What are functional groups in organic chemistry?

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In organic chemistry, they are the key to understanding the behavior of different compounds. Common examples include hydroxyl groups (-OH), carboxyl groups (-COOH), and amino groups (-NH2). Understanding functional groups is essential for students in the IB MYP Science curriculum as they form the basis for predicting the properties and reactions of organic compounds.

Why are functional groups important in the study of organic chemistry?

Functional groups are crucial in organic chemistry because they determine the properties and reactivity of organic molecules. They allow chemists to classify compounds and predict how they will react in chemical reactions. For IB MYP students, recognizing functional groups helps in understanding the structure-function relationship in organic compounds, which is a fundamental concept in science.

How do functional groups affect the physical properties of organic compounds?

Functional groups significantly influence the physical properties of organic compounds, such as boiling point, solubility, and melting point. For instance, the presence of a hydroxyl group (-OH) can increase a compound's solubility in water due to hydrogen bonding. In the IB MYP Science curriculum, students learn how these groups affect molecular interactions and the resulting physical properties, which is essential for predicting the behavior of substances.

Can you give examples of common functional groups and their chemical properties?

Yes, some common functional groups include: Hydroxyl group (-OH), which is polar and can form hydrogen bonds, increasing solubility in water; Carboxyl group (-COOH), which is acidic and can donate a hydrogen ion in solutions; Amino group (-NH2), which is basic and can accept a hydrogen ion. Understanding these properties helps IB MYP students predict how these groups will interact in chemical reactions.

How do functional groups influence the naming of organic compounds?

Functional groups play a critical role in the nomenclature of organic compounds. They often determine the suffix or prefix of the compound's name. For example, alcohols contain the hydroxyl group and have names ending in '-ol', like ethanol. In the IB MYP Science curriculum, learning the IUPAC naming conventions helps students systematically name and identify organic compounds based on their functional groups.

Why is "Saying the alkane turns bromine water colourless." a common mistake in Functional Groups?

Why it happens: Confusion about which way the test works. How to avoid it: REMEMBER: bromine adds across the C=C. So UNSATURATED (has C=C) → DECOLOURISES. Saturated → no change. 'C=C eats the colour'.

Why is "Saying alkane is CₙH₂ₙ." a common mistake in Functional Groups?

Why it happens: Both alkanes and alkenes have similar formulae. How to avoid it: Alkane = CₙH₂ₙ₊₂ (more H, no double bond). Alkene = CₙH₂ₙ (fewer H, one double bond). Each C=C removes 2 H.

Organic chemistryIB MYP Sciences (Years 1-5)

Functional Groups

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Functional Groups — IB MYP Sciences: the atoms or bonds that give an organic compound its character

An organic compound's properties are mostly controlled by its 'functional group' — a particular atom or bond pattern that drives the chemistry. This MYP Sciences note covers the main MYP-level groups: C=C double bonds, –OH, –COOH, and how they relate to homologous series.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Identify the main functional groups in a structural formula.
MYP Sciences A — Match each functional group to its family name.
MYP Sciences A — State the general formulae for alkanes and alkenes.
MYP Sciences D — Predict which family will react with bromine water and explain why.

What is a functional group?

A small atomic 'badge' that decides how the molecule reacts.

Organic chemistry is a vast subject — there are tens of millions of known compounds. To organise them, chemists look at the FUNCTIONAL GROUP: a small arrangement of atoms (or a specific bond pattern) that gives the molecule its characteristic reactions.

If two molecules have the same functional group, they react in similar ways — regardless of how long the carbon chain is. This is the basis for homologous series (families of organic compounds).

The main MYP functional groups:

Group	Suffix	Family	Example
–C–C– (single bonds only)	-ane	Alkane	propane (CH₃CH₂CH₃)
C=C (double bond)	-ene	Alkene	propene (CH₃CH=CH₂)
–OH	-ol	Alcohol	propan-1-ol (CH₃CH₂CH₂OH)
–COOH	-oic acid	Carboxylic acid	propanoic acid (CH₃CH₂COOH)

The functional group is the "active" part. The rest of the molecule (the unreactive carbon chain) is sometimes called the "spectator" part.

Functional group = reactive part of the molecule.
Same group → similar chemistry across the family.
Four main MYP groups: single C-C, C=C, –OH, –COOH.

Homologous series

A family of compounds with the same functional group.

Members of a homologous series share:

The same FUNCTIONAL GROUP.
The same GENERAL FORMULA (e.g. CₙH₂ₙ₊₂ for alkanes).
Similar CHEMICAL reactions.
A GRADUAL change in physical properties (MP, BP, density) as the chain gets longer.
Each member differs from the next by ONE CH₂ unit.

Alkanes (CₙH₂ₙ₊₂): methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀ ... Note that each one has 2 more H and 1 more C than the previous.

Alkenes (CₙH₂ₙ): ethene C₂H₄, propene C₃H₆, butene C₄H₈ ...

Alcohols (CₙH₂ₙ₊₁OH or general CₙH₂ₙ₊₂O): methanol CH₃OH, ethanol C₂H₅OH, propanol C₃H₇OH ...

As you go up a homologous series, the BOILING POINT INCREASES (longer molecules have stronger intermolecular forces). For example: methane is a gas at room temp; ethane and propane are gases; butane just liquefies under pressure; pentane and beyond are liquids; about C18+ becomes solid (paraffin wax).

Homologous series = same functional group, same general formula, similar reactions.
Successive members differ by CH₂.
Boiling point INCREASES along the series (chain length → stronger intermolecular forces).

Saturated vs unsaturated

All single bonds = saturated. Any C=C = unsaturated.

Important distinction within hydrocarbons:

Saturated: ALL carbon-carbon bonds are SINGLE bonds. The molecule contains as many hydrogens as it possibly can. Alkanes are saturated.

Unsaturated: contains AT LEAST ONE C=C double bond (or triple bond). 'Unsaturated' because you could in principle add more H atoms across the double bond to make it saturated. Alkenes are unsaturated.

How to TEST whether a sample is saturated or unsaturated: bromine water.

Bromine water (orange) shaken with an UNSATURATED compound → DECOLOURISES (goes colourless). Bromine adds across the C=C double bond.
Shaken with a SATURATED compound → stays orange. No reaction.

This is one of the most common MYP tests. Just remember:

ORANGE → COLOURLESS = double bond present = unsaturated.
ORANGE → STAYS ORANGE = no double bond = saturated.

This test is why margarine production (hydrogenating vegetable oils to make solid fats) is followed by checking with bromine water — the more the orange remains, the more saturated the product.

Saturated: only single bonds. Unsaturated: at least one C=C.
Bromine water + unsaturated: orange → colourless.
Bromine water + saturated: stays orange (no reaction).

How it’s examined

Criterion A tests recognition of functional groups and family names. Criterion C may include a bromine-water test for unsaturation.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Functional Groups

Step-by-step solutions to past-paper-style questions on functional groups, written exactly the way a tutor would explain them at the board.

1Identifying the functional group
Getting started• functional group
Question
Identify the functional group and family of each compound: (a) CH₃CH₂COOH, (b) CH₂=CHCH₃, (c) CH₃CH₂CH₂OH.
Step-by-step solution
1. Step 1
  (a) –COOH → carboxylic acid (propanoic acid).
2. Step 2
  (b) C=C → alkene (propene).
3. Step 3
  (c) –OH → alcohol (propan-1-ol).
Answer
(a) carboxylic acid, (b) alkene, (c) alcohol.
2Bromine water test
Building confidence• test for unsaturation
Question
Two clear liquids are tested with orange bromine water. Sample A turns the bromine water COLOURLESS; sample B stays orange. Which is the alkene and why?
Step-by-step solution
1. Step 1
  Bromine adds across a C=C double bond, so unsaturated compounds DECOLOURISE bromine water.
2. Step 2
  Sample A turned colourless → contains C=C → ALKENE.
3. Step 3
  Sample B stayed orange → no C=C → saturated (alkane).
Answer
Sample A is the alkene (decolourises bromine water).
3Boiling point trend
Stretch• homologous series
Question
Methane is a gas at room temp; pentane is a liquid; eicosane (C₂₀H₄₂) is a solid. Explain.
Step-by-step solution
1. Step 1
  All three are alkanes. They have the same kind of bonds inside but differ in chain length.
2. Step 2
  Longer molecules have more SURFACE CONTACT with their neighbours, so the INTERMOLECULAR FORCES are stronger.
3. Step 3
  Stronger intermolecular forces → higher melting and boiling points. So methane (CH₄): gas. Pentane (C₅): liquid. Eicosane (C₂₀): solid wax.
Answer
Longer alkanes have stronger intermolecular forces between molecules → higher MPs/BPs. Methane (short) is gas; pentane (medium) liquid; eicosane (long) solid.

Key Formulae — Functional Groups

The formulae you need to memorise for functional groups on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Alkane general formula
$CₙH₂ₙ₊₂$
$n$
number of carbons
When to use
Use to predict the formula of any straight-chain alkane.
Alkene general formula
$CₙH₂ₙ$
$n$
number of carbons
When to use
For alkenes with one C=C double bond.

Key Definitions and Keywords — Functional Groups

Definitions to memorise and the exact keywords mark schemes credit for functional groups answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Functional group
Examiner keyword
An atom or bond pattern in a molecule that determines its chemical reactions.
Homologous series
Examiner keyword
A family of compounds sharing the same functional group, same general formula and similar chemistry.
Saturated
Examiner keyword
A molecule with only single C-C bonds — no more H atoms can be added.
Unsaturated
Examiner keyword
A molecule with at least one C=C double bond (or C≡C triple bond).

Common Mistakes and Misconceptions — Functional Groups

The traps other students keep falling into on functional groups questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying the alkane turns bromine water colourless.
Why it happens
Confusion about which way the test works.
How to avoid it
REMEMBER: bromine adds across the C=C. So UNSATURATED (has C=C) → DECOLOURISES. Saturated → no change. 'C=C eats the colour'.
✕Saying alkane is CₙH₂ₙ.
Why it happens
Both alkanes and alkenes have similar formulae.
How to avoid it
Alkane = CₙH₂ₙ₊₂ (more H, no double bond). Alkene = CₙH₂ₙ (fewer H, one double bond). Each C=C removes 2 H.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Functional Groups — frequently asked questions

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Functional Groups

Short Study Notes in the form of Slides

Short Study Notes — Functional Groups

Detailed Notes

What you’ll learn

How it’s examined

1Identifying the functional group

2Bromine water test

3Boiling point trend

Alkane general formula

Alkene general formula

Functional group

Homologous series

Saturated

Unsaturated

✕Saying the alkane turns bromine water colourless.

✕Saying alkane is CₙH₂ₙ.

Practice questions

Past paper style quiz

Assess your understanding

Functional Groups — frequently asked questions