Overview

What you'll learn

Describe fuels and crude oil. Compare fossil fuels and biofuels, explain fractional distillation, and tell complete from incomplete combustion.

Use the hydrocarbon families. Work with alkanes (substitution), alkenes (addition and the test for unsaturation), and understand cracking.

React alcohols, acids and esters. Make ethanol two ways, oxidise it to a carboxylic acid, and recognise esterification and the −COO− linkage.

Compare polymerisations. Distinguish addition from condensation — monomers, linkage, by-product and repeating unit — plus recycling and environmental impact.

Tutor's Insight

"The biggest topic — but really just five families and their templates."
Organic Chemistry spans four subtopics (11.1 through 11.4) and is by far the longest topic in the guidebook — Sec 4 content that is heavily examined across all three papers. But each homologous series has one functional group and one reaction template. Once you know the alkane, alkene, alcohol, acid and ester families, you can predict almost any organic question. Drawing structures is non-negotiable.

11.1 Fuels & Crude Oil

11.1 Fuels and Crude Oil

What this subtopic asks of you
  • Compare fossil fuels and biofuels — sources, combustion products, and the carbon-neutral logic of bioethanol.
  • Describe fractional distillation of crude oil — the tower, each fraction's properties and uses.

11.1 Fuels

Two sources, two combustions

Source 01
Fossil fuels
Coal, petroleum and natural gas — formed from the remains of plants and animals over millions of years, so non-renewable. Natural gas is mainly methane (CH₄). Burning fossil fuels releases CO₂, a greenhouse gas.
Source 02
Biofuels
Bioethanol and biodiesel — bioethanol comes from fermentation of sugar cane. Carbon-neutral logic: plants take in the same amount of CO₂ during photosynthesis as the fuel later releases on burning.
Combustion
Complete — plenty of O₂
Hydrocarbon + O₂ → CO₂ + H₂O. Clean flame, maximum energy released.
Combustion
Incomplete — limited O₂
Forms toxic carbon monoxide (CO) and even soot (C). Sooty flame, lower energy, dangerous indoors.
Bioethanol is called carbon-neutral because the CO₂ released on burning was absorbed by the plant while it grew.

11.1 Crude Oil

One mixture, seven fractions

Fraction C atoms / molecule Uses
Petroleum gas 1–4 Fuel for stoves & lighters
Petrol (gasoline) 5–10 Fuel for cars
Naphtha 8–12 Feedstock for chemicals & plastics
Kerosene 10–16 Fuel for jets & lamps
Diesel 14–20 Fuel for buses, lorries, trains
Lubricating oil 20–50 Polishes, waxes, lubricants
Bitumen 50+ Tar for roads & roofs
Top of the tower is cool; bottom is hot. Each fraction condenses at its boiling-point range — small, light molecules rise highest; large, heavy molecules collect at the bottom.

Free Notes · O-Level Pure Chemistry

Read the full chapter

Hydrocarbons, alkanes and alkenes, cracking, the bromine test, alcohols, acids and esters, addition vs condensation polymers, and 4 worked exam questions.

11.2 Hydrocarbons

11.2 Hydrocarbons

What this subtopic asks of you
  • Describe alkanes and alkenes — general formulae, saturation, structures, naming, and the differing reactivity.
  • Use the bromine test and predict cracking — distinguish alkenes from alkanes and understand why cracking matters industrially.

11.2 Hydrocarbons

A homologous series — same formula, same chemistry

Defined by
Same general formula
All members fit one general formula — e.g. alkanes are CₙH₂ₙ₊₂ — and share the same functional group.
Behaves as
Same chemistry
Same functional group → same reactions. Learn the family once. Members differ from one another by a −CH₂− unit.
Gradation
Smooth physical trends
Going up the series: Mᵣ ↑, m.p. and b.p. ↑, viscosity ↑, volatility ↓, flammability ↓.
Naming · the prefixes you need

meth · eth · prop · but · pent · hex · hept · oct — C1 = meth, C2 = eth, C3 = prop, C4 = but, C5 = pent, C6 = hex, C7 = hept, C8 = oct. The suffix tells the family: -ane for alkane, -ene for alkene, -ol for alcohol, -oic acid for carboxylic acid.

11.2 Hydrocarbons · Structures

Six structures worth knowing cold

Alkane · Methane
CH₄
All single bonds — the simplest alkane.
Alkane · Ethane
C₂H₆
Two carbons, all single bonds.
Alkene · Ethene
C₂H₄
Note the C=C double bond — this is what makes it reactive.
Alcohol · Ethanol
C₂H₅OH
The −OH functional group.
Carboxylic acid · Ethanoic acid
CH₃COOH
The −COOH functional group.
Ester · Methyl ethanoate
−COO− linkage
The ester linkage joins an acid to an alcohol.
Drawing structures is non-negotiable — examiners want the carbons, hydrogens and the functional group all shown correctly.

11.2 Hydrocarbons · Alkanes

Alkanes — saturated and unreactive

Formula
CₙH₂ₙ₊₂
All single bonds — every carbon has 4 bonds, every hydrogen has 1. Saturated — no room for more atoms to add.
Members
Methane → butane
CH₄ · C₂H₆ · C₃H₈ · C₄H₁₀. C1–C4 are gases at room temperature; C5–C16 liquids; longer chains solid (waxes, bitumen).
Reactivity
Mostly unreactive
They burn (combustion) and undergo substitution with Cl₂ in UV light: CH₄ + Cl₂ → CH₃Cl + HCl.
Substitution · the conditions

Gases mixed in the presence of UV light. One H is replaced by one Cl, and each step replaces one more H: CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄. The hallmark of substitution — no addition across a double bond, because alkanes don't have one to add across.

11.2 Hydrocarbons · Alkenes & Cracking

Alkenes — unsaturated and reactive

Alkenes
CₙH₂ₙ, with one C=C
Ethene C₂H₄, propene C₃H₆, butene C₄H₈. Addition reactions across the C=C:
• with H₂ — Ni catalyst, 150 °C (hydrogenation).
• with Br₂(aq) — room temperature (the test).
• with H₂O (steam) — H₃PO₄, 300 °C, 60 atm (forms ethanol).
Cracking
Splitting long chains
Long alkane → shorter alkane + alkene (and sometimes H₂). Conditions: ~600 °C, catalyst Al₂O₃ / SiO₂ (zeolite).
e.g. C₁₀H₂₂ → C₈H₁₈ + C₂H₄.
Why bother? Too much long-chain fraction, too little petrol; cracking matches supply to demand and produces alkenes for plastics.
Test for unsaturation: aqueous bromine is decolourised from orange to colourless by an alkene; an alkane leaves it orange.

11.3 Alcohols, Acids & Esters

11.3 Alcohols, Acids & Esters

What this subtopic asks of you
  • Identify and react three functional groups — −OH, −COOH and the ester linkage −COO−.
  • Make ethanol two ways; oxidise it to a carboxylic acid — and recognise esterification (acid + alcohol → ester + water).

11.3 Alcohols

Two routes to ethanol

Route 01 · Industrial
Hydration of ethene
Conditions: H₃PO₄ catalyst, 300 °C, 60 atm.
Equation: C₂H₄ + H₂O → C₂H₅OH
Pros: fast, continuous, pure product.
Cons: ethene comes from crude oil — non-renewable.
Route 02 · Biological
Fermentation of glucose
Conditions: yeast, anaerobic, ~37 °C.
Equation: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
Pros: renewable, low energy.
Cons: batch process, slow, dilute product (≤ 15 %, then distil).

11.3 Acids & Esters

From alcohol to acid to ester

Oxidation of alcohols
Alcohol → carboxylic acid
Oxidise an alcohol with acidified KMnO₄ (purple → colourless) or air + bacteria. Ethanol → ethanoic acid (CH₃COOH).
Carboxylic acids
Weak acids
−COOH group. React like other acids: with reactive metals → salt + H₂; with bases → salt + water; with carbonates → salt + water + CO₂. Salts: ethanoate, propanoate, …
Esters
Sweet-smelling
Made by esterification: acid + alcohol ⇌ ester + water. Conditions: conc. H₂SO₄ catalyst, heat. Ester linkage −COO−.
Distinguishing alcohols from carboxylic acids

Pick the diagnostic test. Acidified KMnO₄: an alcohol decolourises it (gets oxidised); a carboxylic acid does NOT. Carbonate: an acid effervesces (CO₂); an alcohol does not.

11.4 Polymers

11.4 Polymers

What this subtopic asks of you
  • Distinguish addition from condensation polymerisation — monomers, conditions, linkage, by-product.
  • Draw a repeating unit — and name the linkage (amide for nylon, ester for Terylene).

11.4 Polymers · Addition

Alkenes link up, nothing leaves

Monomers
Alkenes (have a C=C)
Ethene → poly(ethene). Propene → poly(propene). Chloroethene → PVC. The double bond opens up to link to the next monomer.
Mechanism
Many become one — nothing else
n CH₂=CH₂ → −(CH₂−CH₂)ₙ−. No small molecule is given off. All the atoms of the monomer end up in the polymer.
Properties
Strong, light, non-biodegradable
Useful — for bottles, bags, pipes — but they persist in the environment. Recycling and biodegradable alternatives are the modern response.
Drawing the repeating unit

Open the C=C, attach square brackets, subscript n. From a monomer CH₂=CHR, the repeating unit is −[CH₂−CHR]ₙ−. Always show the carbons that came from the C=C and the substituents — examiners look for both.

11.4 Polymers · Condensation

Two monomers, water leaves

Nylon · Polyamide
Diacid + diamine
Dicarboxylic acid (−COOH at each end) + diamine (−NH₂ at each end).
Linkage: amide −CONH−.
By-product: H₂O (one molecule lost per linkage).
Uses: clothing, ropes, fishing line.
Terylene · Polyester
Diacid + diol
Dicarboxylic acid (−COOH at each end) + diol (−OH at each end).
Linkage: ester −COO−.
By-product: H₂O.
Uses: bottles (PET), fabrics, films.
Addition: one monomer type, no by-product. Condensation: two monomer types (or one with two different ends), a small molecule (water) leaves at each linkage.

Practice

Exam-style questions

Question 01  ·  ACS(BR) 2024 Prelim
The Question
MCQ

Reading the tower

Petroleum can be separated into fractions by fractional distillation. Which statement about this process is correct?

  • A  The bitumen fraction is obtained above the kerosene fraction.
  • B  The fraction at the top of the column has the highest boiling point.
  • C  The lubricating-oil fraction is a source for polishes and waxes.
  • D  The molecules near the bottom of the column have smaller Mᵣ than those near the top.
Worked Answer
C
  1. A — Bitumen has the highest b.p., so it stays at the BOTTOM, below kerosene. ✗
  2. B — The top of the column is the COOLEST, so it collects the LOWEST b.p. fraction. ✗
  3. C — Lubricating oil (C₂₀–C₅₀) is indeed the source for polishes and waxes. ✓
  4. D — Mᵣ near the bottom is LARGER (long molecules). ✗  Answer: C.
Question 02  ·  ACS(BR) 2024 Prelim
The Question
MCQ

Chlorine + methane — substitution

Which row shows the correct equation and condition for the reaction between chlorine and methane?

  • A  Cl₂ + CH₄ → CH₂Cl₂ + H₂  ·  UV light
  • B  Cl₂ + CH₄ → CH₂Cl₂ + H₂  ·  aqueous chlorine
  • C  Cl₂ + CH₄ → CH₃Cl + HCl  ·  UV light
  • D  Cl₂ + CH₄ → CH₃Cl + HCl  ·  aqueous chlorine
Worked Answer
C
  1. Substitution swaps ONE H for ONE Cl per step, so CH₄ + Cl₂ → CH₃Cl + HCl. This rules out A and B.
  2. The reaction needs UV light to break Cl₂ into reactive chlorine atoms — gases mixed in the presence of UV.
  3. Aqueous chlorine wouldn't work — methane doesn't dissolve and the chlorine radicals aren't generated.
  4. Answer: C.
Question 03  ·  ACS(BR) 2024 Prelim
The Question
MCQ

Two ways to make ethanol

In which row are both statements correct?

  • A  ethene: from cracking  ·  glucose: O₂ also produced
  • B  ethene: H₃PO₄ catalyst  ·  glucose: the reaction is slow
  • C  ethene: uses ~300 °C  ·  glucose: uses ~100 °C
  • D  ethene: in aqueous solution  ·  glucose: gases as reagents
Worked Answer
B
  1. A — Ethene is from cracking ✓, but fermentation gives CO₂, not O₂. ✗
  2. B — Hydration of ethene uses phosphoric(V) acid, H₃PO₄, as catalyst ✓, and fermentation is a slow, batch process ✓. ✓✓
  3. C — Hydration of ethene is ~300 °C ✓; fermentation is ~37 °C, not 100 °C. ✗
  4. D — Hydration uses gases (steam + ethene); fermentation is in aqueous solution. Swapped. ✗  Answer: B.
Question 04
The Question
Structured

Two polymers compared

Poly(ethene) is made from ethene, while nylon is made from a diamine and a dicarboxylic acid.

(a) Name the type of polymerisation for each and state the by-product (if any).

(b) Name the linkage formed in nylon.

(c) Suggest one reason poly(ethene) waste is an environmental problem.

Worked Answer
Addition vs condensation
  1. (a) Poly(ethene): addition polymerisation — the C=C of many ethene monomers opens and joins, with no by-product. Nylon: condensation polymerisation — the diamine and diacid join and water (H₂O) is lost at each linkage.
  2. (b) Nylon contains the amide linkage, −CONH−.
  3. (c) Poly(ethene) is non-biodegradable, so it persists in the environment (landfill / litter) instead of breaking down. (Recycling or biodegradable alternatives reduce this.)

Frequently Asked Questions

Organic Chemistry — FAQ

What is Organic Chemistry in the O-Level Chemistry syllabus (6092)?
Organic Chemistry (Topic 11 of Syllabus 6092) is the study of carbon compounds. It has four subtopics: 11.1 Fuels & Crude Oil — fossil fuels vs biofuels, combustion and fractional distillation; 11.2 Hydrocarbons — alkanes, alkenes and cracking; 11.3 Alcohols, Acids & Esters — the −OH, −COOH and −COO− functional groups; and 11.4 Polymers — addition vs condensation polymerisation. It is the longest topic in the syllabus and is examined across all three papers.
What is the difference between complete and incomplete combustion?
Complete combustion happens with plenty of oxygen: a hydrocarbon burns to give carbon dioxide and water (hydrocarbon + O₂ → CO₂ + H₂O). Incomplete combustion happens when oxygen is limited: it produces toxic carbon monoxide (CO) and soot (carbon), gives a sooty flame, releases less energy, and is dangerous indoors.
How do you test for an alkene (test for unsaturation)?
Add aqueous bromine at room temperature. An alkene has a C=C double bond and reacts by addition, decolourising the bromine from orange to colourless. An alkane has no C=C, so it does not react and the bromine stays orange. Decolourisation of aqueous bromine is the standard test for unsaturation.
What are the two ways to make ethanol?
Hydration of ethene (industrial): C₂H₄ + H₂O → C₂H₅OH with an H₃PO₄ catalyst at ~300 °C and 60 atm — fast and pure, but ethene comes from non-renewable crude oil. Fermentation of glucose (biological): C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ using yeast, anaerobic, ~37 °C — renewable and low-energy, but slow, a batch process giving dilute product that must be distilled.
Why is crude oil separated by fractional distillation?
Crude oil is a mixture of hydrocarbons of different chain lengths and boiling points. In the tower the temperature is hot at the bottom, cool at the top, so each fraction condenses at its own boiling-point range. Small, light molecules (petroleum gas, petrol) rise highest; large, heavy molecules (lubricating oil, bitumen) collect at the bottom — giving useful fractions like petrol, kerosene, diesel and bitumen.
What is the difference between addition and condensation polymerisation?
In addition polymerisation many alkene monomers with a C=C join so that all the atoms end up in the polymer and no small molecule is given off (e.g. n CH₂=CH₂ → −(CH₂−CH₂)ₙ−). In condensation polymerisation two monomers (each with two reactive ends) join and a small molecule — usually water — is lost at each linkage, forming an amide linkage (−CONH−) in nylon or an ester linkage (−COO−) in Terylene.

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