Edexcel International A Level Chemistry: Introductory organic chemistry and alkanes (YCH11) – Revision Notes

These notes condense Topic 4, Introductory Organic Chemistry and Alkanes, of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017. They cover outcomes 4.1 to 4.18, all in Unit 1: Structure, Bonding and Introduction to Organic Chemistry, an International AS (IAS) unit. For full explanations and worked examples, use the study guide.

Test yourself afterwards with the practice questions. The course hub and printable checklist place this topic in the course; use the free 10-minute diagnostics to spot weak areas.

Definitions to learn word for word (4.1, 4.4, 4.7–4.10)

Term	Definition
Hazard	Something with the potential to cause harm
Risk	The likelihood that a hazard causes harm under the conditions of use
Homologous series	Members share a functional group and a general formula; neighbouring members differ by CH₂
Functional group	The part of a molecule that determines its characteristic reactions
Hydrocarbon	A compound of carbon and hydrogen only
Saturated	Contains single bonds only
Free radical	A species with an unpaired electron
Electrophile	A species that accepts a pair of electrons to form a new covalent bond
Homolytic fission	Each bonded atom keeps one electron of the shared pair; free radicals form
Heterolytic fission	One bonded atom keeps both shared electrons; ions form
Structural isomers	Same molecular formula, different structural formula
Carbon-neutral	No net CO₂ added to the atmosphere over the fuel’s whole life cycle

Reducing risk (4.2–4.3)

Organic liquids are often volatile, flammable and sometimes toxic, so every practical needs a risk assessment. The three routes the specification lists:

• smaller scale (a few cm³ or a few grams)
• precautions matched to the hazard (fume cupboard for toxic vapour; water bath or electric heater, no flame, for flammable liquids; gloves and goggles for corrosives)
• a different method using less hazardous substances.

Always link the precaution to the named hazard. “Wear goggles” alone does not answer a question about a flammable solvent.

Naming, drawing and classifying (4.5–4.6)

Stems: meth- C₁, eth- C₂, prop- C₃, but- C₄, pent- C₅, hex- C₆, hept- C₇, oct- C₈, non- C₉, dec- C₁₀.

Method in steps: naming a branched alkane

1. Find the longest chain, including any route through a branch.
2. Number from the end that gives the lowest locants.
3. Name branches with positions; di-, tri- for repeats; alphabetical order.
4. Check: no branch should be on carbon 1, and no ethyl should be on carbon 2.

Three formula types

Type	What it shows
Structural	Groups in order, e.g. CH₃CH₂CH(CH₃)₂
Displayed	Every atom and every bond
Skeletal	Zigzag of C–C bonds; carbons at corners and ends; no C–H drawn

Reaction types

Type	Signal in the equation
Addition	Two reactants, one product
Substitution	Atom or group swapped; two products
Oxidation	O gained or H lost
Reduction	H gained or O lost
Polymerisation	n monomers → one long chain

Alkanes and isomers (4.9–4.11)

Series	General formula	First member
Alkanes	CₙH₂ₙ₊₂	methane, CH₄
Cycloalkanes	CₙH₂ₙ	cyclopropane, C₃H₆

Mᵣ check: alkane Mᵣ = 14n + 2; cycloalkane Mᵣ = 14n.

Isomer counts to know (structural only)

• C₄H₁₀: butane, 2-methylpropane
• C₅H₁₂: pentane, 2-methylbutane, 2,2-dimethylpropane
• C₆H₁₄: five isomers (hexane, two methylpentanes, two dimethylbutanes)
• Cycloalkanes C₅H₁₀: cyclopentane, methylcyclobutane, ethylcyclopropane, 1,1-dimethylcyclopropane, 1,2-dimethylcyclopropane

Method in steps: finding every isomer

1. Draw the straight chain.
2. Remove one carbon and place it as a methyl on each possible position (not the end).
3. Remove two carbons: two methyls, or one ethyl if the chain is long enough.
4. Name each one and cross out any duplicates with the same name.
5. For cycloalkanes, repeat for each ring size from the full ring down to three carbons.

Crude oil and fuels (4.12)

Process	What it does	Example equation
Fractional distillation	Separates crude oil into fractions by boiling temperature	Physical, no equation
Cracking	Long alkane → shorter alkane + alkene	C₁₂H₂₆ → C₆H₁₄ + 2C₃H₆
Reforming	Straight chain → branched or cyclic, for better combustion in engines	C₆H₁₄ → C₆H₁₂ (cyclohexane) + H₂

Why crack: demand for short-chain fuels like petrol exceeds supply, and alkenes are needed for polymers.

Cracking practical (further suggested): heat aluminium oxide strongly, let heat reach paraffin on mineral wool, collect gas over water, test with bromine water (decolourised). Remove the delivery tube from the water before heating stops to prevent suck-back.

Combustion and pollutants (4.13–4.14, 4.17)

Method in steps: balancing combustion of CₓHᵧ

1. Carbon: x CO₂ (or x CO for incomplete combustion).
2. Hydrogen: y/2 H₂O.
3. Count oxygen atoms on the right; halve for O₂.
4. A half-integer amount of O₂ means you multiply every coefficient by two.

Example: cyclopentane, 2C₅H₁₀ + 15O₂ → 10CO₂ + 10H₂O.

Pollutant	Source	Problem
Carbon monoxide	Incomplete combustion	Binds to haemoglobin more strongly than O₂, reducing oxygen transport
Oxides of nitrogen	N₂ + O₂ at high engine temperature	Acidic: NO₂ forms nitric acid, so acid rain
Oxides of sulfur	Sulfur impurities in fuel	Acidic: SO₂ + H₂O → H₂SO₃, so acid rain
Carbon particulates	Incomplete combustion	Soot
Unburned hydrocarbons	Fuel that does not burn	Released into the air

The specification limits the “problems” to CO toxicity and the acidity of the nitrogen and sulfur oxides, so learn those two explanations in full.

Alternative fuels (4.15–4.16)

Reasons to develop them: crude oil is finite (sustainability), and burning fossil fuels adds CO₂, which is linked to climate change.

Fuel	Carbon-neutral?	Key point
Petrol	No	Releases fossil carbon locked away for millions of years
Bioethanol	Nearly, not fully	CO₂ from fermentation and combustion was taken in by photosynthesis; farming, processing and transport add CO₂
Hydrogen	Depends on production	Burns to water only; made from methane it releases carbon oxides; from electrolysis it is only as clean as the electricity

Free radical substitution (4.17–4.18)

Conditions: alkane + Cl₂ or Br₂, ultraviolet light. Example: cyclohexane and chlorine.

Stage	Equation	Feature
Initiation	Cl₂ → 2Cl•	Homolytic fission by UV; two curly half-arrows
Propagation 1	Cl• + C₆H₁₂ → C₆H₁₁• + HCl	Radical in, radical out
Propagation 2	C₆H₁₁• + Cl₂ → C₆H₁₁Cl + Cl•	Cl• regenerated
Termination	Cl• + Cl• → Cl₂; C₆H₁₁• + Cl• → C₆H₁₁Cl; 2C₆H₁₁• → C₁₂H₂₂	Two radicals pair up

Limited use in synthesis: the product can be substituted further (C₆H₁₀Cl₂ and beyond), and termination gives side products, so you get a mixture and a low yield of any one compound.

Must-know distinctions

• Hazard vs risk: the property versus the likelihood of harm.
• Homolytic vs heterolytic: radicals versus ions; half-arrows versus full arrows.
• Alkane vs cycloalkane: CₙH₂ₙ₊₂ versus CₙH₂ₙ; a cycloalkane shares its formula with an alkene.
• Cracking vs reforming: cracking shortens chains and makes alkenes; reforming keeps the carbon count and changes shape.
• Complete vs incomplete combustion: CO₂ versus CO or C.

Quick self-test

1. Is “ethoxyethane is extremely flammable” a hazard or a risk?
2. Give the stem for nine carbons.
3. An alkane has Mᵣ = 170. Give its molecular formula.
4. A cycloalkane has eight hydrogen atoms. Give its formula and Mᵣ.
5. Name CH₃CH(CH₃)CH₂CH₂CH₂CH₃.
6. Classify C₂H₄ + HBr → C₂H₅Br.
7. How many moles of O₂ burn one mole of heptane, C₇H₁₆, completely?
8. Write an equation for cracking decane into butane and ethene only.
9. Explain in one sentence why carbon monoxide is toxic.
10. Which stage of free radical substitution forms two radicals from one molecule?
11. Calculate the maximum mass of chlorocyclohexane from 8.40 g of cyclohexane (C = 12.0, H = 1.0, Cl = 35.5).
12. Why is hydrogen not automatically a carbon-neutral fuel?

Answers

1. Hazard (a property of the substance).
2. Non-.
3. 14n + 2 = 170, so n = 12: C₁₂H₂₆.
4. C₄H₈; Mᵣ = 56.
5. 2-methylhexane.
6. Addition.
7. 7 + 16/4 = 11 mol.
8. C₁₀H₂₂ → C₄H₁₀ + 3C₂H₄.
9. It binds to haemoglobin more strongly than oxygen, so the blood carries less oxygen.
10. Initiation.
11. 8.40 ÷ 84.0 = 0.100 mol; 0.100 × 118.5 = 11.9 g.
12. Its production may release CO₂, for example when it is made from methane or by electrolysis using fossil-fuel electricity.

Where marks are usually lost

• Writing “risk is how dangerous a chemical is”. That describes a hazard.
• Giving a generic precaution (goggles) when the question names a different hazard (flammability).
• Naming from the row as drawn when a longer chain runs through a branch.
• Giving 1-methylbutane or 2-ethylpropane: both mean the longest chain was missed.
• Leaving out alkenes as products when balancing a cracking equation.
• Saying CO “reduces oxygen in the air” rather than linking it to haemoglobin.
• Saying oxides of nitrogen “come from the fuel”. They form from atmospheric nitrogen at high temperature.
• Using full curly arrows for homolytic fission, or putting the dot on the wrong atom of an alkyl radical.
• Stating that bioethanol is carbon-neutral without the life-cycle qualification.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, Pearson Education Limited: Unit 1, Topic 4: Introductory Organic Chemistry and Alkanes, outcomes 4.1 to 4.18.