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How are simple organic compounds classified?
Identify and apply IUPAC nomenclature to simple organic compounds (alkanes, alkenes, alkynes, alcohols, carboxylic acids) and recognise their functional groups
A focused answer to the VCE Chemistry Unit 1 dot point on organic functional groups. Identifies the alkane, alkene, alkyne, alcohol and carboxylic-acid families, names compounds up to six carbons, and works the VCAA-style "identify functional groups in this molecule" task.
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What this dot point is asking
VCAA wants you to identify the functional groups of simple organic compounds and apply IUPAC nomenclature to compounds with up to six carbons.
Carbon chain root names
| Chain length | Prefix |
|---|---|
| 1 | meth- |
| 2 | eth- |
| 3 | prop- |
| 4 | but- |
| 5 | pent- |
| 6 | hex- |
Functional groups
| Family | Functional group | Suffix | Example |
|---|---|---|---|
| Alkane | C-C single bonds | -ane | CH methane, CH ethane |
| Alkene | C=C double bond | -ene | CH ethene |
| Alkyne | CC triple bond | -yne | CH ethyne |
| Alcohol | -OH (hydroxyl) | -ol | CHOH methanol |
| Aldehyde | -CHO | -al | HCHO methanal |
| Ketone | C=O (within chain) | -one | propanone |
| Carboxylic acid | -COOH | -oic acid | CHCOOH ethanoic acid |
| Ester | -COOR | -oate | methyl ethanoate |
| Amine | -NH IMATH_10 | -amine | methanamine |
Naming with substituents
For longer chains with branches or functional groups:
- Identify the longest carbon chain containing the highest-priority functional group.
- Number to give the highest-priority group the lowest locant.
- Name substituents with locants in alphabetical order.
Priority order (for numbering): carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkyne > alkane.
Worked examples
CHCH=CHCH. Four carbons (but-), double bond between C2 and C3 (numbered from the end nearer the bond): but-2-ene.
CHCHOH. Two carbons, hydroxyl on C1: ethan-1-ol (commonly written ethanol).
CHCH(CH)CHCH. Four-carbon chain with methyl branch at C2: 2-methylbutane.
Constitutional (structural) isomers
Same molecular formula, different connectivity.
CH has two structural isomers: butane (straight) and 2-methylpropane (branched).
CHO has two isomers: ethanol (CHCHOH) and methoxymethane (CHOCH, an ether).
Cis-trans (geometric) isomers in alkenes
The double bond prevents rotation. If each carbon of the double bond bears different substituents, two isomers exist: cis (same side) and trans (opposite sides).
But-2-ene exists as cis-but-2-ene and trans-but-2-ene.
Common traps
Wrong numbering direction. Give the highest-priority functional group the lowest locant; if there's a tie, lowest locants for substituents.
Missing the lowest-locant rule. 2-methyl... is preferred to 3-methyl... in a four-carbon chain.
Treating -OH as just an OH group without considering acidity. Alcohols are not acidic in the way carboxylic acids are.
Forgetting prefixes for multiple groups. Two double bonds in a six-carbon chain: hexa-2,4-diene.
In one sentence
Simple organic compounds are classified by their functional groups (alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, carboxylic acids, esters, amines) and named using IUPAC conventions with the root name from the longest carbon chain containing the highest-priority group, the suffix from the functional group, and locants chosen to give the highest-priority group the lowest number.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
Year 11 SAC3 marksName CH$_3$CH$_2$CH(OH)CH$_2$COOH.Show worked answer →
Identify functional groups: hydroxyl (-OH) on C3, carboxylic acid (-COOH) on C1.
Numbering from the carboxylic acid (highest-priority functional group): C1 is the carboxyl carbon, C3 has the OH.
Five-carbon chain (pent-). Carboxylic acid (-oic acid). Hydroxyl on C3 (3-hydroxy-).
Name: 3-hydroxypentanoic acid.
Markers reward functional group identification, correct priority of carboxylic acid for numbering, and IUPAC name format.
Related dot points
- Apply IUPAC nomenclature to name and write formulae for ionic, covalent and simple organic compounds
A focused answer to the VCE Chemistry Unit 1 dot point on nomenclature. Applies IUPAC rules to ionic compounds (cation followed by anion, balanced charges), covalent compounds (numerical prefixes), and simple organic compounds (root, suffix), and works the VCAA SAC-style name-the-compound task.
- Determine empirical and molecular formulae from mass-composition or percentage-composition data, and from combustion analysis
A focused answer to the VCE Chemistry Unit 1 dot point on formulae. Walks through the standard percent-composition-to-empirical-formula procedure (divide by atomic mass, divide by smallest, multiply for integers), uses molar mass to find molecular formula, and works the VCAA-style combustion-analysis question.
- the nature of covalent bonding, the construction of Lewis (electron-dot) structures, and the use of valence shell electron pair repulsion (VSEPR) theory to predict the shapes and polarity of simple molecules
A focused VCE Chemistry Unit 1 answer on covalent bonding. Covers the formation of covalent bonds, Lewis (electron-dot) structures including for ions, VSEPR-based shape prediction for the common geometries up to six electron pairs, and how shape plus electronegativity decide overall molecular polarity.