Organic Chemistry (Alkanes, Alkenes, Polymers, Alcohols)

Quick-Fire Definitions

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.

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.

Fractional Distillation

Crude oil is separated into useful fractions by fractional distillation.

Crude oil is heated until it vaporises.
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.

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

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.

Alcohols Chemistry Only

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.

Carboxylic Acids Chemistry Only

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

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.

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.

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 Chemistry Only

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.

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