QCE Chemistry - Unit 4 - Properties and structure of organic materials

Structure of Organic Compounds | QCE Chemistry

Revise homologous series, functional groups, naming and structural representations for QCE Chemistry organic compounds.

Updated 2026-05-17 - 5 min read

Organic chemistry can feel like a new language at first, but most of it comes down to two questions:

1. What carbon skeleton does the molecule have?
2. What functional group is attached to it?

The carbon skeleton tells you the size and shape of the molecule. The functional group tells you the part that usually controls reactivity, polarity, naming priority and many physical properties.

Carbon skeletons and homologous series

A hydrocarbon contains only carbon and hydrogen. The main hydrocarbon families are:

| Family | General feature | Example | | --- | --- | --- | | Alkane | only single carbon-carbon bonds | ethane, $\mathrm{CH_3CH_3}$ | | Alkene | at least one carbon-carbon double bond | ethene, $\mathrm{CH_2=CH_2}$ | | Alkyne | at least one carbon-carbon triple bond | ethyne, $\mathrm{HCCH}$ | | Aromatic compound | benzene ring or related aromatic system | benzene, methylbenzene |

A homologous series is a family of compounds with the same functional group and similar chemical reactions. Consecutive members usually differ by $\mathrm{CH_2}$. For example, methane, ethane, propane and butane are all alkanes.

This matters because you can often predict trends. As the carbon chain grows, boiling point usually increases. As the same functional group appears in different molecules, similar reaction patterns appear.

Functional groups

Common QCE functional groups include:

| Functional group | Pattern to recognise | Example name | | --- | --- | --- | | Alkene | $\mathrm{C=C}$ | propene | | Haloalkane | $\mathrm{C-X}$, where $\mathrm{X}$ is F, Cl, Br or I | 1-bromopropane | | Alcohol | $\mathrm{-OH}$ on a carbon chain | propan-1-ol | | Aldehyde | terminal $\mathrm{-CHO}$ | propanal | | Ketone | internal $\mathrm{C=O}$ | propanone | | Carboxylic acid | $\mathrm{-COOH}$ | propanoic acid | | Ester | $\mathrm{-COO^-}$ between two carbon groups | ethyl ethanoate | | Amine | $\mathrm{-NH_2}$, $\mathrm{-NHR}$ or $\mathrm{-NR_2}$ | ethylamine | | Amide | $\mathrm{-CONH_2}$, $\mathrm{-CONHR}$ or $\mathrm{-CONR_2}$ | ethanamide |

Start by identifying the highest-priority functional group. For many school-level naming questions, the suffix gives this away: $\mathrm{-ol}$, $\mathrm{-al}$, $\mathrm{-one}$, $\mathrm{-oic\ acid}$, $\mathrm{-oate}$, $\mathrm{-amine}$ or $\mathrm{-amide}$.

Representations

The same molecule can be represented in several ways:

| Representation | What it shows well | Example for propan-1-ol | | --- | --- | --- | | Molecular formula | total numbers of each atom | $\mathrm{C_3H_8O}$ | | Structural formula | atoms and bonds clearly | $\mathrm{CH_3-CH_2-CH_2-OH}$ | | Condensed structural formula | structure in compact text form | $\mathrm{CH_3CH_2CH_2OH}$ | | Skeletal formula | carbon framework efficiently | line-angle drawing |

In a skeletal formula:

• every line end and vertex is a carbon atom unless another atom is written
• hydrogens attached to carbon are not usually shown
• hydrogens attached to heteroatoms, such as $\mathrm{O-H}$ or $\mathrm{N-H}$, are usually shown
• multiple bonds are drawn explicitly

For example, a three-carbon zigzag with $\mathrm{OH}$ at the end represents propan-1-ol, not ethanol. The line has two ends and one bend, so it has three carbon atoms.

Naming organic compounds

A reliable naming method is:

1. Find the highest-priority functional group.
2. Choose the longest carbon chain containing that group where required.
3. Number the chain so the main functional group gets the lowest possible number.
4. Name substituents, such as methyl, ethyl, fluoro, chloro, bromo or iodo.
5. Put the name together with position numbers.

For example, $\mathrm{CH_3CH(OH)CH_2CH_3}$ is butan-2-ol. The parent chain has four carbons, so it is based on butane. The $\mathrm{-OH}$ group is on carbon 2, so the suffix becomes $\mathrm{-2-ol}$.

Isomers

Isomers have the same molecular formula but different arrangements of atoms.

Structural isomers have different connectivity. For example, $\mathrm{C_3H_8O}$ can be propan-1-ol, propan-2-ol or methoxyethane. Same formula, different bonding pattern.

Stereoisomers have the same connectivity but different three-dimensional arrangement. In QCE Chemistry, you may meet geometric isomers around a restricted bond, such as a carbon-carbon double bond, where rotation is not free.

How to describe structure in exam answers

When explaining a structure, avoid vague phrases like "it has oxygen" or "it is organic". Be specific:

• "The molecule contains a carboxylic acid functional group, $\mathrm{-COOH}$."
• "It is a secondary alcohol because the carbon bonded to $\mathrm{-OH}$ is attached to two other carbon atoms."
• "The molecule has a carbon-carbon double bond, so it is unsaturated."

These sentences give the marker chemical evidence, not just labels.

Quick check

Exam traps worth knowing

• Counting carbons incorrectly in skeletal formulas.
• Naming the longest chain before checking the highest-priority functional group.
• Treating molecules with the same formula as identical without checking structure.
• Forgetting that the carbon in $\mathrm{-CHO}$, $\mathrm{-COOH}$, $\mathrm{-COO^-}$ or $\mathrm{-CONH_2}$ is part of the parent chain when naming many compounds.

Sources

• QCAA Chemistry subject page
• QCAA Chemistry 2025 syllabus
• OpenStax Chemistry 2e: Hydrocarbons
• OpenStax Chemistry 2e: Alcohols and Ethers
• OpenStax Chemistry 2e: Aldehydes, Ketones, Carboxylic Acids, and Esters
• OpenStax Chemistry 2e: Amines and Amides