Nomenclature and isomerism: TCE Chemistry (Tasmania)

What this dot point is asking

TASC expects you to name organic compounds systematically and to identify and draw their structural and geometric isomers.

IUPAC naming rules

Build the name in three parts:

• Stem: the number of carbons in the longest chain that contains the main functional group (meth, eth, prop, but, pent, hex, and so on).
• Suffix: the main functional group ($-ane$, $-ene$, $-ol$, $-al$, $-one$, $-oic\ acid$, $-oate$, $-amine$, $-amide$).
• Prefixes: branches and other substituents, each with a locant (number) showing its position.

Number the chain from the end that gives the main functional group the lowest locant. List substituents alphabetically, and use di, tri and tetra for repeated groups (these multiplying prefixes are ignored when alphabetising).

Structural isomerism

For example, $\text{C}_4\text{H}_{10}$ exists as butane (straight chain) and 2-methylpropane (branched). Chain branching lowers the boiling point because branched molecules pack less closely, so the dispersion forces between them are weaker. Functional-group isomerism is why $\text{C}_2\text{H}_6\text{O}$ can be either ethanol (an alcohol) or methoxymethane (an ether), with very different properties.

Cis-trans (geometric) isomerism

A carbon-carbon double bond cannot rotate, so the groups attached to it are fixed in position. When each double-bond carbon carries two different groups, two arrangements exist: cis (matching groups on the same side) and trans (on opposite sides). These geometric isomers can have different physical properties, such as melting and boiling points, because their shapes and polarities differ; cis-2-butene, for instance, is slightly polar while trans-2-butene is non-polar.

Priority of functional groups

When a molecule contains more than one functional group, only one is the principal group (named as the suffix) and the others become prefixes. A simplified priority order, highest first, is: carboxylic acid, then ester, then aldehyde, then ketone, then alcohol, then amine. For example, a compound containing both a hydroxyl and a carboxyl group is named as a hydroxy-substituted acid (the acid takes the suffix), as in 2-hydroxypropanoic acid (lactic acid). Getting the priority right ensures the chain is numbered to give the most senior group the lowest locant.

How isomerism affects properties

Isomers can behave very differently despite sharing a molecular formula. Chain isomers differ in boiling point because branching changes how closely molecules pack and therefore the strength of dispersion forces. Positional and functional-group isomers can differ in chemical reactivity: a primary alcohol and a secondary alcohol of the same formula give different oxidation products, and an alcohol and its isomeric ether differ enormously in boiling point because only the alcohol can hydrogen bond. Recognising the isomer present is therefore essential before predicting reactions, and analytical techniques such as NMR and mass spectrometry are used to tell isomers apart.

In the exam, identify and number the longest chain through the main functional group, name substituents alphabetically with locants, and check for chain, positional, functional-group and geometric isomers whenever you are asked for isomers of a formula.