Summary — What This Chapter Covers

Before we dive in, here's the map. This chapter builds the "vocabulary and grammar" of organic chemistry — the naming rules, hazard awareness, and classification systems you'll use in every single organic topic from here on. Get this chapter rock-solid and the rest of the course gets dramatically easier.

1. Hazards & Risks

This sounds like a "boring safety" topic, but examiners LOVE testing it because students consistently mix up two words that sound almost identical: hazard and risk. Get the distinction crystal clear and you'll never lose a mark on it again.

Hazard vs. Risk — the core distinction

Think of it like this: a shark in the ocean is a hazard — it has sharp teeth and could hurt you, no matter where it is or what you're doing. Whether you're actually in danger from it depends on the risk — are you swimming in that ocean right now, how close is it, are you doing anything to protect yourself? Same shark, wildly different risk depending on the situation.

The textbook's own example is a great one: hydrochloric acid is corrosive — that's its hazard, and it's true whether the acid is sealed in a cupboard or splashed across a bench. Wearing eye protection doesn't make the acid any less corrosive (the hazard is unchanged) — but it massively lowers the chance the acid actually reaches your eyes (the risk goes down).

Hazard warning symbols

You need to recognise these on sight — exams will show you the diamond symbol and expect you to name the hazard and explain what it means in context.

Risk assessments

A risk assessment is essentially a plan that looks at the hazards of the chemicals involved, then works out control measures to reduce the risk of using them. It considers things like:

• The amount of the substance being used
• The age and experience of the person using it
• Whether it will be heated
• Whether ventilation or a fume cupboard is needed

From this, control measures are written — the type of eye protection, whether gloves are needed, keeping bottles capped, keeping substances away from heat sources, and what to do if a spill happens.

2. Functional Groups & Homologous Series

What is a homologous series?

Imagine a family of siblings who all share the same signature accessory — say, the same style of hat — but each sibling is a little taller than the last. That's basically a homologous series: every member has the same functional group (their "signature hat"), and each successive member is just one CH₂ unit longer than the one before.

Take ethanol and propanol as the textbook's example. Both have an –OH (alcohol) functional group. Both fit the general formula CₙH₂ₙ₊₁OH. The only difference is propanol has one extra CH₂ group tucked into its chain:

Because each member is just a slightly bigger version of the last, physical properties (boiling point, melting point, density) change gradually and predictably as you go up the series — but the chemical properties stay basically the same, since chemistry is governed by the functional group, and that never changes within the series.

Homologous series of alkanes — the pattern in action

Notice the boiling point climbs steadily as chain length increases — more carbons means more surface contact between molecules, stronger van der Waals forces, and so a higher boiling point. This is exactly the "gradually changing physical property" idea from the rule box above, made visible.

Functional groups you must recognise

R in the table below just means "any other atom or group of atoms except hydrogen" — it's a placeholder for "the rest of the molecule."

Reaction classification — the five verbs of organic chemistry

Once you know the functional groups, the next skill is describing what kind of change happens to them in a reaction. Examiners expect precise vocabulary here — vague words like "changes into" won't cut it.

Oxidation & reduction — the [O] and [H] shorthand

In organic chemistry, we rarely track electrons directly. Instead, we use a simplified, exam-friendly shorthand:

This is a genuinely useful memory hook: alcohol → aldehyde is oxidation (losing hydrogen); aldehyde → alcohol is reduction (gaining hydrogen). The direction of the arrow between [O] and [H] tells you exactly which reaction you're looking at.

Polymerisation (brief intro)

At this stage, all the polymerisation you'll meet is addition polymerisation: many small monomer molecules, each containing at least one C=C double bond, join together to form one long polymer chain — and crucially, the polymer is the only product (no small molecule is lost, unlike condensation reactions).

3. Nomenclature & Classification

Why systematic names exist

Every organic compound could technically be given a random nickname, but that would be chaos — you'd have to memorise millions of arbitrary names. Instead, chemists use IUPAC (systematic) nomenclature: a rule-based naming system where the name itself tells you the exact structure. Learn the rules once, and you can name — or draw — literally any organic molecule you'll meet at this level.

Step 1 — The stem tells you the chain length

The alkane names form the backbone of the entire system. The "stem" (the part before -ane, -ene, -ol, etc.) always tells you how many carbons are in the longest continuous chain.

Step 2 — Numbering the chain correctly

If there's a side-chain or functional group, you number the longest chain's carbons starting from whichever end gives the lowest possible numbers in the final name. This is one of the most commonly tested rules — and one of the easiest to get backwards under exam pressure.

Step 3 — Multiple identical side-chains: di-, tri-, tetra-

If the same alkyl group appears more than once, use a multiplying prefix — di- (2), tri- (3), tetra- (4) — placed in front of the group's name. Each occurrence still needs its own locant number, separated from other numbers by commas, and separated from words by a hyphen.

Notice the pattern in the numbers: "3,3,4" — comma between numbers, hyphen before the word "trimethylhexane." That punctuation is exam-marked, so get it exactly right.

Step 4 — Multiple DIFFERENT side-chains: alphabetical order

When there's more than one type of alkyl group, list them alphabetically in the name (not in the order you find them on the chain).

Naming compounds with functional groups

Once a functional group is present, its suffix (or prefix, for halogens) replaces or combines with the "-ane" ending.

4. Structural Isomerism

Here's a genuinely mind-bending idea when you first meet it: two molecules can have the exact same molecular formula — same number of every atom — but be completely different compounds, with different shapes, different names, and sometimes even different chemical behaviour. These are called structural isomers.

Classic example: propene and cyclopropane. Both are C₃H₆ — three carbons, six hydrogens, nothing more, nothing less. But propene has a C=C double bond in an open chain, while cyclopropane has all single bonds arranged in a closed triangle ring. Completely different molecules hiding behind an identical formula.

You need to know two flavours of structural isomerism for this chapter:

Type 1 — Chain isomerism

This happens when the molecular formula is fixed but the longest carbon chain differs — caused by branching. Think of it like the same length of rope: you can lay it out in a straight line, or you can coil part of it into a side-loop. Same total rope, different "longest straight stretch."

Both molecules are C₅H₁₂. Pentane's longest chain runs straight through all 5 carbons. 2,2-dimethylpropane's longest chain is only 3 carbons — the other two carbons are tucked away as branches. Same formula, genuinely different structures (and different physical properties too — branched isomers tend to have lower boiling points than their straight-chain twins, because branching reduces surface contact between molecules).

Type 2 — Positional isomerism

This happens when the molecular formula and the carbon skeleton are the same, but the functional group sits on a different carbon. Think of it as the same house frame, but the front door installed on a different wall.

Both are C₄H₁₀O, both are alcohols, both have exactly one –OH group — the only difference is which carbon that –OH is attached to. That's a positional isomer.

Worked example — systematically finding ALL isomers

This is a favourite exam question style: "How many structural isomers does C₄H₁₀O have?" or similar. Here's the reliable step-by-step method used in the source material:

1. Step 1: Draw the structural formula of the given compound.
2. Step 2: Decide whether it's stereoisomerism or structural isomerism (structural = no restricted rotation issue, just different connectivity).
3. Step 3: Systematically check — is it a functional group isomer? A chain isomer? A positional isomer? Work through each possibility methodically rather than guessing.

Worked example from the textbook — dibromopropane, C₃H₆Br₂. Working through Step 3: there's no other functional group possible (Br is the only option), the longest chain can only ever be 3 carbons (so no chain isomerism), but the two Br atoms can sit on different carbons — so this is positional isomerism. Systematically placing the two Br atoms on every unique combination of carbons gives 4 structural isomers: 1,1-dibromopropane, 1,2-dibromopropane, 1,3-dibromopropane, and 2,2-dibromopropane.

A second worked example: how many isomers does C₄H₁₀ have? Here there's no functional group at all (it's a plain alkane), so functional group and positional isomerism are both impossible — the only option is chain isomerism. That gives exactly 2 isomers: butane (straight chain) and 2-methylpropane (branched, sometimes called isobutane).

What to Memorise

Concepts Checklist

• I can explain the difference between a hazard and a risk, with an example.
• I can identify and describe the meaning of the main hazard warning symbols.
• I can list the factors a risk assessment considers and give hazard-specific precautions (not generic ones).
• I can define a homologous series and state its three defining features.
• I can identify all nine functional groups from the table (alkene, halogenoalkane, alcohol, aldehyde, ketone, carboxylic acid, ester, amine, nitrile) from a structure.
• I can tell the difference between an aldehyde and a ketone by looking at the position of the C=O group.
• I can classify a reaction as addition, substitution, elimination, hydrolysis, or condensation.
• I can use [O] and [H] shorthand correctly to represent oxidation and reduction reactions.
• I know that addition polymerisation forms a polymer as the only product from monomers with a C=C bond.
• I can name straight-chain and branched alkanes using IUPAC rules, including the lowest-locant rule.
• I can use di-, tri-, tetra- prefixes correctly, with correct comma/hyphen punctuation.
• I can name compounds with multiple different side-chains in alphabetical order.
• I can name compounds containing a functional group (alkene, alcohol, aldehyde, etc.) using the correct suffix/prefix.
• I can define structural isomerism and distinguish it from stereoisomerism.
• I can identify and draw chain isomers.
• I can identify and draw positional isomers.
• I can systematically work out ALL structural isomers of a given molecular formula without missing any.

Exam Tips & Common Mistakes