Isomerism: structural isomerism and stereoisomerism

· Isomers = compounds with the same molecular formula but different arrangements of atoms.
· Two main types required: structural isomerism and stereoisomerism.
· In exam questions, always check whether atoms are connected differently first; if not, check whether the 3D arrangement is different.
· You may need to identify, draw, or deduce possible isomers from a given molecular formula.

Structural isomerism

· Structural isomers have the same molecular formula but different structural formulae.
· This means the atoms are connected in a different order.
· Three types required: chain isomerism, positional isomerism, and functional group isomerism.
· Structural isomers usually have different chemical and physical properties because their structures and/or functional groups differ.

These diagrams compare structural isomers by showing how the same atoms can be connected in different ways. They are useful for practising how to recognise chain, positional, and functional group isomerism. Source

Chain isomerism

· Chain isomers have the same molecular formula but a different carbon skeleton.
· Usually caused by straight-chain vs branched-chain arrangements.
· Example pattern: one isomer has a longer unbranched chain, another has a shorter chain with a branch.
· Exam tip: when drawing chain isomers, keep the same number of carbon and hydrogen atoms.

Positional isomerism

· Positional isomers have the same carbon skeleton and same functional group, but the functional group is in a different position.
· Common examples involve changing the position of: –OH, halogen, or C=C.
· Example pattern: but-1-ene and but-2-ene are positional isomers.
· Exam tip: number the carbon chain carefully to show the lowest possible locant for the functional group or double bond.

Functional group isomerism

· Functional group isomers have the same molecular formula but different functional groups.
· This means they belong to different homologous series.
· Example pattern: aldehydes and ketones can be functional group isomers.
· Exam tip: check whether the molecule could be drawn with a completely different functional group while keeping the same molecular formula.

Stereoisomerism

· Stereoisomers have the same structural formula but a different arrangement of atoms in space.
· Two types required: geometrical isomerism and optical isomerism.
· Stereoisomers have the same atom connectivity, so do not confuse them with structural isomers.
· Exam tip: stereoisomerism often depends on restricted rotation or the presence of a chiral centre.

Geometrical isomerism: cis/trans isomerism

· Geometrical isomerism is a type of stereoisomerism caused by restricted rotation.
· In alkenes, restricted rotation is caused by the π bond in the C=C double bond.
· cis/trans isomerism occurs when each carbon in the C=C bond has two different groups attached.
· cis = similar or priority groups are on the same side of the double bond.
· trans = similar or priority groups are on opposite sides of the double bond.
· E/Z nomenclature is acceptable but not required for this syllabus topic.
· Exam tip: if one carbon of the C=C has two identical groups, cis/trans isomerism is not possible.

Geometrical isomerism in cyclic compounds

· Cyclic compounds can also show cis/trans isomerism because rotation is restricted by the ring structure.
· cis = substituents are on the same side of the ring.
· trans = substituents are on opposite sides of the ring.
· Exam tip: when given a cyclic structural formula, check whether two groups can be arranged above/below the ring in different ways.

Optical isomerism

· Optical isomerism occurs when a molecule has a chiral centre.
· A chiral centre is usually a carbon atom bonded to four different groups.
· A chiral centre gives rise to two optical isomers, called enantiomers.
· Enantiomers are non-superimposable mirror images.
· Compounds can contain more than one chiral centre, but meso compounds and diastereoisomer nomenclature are not required.
· Exam tip: mark each carbon attached to four single bonds, then check whether the four attached groups are all different.

This diagram shows the key idea of optical isomerism: a chiral centre can produce two mirror-image forms. These mirror images are enantiomers and cannot be superimposed exactly. Source

Identifying chiral centres

· Look for a tetrahedral carbon bonded to four different groups.
· Do not count a carbon as chiral if it has two identical atoms or groups attached.
· In displayed or structural formulae, compare the entire group attached to the carbon, not just the first atom.
· Chiral centres may appear in straight-chain, branched, or cyclic compounds.
· Exam tip: a carbon in a C=O or C=C bond is not normally a chiral centre because it is not bonded to four different groups.

Deducing possible isomers from a molecular formula

· Start by identifying the functional group possibilities from the formula.
· Draw possible carbon skeletons first to check for chain isomerism.
· Move the functional group or double bond to check for positional isomerism.
· Consider whether a different functional group is possible to check for functional group isomerism.
· Finally, check each structure for cis/trans isomerism and chiral centres.
· Exam tip: avoid duplicates by naming each structure or checking whether rotations/flips give the same molecule.

Common exam mistakes to avoid

· Do not call molecules stereoisomers if they have different structural formulae.
· Do not assume every alkene has cis/trans isomerism; each C of the C=C must have two different groups.
· Do not identify a chiral centre unless the carbon has four different groups.
· Do not confuse cis/trans isomerism with positional isomerism of a double bond.
· Do not include advanced terms such as meso compounds or diastereoisomers unless the question gives enough context; they are not required here.