Organic Chemistry | Edexcel IGCSE Section 4

Key Definitions

Hydrocarbon: A compound containing only carbon and hydrogen atoms.
Homologous Series: A family of compounds with the same general formula, similar chemical properties, and a trend in physical properties.
Functional Group: An atom or group of atoms that determines the chemical properties of an organic compound.
Isomers: Compounds with the same molecular formula but different structural formulae.
Saturated: An organic compound containing only single covalent bonds.
Unsaturated: An organic compound containing one or more double (or triple) covalent bonds.
Esterification: The reaction between an alcohol and a carboxylic acid to form an ester and water.
Polymer: A large molecule made of many repeating units (monomers) joined together.

Introduction to Organic Chemistry

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.

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.

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

Homologous Series, Functional Groups & Isomerism

Organic chemistry is the study of carbon compounds. To make this vast field manageable, compounds are grouped into families.

Homologous Series: A family of organic compounds that share the same general formula, similar chemical properties, and show a gradual trend in physical properties (e.g. boiling point).
Functional Group: An atom or group of atoms that determines the chemical properties of an organic compound (e.g., -OH for alcohols, -COOH for carboxylic acids).
Structural Isomerism: Isomers are molecules that have the same molecular formula but different structural formulae (they are arranged differently).

Example of Isomerism: Butane vs Methylpropane (C₄H₁₀)

Both butane and methylpropane have the molecular formula C₄H₁₀, but they have different structures:

Butane (Straight-chain):

Methylpropane (Branched):

Crude Oil

Fractional Distillation

Crude oil is separated into useful fractions by fractional distillation.

Crude oil is heated until it forms a mixture of liquid and vapour.
The vapour enters a fractionating column which is hot at the bottom and cool at the top.
Hydrocarbons with high boiling points condense near the bottom; those with low boiling points rise higher before condensing.
Each fraction is collected at a different level.

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).

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:

C_nH_2n+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

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.

Reaction of Alkanes with Halogens

Alkanes are generally unreactive. However, they react with halogens (such as bromine and chlorine) in the presence of ultraviolet (UV) radiation (light).

This is a substitution reaction because a hydrogen atom in the alkane is replaced by a halogen atom.
For example, methane reacts with bromine to form bromomethane and hydrogen bromide gas:
CH₄ + Br₂ → CH₃Br + HBr
Observation: The orange colour of bromine fades slowly in the presence of UV light.

Alkenes

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

General formula: C_nH_2n

The First Three Alkenes

Ethene: C₂H₄
Propene: C₃H₆
Butene: C₄H₈

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

C₂H₄ + Br₂ → C₂H₄Br₂

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

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

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).

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

Reaction of Alkenes with Halogens

Alkenes are much more reactive than alkanes because of their carbon-carbon double bond (C=C).

They undergo addition reactions where the double bond opens up and the reactant atoms add across it.
For example, ethene reacts with bromine to form 1,2-dibromoethane:
CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br

The Test for Unsaturation (Alkenes vs Alkanes)

Bromine water can be used to distinguish between a saturated compound (alkane) and an unsaturated compound (alkene):

Add orange bromine water to the sample and shake.
With an Alkene: The double bond reacts immediately with bromine. The solution turns from orange to colourless (decolourises).
With an Alkane: No reaction occurs because there are no double bonds. The solution remains orange (unless left in UV light, where a slow substitution occurs).

Alcohols

Alcohols Extended

Alcohols contain the functional group –OH.

General formula: C_nH_2n+1OH

The First Three Alcohols

Methanol: CH₃OH
Ethanol: C₂H₅OH
Propanol: C₃H₇OH

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)

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.

Oxidation of Ethanol

Ethanol can be oxidized in three ways:

Complete Combustion: Burning ethanol in oxygen to produce carbon dioxide and water:
CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O
Microbial Oxidation: Oxidation by oxygen in the air in the presence of microorganisms (bacteria). This occurs when beer or wine is left open, turning it into vinegar (ethanoic acid):
CH₃CH₂OH + O₂ → CH₃COOH + H₂O
Chemical Oxidation: Heating ethanol with an oxidizing agent: potassium dichromate(VI) in dilute sulfuric acid.
The solution changes colour from orange to green, indicating the ethanol has been oxidized to ethanoic acid.

Manufacture of Ethanol

There are two industrial processes for manufacturing ethanol:

Feature	Method 1: Hydration of Ethene	Method 2: Fermentation of Glucose
Raw Materials	Ethene (from crude oil) and Steam	Glucose (from sugar cane/plants)
Catalyst/Agent	Phosphoric acid (H₃PO₄)	Yeast enzymes
Conditions	300°C, 60-70 atm pressure	Warm (~30-40°C), anaerobic (no oxygen)
Type of Process	Continuous process (fast, automated)	Batch process (slow, needs cleaning between runs)
Purity of Product	Very pure ethanol	Impure (requires fractional distillation to purify)
Sustainability	Non-renewable (uses crude oil)	Renewable (uses crops)

Why must fermentation be anaerobic? If oxygen is present, yeast will respire aerobically to produce carbon dioxide and water instead of ethanol. Furthermore, any ethanol produced would be oxidized by bacteria to ethanoic acid (vinegar).

Carboxylic Acids

Carboxylic Acids Extended

Carboxylic acids contain the functional group –COOH.

General formula: C_nH_2n+1COOH

The First Three Carboxylic Acids

Methanoic acid: HCOOH
Ethanoic acid: CH₃COOH (vinegar)
Propanoic acid: C₂H₅COOH

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 (esterification) → ester + water

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.

Naming an ester

Ethanol reacts with ethanoic acid. Name the ester produced and give the equation.

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

Answer: Ethanol + ethanoic acid → ethyl ethanoate + water.

Equation: C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O

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

Reactions of Carboxylic Acids

Carboxylic acids behave as typical weak acids. They react slowly with metals and metal carbonates:

Reaction with Metals: Reacts to form a salt and hydrogen gas. For example, ethanoic acid reacts with magnesium:
2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g)
Product name: Magnesium ethanoate.
Reaction with Carbonates: Reacts to form a salt, carbon dioxide gas, and water. For example, ethanoic acid reacts with sodium carbonate:
2CH₃COOH(aq) + Na₂CO₃(aq) → 2CH₃COONa(aq) + CO₂(g) + H₂O(l)
Product name: Sodium ethanoate.

Esters Extended

Esters are a homologous series of organic compounds containing the ester functional group: -COO- (ester linkage).

Properties of Esters:

They are highly volatile (evaporate easily).
They have distinctive, sweet, fruity smells.
They are used as food flavourings and in perfumes/cosmetics.

Preparation of Esters (Esterification)

Esters are produced by reacting an alcohol with a carboxylic acid in a condensation reaction (a molecule of water is eliminated). The reaction is reversible and is catalyzed by concentrated sulfuric acid.

Alcohol + Carboxylic Acid ⇌ Ester + Water

Preparation of Ethyl Ethanoate

Reacting ethanol with ethanoic acid produces the ester ethyl ethanoate:

CH₃CH₂OH + CH₃COOH ⇌ CH₃COOCH₂CH₃ + H₂O

Structure of Ethyl Ethanoate:

Notice that the prefix of the name (ethyl-) comes from the alcohol (ethanol), and the suffix (-ethanoate) comes from the carboxylic acid (ethanoic acid).

Naming and Structures of Esters

Ester Name	Alcohol Source	Acid Source	Structural Formula
Methyl Ethanoate	Methanol	Ethanoic Acid	CH₃COOCH₃
Ethyl Ethanoate	Ethanol	Ethanoic Acid	CH₃COOCH₂CH₃
Propyl Ethanoate	Propanol	Ethanoic Acid	CH₃COOCH₂CH₂CH₃
Methyl Propanoate	Methanol	Propanoic Acid	CH₃CH₂COOCH₃

Synthetic Polymers

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: 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).

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.

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 Extended

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).

Natural Condensation Polymers

Proteins: Made from amino acid monomers.
DNA: Made from nucleotide monomers.
Starch/cellulose: Made from sugar monomers.

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

Condensation Polymers & Polyesters

In condensation polymerisation, monomers with two functional groups react together, linking up and eliminating a small molecule (usually water) for each linkage formed.

Formation of a Polyester

A polyester is formed by reacting a dicarboxylic acid (containing two -COOH groups) with a diol (containing two -OH groups):

Dicarboxylic Acid + Diol → Polyester + Water

If we represent the dicarboxylic acid as HOOC-█-COOH and the diol as HO-█-OH, the reaction is:

n HOOC-█-COOH + n HO-█-OH → [-O-█-O-CO-█-CO-]_n + 2n H₂O

Displayed Formula of Polyester Repeat Unit:

    O         O
    ||        ||
  [ C - █ - C - O - █ - O ]_n

Biodegradable Polyesters (Biopolyesters)

Traditional addition polymers are inert, non-biodegradable, and persist in landfill sites for hundreds of years. However, many condensation polyesters are biodegradable:

These are called biopolyesters.
They can be broken down by microorganisms in the environment over time.
This breakdown produces harmless products like water and carbon dioxide, greatly reducing plastic pollution.