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FOLLOW THE CARBON.

Organic Chemistry

Explore hydrocarbons, fuels and the carbon compounds used to make everyday materials.

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Quick reference: key definitions
Key Definitions

Topic Core Terms

Hydrocarbon
A molecule made of hydrogen and carbon atoms only.
Homologous series
A family of compounds with the same general formula, similar chemical properties, and a gradual trend in physical properties.
Saturated
Contains only single C-C bonds (e.g. Alkanes). Cannot react by addition.
Unsaturated
Contains at least one C=C double bond (e.g. Alkenes). Can react by addition.
Functional group
The atom or group of atoms responsible for the characteristic reactions of a homologous series (e.g. -OH in alcohols, C=C in alkenes).
Fraction
A group of hydrocarbons with similar chain lengths and boiling points, separated by fractional distillation.
Monomer
A small molecule that joins with many others to form a polymer.
Polymer
A very large molecule made by joining many small monomer molecules together.
CHAPTER 01 · 4 sectionsCrude oil and fuelsHydrocarbons, fractions, alkanes and combustion

Crude Oil & Hydrocarbons

Crude oil is a finite resource formed over millions of years from the remains of ancient marine organisms buried under layers of rock. It is a mixture of many different hydrocarbons.

Key Concept

Key Fact

A hydrocarbon is a molecule made of only hydrogen and carbon atoms. There are no other elements.

Crude oil is an important feedstock for the petrochemical industry - it provides fuels and raw materials for many products.

What is a hydrocarbon? Propane contains only carbon and hydrogen atoms CHHCHHCHHHH propane, C₃H₈ Hydrocarbon a compound of hydrogen and carbon atoms only Ccarbon atom Hhydrogen atom O N no other elements

A hydrocarbon is a molecule made of only carbon and hydrogen atoms. No other elements are present.

Fractional Distillation

Crude oil is separated into useful fractions by fractional distillation.

  1. Crude oil is heated until it forms a mixture of liquid and vapour.
  2. The vapour enters a fractionating column which is hot at the bottom and cool at the top.
  3. Hydrocarbons with high boiling points condense near the bottom; those with low boiling points rise higher before condensing.
  4. Each fraction is collected at a different level.
Fractional distillation of crude oil refinery gases C₁ to C₄: bottled gas for heating and cooking petrol C₅ to C₈: fuel for cars naphtha C₈ to C₁₂: making chemicals and plastics kerosene C₁₂ to C₁₆: fuel for aircraft diesel oil C₁₆ to C₂₀: fuel for lorries and trains heavy fuel oil C₂₀ to C₄₀: fuel for ships and power stations bitumen more than C₄₀: surfacing roads and roofs crude oil heated: vaporises vapour cool about 25 °C hot about 400 °C longer chains, higher boiling point

Fractional distillation separates crude oil into useful mixtures (fractions) based on their boiling points. Smaller molecules rise to the top, while larger molecules condense nearer the bottom.

Trends in Properties

As the chain length of hydrocarbons increases:

  • Boiling point increases (stronger intermolecular forces).
  • Viscosity increases (thicker/stickier).
  • Flammability decreases (harder to ignite).
Exam Tip

Exam Technique

The key trend to explain: longer chains have more intermolecular forces between molecules, so more energy is needed to separate them → higher boiling points.

Fraction Properties

Fraction Carbon atoms Use Boiling point
Gases (LPG) 1-4 Domestic heating, cooking Below 25°C
Petrol (gasoline) 5-8 Car fuel 25-75°C
Naphtha 8-12 Chemical feedstock 75-150°C
Kerosene 12-16 Jet fuel 150-240°C
Diesel 16-20 Diesel engines, trains 240-350°C
Fuel oil 20-40 Ships, power stations 350-500°C
Bitumen 40+ Roads, roofing Above 500°C

Alkanes

Alkanes are a homologous series of saturated hydrocarbons with the general formula:

CnH2n+2

"Saturated" means they contain only single covalent bonds (C-C and C-H) - no double bonds.

The First Four Alkanes

  • Methane: CH₄
  • Ethane: C₂H₆
  • Propane: C₃H₈
  • Butane: C₄H₁₀
Ball and stick models of Methane, Ethane, Propane, and Butane CHHHH methane, CH₄ CHHCHHHH ethane, C₂H₆ CHHCHHCHHHH propane, C₃H₈ CHHCHHCHHCHHHH butane, C₄H₁₀

The complete structural formulas (displayed formulas) of the first four alkanes: methane, ethane, propane, and butane.

Combustion of Hydrocarbons

Complete Combustion

When a hydrocarbon burns in plenty of oxygen, it produces carbon dioxide and water.

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete Combustion

When there is a limited supply of oxygen, incomplete combustion occurs. This can produce carbon monoxide (CO) and/or carbon (soot) instead of CO₂.

2CH₄ + 3O₂ → 2CO + 4H₂O

Carbon monoxide is toxic - it binds to haemoglobin in red blood cells, preventing them from carrying oxygen.

Pollutants from Fuels

  • CO₂: Greenhouse gas → climate change.
  • CO: Toxic and odourless.
  • Sulfur dioxide (SO₂): From sulfur impurities → acid rain.
  • Nitrogen oxides (NOₓ): From N₂ + O₂ at high engine temps → acid rain, smog.
  • Particulates (soot): Respiratory problems, global dimming.
CHAPTER 02 · 2 sectionsCracking and alkenesMaking smaller and more useful molecules

Cracking

Cracking breaks down long-chain hydrocarbons into shorter, more useful ones. This produces shorter alkanes (fuels) and alkenes (for making polymers).

Catalytic Cracking

Hydrocarbon vapour is passed over a hot zeolite catalyst (aluminium oxide/silicon dioxide) at about 600-700°C.

Steam Cracking

Hydrocarbon vapour is mixed with steam and heated to very high temperatures (over 800°C). No catalyst needed.

Cracking decane into octane and ethene CCCCCCCCCCHHHHHHHHHHHHHHHHHHHHHH this C-C bond breaks decane, C₁₀H₂₂: long-chain alkane catalytic cracking vapour passed over a hot catalyst CCCCCCCCHHHHHHHHHHHHHHHHHH + CCHHHH octane, C₈H₁₈: shorter alkane ethene, C₂H₄: alkene C=C

Cracking equation: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (Decane → Octane + Ethene)

Exam Tip

Exam Technique

In cracking equations, check that the number of carbon and hydrogen atoms balances on both sides. One product will be an alkane (fuel) and at least one will be an alkene (for polymers).

Worked Example

Balancing a cracking equation

Dodecane (C₁₂H₂₆) is cracked to produce octane and one other product. Write the balanced equation and identify the other product.

Step 1: C₁₂H₂₆ → C₈H₁₈ + ?

Step 2: Carbon: 12 − 8 = 4 carbons remaining.
Hydrogen: 26 − 18 = 8 hydrogens remaining.

Step 3: The other product is C₄H₈ - check: C₄H₂(₄) = C₄H₈ → this is butene (an alkene, CₙH₂ₙ).

Balanced: C₁₂H₂₆ → C₈H₁₈ + C₄H₈

Alkenes

Alkenes are an homologous series of unsaturated hydrocarbons containing a C=C double bond.

General formula: CnH2n

The First Four Alkenes

  • Ethene: C₂H₄
  • Propene: C₃H₆
  • Butene: C₄H₈
  • Pentene: C₅H₁₀

Testing for Alkenes

Add bromine water to the substance. If it's an alkene, the bromine water changes from orange to colourless as an addition reaction occurs across the double bond.

The bromine water test for alkenes alkane, e.g. ethane alkene, e.g. ethene CCHHHHHH CCHHHH shake bromine water shake bromine water after after colourless stays orange no reaction: alkanes are saturated orange → colourless bromine adds across the C=C bond

The Bromine Water test physically distinguishes between saturated alkanes and unsaturated alkenes based on whether an addition reaction can occur.

C₂H₄ + Br₂ → C₂H₄Br₂
Key Concept

Key Fact

Alkanes do NOT decolourise bromine water because they have no double bond - they are saturated.

Addition Reactions of Alkenes

Alkenes are far more reactive than alkanes because of the C=C double bond. In addition reactions, the C=C double bond opens up, allowing new atoms to add across the carbon atoms to form a saturated compound.

1. Hydrogenation (Addition of Hydrogen)

Alkenes react with hydrogen gas (H₂) in the presence of a nickel catalyst at 150°C. The double bond opens up to form the corresponding alkane.

C₂H₄(g) + H₂(g) → C₂H₆(g) (Ethene + Hydrogen → Ethane)

This process is used industrially to harden vegetable oils to make margarine.

2. Hydration (Addition of Water / Steam)

Alkenes react with steam (H₂O) at high temperature (300°C), high pressure (60 atm), and with a phosphoric acid catalyst to produce an alcohol.

C₂H₄(g) + H₂O(g) → C₂H₅OH(g) (Ethene + Steam → Ethanol)

3. Incomplete Combustion of Alkenes

Alkenes burn in air with a smoky, yellow flame due to incomplete combustion. Because alkenes have a higher carbon-to-hydrogen ratio than alkanes, unburnt carbon (soot) is produced along with carbon monoxide, carbon dioxide, and water.

CHAPTER 03 · 2 sectionsAlcohols and carboxylic acidsMore organic families for separate chemistry

Alcohols Chemistry Only

This section is only required for Separate Science (Chemistry GCSE) students, not Combined Science.

Alcohols contain the functional group -OH.

General formula: CnH2n+1OH

The First Four Alcohols

  • Methanol: CH₃OH
  • Ethanol: C₂H₅OH
  • Propanol: C₃H₇OH
  • Butanol: C₄H₉OH

Reactions of Alcohols

  • Combustion: Burn to produce CO₂ and H₂O - used as fuels.
  • With sodium: React gently to produce hydrogen gas.
  • With water: Dissolve in water to form neutral solutions.
  • Oxidation: Can be oxidised to carboxylic acids (e.g. Ethanol → ethanoic acid when left in air).

Making Ethanol

Ethanol can be produced by two methods:

Fermentation Hydration of ethene
Reactants Glucose + yeast Ethene + steam
Conditions ~37°C, anaerobic (no oxygen) 300°C, 60 atm, phosphoric acid catalyst
Equation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ C₂H₄ + H₂O → C₂H₅OH
Rate Slow (batch process) Fast (continuous process)
Purity Impure - needs distillation Pure product
Carbon neutral? Yes - sugar from plants (renewable) No - ethene from crude oil (non-renewable)
Exam Tip

Exam Technique

A common 6-mark question is "Compare fermentation and hydration of ethene for producing ethanol." Cover: raw materials, conditions, rate, purity, sustainability, and atom economy.

Carboxylic Acids Chemistry Only

This section is only required for Separate Science (Chemistry GCSE) students, not Combined Science.

Carboxylic acids contain the functional group -COOH.

General formula: CnH2n+1COOH   (where n ≥ 0 is the number of alkyl carbons)

For methanoic acid (HCOOH), n = 0 (C₀H₁COOH ≡ HCOOH). Alternatively, using total carbon atoms (n ≥ 1), the molecular formula is CnH2nO₂.

The First Four Carboxylic Acids

  • Methanoic acid: HCOOH
  • Ethanoic acid: CH₃COOH (vinegar)
  • Propanoic acid: C₂H₅COOH
  • Butanoic acid: C₃H₇COOH

Properties

  • Dissolve in water to form acidic solutions.
  • They are weak acids, meaning they only partially ionise in solution.
  • They have a higher pH than strong acids of the same concentration.

Reactions

Carboxylic acids react like typical acids, but less vigorously because they are weak acids:

  • With carbonates → salt + water + CO₂
  • With metals (e.g. magnesium) → salt + hydrogen
  • With metal oxides → salt + water
  • With alcohols (in the presence of an acid catalyst) ⇌ ester + water
Exam Tip

Exam Technique

Carboxylic acids are weak acids. Compared to a strong acid (like HCl) at the same concentration, they have a higher pH and react less vigorously. This is because they only partially dissociate into ions in solution.

Worked Example

Naming an ester

Ethanoic acid reacts with ethanol in the presence of a concentrated sulfuric acid catalyst. Name the ester produced and give the equation.

Rule: The ester name comes from: alcohol part (-yl) + acid part (-anoate).

Answer: Ethanoic acid + ethanol ⇌ ethyl ethanoate + water.

Equation: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l)

Esters have fruity smells and are used in flavourings and perfumes.

CHAPTER 04 · 2 sectionsBuilding polymersFrom small molecules to long chains

Addition Polymers

Many small alkene monomers join together to form a long-chain polymer. The C=C double bond opens up so each monomer can link to the next.

Addition polymerisation of ethene to poly(ethene) n CCHHHH CCHHHH n ethene (monomer) poly(ethene) (polymer) The C=C double bond opens up, so the monomers join in a long chain of single C-C bonds.

Addition polymerisation: The C=C double bonds in the monomers open up to form a long, continuous chain (the polymer).

n C₂H₄ → (C₂H₄)n

No other product is formed - only the polymer. This is why it’s called addition polymerisation.

Examples: Poly(ethene), poly(propene), poly(chloroethene) (PVC).

Worked Example

Drawing the repeat unit from the monomer

Given the monomer propene (CH₂=CHCH₃), draw the repeat unit of poly(propene).

Step 1: Open the C=C double bond to make two single bonds (one on each side).

Step 2: The repeat unit becomes: -CH₂-CH(CH₃)- with extending bonds on each side.

Key rule: To go from polymer back to monomer, replace the extending single bonds with a C=C double bond.

Key Concept

Key Fact

Most addition polymers are non-biodegradable - they persist in landfill and the environment for hundreds of years. This is a significant environmental concern.

Condensation Polymers Chemistry Only

This section is only required for Separate Science (Chemistry GCSE) students, not Combined Science.

In condensation polymerisation, monomers join together and a small molecule (usually water) is released as a by-product.

Two types of monomer are needed - typically a dicarboxylic acid and a diol (polyester) or a dicarboxylic acid and a diamine (polyamide/nylon).

Polyester Formation

Polyesters are formed when a dicarboxylic acid monomer (containing two -COOH groups) reacts with a diol monomer (containing two -OH groups). Each time an ester bond forms, a molecule of water (H₂O) is eliminated.

n HO-C(=O)-R-C(=O)-OH + n HO-R'-OH → [-C(=O)-R-C(=O)-O-R'-O-]ₙ + 2n H₂O

Amino Acids & Polypeptides

Amino acids contain two distinct functional groups in the same molecule:

  • Amine group: -NH₂ (basic)
  • Carboxylic acid group: -COOH (acidic)

Glycine (H₂N-CH₂-COOH) is the simplest amino acid. Amino acids react by condensation polymerisation to form polypeptides. The amino acid monomers join via peptide links, eliminating a water molecule at each step. Long polypeptide chains fold to form proteins.

Natural Condensation Polymers

  • Proteins: Made from different amino acid monomers joined by peptide bonds.
  • DNA (Deoxyribonucleic Acid): Essential for life. DNA consists of two polymer chains coiled into a double helix. The chains are made from four different nucleotide monomers (containing phosphate, sugar, and organic bases A, T, C, G).
  • Starch & Cellulose: Complex carbohydrates made by joining glucose (sugar) monomers together.
Exam Tip

Exam Technique

Key difference: addition polymerisation produces only the polymer (no by-product). Condensation polymerisation produces a small molecule (H₂O) as a by-product.

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