Homologous Series, Functional Groups and Isomerism

Organic chemistry is organised into families. Learn the four features of a homologous series and the four functional groups, and the whole of organic chemistry becomes predictable.

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1. What a homologous series is

A HOMOLOGOUS SERIES is a family of compounds with: 1. The same GENERAL FORMULA 2. The same FUNCTIONAL GROUP 3. SIMILAR CHEMICAL properties 4. A GRADUAL CHANGE in PHYSICAL properties (boiling point rises with chain length) **5. Each member differs from the next by –CH₂–

Give at least three of these features if asked to define a homologous series — it is usually worth 2–3 marks.

Chemical properties are SIMILAR; physical properties change GRADUALLY. Mixing these up is the standard error — the functional group determines chemistry, so it stays the same; the chain length determines physical properties, so those trend.

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2. The four series

Series	General formula	Functional group	Ending
Alkanes	CₙH₂ₙ₊₂	none (all single bonds)	-ane
Alkenes	CₙH₂ₙ	C=C double bond	-ene
Alcohols	CₙH₂ₙ₊₁OH	–OH hydroxyl	-ol
Carboxylic acids	CₙH₂ₙ₊₁COOH	–COOH carboxyl	-oic acid

Learn the general formulae exactly. Incorrectly identifying the homologous series, and giving the wrong general formula, were both recorded.

The stems by carbon number:

Carbons	Stem
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-

So: methane, ethene, propanol, butanoic acid.

Read the ending to identify the family. Ethane (-ane) is an alkane; ethene (-ene) is an alkene; ethanol (-ol) is an alcohol; ethanoic acid (-oic acid) is a carboxylic acid. These four look and sound similar, and confusing them is the commonest slip in organic chemistry.

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3. Types of formula

Type	What it shows	Example (ethanol)
Molecular	the numbers of each atom	C₂H₆O
Structural	the arrangement, condensed	CH₃CH₂OH
Displayed	every atom and every bond drawn	full diagram with C–H and O–H bonds shown
Empirical	simplest ratio	C₂H₆O

A DISPLAYED formula must show EVERY bond, including all the C–H bonds. Difficulty with structural and displayed formulae was recorded — the commonest failure is leaving hydrogens off or not drawing their bonds.

Check every carbon has exactly FOUR bonds. This single check catches almost every drawing error.

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4. Isomerism

ISOMERS are compounds with the SAME MOLECULAR FORMULA but a DIFFERENT STRUCTURAL ARRANGEMENT of atoms.

Example — C₄H₁₀ has two isomers:

• butane — a straight chain of four carbons
• methylpropane — a three-carbon chain with a CH₃ branch

Example — C₃H₇OH:

• propan-1-ol — the –OH on the end carbon
• propan-2-ol — the –OH on the middle carbon

Isomers must have the SAME molecular formula. If the formulae differ, they are different compounds, not isomers.

The position number tells you where the functional group is — propan-1-ol vs propan-2-ol. Include it whenever there is a choice.

Isomers have the same molecular formula but different physical properties (such as boiling point), because of their different shapes.

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5. Trends within a series

As the chain gets longer:

Boiling point INCREASES (stronger intermolecular forces between larger molecules) Viscosity INCREASES Volatility DECREASES Flammability DECREASES

Chemical reactions stay the same throughout the series, because the functional group is unchanged — which is why you can predict the reactions of butanol from those of ethanol.

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6. Identifying an unknown compound

Use the tests:

Test	Result	Family
Bromine water	decolourised	alkene (unsaturated)
Bromine water	stays orange	alkane (saturated)
Universal indicator / pH	acidic	carboxylic acid
Add a carbonate	effervescence (CO₂)	carboxylic acid
Neutral, burns cleanly		alcohol

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7. Mistakes that cost marks

Wrong general formula for a series.

Confusing -ane with -ene, or -ol with -oic acid.

Saying physical properties are similar and chemical ones vary — it is the reverse.

Incomplete displayed formulae — missing hydrogens or bonds.

Carbons with the wrong number of bonds.

Calling compounds isomers when the molecular formulae differ.

Omitting the position number when naming.

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Frequently asked questions

What is a homologous series? A family of compounds with the same general formula and functional group, similar chemical properties and a gradual change in physical properties.

What is a functional group? The group of atoms that gives a series its characteristic chemical reactions.

What is the general formula for alkanes? CₙH₂ₙ₊₂. Alkenes are CₙH₂ₙ.

What is the functional group in alcohols? –OH.

What ending do carboxylic acids have? -oic acid — and the group is –COOH.

What is a displayed formula? One showing every atom and every bond.

What are isomers? Compounds with the same molecular formula but a different structure.

How many isomers does C₄H₁₀ have? Two — butane and methylpropane.

Why do members of a series react similarly? They share the same functional group.

How do I test for an alkene? Bromine water turns from orange to colourless.

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Quick revision checklist

• I can define a homologous series with at least three features
• I know chemical properties are similar and physical ones trend
• I know all four general formulae
• I know all four functional groups
• I know the stems meth-, eth-, prop-, but-
• I can identify the family from the ending
• I can write molecular, structural and displayed formulae
• I check every carbon has four bonds
• I can define and draw isomers
• I include position numbers when naming
• I know the trends with chain length
• I can identify a family using the chemical tests

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These notes cover homologous series, functional groups and isomerism in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.