Organic chemistry

Welcome to Organic Chemistry!

Welcome to one of the most exciting parts of your GCSE Chemistry course! Organic chemistry is simply the study of carbon compounds. Carbon is a bit of a "superstar" element because it can form four strong bonds with other atoms, allowing it to build the complex chains and rings that make up everything from the fuel in your car to the DNA in your body.
Don't worry if this seems like a lot of new names and shapes at first—once you spot the patterns, it becomes much easier! Since this is part of your "Global Challenges" section, we will also look at how we use these chemicals as resources and how we can manage them sustainably.

1. The "Families" of Organic Chemistry

In organic chemistry, we group chemicals into "families" called homologous series. Members of the same family react in very similar ways because they have the same functional group (a specific atom or group of atoms that decides how the molecule behaves).

The "Naming" Rule

To name these molecules, we look at how many carbon atoms are in the main chain. Use this mnemonic to remember the first four:

• Meth- = 1 Carbon (Monkeys)
• Eth- = 2 Carbons (Eat)
• Prop- = 3 Carbons (Peeled)
• But- = 4 Carbons (Bananas)

The Four Main Families

1. Alkanes: The simplest hydrocarbons (only carbon and hydrogen). They have only single bonds. They are "saturated."
2. Alkenes: Hydrocarbons with at least one C=C double bond. They are "unsaturated."
3. Alcohols: These contain the -OH functional group.
4. Carboxylic Acids: These contain the -COOH functional group.

Quick Review: Think of a homologous series like a family of cars. They all have the same engine type (functional group) and work the same way, but some are longer than others (different number of carbons).

2. Hydrocarbons: Alkanes and Alkenes

Alkanes are mainly used as fuels. Their general formula is \(C_nH_{2n+2}\). For example, if an alkane has 3 carbons (Propane), it must have \((2 \times 3) + 2 = 8\) hydrogens: \(C_3H_8\).

Alkenes are more reactive because of their double bond. Their general formula is \(C_nH_{2n}\). They are used to make plastics.

Testing for Alkenes

How do we tell them apart in a lab? We use bromine water (which is orange):

• Add bromine water to an Alkane \(\rightarrow\) It stays orange.
• Add bromine water to an Alkene \(\rightarrow\) It turns colorless.

Common Reactions

• Combustion: Burning hydrocarbons in oxygen produces carbon dioxide and water.
• Addition: Alkenes can "open" their double bond to add other atoms, like hydrogen (to make an alkane) or bromine.

Key Takeaway: Alkanes are "full" (saturated), while Alkenes have a double bond that can "open up" to react (unsaturated).

3. Alcohols and Carboxylic Acids

Alcohols (like Ethanol) are used as solvents and fuels. They can be made by fermenting sugars or reacting alkenes with steam.

Carboxylic Acids (like Ethanoic acid, which is vinegar) are weak acids.
Important Process: You can turn an alcohol into a carboxylic acid by oxidation. In the lab, we often use potassium manganate(VII) as the oxidising agent for this.

Did you know? When a bottle of wine is left open, it turns into vinegar because the ethanol reacts with oxygen in the air to become ethanoic acid!

4. Crude Oil and Fractional Distillation

Crude oil is a thick, black liquid found underground. It is a finite resource, meaning once we use it all, it’s gone. It is a mixture of many different hydrocarbons.

Separating the Mixture

We use fractional distillation to separate crude oil into useful "fractions" (groups of molecules with similar sizes).

1. The oil is heated until it turns into gas (vapour).
2. The vapour enters a fractionating column which is hot at the bottom and cool at the top.
3. Long-chain molecules have high boiling points because they have stronger intermolecular forces. They condense back into liquid near the bottom.
4. Short-chain molecules have low boiling points and weak intermolecular forces. They rise up and condense near the top.

Cracking: Making it Useful

The world needs more short-chain hydrocarbons (like petrol) than long-chain ones (like bitumen for roads). Cracking is a process that breaks long alkanes into a smaller, more useful alkane and an alkene.

Quick Review: Fractional distillation separates molecules by size. Cracking breaks big ones into small ones.

5. Polymers (Plastics)

Polymers are giant molecules made by joining thousands of small molecules called monomers together.

Addition Polymerisation

This happens with alkenes. The double bond opens up, and the molecules join together like a long chain of people holding hands.
Example: Ethene monomers join to make poly(ethene).

Condensation Polymerisation

This is slightly different. It usually involves two different monomers, each with two functional groups. As they join, a small molecule (usually water) is "spit out" or lost. This creates materials like polyesters and polyamides (nylon).

Natural Polymers

Nature was making polymers long before humans! You need to know these three:

• DNA: A polymer made from four different monomers called nucleotides.
• Proteins: Polymers made from amino acid monomers.
• Starch/Cellulose: Polymers made from sugar monomers.

Key Takeaway: Polymers are long chains. Addition polymerisation keeps all atoms; condensation polymerisation loses a small molecule like water.

6. Chemical Cells and Fuel Cells

As part of our global challenge to find "cleaner" energy, we use chemical reactions to produce electricity.

• Chemical Cells: Use chemical reactions to create a voltage (potential difference). They stop working once the reactants are used up.
• Hydrogen Fuel Cells: These react hydrogen with oxygen to produce electricity and water.

Hydrogen Fuel Cells: Pros and Cons

Advantages:

• The only waste product is water (no CO2!).
• They can be refuelled quickly compared to charging a battery.

Disadvantages:

• Hydrogen is a gas and is very difficult and dangerous to store.
• Making the hydrogen often requires energy from fossil fuels, which produces CO2 elsewhere.

Common Mistake to Avoid: Don't say fuel cells are "renewable" energy sources. They are efficient and clean at the point of use, but the hydrogen must be manufactured first!

Quick Summary of the Chapter:
1. Carbon forms 4 bonds, leading to diverse molecules.
2. Functional groups determine the family (homologous series).
3. Crude oil provides the raw materials (feedstock) for fuels and plastics.
4. Polymers are long chains made from monomers (Addition vs. Condensation).
5. Future energy challenges involve using hydrogen fuel cells for cleaner transport.