Study Notes: Organic Chemistry (International GCSE Double Award)
Hello future chemist! Welcome to the world of Organic Chemistry. Don't worry if the name sounds intimidating—it’s simply the study of compounds that contain carbon. Carbon is a super-versatile element, and understanding how it bonds is the key to understanding everything from fuels to plastics and even the molecules in your own body! You've got this!
1. The Foundations of Organic Chemistry
1.1 What is Organic Chemistry?
Organic chemistry is defined as the study of carbon compounds, primarily those containing carbon and hydrogen (called hydrocarbons), and their derivatives.
- Why Carbon? Carbon atoms are special because they can form four strong covalent bonds. They can link together in long chains, branched structures, and rings, leading to millions of possible compounds.
- Source: Most organic compounds we study come originally from crude oil (petroleum), which is a complex mixture of hydrocarbons.
1.2 Naming Conventions (Nomenclature)
To name organic molecules, we count the number of carbon atoms in the longest chain. You only need to memorize the first four names for the IGCSE curriculum:
| Number of Carbons (n) | Prefix | Example |
| 1 | Meth- | Methane |
| 2 | Eth- | Ethane |
| 3 | Prop- | Propane |
| 4 | But- | Butane |
✨ Memory Aid: Use the mnemonic: Most Excellent Professors Bake (Meth, Eth, Prop, But).
Quick Review: Key Terms
- Hydrocarbon: A compound containing only hydrogen and carbon atoms.
- Homologous Series: A family of organic compounds that have the same general formula, similar chemical properties, and show a gradual change in physical properties (like boiling point) as the chain length increases.
2. Alkanes: The Saturated Hydrocarbons
2.1 Structure and General Formula
Alkanes are the simplest type of hydrocarbon. They are known as saturated hydrocarbons because they contain only single covalent bonds between carbon atoms.
General Formula: \(C_n H_{2n+2}\)
- If n=1 (Methane), the formula is \(C_1 H_{2(1)+2}\) = \(CH_4\).
- If n=2 (Ethane), the formula is \(C_2 H_{2(2)+2}\) = \(C_2 H_6\).
Analogy: Think of a saturated sponge. It is completely full of water and cannot absorb any more. Similarly, a saturated alkane is "full" of hydrogen atoms and cannot hold any more without breaking a C-C bond.
2.2 Physical Properties
As the chain length increases (n increases):
- The boiling point increases (more energy needed to overcome the stronger intermolecular forces).
- The compounds become less volatile (less easily turned into a gas).
- Viscosity (thickness) increases.
Example: Methane and ethane are gases, while octane (C8H18) is a liquid found in petrol.
2.3 Reactions of Alkanes
A. Combustion (Burning)
Alkanes are excellent fuels. They react with oxygen to release heat (exothermic reaction).
1. Complete Combustion (Plenty of oxygen):
\(Alkane + Oxygen \rightarrow Carbon\ dioxide + Water\)
Example (Methane): \(CH_4 (g) + 2O_2 (g) \rightarrow CO_2 (g) + 2H_2O (l)\)
2. Incomplete Combustion (Limited oxygen):
This happens when oxygen supply is restricted, producing dangerous products:
\(Alkane + Limited\ oxygen \rightarrow Carbon\ monoxide + Carbon + Water\)
- Carbon monoxide (CO) is a toxic, colourless, odourless gas that prevents red blood cells from carrying oxygen.
- Carbon (C), or soot, is produced, causing dark smoke.
B. Substitution Reaction
Since alkanes are saturated, they can only react by substitution—where one atom replaces another.
- This reaction typically happens between an alkane and a halogen (like chlorine, \(\text{Cl}_2\), or bromine, \(\text{Br}_2\)).
- It requires ultraviolet (UV) light or high temperature to provide the energy needed to start the reaction.
Step-by-Step Example (Methane and Chlorine):
1. UV light splits the chlorine molecule into highly reactive free radicals.
2. A chlorine atom replaces one hydrogen atom on the methane molecule.
3. This creates a halogenoalkane and hydrogen chloride gas.
\(CH_4 + Cl_2 \xrightarrow{UV\ light} CH_3Cl + HCl\)
Key Takeaway for Alkanes: Alkanes are saturated (single bonds only), have the formula \(C_n H_{2n+2}\), and undergo combustion and substitution reactions.
3. Alkenes: The Unsaturated Hydrocarbons
3.1 Structure and General Formula
Alkenes are unsaturated hydrocarbons because they contain at least one carbon-carbon double covalent bond (\(C=C\)). This double bond is the functional group for the alkene series.
General Formula: \(C_n H_{2n}\)
- Since you need at least two carbon atoms to form a double bond, the smallest alkene is Ethene (n=2).
- If n=2 (Ethene), the formula is \(C_2 H_{2(2)}\) = \(C_2 H_4\).
Don't worry if this seems tricky at first: The presence of the double bond means the molecule is 'unsaturated.' Unlike alkanes, alkenes have 'space' to open up that double bond and add more atoms directly—making them much more reactive!
3.2 Isomerism (Structural Isomers)
Isomers are molecules that have the same molecular formula but different structural formulas.
- Example: Butane (\(C_4 H_{10}\)) and Methylpropane (\(C_4 H_{10}\)). Both have the same number of C and H atoms, but one is a straight chain and the other is branched.
- This concept shows how important structure is in chemistry!
3.3 Reactions of Alkenes: Addition Reactions
Because of the double bond, alkenes typically undergo addition reactions, where atoms are added across the double bond, turning it into a single bond.
A. Addition with Hydrogen (Hydrogenation)
Hydrogen gas is added to an alkene to form an alkane. This requires a nickel (Ni) catalyst and a temperature of about 150°C.
\(Alkene + H_2 \xrightarrow{Ni,\ heat} Alkane\)
Real-world connection: This process is used industrially to convert unsaturated vegetable oils (liquids) into saturated fats (solids, like margarine).
B. Addition with Halogens (Bromine/Chlorine)
The halogen adds across the double bond to form a dihalogenoalkane.
The crucial test for unsaturation is the reaction with Bromine water:
1. Bromine water is naturally orange-brown.
2. When added to an alkene, the double bond opens up, and the bromine adds across it. The colour disappears (decolourises).
3. When added to an alkane, nothing happens (no reaction without UV light), and the colour remains orange-brown.
This is a standard practical test you must know!
Reaction (Ethene and Bromine): \(C_2 H_4 + Br_2 \rightarrow C_2 H_4 Br_2\)
C. Addition with Steam (Hydration)
Steam (\(H_2 O\)) can be added to ethene to make ethanol (an alcohol).
- Conditions: High temperature (around 300°C), high pressure (around 60 atm), and a phosphoric(V) acid catalyst.
- \(\text{Ethene} + \text{Steam} \rightleftharpoons \text{Ethanol}\)
- \(C_2 H_4 (g) + H_2 O (g) \rightleftharpoons C_2 H_5 OH (g)\)
Quick Review: Alkanes vs. Alkenes
| Alkanes | Alkenes | |
| Saturation | Saturated (single C-C bonds) | Unsaturated (C=C double bond) |
| General Formula | \(C_n H_{2n+2}\) | \(C_n H_{2n}\) |
| Main Reaction Type | Substitution (requires UV light) | Addition (happens easily) |
| Bromine Test | No reaction; colour stays orange | Decolourises instantly |
4. Cracking
4.1 The Need for Cracking
Crude oil contains a large amount of long-chain hydrocarbons (heavy fractions) but the demand is highest for short-chain hydrocarbons (light fractions, like petrol/gasoline).
- Cracking is the process of breaking down large, complex hydrocarbon molecules into smaller, more useful molecules.
4.2 The Cracking Process
Cracking is a form of thermal decomposition (breaking down using heat).
Conditions: High temperature (around 600–700°C) and often a catalyst (like porcelain or aluminium oxide/silicon dioxide).
What is produced? Cracking always produces a mixture of a shorter alkane and at least one alkene.
Example: Breaking a long alkane (\(C_{10} H_{22}\)) might produce an octane (\(C_8 H_{18}\)) and ethene (\(C_2 H_4\)).
\(C_{10} H_{22} \rightarrow C_8 H_{18} + C_2 H_4\)
Why are alkenes important products of cracking? They are highly reactive and are essential raw materials for making plastics (polymers).
Key Takeaway for Cracking: Cracking turns less useful large hydrocarbons into highly useful smaller alkanes (fuel) and alkenes (plastics) using heat and/or a catalyst.
5. Polymers and Polymerisation
5.1 Defining Polymers
A polymer is a very long molecule made up of many small, identical repeating units called monomers.
- Analogy: If a monomer is a single LEGO brick, the polymer is the giant structure you build with thousands of bricks.
- Most common polymers are plastics.
5.2 Addition Polymerisation
This type of polymerisation only happens with alkenes (monomers must have a C=C double bond).
During the reaction, the double bond in the alkene monomer breaks open, allowing the molecules to link together in a long chain.
The Polymerisation of Ethene:
The monomer ethene (\(C_2 H_4\)) links together to form the polymer poly(ethene), commonly known as polythene or polyethylene.
Step 1: Monomer: Ethene (\(H_2 C=CH_2\))
Step 2: Reaction: High pressure, moderate temperature, and a catalyst cause the double bonds to break.
Step 3: Polymer: The chains link together. We represent the long chain using an 'n' subscript to show many repeating units.
$$n \left( \begin{array}{c} H \\ | \\ C=C \\ | \\ H \end{array} \right) \rightarrow \left( \begin{array}{c} H \\ | \\ -C-C- \\ | \\ H \end{array} \right)_n $$
Naming Rule: The polymer name is always "poly" followed by the name of the monomer (in brackets).
- Monomer: Propene \(\rightarrow\) Polymer: Poly(propene)
- Monomer: Chloroethene (Vinyl chloride) \(\rightarrow\) Polymer: Poly(chloroethene) or PVC
5.3 Disposal of Polymers
Plastics are very useful because they are chemically unreactive and do not biodegrade easily.
- Problem: This lack of reactivity leads to massive landfill problems as plastics persist in the environment for hundreds of years.
- Solution: The primary methods of dealing with plastic waste are recycling (re-melting and reshaping) and burning (incineration) to generate energy.
- Common Mistake: Burning plastics can release toxic gases (especially from PVC), so incineration must be done under strict controlled conditions.
Key Takeaway for Polymers: Monomers with double bonds link together via addition polymerisation to form long-chain polymers like poly(ethene). Disposal is difficult because they are chemically inert and non-biodegradable.