Welcome to Organic Chemistry!
Welcome to one of the most exciting areas of chemistry! Organic chemistry is simply the study of carbon compounds. Carbon is a special element because its atoms can join together in long chains and rings, forming millions of different substances. From the petrol that powers cars to the plastics in our phones, organic chemistry is all around us.
Don't worry if this seems tricky at first! We will break everything down into bite-sized, easy-to-understand steps with plenty of helpful memory tricks.
1. What is a Hydrocarbon?
Let's start with a foundational definition that comes up all the time in exams.
A hydrocarbon is a compound made up of hydrogen and carbon ONLY.
Common Mistake to Avoid: Never forget the word "only"! If a molecule contains hydrogen, carbon, and oxygen, it is not a hydrocarbon.
2. Crude Oil and Fractional Distillation
Crude oil is a thick, dark, sticky liquid found underground in rocks. It is a finite resource (meaning it will eventually run out) and is formed over millions of years from the remains of ancient sea creatures under high heat and pressure.
Crude oil itself isn't very useful straight out of the ground because it is a mixture of many different hydrocarbons of different chain lengths. To make it useful, we must separate it using a process called fractional distillation.
How Fractional Distillation Works:
Fractional distillation separates crude oil based on the different boiling points of the hydrocarbons:
1. Crude oil is heated until it turns into a gas (vaporises) and enters the bottom of a fractionating column.
2. The column is hot at the bottom and gets progressively cooler towards the top (a temperature gradient).
3. The gases rise up the column. Different hydrocarbons condense into liquids at different levels when they reach their boiling point.
4. Small molecules with low boiling points rise all the way to the top before condensing (or leave as gases).
5. Large molecules with high boiling points condense near the bottom where it is hottest.
Main Fractions and Their Uses (from top to bottom):
• Refinery gases: Bottled gas for camping and domestic heating/cooking.
• Petrol (Gasoline): Fuel for cars.
• Kerosene: Fuel for aircraft.
• Diesel: Fuel for heavy vehicles like lorries, buses, and some cars.
• Fuel oil: Fuel for ships and power stations.
• Bitumen: Thick, sticky residue used for surfacing roads and roofing.
Key Takeaway: Short-chain hydrocarbons have low boiling points, run runny (low viscosity), and ignite easily. Long-chain hydrocarbons have high boiling points, are thick and sticky (high viscosity), and are harder to burn.
3. The Alkanes (Saturated Hydrocarbons)
The alkanes are the simplest family (homologous series) of hydrocarbons.
Alkanes are described as saturated hydrocarbons because all the bonds between carbon atoms are single covalent bonds (\(C-C\)). Every carbon atom is bonded to the maximum possible number of hydrogen atoms.
The general formula for all alkanes is: \(C_n H_{2n+2}\)
(Where \(n\) is the number of carbon atoms).
The First Four Alkanes:
Here is a handy mnemonic to remember the order of the first four alkanes: Monkeys Eat Peanut Butter!
1. Methane (\(n = 1\)):
Molecular formula: \(CH_4\)
Structural formula: One central \(C\) bonded to 4 \(H\) atoms.
2. Ethane (\(n = 2\)):
Molecular formula: \(C_2H_6\)
Structural formula: Two \(C\) atoms linked by a single bond, surrounded by 6 \(H\) atoms.
3. Propane (\(n = 3\)):
Molecular formula: \(C_3H_8\)
Structural formula: Three \(C\) atoms in a chain linked by single bonds, with 8 \(H\) atoms.
4. Butane (\(n = 4\)):
Molecular formula: \(C_4H_{10}\)
Structural formula: Four \(C\) atoms in a chain linked by single bonds, with 10 \(H\) atoms.
Maths Trick: To find the number of hydrogens, double the number of carbons and add 2! For example, if an alkane has 5 carbons (\(C_5\)), it must have \((2 \times 5) + 2 = 12\) hydrogens (\(C_5H_{12}\)).
4. The Alkenes (Unsaturated Hydrocarbons)
Alkenes are another family of hydrocarbons. They contain at least one carbon-carbon double covalent bond (\(C=C\)).
Because they have a double bond, they have fewer hydrogen atoms than an alkane with the same number of carbons. Therefore, we call them unsaturated hydrocarbons.
The general formula for alkenes is: \(C_n H_{2n}\)
The First Two Alkenes:
Note: There is no 1-carbon alkene because you need at least two carbon atoms to make a \(C=C\) double bond!
1. Ethene (\(n = 2\)):
Molecular formula: \(C_2H_4\)
Structure: Two carbons joined by a double bond (\(CH_2=CH_2\)).
2. Propene (\(n = 3\)):
Molecular formula: \(C_3H_6\)
Structure: Three carbons in a chain with one double bond (\(CH_2=CH-CH_3\)).
Testing for Alkenes (Bromine Water Test):
How can you tell the difference between an alkane and an alkene in a science lab? Use bromine water!
• Alkane + Bromine Water: Stays orange/brown (no reaction because alkanes are saturated).
• Alkene + Bromine Water: Turns from orange/brown to colourless (decolourises). The double bond breaks open and bonds with bromine atoms.
Key Takeaway: Alkanes have only single bonds (saturated, formula \(C_n H_{2n+2}\)). Alkenes have a double bond (unsaturated, formula \(C_n H_{2n}\)) and turn bromine water colourless.
5. Combustion of Hydrocarbons
Hydrocarbons are widely used as fuels because they release large amounts of heat energy when they burn. This reaction is called combustion (burning in oxygen).
A. Complete Combustion (Plenty of Oxygen)
When hydrocarbons burn with plenty of oxygen available, they burn cleanly and completely.
Word Equation:
Hydrocarbon + Oxygen \(\rightarrow\) Carbon Dioxide + Water (+ Energy)
Example (Methane):
\(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)
Testing the Products of Combustion:
• Carbon Dioxide (\(CO_2\)): Bubble the gas through limewater. It turns from colourless to milky/cloudy.
• Water (\(H_2O\)): Turns blue cobalt chloride paper to pink (or turns white anhydrous copper sulfate to blue).
B. Incomplete Combustion (Limited Oxygen)
When there is not enough oxygen (for example, in a faulty boiler or poorly ventilated room), hydrocarbons undergo incomplete combustion.
Instead of only producing carbon dioxide, incomplete combustion produces:
1. Carbon Monoxide (\(CO\)): A colourless, odourless, and highly toxic gas. It binds to haemoglobin in red blood cells, preventing them from carrying oxygen around the body.
2. Soot / Carbon (\(C\)): Tiny unburnt carbon particles that make surfaces black and can cause respiratory (breathing) problems.
6. Polymers and Plastics
Polymers are very long-chain molecules made by joining thousands of small molecules together. The small individual molecules are called monomers.
Addition Polymerisation of Ethene:
When many ethene molecules (monomers) are heated under high pressure with a catalyst, their double bonds open up and join together to form a long chain called poly(ethene) (commonly known as polythene).
• Monomer: Ethene (\(C_2H_4\))
• Polymer: Poly(ethene)
Analogy: Think of a monomer as a single paperclip. Polymerisation is linking thousands of paperclips together to make a long paperclip chain (the polymer)!
Environmental Problems with Polymers:
Most synthetic polymers (plastics) are non-biodegradable, meaning microorganisms cannot break them down naturally. This creates serious waste disposal challenges:
• Landfill: Plastics are buried in giant dumps. They take hundreds of years to break down and take up valuable space.
• Incineration (Burning): Plastics are burned to release energy, but this releases carbon dioxide (\(CO_2\)), which causes global warming, and can produce toxic gases.
• Recycling: Melting down used plastics to make new items. This saves crude oil and reduces landfill waste, but sorting different plastics can be difficult and expensive.
Quick Review: Essential Terms & Formulas
• Hydrocarbon: Contains hydrogen and carbon only.
• Alkane formula: \(C_n H_{2n+2}\) (Saturated, single bonds \(C-C\)).
• Alkene formula: \(C_n H_{2n}\) (Unsaturated, double bond \(C=C\)).
• Bromine water test: Orange to colourless with alkenes.
• Complete combustion: Makes \(CO_2\) and \(H_2O\).
• Incomplete combustion: Makes \(CO\) (toxic carbon monoxide) and soot (\(C\)).
• Monomer to Polymer: Ethene \(\rightarrow\) Poly(ethene).