alkenes
If you want to know if a liquid is an alkene, you use bromine water:
- When orange bromine water is added to an alkane, it stays orange.
- When orange bromine water is added to an alkene, it turns colourless (it decolourises).
Memory Trick: Alkene makes the bromine water empty of colour!
Key Takeaway: Cracking breaks big molecules into smaller ones using heat. It produces alkanes (for fuel) and alkenes (for plastic).
Summary Checklist for Success
- Can you define "hydrocarbon"?
- Do you know the names and formulas of Methane, Ethane, Propane, and Butane?
- Can you explain why the temperature in the fractionating column is different at the top and bottom?
- Do you know the products of complete combustion?
- Can you describe the difference between catalytic cracking and steam cracking?
- Do you remember the colour change for the bromine water test?
Don't worry if this seems like a lot to take in at first! Just remember that organic chemistry is all about patterns. Once you spot the patterns in the carbon chains, the rest starts to fall into place.