Homologous series (alkanes, alkenes, haloalkanes, alcohols, aldehydes, ketones, acids, esters, amides, amines)

Q: How are haloalkanes classified and what is their characteristic chemical reactivity?

Haloalkanes are classified as primary (1°), secondary (2°), or tertiary (3°) based on alkyl groups attached to the C-X carbon; they undergo nucleophilic substitution and elimination reactions due to the polar C-X bond.

Q: How does the hydroxyl group affect the physical and oxidation properties of alcohols?

The \( -\text{OH} \) group allows intermolecular hydrogen bonding, causing high boiling points and water solubility; primary and secondary alcohols can be oxidised, whereas tertiary alcohols resist oxidation.

Q: How do aldehydes and ketones differ in structure, naming, and oxidisability?

Aldehydes have a terminal carbonyl group (\( -\text{CHO} \), suffix -al ) and oxidise easily, whereas ketones have an internal carbonyl group (\( >\text{C}=\text{O} \), suffix -one ) and resist mild oxidation.

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Chemistry · Senior Secondary (HKDSE) · 8 min read · Updated: 21 Aug 2026

Study Notes: Homologous Series in Organic Chemistry

Hello! Welcome to the fascinating world of organic chemistry. Think of it like learning about different families. In chemistry, these families are called homologous series. Each family has a unique "surname" (its functional group) and its members share a strong family resemblance (similar chemical properties).

In these notes, we'll explore the main families of carbon compounds in the HKDSE syllabus. We'll learn how to identify them, represent their structures, give them their proper IUPAC names, and understand why they behave the way they do.

The Basics: What are Homologous Series?

So, what exactly is a Homologous Series?

A homologous series is a group or 'family' of organic compounds that follow a set of simple rules. To be in the same family, all members must have:

The same functional group (responsible for characteristic chemical properties).
The same general formula.
Each successive member differs from the previous one by a -CH₂- unit (a molar mass difference of \( 14\text{ g mol}^{-1} \)).
Similar chemical properties due to having the same functional group.
A gradual gradation in physical properties (e.g., boiling point increases as molecular size and London dispersion forces increase).

What is a Functional Group?

A functional group is an atom or group of atoms in a molecule that is largely responsible for the characteristic chemical reactions of that compound.

Representing Organic Molecules

In HKDSE Chemistry, you need to be comfortable interpreting and drawing carbon compounds in several standard representations:

Displayed Formula: Shows every atom and all the covalent bonds between them.
Condensed Structural Formula: Shows the arrangement of atoms group-by-group without showing all individual single bonds (e.g., \( \text{CH}_3\text{CH}_2\text{OH} \) or \( \text{CH}_3(\text{CH}_2)_2\text{CH}_3 \)).
Skeletal Formula: Shows only the carbon framework as lines (vertices and line ends represent carbon atoms with attached hydrogens omitted); functional groups and heteroatoms (like -OH, -Cl) are explicitly shown.

Naming Organic Compounds: The IUPAC System

Chemists use a systematic IUPAC nomenclature system to name carbon compounds accurately.

The 3-Step Guide to Naming

Step 1: Find the 'Stem' (the parent chain)

Find the longest continuous carbon chain containing the principal functional group:

1 Carbon: Meth-
2 Carbons: Eth-
3 Carbons: Prop-
4 Carbons: But-
5 Carbons: Pent-
6 Carbons: Hex-
7 Carbons: Hept-
8 Carbons: Oct-

Step 2: Find the 'Suffix' or 'Prefix' (the functional group)

Identify the principal functional group to assign the suffix (e.g., -ane, -ene, -ol, -al, -one, -oic acid) or substituent prefixes (e.g., chloro-, bromo-, methyl-, ethyl-).

Step 3: Number the Chain

Number the carbon backbone from the end that gives the principal functional group (or substituents) the lowest possible locant number.

The 10 Homologous Series Families

1. The Alkanes (Saturated Hydrocarbons)

These are saturated hydrocarbons containing only single covalent bonds between carbon and hydrogen atoms.

Functional Group:

None (no specific reactive functional group; only C-C and C-H single bonds)

General Formula:

\( \text{C}_n\text{H}_{2n+2} \)

Naming (Suffix):

Ends in -ane (e.g., methane, propane, 2-methylbutane).

Physical Properties:

Non-polar molecules held by van der Waals' forces (dispersion forces). Boiling points increase with chain length and decrease with chain branching (due to reduced surface contact area).

Chemical Properties:

Generally unreactive, but undergo complete/incomplete combustion and free radical substitution with halogens under UV light.

2. The Alkenes (Unsaturated Hydrocarbons)

Hydrocarbons that contain at least one carbon-carbon double bond (\( >\text{C}=\text{C}< \)).

Functional Group:

Carbon-carbon double bond (>C=C<)

General Formula:

\( \text{C}_n\text{H}_{2n} \) (for acyclic alkenes with one double bond)

Naming (Suffix):

Ends in -ene (e.g., ethene, propene, but-1-ene, but-2-ene).

Chemical Properties:

The electron-rich double bond readily undergoes electrophilic addition reactions (e.g., addition of \( \text{Br}_2 \), \( \text{H}_2 \), \( \text{HX} \), and \( \text{H}_2\text{O} \)) and addition polymerisation.

3. The Haloalkanes

Alkanes with one or more hydrogen atoms replaced by halogen atoms (F, Cl, Br, I).

Functional Group:

Halogen atom (-X)

General Formula:

\( \text{C}_n\text{H}_{2n+1}\text{X} \) (for mono-haloalkanes)

Classification:

Primary (1°): Halogen-bearing carbon is bonded to 1 alkyl group (or 0, for \( \text{CH}_3\text{X} \)). E.g., 1-chloropropane.
Secondary (2°): Halogen-bearing carbon is bonded to 2 alkyl groups. E.g., 2-chloropropane.
Tertiary (3°): Halogen-bearing carbon is bonded to 3 alkyl groups. E.g., 2-chloro-2-methylpropane.

Naming (Prefix):

Uses prefixes: fluoro-, chloro-, bromo-, iodo- with position locants (e.g., 2-bromobutane).

Chemical Properties:

The polar C-X bond makes haloalkanes susceptible to nucleophilic substitution (e.g., with aqueous \( \text{NaOH} \) to form alcohols) and elimination to form alkenes.

4. The Alcohols (Alkanols)

Compounds containing a hydroxyl group bonded to a saturated carbon atom.

Functional Group:

Hydroxyl group (-OH)

General Formula:

\( \text{C}_n\text{H}_{2n+1}\text{OH} \)

Classification:

Primary (1°): -OH carbon is attached to 1 alkyl group. E.g., propan-1-ol. (Oxidises to aldehyde, then carboxylic acid).
Secondary (2°): -OH carbon is attached to 2 alkyl groups. E.g., propan-2-ol. (Oxidises to ketone).
Tertiary (3°): -OH carbon is attached to 3 alkyl groups. E.g., 2-methylpropan-2-ol. (Resistant to oxidation with acidified \( \text{K}_2\text{Cr}_2\text{O}_7 \) or \( \text{KMnO}_4 \)).

Naming (Suffix):

Ends in -ol (e.g., ethanol, butan-1-ol, butan-2-ol).

Physical Properties:

Intermolecular hydrogen bonding gives alcohols significantly higher boiling points than alkanes of comparable molecular mass. Small alcohols (\( C_1 - C_3 \)) are completely miscible with water.

5. The Aldehydes

Carbonyl compounds where the carbonyl carbon is at the terminal position of the chain, bonded to at least one hydrogen atom.

Functional Group:

Aldehyde group (-CHO)

Naming (Suffix):

Ends in -al (e.g., methanal, ethanal, propanal). No number locant is needed because -CHO is always carbon-1.

Chemical Properties:

Easily oxidised by acidified \( \text{K}_2\text{Cr}_2\text{O}_7 \) or \( \text{KMnO}_4 \) to form carboxylic acids, and reduced by \( \text{NaBH}_4 \) or \( \text{LiAlH}_4 \) to primary alcohols.

6. The Ketones

Carbonyl compounds where the carbonyl group is located between two carbon atoms within the carbon chain.

Functional Group:

Carbonyl group (>C=O)

Naming (Suffix):

Ends in -one. Propanone and butanone do not require numbering, but for chains of 5 carbons or more, numbering is required to prevent ambiguity (e.g., pentan-2-one vs. pentan-3-one).

Chemical Properties:

Resistant to mild oxidation, but can be reduced to secondary alcohols using reducing agents like \( \text{NaBH}_4 \).

7. The Carboxylic Acids

Organic acids containing the carboxyl group at the end of the carbon chain.

Functional Group:

Carboxyl group (-COOH)

General Formula:

\( \text{C}_n\text{H}_{2n+1}\text{COOH} \) (or \( \text{R-COOH} \))

Naming (Suffix):

Ends in -oic acid (e.g., methanoic acid, ethanoic acid).

Physical Properties:

Exhibit very high boiling points because pairs of molecules form stable hydrogen-bonded dimers.

Chemical Properties:

Weak monobasic acids that react with bases, carbonates, and reactive metals. React with alcohols in the presence of concentrated \( \text{H}_2\text{SO}_4 \) catalyst to form esters (esterification).

8. The Esters

Volatile, sweet/fruity smelling compounds formed by condensing a carboxylic acid with an alcohol.

Functional Group:

Ester group (-COO-)

Naming:

Consists of two parts:

Alkyl group from the parent alcohol: ending in -yl.
Carboxylate group from the parent acid: ending in -oate.

Example: Ethanol + Propanoic acid \( \rightarrow \) ethyl propanoate + \( \text{H}_2\text{O} \).

Chemical Properties:

Undergo acid hydrolysis (reversible, yielding acid + alcohol) or alkaline hydrolysis / saponification (irreversible, yielding carboxylate salt + alcohol).

9. The Amides (Primary / Unsubstituted)

Carboxylic acid derivatives where the -OH group is replaced by an amino group (-NH₂).

Functional Group:

Amide group (-CONH₂)

Naming (Suffix):

Ends in -amide (e.g., methanamide, ethanamide).

Chemical Properties:

Can undergo hydrolysis upon prolonged heating with strong aqueous acid or alkali to yield carboxylic acid/carboxylate and ammonium/ammonia.

10. The Amines

Basic organic nitrogen compounds derived from ammonia (\( \text{NH}_3 \)) by replacing one or more hydrogen atoms with alkyl groups.

Functional Group:

Amino group (-NH₂ for primary amines)

Classification:

Primary (1°) amine: One alkyl group attached to nitrogen (\( \text{R-NH}_2 \)).
Secondary (2°) amine: Two alkyl groups attached to nitrogen (\( \text{R}_2\text{NH} \)).
Tertiary (3°) amine: Three alkyl groups attached to nitrogen (\( \text{R}_3\text{N} \)).

General Formula (Primary aliphatic amines):

\( \text{C}_n\text{H}_{2n+1}\text{NH}_2 \)

Naming (Suffix):

Ends in -amine (e.g., methanamine, ethanamine, propan-1-amine).

Chemical Properties:

Act as Lewis bases and Brønsted-Lowry bases due to the lone pair of electrons on the nitrogen atom, reacting with acids to form alkylammonium salts.

Summary of Homologous Series

Here is a quick reference guide to the 10 key homologous series:

Series: Alkanes
Functional Group: None (saturated C-C, C-H)
Suffix / Prefix: -ane
Example: Ethane (\( \text{CH}_3\text{CH}_3 \))

Series: Alkenes
Functional Group: >C=C<
Suffix / Prefix: -ene
Example: Ethene (\( \text{CH}_2=\text{CH}_2 \))

Series: Haloalkanes
Functional Group: -X (where X = F, Cl, Br, I)
Suffix / Prefix: halo- (e.g., chloro-)
Example: Chloroethane (\( \text{CH}_3\text{CH}_2\text{Cl} \))

Series: Alcohols
Functional Group: -OH
Suffix / Prefix: -ol
Example: Ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \))

Series: Aldehydes
Functional Group: -CHO
Suffix / Prefix: -al
Example: Ethanal (\( \text{CH}_3\text{CHO} \))

Series: Ketones
Functional Group: >C=O
Suffix / Prefix: -one
Example: Propanone (\( \text{CH}_3\text{COCH}_3 \))

Series: Carboxylic Acids
Functional Group: -COOH
Suffix / Prefix: -oic acid
Example: Ethanoic acid (\( \text{CH}_3\text{COOH} \))

Series: Esters
Functional Group: -COO-
Suffix / Prefix: -yl ... -oate
Example: Methyl ethanoate (\( \text{CH}_3\text{COOCH}_3 \))

Series: Amides
Functional Group: -CONH₂
Suffix / Prefix: -amide
Example: Ethanamide (\( \text{CH}_3\text{CONH}_2 \))

Series: Amines
Functional Group: -NH₂ (primary)
Suffix / Prefix: -amine
Example: Ethanamine (\( \text{CH}_3\text{CH}_2\text{NH}_2 \))

Well done for working through all these families! Practice identifying functional groups, sketching skeletal and displayed structures, and naming branched chains to master HKDSE organic chemistry questions. You've got this!

References

Science Education - Chemistry (Curriculum Resources & Guides) — Education Bureau (EDB), HKSAR Government

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What are the key defining characteristics of a homologous series?

Members of a homologous series share the same functional group and general formula, have similar chemical properties, show a gradation in physical properties, and differ successively by a \( -\text{CH}_2- \) unit (\( 14\text{ g mol}^{-1} \)).

How are organic molecules represented using displayed, condensed, and skeletal formulas?

Displayed formulas show every atom and bond; condensed structural formulas group atoms together without showing all single bonds; skeletal formulas show only the carbon framework as vertices/lines with heteroatoms explicitly drawn.

What are the three core steps in the IUPAC systematic naming of organic compounds?

1. Find the longest continuous carbon chain containing the principal functional group (stem).
2. Identify functional groups and substituents to determine suffix/prefixes.
3. Number the carbon chain to give the principal group or substituents the lowest possible locants.

What are the key structural features and chemical properties of alkanes?

Alkanes are saturated hydrocarbons with the general formula \( \text{C}_n\text{H}_{2n+2} \) containing only single C-C and C-H bonds, undergoing combustion and free radical substitution under UV light.

What is the functional group of alkenes, and why do they readily undergo addition reactions?

Alkenes contain a carbon-carbon double bond (\( >\text{C}=\text{C}< \)), whose high electron density makes them susceptible to electrophilic addition reactions.

How are haloalkanes classified and what is their characteristic chemical reactivity?

Haloalkanes are classified as primary (1°), secondary (2°), or tertiary (3°) based on alkyl groups attached to the C-X carbon; they undergo nucleophilic substitution and elimination reactions due to the polar C-X bond.

How does the hydroxyl group affect the physical and oxidation properties of alcohols?

The \( -\text{OH} \) group allows intermolecular hydrogen bonding, causing high boiling points and water solubility; primary and secondary alcohols can be oxidised, whereas tertiary alcohols resist oxidation.

How do aldehydes and ketones differ in structure, naming, and oxidisability?

Aldehydes have a terminal carbonyl group (\( -\text{CHO} \), suffix -al) and oxidise easily, whereas ketones have an internal carbonyl group (\( >\text{C}=\text{O} \), suffix -one) and resist mild oxidation.

Why do carboxylic acids have high boiling points and how do they react with alcohols?

Carboxylic acids form stable hydrogen-bonded dimers that elevate boiling points, and they react with alcohols via esterification in the presence of concentrated \( \text{H}_2\text{SO}_4 \) to form esters.

How are esters named, and what are the two main types of ester hydrolysis?

Esters are named as 'alkyl alkanoate' (alkyl from alcohol, alkanoate from carboxylic acid); they undergo reversible acid hydrolysis or irreversible alkaline hydrolysis (saponification).

What is the functional group of primary amides, and how are they structurally related to carboxylic acids?

Primary amides contain the \( -\text{CONH}_2 \) functional group (suffix -amide), formed conceptually by replacing the \( -\text{OH} \) of a carboxylic acid with an amino group (\( -\text{NH}_2 \)).

Why do amines act as bases, and how are they classified?

Amines act as bases due to the lone pair of electrons on the nitrogen atom which can accept a proton; they are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of alkyl groups on nitrogen.

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