Organic Chemistry

Homologous series

A family of organic compounds with the same functional group, similar chemical properties and a trend in physical properties; each member differs by CH₂.

Functional group

The atom or group of atoms responsible for the characteristic reactions of an organic compound.

Hydrocarbon

A compound containing only carbon and hydrogen atoms.

Saturated hydrocarbon

A hydrocarbon with only single covalent bonds between carbon atoms (e.g. alkanes).

Unsaturated hydrocarbon

A hydrocarbon containing at least one carbon–carbon double bond (e.g. alkenes).

General formula of alkanes

CₙH₂ₙ₊₂ for a straight-chain alkane.

General formula of alkenes

CₙH₂ₙ for a straight‑chain alkene with one C=C bond.

General formula of alcohols

CₙH₂ₙ₊₁OH for a straight‑chain alcohol.

General formula of carboxylic acids

CₙH₂ₙO₂ for a straight-chain carboxylic acid.

Alkane functional group

No specific functional group; single C–C bonds only.

Alkene functional group

Carbon–carbon double bond, C=C.

Alcohol functional group

Hydroxyl group, –OH, attached to a carbon chain.

Carboxylic acid functional group

Carboxyl group, –COOH.

First four alkanes

Methane, ethane, propane, butane.

First four alkenes

Ethene, propene, butene, pentene.

First four alcohols

Methanol, ethanol, propanol, butanol.

First four carboxylic acids

Methanoic acid, ethanoic acid, propanoic acid, butanoic acid.

Displayed formula

A formula showing all atoms and all bonds in a molecule.

Structural formula

A formula showing the arrangement of atoms in a molecule without all bonds.

Balanced symbol equation

A chemical equation with the same number of each atom on both sides.

Crude oil

A finite mixture of many different hydrocarbons, mainly alkanes.

Fractional distillation of crude oil

Separation of crude oil into fractions with similar boiling points in a fractionating column.

Fraction

A group of hydrocarbons with similar boiling points collected at a certain level in the fractionating column.

Trend in boiling point of alkanes

Boiling point increases as the length of the carbon chain increases.

Viscosity trend in alkanes

Longer-chain hydrocarbons are more viscous (thicker).

Flammability trend in alkanes

Shorter-chain hydrocarbons are more flammable and burn more easily.

Cleanliness of flame trend

Shorter-chain hydrocarbons burn with cleaner (less smoky) flames.

Use of short-chain hydrocarbons

Good fuels; low boiling point, very flammable.

Cracking definition

Breaking long-chain alkanes into shorter-chain alkanes and alkenes.

Reason for cracking

To meet demand for fuels like petrol and to make alkenes for polymers.

Two types of cracking

Catalytic cracking and steam cracking.

Catalytic cracking conditions

High temperature and a hot catalyst, no steam.

Steam cracking conditions

Very high temperature and steam, no catalyst.

Example cracking equation

C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane to octane and ethene).

Complete combustion of a hydrocarbon

Hydrocarbon + oxygen → carbon dioxide + water.

Word equation for complete combustion of methane

Methane + oxygen → carbon dioxide + water.

Incomplete combustion products

Carbon monoxide, carbon (soot) and water.

Why incomplete combustion is dangerous

Produces toxic carbon monoxide and less energy per mole.

Carbon monoxide danger

Combines with haemoglobin, reducing oxygen transport in blood.

Addition reaction (organic)

A reaction where atoms add across a C=C double bond, turning an alkene into a saturated compound.

Hydrogenation of alkenes

Alkene + hydrogen → alkane, using a catalyst and moderate temperature.

Hydration of alkenes

Alkene + steam → alcohol, using a catalyst, high temperature and pressure.

Test for alkenes (bromine water)

Orange bromine water is decolourised by alkenes but not by alkanes.

Combustion of alkenes

Alkenes burn in air with a smoky flame, often incompletely.

Polymerisation of alkenes

Many alkene monomers join to form an addition polymer.

Monomer definition (addition polymer)

A small unsaturated molecule (with C=C) that can join to form a polymer.

Polymer definition

Very long molecule made from many repeating monomer units.

Example addition polymer

Poly(ethene) formed from many ethene molecules.

Alcohol definition

An organic compound containing the –OH functional group.

Methanol formula

CH₃OH.

Ethanol formula

C₂H₅OH.

Propanol formula (1-propanol)

C₃H₇OH.

Butanol formula (1-butanol)

C₄H₉OH.

Key properties of alcohols

Flammable, soluble in water with neutral pH, form solutions that conduct electricity poorly.

Combustion of alcohols

Alcohols burn in air to produce carbon dioxide and water.

Word equation for combustion of ethanol

Ethanol + oxygen → carbon dioxide + water.

Reaction of sodium with alcohols

Alcohol + sodium → sodium alkoxide + hydrogen gas.

Solubility of small alcohols

First four alcohols are soluble in water to form neutral solutions.

Uses of ethanol

Solvent, fuel, and in alcoholic drinks.

Fermentation conditions

Yeast, around 30 °C, anaerobic, aqueous sugar solution.

Fermentation word equation

Glucose → ethanol + carbon dioxide (using yeast).

Hydration of ethene to ethanol

Ethene + steam → ethanol, using a phosphoric acid catalyst at high temperature and pressure.

Comparing fermentation vs hydration

Fermentation is renewable but slow and batch; hydration is continuous, faster and gives pure ethanol but uses non‑renewable ethene.

Oxidation of alcohols (GCSE)

Alcohols can be oxidised to carboxylic acids using oxidising agents.

Ethanol oxidation product

Ethanol is oxidised to ethanoic acid.

Laboratory oxidising agent for alcohols

Acidified potassium dichromate(VI) is commonly used (beyond GCSE spec but useful).

Carboxylic acid definition

An organic acid containing the –COOH functional group.

Methanoic acid formula

HCOOH.

Ethanoic acid formula

CH₃COOH.

Propanoic acid formula

C₂H₅COOH.

Butanoic acid formula

C₃H₇COOH.

Aqueous carboxylic acids

Form acidic solutions with pH less than 7 in water.

Strength of carboxylic acids (GCSE)

Weak acids; they do not ionise completely in water.

Reaction of carboxylic acids with carbonates

Carboxylic acid + metal carbonate → salt + carbon dioxide + water.

Observation with carbonates

Fizzing from CO₂ gas; carbonate gradually dissolves.

Carboxylic acid salts naming

Carboxylic acid + metal → metal + carboxylate (e.g. ethanoate).

Reaction of carboxylic acids with metals

Carboxylic acid + metal → salt + hydrogen gas.

Reaction of carboxylic acids with bases

Carboxylic acid + base → salt + water.

Odour of carboxylic acids

Often have sharp, sour or vinegar-like smells.

Ethanoic acid everyday example

Main acid in vinegar (dilute ethanoic acid).

Why carboxylic acids are weak acids

Only a small fraction of molecules ionise to release H⁺ ions in water.

Ester definition

Organic compound formed from a carboxylic acid and an alcohol; contains –COO– group.

Formation of esters word equation

Carboxylic acid + alcohol ⇌ ester + water (with acid catalyst).

Catalyst for esterification

Concentrated sulfuric acid.

Example ester at GCSE

Ethyl ethanoate from ethanol and ethanoic acid.

Uses of esters

Perfumes, flavourings, solvents and plasticisers.

Smell of esters

Often have sweet, fruity smells.

Comparing alkanes and alkenes

Alkanes are saturated and less reactive; alkenes are unsaturated and more reactive, decolourising bromine water.

Comparing alcohols and carboxylic acids

Alcohols are neutral, contain –OH; carboxylic acids are acidic, contain –COOH and react with carbonates and metals.

Why organic compounds show trends

Members of a homologous series have similar structures, so their chemical properties are similar and physical properties change gradually.