Topic 7 of 10

Organic Chemistry

Explore the chemistry of carbon compounds — from crude oil and hydrocarbons to the reactions of alkenes, alcohols, carboxylic acids, and the world of polymers.

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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.

Fractional Distillation

Crude oil is separated into useful fractions by fractional distillation.

  1. Crude oil is heated until it vaporises.
  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.

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.

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

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.

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.

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).

Alkenes

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

General formula: CnH2n

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.

C₂H₄ + Br₂ → C₂H₄Br₂
Alkanes do NOT decolourise bromine water because they have no double bond — they are saturated.

Alcohols (HT)

Alcohols contain the functional group –OH.

General formula: CnH2n+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.

Carboxylic Acids (HT)

Carboxylic acids contain the functional group –COOH.

General formula: CnH2n+1COOH

The First Three Carboxylic Acids

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

Reactions

Carboxylic acids are weak acids — they partially ionise in water. They react typically like acids:

  • With carbonates → salt + water + CO₂
  • With alcohols (esterification) → ester + water
Carboxylic acids are weak acids, so they have a higher pH (less acidic) than strong acids of the same concentration. Less vigorous reactions too.

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.

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).

Condensation Polymers (HT)

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.