# Introductory Organic Chemistry and Alkanes

> Edexcel International A-Level Chemistry · Edexcel IAL Chemistry U1
> Source: https://www.owlsprep.com/study/edexcel-ial-chemistry-u1-introductory-organic-chemistry-and-alkanes/

This guide covers all Edexcel IAL Unit 1 content for introductory organic chemistry and alkanes, including hazard assessment, IUPAC naming, reaction classification, alkane properties, pollutants, and the free-radical substitution mechanism.

**Prerequisites:** [Covalent bonding and structure (Edexcel IAL Chemistry U1 Topic 2)](https://www.owlsprep.com/study/edexcel-ial-chemistry-u1-covalent-bonding/); [Balanced chemical equations (Edexcel IAL Chemistry U1 Topic 1)](https://www.owlsprep.com/study/edexcel-ial-chemistry-u1-amount-of-substance/)

## Learning objectives

- Distinguish between hazard and risk, and outline risk reduction measures for organic practicals
- Define homologous series, functional groups, and apply IUPAC rules to name/draw alkanes, cycloalkanes and their isomers up to C6
- Classify organic reactions and describe homolytic/heterolytic fission, free radicals and electrophiles
- Explain alkane industrial processing, combustion, associated pollutants and alternative fuel sustainability
- Outline the full free-radical substitution mechanism of alkanes with halogens, including all steps and correct notation

## Foundations of Organic Chemistry & Risk Assessment

Organic chemistry is the study of carbon-containing compounds, forming over half of the Edexcel IAL Chemistry specification. Core safety and classification principles apply to all organic reactions, and are frequently tested in Unit 1.

**Hazard vs Risk** — A **hazard** is a substance/procedure with the potential to cause harm. **Risk** is the likelihood that the hazard will cause harm under specific conditions.

*Example:* Hazard: Methanol is toxic. Risk: Risk of poisoning is low if you wear gloves and work in a fume hood.

> **tip**
>
> Risk assessment questions almost always ask for two reduction measures: common valid answers include fume hood use, wearing PPE, using small volumes, or less toxic alternatives.

**Check your understanding**

1. Which of the following is a risk, not a hazard?

   - Ethanol is flammable
   - Ethanol could catch fire near a Bunsen burner
   - Ethanol causes eye irritation

   *Answer:* Ethanol could catch fire near a Bunsen burner

   *Why:* A risk describes the *likelihood* of harm occurring, not the inherent potential for harm.

**Homologous Series** — A family of organic compounds with the same functional group, same general formula, similar chemical properties, and gradual trends in physical properties (e.g. boiling point). Each member differs by a CH₂ unit from the next.

- **Addition**: Two molecules join to form one single product
- **Substitution**: One atom/group is replaced by another atom/group
- **Oxidation**: Gain of oxygen / loss of hydrogen / loss of electrons
- **Reduction**: Loss of oxygen / gain of hydrogen / gain of electrons
- **Polymerisation**: Many small monomer molecules join to form one large polymer

**Worked example:** Classify the reaction between methane and chlorine to form chloromethane and hydrogen chloride.

1. Step 1: Identify the change to the reactant molecules
2. Step 2: One hydrogen atom on methane is replaced by a chlorine atom
3. Step 3: This matches the definition of a substitution reaction

> **Exam tip:** You will be asked to classify both inorganic and organic reactions in Unit 1, so memorise these 5 classification definitions carefully.

## IUPAC Naming, Formulae and Structural Isomerism

Edexcel expects you to name and draw three types of formula for alkanes and cycloalkanes up to C10: displayed (all bonds shown), structural (atoms grouped by carbon), and skeletal (lines represent bonds, vertices represent carbons).

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

**Structural Isomerism** — Compounds with the same molecular formula but different structural arrangements of atoms. Isomers have different physical properties and may have different chemical properties.

**Worked example:** Draw and name all structural isomers of C₅H₁₂.

1. Step 1: Draw the unbranched straight chain isomer: 5 carbons in a row = pentane
2. Step 2: Draw the isomer with one methyl branch: 4 carbon main chain, methyl group on carbon 2 = 2-methylbutane
3. Step 3: Draw the isomer with two methyl branches: 3 carbon main chain, two methyl groups on carbon 2 = 2,2-dimethylpropane
4. Step 4: Verify all have molecular formula C₅H₁₂ and no duplicates exist

> **Exam tip:** When naming branched alkanes, always number the carbon chain from the end that gives the branch points the lowest possible locant numbers.

## Bond Fission, Free Radicals and Electrophiles

Organic reactions proceed via the breaking of covalent bonds, which can occur in two ways: homolytic fission and heterolytic fission.

**Homolytic Fission** — Covalent bond breaks evenly, each atom retains one electron from the bonded pair, forming two free radicals. Represented with single-barbed (fishhook) curly arrows.

**Free Radical** — A species with an unpaired electron, very reactive. Denoted with a single dot next to the atom with the unpaired electron, e.g. Cl•.

**Worked example:** Show the homolytic fission of a Cl-Cl bond to form two chlorine free radicals.

1. $$Cl-Cl \xrightarrow{h\nu} Cl\cdot + \cdot Cl$$
2. Note the single barbed arrows showing movement of one electron to each Cl atom, and the dots representing unpaired electrons.

> **Exam tip:** Never use double-barbed arrows for homolytic fission or free radical steps: marks are explicitly awarded for correct arrow type in mechanism questions.

## Alkanes: Properties, Fuels and Environmental Impact

Alkanes are saturated hydrocarbons: they contain only single C-C and C-H bonds, no double or triple bonds. The general formula for alkanes is $C_nH_{2n+2}$, and for cycloalkanes (ring structure) is $C_nH_{2n}$.

- **Fractional distillation**: Separates crude oil into fractions by boiling point
- **Cracking**: Breaks long chain alkanes into shorter chain alkanes and alkenes, using high temperature or catalyst
- **Reforming**: Converts straight chain alkanes into branched/cyclic alkanes for cleaner, more efficient fuels

- CO: Toxic gas that binds to haemoglobin, reducing oxygen transport in blood
- NOₓ: Formed at high engine temperatures, causes acid rain and photochemical smog
- SOₓ: Formed from sulfur impurities in fuel, causes acid rain
- Particulates: Small carbon particles, cause respiratory disease and global dimming
- Unburnt hydrocarbons: Contribute to photochemical smog

**Worked example:** Write a balanced equation for the complete combustion of butane (C₄H₁₀), including state symbols.

1. Step 1: Write unbalanced equation: $C_4H_{10}(g) + O_2(g) \rightarrow CO_2(g) + H_2O(l)$
2. Step 2: Balance C: 4 CO₂ on product side, balance H: 5 H₂O on product side
3. Step 3: Balance O: 13/2 O₂ on reactant side, multiply all by 2 to eliminate fraction
4. $$2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(l)$$

> **Exam tip:** Always check whether the question asks for complete or incomplete combustion: incomplete combustion produces CO or C instead of CO₂.

## Free-Radical Substitution of Alkanes with Halogens

The only reaction of alkanes (other than combustion) assessed in Unit 1 is free-radical substitution with halogens (Cl₂ or Br₂) in the presence of UV radiation. The mechanism has three steps: initiation, propagation and termination.

**Worked example:** Outline the full free-radical substitution mechanism for the reaction between methane and chlorine to form chloromethane.

1. **Initiation**: UV radiation breaks Cl-Cl bond via homolytic fission, forming two Cl free radicals
2. $$Cl-Cl \xrightarrow{h\nu} Cl\cdot + \cdot Cl$$
3. **Propagation step 1**: Cl• attacks methane, removing a H atom to form HCl and a methyl radical
4. $$Cl\cdot + CH_4 \rightarrow HCl + \cdot CH_3$$
5. **Propagation step 2**: Methyl radical attacks a Cl₂ molecule, forming chloromethane and another Cl•, which repeats the cycle
6. $$\cdot CH_3 + Cl_2 \rightarrow CH_3Cl + Cl\cdot$$
7. **Termination**: Two free radicals combine to form a stable molecule, ending the chain reaction. Common termination steps:
8. $$Cl\cdot + Cl\cdot \rightarrow Cl_2$$
9. $$Cl\cdot + \cdot CH_3 \rightarrow CH_3Cl$$
10. $$\cdot CH_3 + \cdot CH_3 \rightarrow C_2H_6$$

> **warning**
>
> Free-radical substitution produces a mixture of products (e.g. di-, tri-, tetrachloromethane with excess Cl₂), so it has limited synthetic use for making pure halogenoalkanes.

> **Exam tip:** You must show the single dot on every free radical, and use only single-barbed curly arrows for all steps to score full marks.

## Common pitfalls

- **Wrong:** Using double-barbed curly arrows for free-radical reaction steps
  - Why it fails: Double-barbed arrows show movement of two electrons, only correct for ionic reactions. Free-radical steps involve movement of one electron
  - Correct: Always use single-barbed (fishhook) curly arrows for homolytic fission, initiation, propagation and termination steps of free-radical reactions
- **Wrong:** Numbering alkane chains from the end furthest from branch points, leading to high locant numbers
  - Why it fails: IUPAC rules require the lowest possible numbers for branch positions
  - Correct: Number the carbon main chain from the end that gives the branch points the smallest sum of locant numbers
- **Wrong:** Omitting state symbols in balanced combustion equations
  - Why it fails: Edexcel explicitly awards marks for correct state symbols in all Unit 1 balanced equation questions
  - Correct: Include (g) for gases, (l) for liquids, (s) for solids, (aq) for aqueous solutions in all balanced equations
- **Wrong:** Confusing hazard and risk in safety questions
  - Why it fails: Students often mix up the two definitions, leading to lost marks on risk assessment questions
  - Correct: Hazard = potential to cause harm; Risk = likelihood of harm occurring in a specific scenario
- **Wrong:** Writing termination steps as reactions between radicals and stable molecules
  - Why it fails: Termination steps only occur when two free radicals collide and combine to form a stable molecule
  - Correct: All termination steps must have two free radical species on the reactant side, and one stable molecule on the product side

## Cheatsheet

| Concept | Key Details |
| --- | --- |
| Hazard vs Risk | Hazard = potential harm; Risk = likelihood of harm |
| Alkane General Formula | CₙH₂ₙ₊₂; Cycloalkanes = CₙH₂ₙ |
| IUPAC Prefixes (C1-C6) | meth, eth, prop, but, pent, hex |
| Free-Radical Substitution Steps | 1. Initiation (UV, homolytic fission) 2. Propagation (chain reaction) 3. Termination (radical combination) |
| Common Pollutants | CO (toxic), NOₓ/SOₓ (acid rain), particulates, unburnt HC |
| Carbon Neutral | CO₂ released = CO₂ absorbed during production |

## What's next

Now you have mastered the core introductory organic chemistry and alkane content for Edexcel IAL Unit 1, you are ready to move on to the next organic topic: alkenes, which covers electrophilic addition reactions and polymerisation. This topic builds directly on your knowledge of functional groups, reaction classification and bond fission you have learned here. You should also practice past paper questions on free-radical substitution and IUPAC naming, as these are high-frequency question types in Unit 1 exams. Make sure you can correctly draw mechanisms with single-barbed arrows and name isomers up to C6 without referring to notes, as these are often tested in extended response questions worth 4-6 marks.

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