Identify and apply IUPAC nomenclature to simple organic compounds (alkanes, alkenes, alkynes, alcohols, carboxylic acids) and recognise their functional groups

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

Functional groups

Naming with substituents

For longer chains with branches or functional groups:

1. Identify the longest carbon chain containing the highest-priority functional group.
2. Number to give the highest-priority group the lowest locant.
3. 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

CH$_3$CH=CHCH$_3$. Four carbons (but-), double bond between C2 and C3 (numbered from the end nearer the bond): but-2-ene.

CH$_3$CH$_2$OH. Two carbons, hydroxyl on C1: ethan-1-ol (commonly written ethanol).

CH$_3$CH(CH$_3$)CH$_2$CH$_3$. Four-carbon chain with methyl branch at C2: 2-methylbutane.

Constitutional (structural) isomers

Same molecular formula, different connectivity.

C$_4$H$_{10}$ has two structural isomers: butane (straight) and 2-methylpropane (branched).

C$_2$H$_6$O has two isomers: ethanol (CH$_3$CH$_2$OH) and methoxymethane (CH$_3$OCH$_3$, 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.

Examples in context

Example 1. Mornington Peninsula chardonnay aroma chemistry. Winemakers at Ten Minutes by Tractor and other Mornington Peninsula wineries identify aroma compounds using their functional groups. Ethanol, $\text{CH}_3 \text{CH}_2 \text{OH}$, is a primary alcohol producing the "alcoholic" note. Ethyl ethanoate, $\text{CH}_3 \text{COOCH}_2 \text{CH}_3$, is an ester with the characteristic fruity, nail-polish aroma at threshold around $7 \, \text{mg/L}$. Acetaldehyde (ethanal), $\text{CH}_3 \text{CHO}$, is an aldehyde from oxidation, giving green-apple notes at low levels but a sherry-like off-flavour above $125 \, \text{mg/L}$. The hexanoic acid in over-extracted whites is a $\text{C}_6$ carboxylic acid with cheese-like character. Each class of compound is recognised by its IUPAC suffix.

Example 2. Geelong Refinery (closed 2021) petrochemical streams. The Viva Energy Geelong Refinery historically separated crude oil into streams classified by carbon number and functional group. The straight-chain alkanes pentane through octane ($\text{C}_5 \text{H}_{12}$ to $\text{C}_8 \text{H}_{18}$) formed the petrol fraction. Cracking converted long alkanes to short alkenes such as ethene $\text{CH}_2 = \text{CH}_2$ for plastics. The naphtha reformer produced cyclic and branched hydrocarbons with higher octane ratings. Refinery wastewater was monitored for phenol (an alcohol on a benzene ring) and benzoic acid (a carboxylic acid). Knowing the functional-group suffix on each component (-ane, -ene, -ol, -oic acid) let chemists assign each peak in a GC-MS chromatogram.

Try this

Q1. Name each compound: (a) $\text{CH}_3 \text{CH}_2 \text{CH}_2 \text{OH}$, (b) $\text{CH}_3 \text{CH}_2 \text{COOH}$, (c) $\text{CH}_2 = \text{CHCH}_3$. [3 marks]

• Cue. (a) Propan-1-ol. (b) Propanoic acid. (c) Propene.

Q2. A four-carbon compound has formula $\text{C}_4 \text{H}_8 \text{O}_2$. (a) Suggest two structural isomers from different functional-group classes. (b) Name each. [4 marks]

• Cue. (a, b) Butanoic acid $\text{CH}_3 \text{CH}_2 \text{CH}_2 \text{COOH}$ (carboxylic acid); methyl propanoate $\text{CH}_3 \text{CH}_2 \text{COOCH}_3$ (ester); or ethyl ethanoate $\text{CH}_3 \text{COOCH}_2 \text{CH}_3$.

Q3. Consider but-2-ene and butan-2-ol. (a) Draw each structure. (b) Identify each functional group. (c) Explain why butan-2-ol has a higher boiling point. [2+2+2 marks]

• Cue. (a) $\text{CH}_3 \text{CH} = \text{CHCH}_3$ and $\text{CH}_3 \text{CH(OH)CH}_2 \text{CH}_3$. (b) Alkene; alcohol. (c) Hydrogen bonding in alcohol; only dispersion in alkene.