O-Level Chemistry: Organic Chemistry Basics

Let’s be real — O-Level Chemistry organic chemistry can feel like a totally different subject. One week your child is doing mole calculations, the next they’re staring at structures full of lines and letters, wondering how anyone remembers what’s what.

Here’s the thing: organic chemistry is less about memorising random facts and more about spotting patterns. Once your child learns to group compounds into a few “families” and recognise the key functional groups, many questions become predictable — even when they look unfamiliar.

This guide sits under O-Level Sciences, so you can place organic chemistry in the wider Sec 3–4 science picture. We’ll cover the basics that unlock marks fast: homologous series, functional groups, simple naming, and the exam traps that cause easy marks to slip away.

Where this topic fits in the O-Level Chemistry picture

Organic chemistry usually comes after your child has learned the “building blocks” of Chemistry: particle ideas, bonding, acids and bases, and energy changes. That’s why it can feel like a jump — the questions are more visual (structures) and more language-heavy (naming), even though the same foundations are still running underneath.

It’s also where Chemistry starts to feel more “real”: fuels, plastics, air quality, and how science links to sustainability. If your child is in Sec 3, treat organic as a foundation topic — get the families and naming right now, and Sec 4 practice becomes much more manageable. If they’re already in Sec 4, this is a good “reset” chapter before full-paper practice across the O-Level Complete Guide journey.

Organic chemistry in one sentence

In O-Level Chemistry, organic chemistry is mainly the study of carbon-based compounds organised into a small number of families that behave in predictable ways.

Those families are called homologous series. Members of the same series share:

a similar functional group (the “badge” that tells you how it reacts), and
a similar general formula (a pattern in their atom counts).

So instead of learning hundreds of separate molecules, students learn how to:

identify the family from a structure,
name it reliably, and
predict common reactions and properties from the family.

The big idea: homologous series (the “families”)

A homologous series is a family of organic compounds that follow the same pattern. If your child understands this concept properly, organic stops feeling like a long list.

A homologous series has four key features:

1) Same functional group

The functional group drives the family’s typical reactions:

alkenes: C=C
alcohols: –OH
carboxylic acids: –COOH

2) Same general formula

This helps students check whether an answer “makes sense”:

alkanes: (C_nH_{2n+2})
alkenes: (C_nH_{2n})

3) Each member differs by one –CH₂– unit

Example: ethane (C_2H_6) → propane (C_3H_8) → butane (C_4H_{10}).
That’s why “next member of the series” questions are common.

4) Similar chemical properties, gradual physical changes

Chemical properties are similar (same functional group), while physical properties (like boiling point) change gradually as the chain length increases.

A simple exam habit: get your child to say the series out loud first (“This is an alkene / alcohol / acid…”) before they write anything. It sounds small, but it reduces careless slips.

Functional groups that show up again and again

If homologous series are the families, functional groups are the family badge — they tell you what the compound is and how it usually behaves.

Here’s a simple spotting guide your child can use in most Paper 2 questions:

Alkane (–ane)

Functional group: none (only single C–C bonds)
Key idea: saturated
Typical reactions: combustion; sometimes substitution with chlorine (UV often mentioned)

Alkene (–ene)

Functional group: C=C
Key idea: unsaturated
Classic test: decolourises aqueous bromine
Typical reactions: addition; polymerisation (plastics)

Alcohol (–ol)

Functional group: –OH
Typical reactions: combustion; oxidation idea (alcohol → carboxylic acid)

Carboxylic acid (–oic acid)

Functional group: –COOH
Key idea: weak acid behaviour (reacts like acids your child already knows)
Typical reactions: forms esters with alcohols (esterification)

Ester (–oate)

Functional group: –COO–
Key idea: made from carboxylic acid + alcohol
Typical context: flavourings/fragrances; recognising the pattern and naming

Mark-saving habit: before answering, circle the functional group on the structure. It turns a messy diagram into a routine question.

How to write organic compounds properly (and not lose easy marks)

Many students lose marks in organic not because they don’t understand, but because their formulae and structures are unclear.

1) Know which formula the question wants

Molecular formula: atom counts only (e.g. (C_2H_6O)). It doesn’t show structure.
Displayed (structural) formula: shows all bonds. Common in organic questions.
Condensed structural formula: shorter writing style (e.g. CH₃CH₂OH), still shows the order of atoms.

Exam trap: writing a molecular formula when the question asks you to draw a displayed formula. Train your child to underline the command word: state, draw, write the displayed formula, write the structural formula.

2) Make the carbon chain obvious (especially for C1–C4)

The first four prefixes show up constantly:

meth- (C1), eth- (C2), prop- (C3), but- (C4)

When drawing, make the carbon chain clear first, then add the functional group. Messy chains are where missing bonds and wrong hydrogens sneak in.

3) A 10-second checklist before moving on

Does every carbon have 4 bonds in total?
If there’s a C=C, did the hydrogen count drop correctly?
Is the functional group correct (–OH vs –COOH vs –COO–)?
If needed: does the molecular formula match the structure (count C and H once)?

Naming organic compounds: the O-Level “minimum viable” system

Your child doesn’t need to become a chemist — they need a method that works reliably for the common question types.

Step 1: Count the carbon atoms in the main chain

1 carbon: meth-
2 carbons: eth-
3 carbons: prop-
4 carbons: but-

Step 2: Identify the family from the functional group

This decides the suffix:

alkane: -ane
alkene: -ene
alcohol: -ol
carboxylic acid: -oic acid

Step 3: Combine prefix + suffix

Examples they should know confidently:

Alkanes

CH₄ → methane
C₂H₆ → ethane
C₃H₈ → propane
C₄H₁₀ → butane

Alkenes

C₂H₄ → ethene
C₃H₆ → propene
C₄H₈ → butene

Alcohols

CH₃OH → methanol
C₂H₅OH → ethanol
C₃H₇OH → propanol

Carboxylic acids

HCOOH → methanoic acid
CH₃COOH → ethanoic acid
C₂H₅COOH → propanoic acid

What about esters?

At “basics” level, the key exam move is recognising the pattern: acid + alcohol → ester + water. Ester naming is usually taught as a separate pattern to practise once families and prefixes/suffixes are secure.

Sanity check habit: prefix matches carbon count; suffix matches functional group; and for alkenes, remember (C_nH_{2n}) (not (C_nH_{2n+2})).

Isomerism: same formula, different molecule

Isomers have the same molecular formula but different structures. That means the atoms are connected differently, and the compound is genuinely different.

Simple example: C₄H₁₀

(C_4H_{10}) can form:

butane (straight chain)
2-methylpropane (branched)

Same formula, different connectivity — so they’re isomers.

How to check if two drawings are really different

Compare the connectivity, not the orientation on the page.
Check for branching vs a straight chain.
Make sure you haven’t just “flipped” the same structure.

A “must-know” reaction map you’ll keep seeing

At O-Level, organic reactions aren’t meant to be an endless list. They’re a small set of common routes tied to each family.

Alkanes: mainly fuels

Complete combustion: alkane + oxygen → carbon dioxide + water
Incomplete combustion (idea level): carbon monoxide/soot if oxygen is limited
Substitution with chlorine (sometimes): alkane + chlorine → chloroalkane + hydrogen chloride (UV often stated)

Alkenes: reactive because of C=C

Addition reactions: double bond opens, atoms add across
Polymerisation: many alkene molecules join to form a polymer
Test for unsaturation: decolourises aqueous bromine (alkanes: no change)

Alcohols: can be converted into acids

Combustion: alcohol + oxygen → carbon dioxide + water
Oxidation pathway (concept): alcohol → carboxylic acid

Carboxylic acids + alcohols: esters

Esterification: carboxylic acid + alcohol → ester + water (schools often teach this with conditions/catalyst wording)

Memory-friendly map

Alkane: burn it; sometimes substitute
Alkene: add to it; polymerise; bromine test
Alcohol: burn it; can become an acid
Acid + alcohol: ester + water

Skills you’ll use across Chemistry (and why organic punishes weak fundamentals)

Organic chemistry looks wordy, but it still rewards core skills — and it exposes weak ones quickly.

Balancing equations (especially combustion)

Combustion appears everywhere (fuels, alcohols, pollution). A helpful routine: balance C, then H, then O last.

Interpreting formulae and ratios

Students often move between:

displayed formula (picture),
molecular formula (numbers),
general formula (pattern like (C_nH_{2n+2})).

This is where many slip: they can spot the functional group, but their counting (especially hydrogen) is off.

Mole concept still shows up

Organic questions can involve combustion data, formula deduction, or comparing amounts of reactants/products. If this area is shaky, revise systematically: O-Level Chemistry Mole Concept: Step-by-Step Guide.

Parent reality check: when a child says “I hate organic”, sometimes it’s not the structures — it’s that organic makes it harder to hide weak fundamentals. Fix those, and organic often improves faster than expected.

Common exam traps (and quick fixes)

Trap 1: Wrong general formula

Fix: alkanes (C_nH_{2n+2}); alkenes (C_nH_{2n}).
If there’s C=C, hydrogen count drops.

Trap 2: Mixing up saturated vs unsaturated

Fix: saturated = single bonds only; unsaturated = has C=C.

Trap 3: Messy structures

Fix: carbon has 4 bonds; functional group correct; count C and H once.

Trap 4: Naming slips (prefix/suffix mismatch)

Fix: prefix = carbon count; suffix = functional group.

Trap 5: Vague test answers

Fix: write it precisely:
- alkene + aqueous bromine: orange/brown → colourless
- alkane: no change (at room conditions)

Parent-friendly check after practice:

“What family is it?”
“What’s the functional group?”
“What test/reaction do you expect?”
If they can answer those three, most organic questions become routine.

How to study organic chemistry effectively (without cramming)

Organic chemistry is one of those topics where rereading notes doesn’t move the needle much. What works better is short, repeated practice that trains pattern recognition.

1) Build a one-page “spotting sheet”

Keep it simple:

functional groups (C=C, –OH, –COOH, –COO–)
general formula for alkanes vs alkenes
prefixes (meth-/eth-/prop-/but-)
bromine test outcome

Use it as a warm-up sheet — not something to rewrite endlessly.

2) Use the same routine for every question

circle the functional group
name the homologous series
predict likely reaction/test
then write the answer

This reduces panic-guessing and careless slips.

3) Do structured, small practice sets

daily: 10–15 minutes on identifying + naming
every few days: reaction/products practice
weekly: mixed questions

If your child is juggling multiple subjects, a realistic plan matters more than a perfect one. For broader study habits: Study Hacks Every Secondary School Student Should Know.

4) When tuition helps (and when it doesn’t)

Tuition helps most when a student:

keeps mixing up families/functional groups,
can’t name reliably,
blanks out in reaction questions even after revising.

It helps less if they simply haven’t done enough practice yet — so start with consistent small sets, then escalate if they’re still stuck after a couple of weeks.

Summary + TutorBee CTA

Organic chemistry doesn’t have to feel like a memory test. At O-Level, it’s really about a few repeatable basics:

homologous series (families),
functional groups (the badge that tells you what it is),
naming rules (carbon count + correct suffix),
and a small reaction map that keeps showing up in exams.

If your child can reliably spot the functional group first, a lot of organic questions stop being scary and start becoming routine.

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If organic chemistry still feels like memorising, submit your request to TutorBee and get matched with a tutor who teaches it using simple patterns (spot the functional group → choose the right name/reaction), so your child makes fewer careless mistakes.

References

Singapore Examinations and Assessment Board (SEAB), Chemistry (Syllabus 6092) for GCE O-Level (2026): https://www.seab.gov.sg/files/O Lvl Syllabus Sch Cddts/2026/6092_y26_sy.pdf
MOE Singapore, Upper Secondary Chemistry Syllabus (Updated 2024): https://www.moe.gov.sg/-/media/files/secondary/syllabuses/science/updated-2024/2023-olevel-chemistry-syllabus-updated-2024.pdf