C11 Organic Chemistry: The Chemistry of Carbon
Hello future scientists! Welcome to Organic Chemistry. This might sound intimidating, but it is simply the study of compounds that contain carbon, often called the "chemistry of life." Understanding these basic building blocks is crucial, as they form everything from fuels to plastics, and even the molecules inside your own body!
Don't worry if this seems tricky at first; we will break down the concepts into manageable, easy-to-understand chunks.
C11.1 Key Terminology: Families and Bonds
What is Organic Chemistry?
Organic compounds are generally derived from living things (although we can make them synthetically now). They all contain carbon atoms, usually bonded to hydrogen atoms.
1. Saturated vs. Unsaturated Compounds (Core)
This terminology describes the type of bonds between the carbon atoms.
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Saturated Compound: A molecule where all the carbon-carbon bonds are single bonds (C–C).
Analogy: Think of a saturated sponge. It's full and cannot absorb any more water. Similarly, a saturated hydrocarbon cannot hold any more hydrogen atoms.
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Unsaturated Compound: A molecule that has one or more carbon-carbon bonds that are not single bonds. This means they contain at least one double bond (C=C) or triple bond.
Analogy: An unsaturated sponge still has space and can absorb more water. These compounds can "open up" their double bonds to add more atoms.
Quick Review: Saturation
Saturated = Single bonds only (Alkanes)
Unsaturated = Double or Triple bonds (Alkenes)
2. Homologous Series (Supplement)
A homologous series is a family of similar compounds that share common characteristics.
General Characteristics of a Homologous Series:
- They have similar chemical properties (they react in similar ways).
- They can be represented by the same general formula (you do not need to recall specific general formulas for this syllabus, but know the concept).
- They show a trend in physical properties (e.g., boiling points get higher as the molecules get bigger).
Did you know? The simplest homologous series are the Alkanes and the Alkenes, which we cover later in this chapter.
Key Takeaway: We classify organic molecules by the bonds they have (saturated/unsaturated) and the family they belong to (homologous series).
C11.2 Organic Fuels and Petroleum
1. Fossil Fuels and Hydrocarbons (Core)
Fossil fuels are non-renewable energy sources formed from ancient biological material. The main fossil fuels are:
- Coal
- Natural gas (The main constituent of natural gas is methane, \(CH_4\))
- Petroleum (Crude oil)
Many of these fuels are made of hydrocarbons: compounds that contain hydrogen and carbon only.
2. Petroleum and Fractional Distillation (Core & Supplement)
Petroleum (crude oil) is a mixture of many different hydrocarbons, all jumbled together. To make them useful, we must separate them using a process called fractional distillation.
The Process of Fractional Distillation (Step-by-Step)
Fractional distillation separates the different hydrocarbons based on their boiling points.
- Heating: Crude oil is heated until most of it vaporizes into gas.
- Rising Vapours: The hot hydrocarbon vapours rise up a tall tower called a fractionating column.
- Condensation: The column is hottest at the bottom and coolest at the top. As the vapours rise, they cool down.
- Collecting Fractions: When a specific hydrocarbon mixture reaches the temperature corresponding to its boiling point, it condenses back into a liquid and is collected as a fraction.
Trends in the Fractionating Column (Supplement)
The properties of the fractions change predictably from the bottom to the top of the column:
- At the Bottom (Hottest part):
- Longer chain length (More C atoms)
- Higher boiling points (Requires more energy to boil)
- More viscous (thicker)
- At the Top (Coolest part):
- Decreasing chain length (Fewer C atoms)
- Lower boiling points (Easy to vaporize)
- Less viscous (runnier, like gas)
Memory Aid: Light things float to the top! The short, light molecules condense at the top. Heavy, long molecules stay near the bottom.
Uses of the Main Fractions (Core)
| Fraction Name | Use |
| Refinery Gas | Gas for heating and cooking |
| Gasoline / Petrol | Fuel used in cars |
| Naphtha | Chemical feedstock (raw material for making plastics and other chemicals) |
| Diesel Oil / Gas Oil | Fuel used in diesel engines |
| Bitumen | Used for making roads and roofing |
Key Takeaway: Petroleum is a mixture separated by boiling point differences in a fractionating column. Longer chains have higher boiling points and are found lower down the column.
C11.3 Alkanes (Saturated Hydrocarbons)
1. Structure and Bonding (Core)
- Alkanes are the simplest family of hydrocarbons.
- The bonding in alkanes is single covalent.
- Alkanes are saturated hydrocarbons because they contain only single C-C bonds.
2. Properties and Reactions (Core)
Alkanes are generally unreactive because their single bonds are strong and stable.
However, they readily undergo combustion (burning) in air or oxygen, making them excellent fuels:
- Complete combustion (plenty of oxygen) produces carbon dioxide and water.
- Incomplete combustion (limited oxygen) produces carbon monoxide (a toxic gas) and/or carbon (soot).
Key Takeaway: Alkanes are stable, saturated, single-bonded hydrocarbons used primarily as fuels.
C11.4 Alkenes (Unsaturated Hydrocarbons)
1. Structure and Bonding (Core)
- The bonding in alkenes includes at least one double carbon-carbon covalent bond (C=C).
- Alkenes are unsaturated hydrocarbons because of the presence of the double bond.
2. Test for Unsaturation (Core)
Because the double bond is reactive, we can use it to distinguish between saturated (alkanes) and unsaturated (alkenes) hydrocarbons.
The Test: Shake the hydrocarbon with aqueous bromine (which is an orange-brown colour).
- Result for Alkanes (Saturated): The bromine water remains orange-brown. (No reaction).
- Result for Alkenes (Unsaturated): The orange-brown bromine water decolorizes rapidly (turns colourless). (Reaction occurs - the bromine atoms add across the double bond).
Analogy: The double bond is like a temporary gap in the chain. The bromine atoms rush in and break the double bond, linking onto the carbons instead.
3. Manufacture by Cracking (Supplement)
Cracking is an important industrial process used to convert large, less useful alkane molecules (like those found in heavy fractions of crude oil) into smaller, more valuable molecules.
Cracking involves using a high temperature and a catalyst to break down larger alkane molecules into:
- Smaller alkanes
- Smaller alkenes (which are highly useful)
- Hydrogen (also a useful product)
4. Properties: Addition Reactions (Supplement)
The key feature of alkenes is their high reactivity due to the C=C double bond. Alkenes undergo addition reactions, where the double bond breaks and other atoms are added across the two carbon atoms.
Alkenes react by addition with:
- Bromine: (This is the test above). The product formed is called a dibromoalkane.
- Hydrogen: The alkene is converted into a saturated alkane. This reaction requires a nickel catalyst and heat (Hydrogenation).
- Steam: The alkene is converted into an alcohol. This requires an acid catalyst (often phosphoric acid) and heat (Hydration).
Key Takeaway: Alkenes have a C=C double bond, making them unsaturated and reactive. They can be manufactured by cracking and react via addition (like decolorizing bromine water).
C11.5 Polymers
1. Definitions (Core)
Organic chemistry is responsible for plastics, which are examples of polymers.
- Monomers: Small, simple molecules.
- Polymers: Very large molecules (macromolecules) built up when many small, simple molecules (monomers) join together in a chain.
Analogy: Monomers are like individual paperclips. A polymer is the long chain formed by linking thousands of paperclips together.
2. Addition Polymerisation (Core)
In addition polymerisation, the monomers join together end-to-end without losing any atoms. This process relies on unsaturated monomers (like alkenes).
The formation of poly(ethene) (commonly known as polythene or polyethylene) is the most common example:
- The monomer used is ethene (an alkene).
- The double bond in the ethene monomer breaks, allowing the monomers to link up to form the long chain polymer, poly(ethene).
Key Takeaway: Polymers are massive chains made from smaller units called monomers. Poly(ethene) is a common plastic formed by linking ethene monomers via addition polymerisation.