Functional Groups Revision Notes for HSC SSCE Chemistry

Functional Groups

Introduction to organic compound families

Organic molecules are organized into groups or families of related compounds. These families share similar chemical properties and behaviours. These characteristics come from specific groups of atoms within the molecules, called functional groups.

When organic compounds have the same functional group but different carbon chain lengths, they form what we call a homologous series. A homologous series is a group of organic chemicals with the same functional group, varying only in the length of the carbon chain. Each homologous series can be represented by a general molecular formula.

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This classification system is extremely valuable for chemists and students alike. Understanding how one member of a family behaves helps you predict the properties and reactions of other members in the same family. Similarly, if you know how to chemically modify one compound in a family, you can apply the same method to modify other members.

What are functional groups?

A functional group is a specific atom or group of atoms that gives a family of organic compounds its characteristic properties. For example, compounds containing the hydroxyl group (—OH) are called alcohols. Alcohols share many common properties because the hydroxyl group is the most reactive part of the molecule. This means compounds within this group undergo similar types of reactions.

In chemistry, we use symbols like R and R' to represent different alkyl groups in general formulas. These R groups can be:

A single hydrogen atom
A carbon chain of any length
A straight chain, branched chain, cyclic, or aromatic structure

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The R group notation is a powerful tool that allows us to write general formulas that apply to entire families of compounds, making it easier to understand and predict their behaviour across different chain lengths and structures.

Major functional groups you need to know

The table below shows eight important functional groups that are part of your chemistry course. Each functional group has:

A specific class name
A characteristic suffix used in naming compounds
A distinctive functional group structure
A general structure showing how it connects to R groups
A specific example with its chemical name

Class	Suffix	Functional group	General structure	Example	Name of example
Haloalkane	–ane	Halogen (F, Cl, Br, I)	$\text{R}—\text{X}$	$\text{CH}_3\text{CH}_2\text{Br}$	Bromoethane
Alcohol	–ol	—OH	$\text{R}—\text{OH}$	$\text{CH}_3\text{CH}_2\text{OH}$	Ethanol
Aldehyde	–al	$—\text{C}(=\text{O})\text{H}$	$\text{R}—\text{C}(=\text{O})\text{H}$	$\text{CH}_3\text{CH}_2\text{CHO}$	Propanal
Ketone	–one	$—\text{C}(=\text{O})—$	$\text{R}—\text{C}(=\text{O})—\text{R}'$	$\text{CH}_3\text{COCH}_3$	Propanone
Carboxylic acid	–oic acid	$—\text{C}(=\text{O})\text{OH}$	$\text{R}—\text{C}(=\text{O})\text{OH}$	$\text{CH}_3\text{COOH}$	Ethanoic acid
Ester	alkyl –oate	$—\text{C}(=\text{O})\text{O}—$	$\text{R}—\text{C}(=\text{O})\text{O}—\text{R}'$	$\text{CH}_3\text{COOCH}_3$	Methyl ethanoate
Amine	–amine	$—\text{NH}_2$	$\text{R}—\text{NH}_2$	$\text{CH}_3\text{CH}_2\text{NH}_2$	Ethanamine
Amide	–amide	$—\text{C}(=\text{O})\text{NH}_2$	$\text{R}—\text{C}(=\text{O})\text{NH}_2$	$\text{HCONH}_2$	Methanamide

Understanding each functional group

Haloalkanes contain halogen atoms (fluorine, chlorine, bromine, or iodine) attached to a carbon chain. They use the suffix –ane in their names.

Alcohols contain the hydroxyl group (—OH) and use the suffix –ol. This functional group makes alcohols polar and able to form hydrogen bonds.

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Distinguishing Aldehydes and Ketones by Position:

Both aldehydes and ketones contain a carbonyl group ( $\text{C}=\text{O}$ ), but their positions differ:

Aldehydes: Carbonyl group at the end of the carbon chain with a hydrogen atom attached ( $—\text{CHO}$ ). Suffix: –al
Ketones: Carbonyl group within the carbon chain, bonded to two carbon atoms. Suffix: –one

This positional difference leads to different chemical properties and reactivities.

Carboxylic acids contain both a carbonyl group and a hydroxyl group combined as —COOH. This gives them acidic properties. They use the suffix –oic acid.

Esters form when a carboxylic acid reacts with an alcohol. They contain the $—\text{C}(=\text{O})\text{O}—$ linkage and use the suffix alkyl –oate.

Amines contain a nitrogen atom bonded to hydrogen atoms and carbon chains. The functional group is $—\text{NH}_2$ , and they use the suffix –amine.

Amides combine a carbonyl group with a nitrogen atom as $—\text{C}(=\text{O})\text{NH}_2$ . They use the suffix –amide.

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Common Mistake to Avoid:

Don't confuse the suffix with the functional group itself! The suffix (like –ol, –al, –one) is used in the compound's name, while the functional group (like —OH, —CHO, —CO—) is the actual atom or group of atoms in the molecule's structure. Always check both the name and the structure when identifying compounds.

Exam tips

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Exam Success Strategies:

When identifying functional groups, look for the characteristic atom or group of atoms — this is your key identifier
Remember that the suffix in the name tells you which functional group is present (–ol for alcohols, –al for aldehydes, etc.)
Practice drawing structural formulas for each functional group to become familiar with their arrangements
Understand that compounds in the same homologous series have similar chemical properties but different physical properties (like boiling points) due to chain length differences
Always double-check the position of functional groups — aldehydes have carbonyl groups at the end, while ketones have them in the middle

Remember!

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Key Points to Remember:

Functional groups are specific atoms or groups of atoms that determine the properties and reactions of organic compounds
A homologous series is a family of compounds with the same functional group but different carbon chain lengths, represented by a general molecular formula
The eight major functional groups are: haloalkanes (—X), alcohols (—OH), aldehydes (—CHO), ketones (—CO—), carboxylic acids (—COOH), esters (—COO—), amines (—NH₂), and amides (—CONH₂)
Each functional group has a characteristic suffix used in naming: –ane, –ol, –al, –one, –oic acid, alkyl –oate, –amine, and –amide respectively
The same functional group means similar chemical properties across the entire homologous series

Functional Groups (HSC SSCE Chemistry): Revision Notes