Apply IUPAC nomenclature, interpret structural, displayed and skeletal formulae, and describe and identify constitutional (structural) isomerism and stereoisomerism (cis-trans and optical isomerism)

What this dot point is asking

SEAB wants you to apply IUPAC nomenclature, interpret the different ways of representing organic molecules (structural, displayed and skeletal formulae), and describe and identify the types of isomerism: constitutional (structural) and stereoisomerism (cis-trans and optical). Drawing isomers and explaining optical isomerism are recurring questions across the organic papers.

The answer

IUPAC nomenclature

Naming follows a system: identify the longest carbon chain (the stem: meth, eth, prop, but, pent...), the functional group (the suffix: -ane, -ene, -ol, -al, -one, -oic acid), and any substituents (prefixes such as methyl-, chloro-) with locants (numbers) chosen to give the lowest set. For example, $\text{CH}_3\text{CH(OH)CH}_3$ is propan-2-ol.

Representing molecules

• Structural (condensed) formula: shows the grouping of atoms, e.g. $\text{CH}_3\text{CH}_2\text{OH}$.
• Displayed formula: shows every atom and every bond.
• Skeletal formula: lines represent C-C bonds, carbons and their hydrogens are implied, and only functional groups and heteroatoms are drawn. This is the fastest representation for larger molecules.

Structural (constitutional) isomerism

Structural isomers have the same molecular formula but a different connectivity of atoms. The three types:

• Chain isomerism: different carbon-chain branching (e.g. butane and 2-methylpropane).
• Positional isomerism: the functional group is on a different carbon (e.g. propan-1-ol and propan-2-ol).
• Functional group isomerism: different functional groups (e.g. an alcohol and an ether, both $\text{C}_2\text{H}_6\text{O}$).

Stereoisomerism

Stereoisomers have the same connectivity but a different arrangement in space. Two kinds at H2 level:

Cis-trans (geometric) isomerism arises about a C=C double bond (no rotation) when each doubly-bonded carbon carries two different groups. The cis isomer has the two like groups on the same side, the trans isomer on opposite sides.

Optical isomerism arises when a molecule has a chiral centre: a carbon bonded to four different groups. The molecule and its mirror image are non-superimposable; the two forms are called enantiomers.

Properties of enantiomers

The two enantiomers are identical in all ordinary physical properties except that they rotate the plane of plane-polarised light by equal amounts in opposite directions. They react identically with achiral reagents but can behave very differently with other chiral species, which is crucial in biology and pharmacy.

Examples in context

Example 1. Why one enantiomer of a drug can be harmful. Many drugs are chiral, and the two enantiomers can have very different effects because the body's enzymes and receptors are themselves chiral and bind only one shape. SEAB uses this pharmaceutical context to test the idea that enantiomers behave differently with chiral biological molecules even though their bulk properties are identical.

Example 2. Identifying isomers from a molecular formula. A common opening to an organic Paper 2 question gives a molecular formula such as $\text{C}_4\text{H}_8$ and asks for all isomers, expecting candidates to find chain, positional and (for alkenes) geometric isomers. Working systematically through the isomer types is exactly the skill being assessed.

Try this

Q1. Name the compound $\text{CH}_3\text{CH}_2\text{CH(CH}_3)\text{CH}_2\text{OH}$. [1 mark]

• Cue. 2-methylbutan-1-ol.

Q2. State which of propan-1-ol and methoxyethane shows functional group isomerism with the other, and give their shared molecular formula. [2 marks]

• Cue. They are functional group isomers (alcohol versus ether), shared formula $\text{C}_3\text{H}_8\text{O}$.

Q3. Identify the chiral centre, if any, in 2-hydroxypropanoic acid (lactic acid), $\text{CH}_3\text{CH(OH)COOH}$. [2 marks]

• Cue. Carbon 2 is bonded to H, OH, $\text{CH}_3$ and COOH (four different groups), so it is a chiral centre; the molecule has two enantiomers.