Introductory Organic Chemistry and Alkanes

Q: What type of arrow do I use for free-radical steps?

You must use single-barbed (fishhook) curly arrows for all free-radical reaction steps, as these represent the movement of one electron, not two. Double-barbed arrows are only used for ionic reactions later in the course.

Edexcel International A-Level Chemistry· 4.1–4.18 (4A–4B)· 25 min read

1. Foundations of Organic Chemistry & Risk Assessment★★☆☆☆⏱ 5 min

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.

📘 Definition

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.

✓ Quick check

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
Reveal answer
1 —
A risk describes the likelihood of harm occurring, not the inherent potential for harm.

📘 Definition

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.

2. IUPAC Naming, Formulae and Structural Isomerism★★★☆☆⏱ 7 min

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-

📘 Definition

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.

3. Bond Fission, Free Radicals and Electrophiles★★★☆☆⏱ 4 min

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

📘 Definition

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.

📘 Definition

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.

4. Alkanes: Properties, Fuels and Environmental Impact★★☆☆☆⏱ 5 min

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 $C_{n} H_{2 n + 2}$ , and for cycloalkanes (ring structure) is $C_{n} H_{2 n}$ .

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: $C_{4} H_{10} (g) + O_{2} (g) \to C O_{2} (g) + H_{2} O (l)$
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.

5. Free-Radical Substitution of Alkanes with Halogens★★★★☆⏱ 6 min

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
$\cdot CH_3 + \cdot CH_3 \rightarrow C_2H_6$

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.

6. Common Pitfalls

Wrong move:

Using double-barbed curly arrows for free-radical reaction steps

Why:

Double-barbed arrows show movement of two electrons, only correct for ionic reactions. Free-radical steps involve movement of one electron

Correct move:

Always use single-barbed (fishhook) curly arrows for homolytic fission, initiation, propagation and termination steps of free-radical reactions

Wrong move:

Numbering alkane chains from the end furthest from branch points, leading to high locant numbers

Why:

IUPAC rules require the lowest possible numbers for branch positions

Correct move:

Number the carbon main chain from the end that gives the branch points the smallest sum of locant numbers

Wrong move:

Omitting state symbols in balanced combustion equations

Why:

Edexcel explicitly awards marks for correct state symbols in all Unit 1 balanced equation questions

Correct move:

Include (g) for gases, (l) for liquids, (s) for solids, (aq) for aqueous solutions in all balanced equations

Wrong move:

Confusing hazard and risk in safety questions

Why:

Students often mix up the two definitions, leading to lost marks on risk assessment questions

Correct move:

Hazard = potential to cause harm; Risk = likelihood of harm occurring in a specific scenario

Wrong move:

Writing termination steps as reactions between radicals and stable molecules

Why:

Termination steps only occur when two free radicals collide and combine to form a stable molecule

Correct move:

All termination steps must have two free radical species on the reactant side, and one stable molecule on the product side

7. Quick Reference 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	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

8. Frequently Asked

Do I need to draw full or skeletal formulae for naming questions?

Unless specified otherwise, displayed, structural or skeletal formulae are all accepted for Edexcel IAL Unit 1, as long as all bonds and branch points are clearly shown.

What type of arrow do I use for free-radical steps?

You must use single-barbed (fishhook) curly arrows for all free-radical reaction steps, as these represent the movement of one electron, not two. Double-barbed arrows are only used for ionic reactions later in the course.

Going deeper

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.