13.4 Structural isomerism and stereoisomerism

Structural isomers have the same molecular formula but different atom connectivity. Chain isomerism changes the carbon skeleton, positional isomerism changes a group or multiple bond position, and functional-group isomerism changes the functional group.

Draw the connectivity before comparing names. The atoms and molecular formula stay constant, but the arrangement changes reactivity and often physical properties.

C₄H₁₀ gives butane and 2-methylpropane (chain isomers). C₃H₈O gives propan-1-ol and propan-2-ol (positional isomers), and also methoxyethane (functional-group isomer).

Different conformations from rotation around a single bond are not automatically different structural isomers.

Stereoisomers have the same atoms joined in the same order but differ in spatial arrangement. Geometrical isomerism occurs when rotation is restricted, while optical isomerism arises from a chiral centre and produces non-superimposable mirror images.

A C=C can give cis/trans forms only when each carbon has two different groups. A tetrahedral carbon is a chiral centre when it is bonded to four different groups.

cis-but-2-ene and trans-but-2-ene are geometrical stereoisomers. A molecule with one carbon attached to H, OH, CH₃ and CH₂CH₃ has an optical stereocentre.

Same molecular formula alone is insufficient. Check connectivity first, then test the structural condition for cis/trans or chirality.

Geometrical isomers have the same connectivity but different groups on either side of a double bond. A π bond requires sideways p-orbital overlap, so rotation would break that overlap and is restricted.

Each carbon of the C=C must have two different groups for cis/trans isomerism. “Cis” places corresponding groups on the same side; “trans” places them on opposite sides.

But-2-ene exists as cis-but-2-ene and trans-but-2-ene. But-1-ene does not give cis/trans forms because one double-bond carbon has two H atoms.

A single bond can rotate, so do not infer geometrical isomerism from any pair of substituents without checking the double-bond condition.

A chiral centre is usually a tetrahedral carbon attached to four different groups. Its mirror-image arrangement cannot be superimposed on the original, giving a pair of optical isomers called enantiomers.

Enantiomers have the same connectivity and most achiral physical properties, but rotate plane-polarised light in opposite directions and can behave differently with chiral environments.

In 2-butanol, the carbon bearing OH is attached to H, OH, CH₃ and CH₂CH₃, so it is chiral and gives two enantiomers.

A carbon with two identical groups is not chiral. Do not add advanced meso or diastereomer terminology when the syllabus does not require it.

To find a chiral centre, inspect each tetrahedral carbon for four different attached groups. To find geometrical isomerism, inspect each C=C—or suitable ring—for restricted rotation and two different groups at each relevant position.

Compare the paths around a ring as well as the atoms directly attached. A drawing can hide equivalence, so trace each substituent until the first point of difference.

A substituted cycloalkane can have cis and trans forms even without a C=C because the ring restricts rotation. A carbon with two identical ring paths is not a chiral centre.

Do not count every wedge/dash bond as a chiral centre, and do not call a double bond geometrically isomeric when one end has identical substituents.

To deduce isomers from a molecular formula, first enumerate different carbon skeletons, then place functional groups or multiple bonds, and finally check for geometrical or optical stereoisomers.

Reject duplicates by drawing the same connectivity in a different orientation. Check valency, the required degree of unsaturation and the syllabus limits before counting a structure.

For C₄H₈, consider butenes and methylpropene, then ask whether but-2-ene has cis/trans forms. A cyclic structure may also satisfy the formula if rings are in scope.

Rotating or redrawing one molecule does not create a new isomer. Count distinct connectivity and spatial arrangements only.