Isomerism
Same formula, different molecule — how identical atoms arranged differently give chain and functional isomers, cis and trans, and the left- and right-handed worlds of chirality
- The split between structural and stereo isomerism.
- The five structural types — chain, position, functional, metamerism, ring-chain — plus tautomerism.
- Geometrical isomerism (cis–trans) and the E–Z system using CIP priority.
- Chirality, the chiral carbon, optical activity, and enantiomers vs diastereomers.
- Racemic mixtures, meso compounds, the \(2^n\) rule, and R–S configuration.
- Conformational isomerism of ethane and butane via Newman projections.
The Two Great Branches
Isomers share a molecular formula but differ in structure, and so in properties. The first division is fundamental: structural (constitutional) isomers differ in which atoms are joined to which; stereoisomers have the same connectivity but a different arrangement in space.
Structural Isomerism
Structural isomers come in several flavours, each differing in where the atoms or groups sit in the connectivity.
| Type | Differs in | Example pair (same formula) |
|---|---|---|
| Chain | carbon skeleton | \(n\)-butane / isobutane (\(\ce{C4H10}\)) |
| Position | position of a group | propan-1-ol / propan-2-ol |
| Functional | functional group itself | ethanol / dimethyl ether (\(\ce{C2H6O}\)) |
| Metamerism | alkyl groups around a hetero-atom | diethyl ether / methyl propyl ether |
| Ring-chain | ring vs open chain | propene / cyclopropane (\(\ce{C3H6}\)) |
Tautomerism
Tautomerism is a special, dynamic structural isomerism: two isomers interconvert by the migration of a hydrogen atom and a double bond, existing together in equilibrium. The classic case is keto–enol tautomerism.
A hydrogen shifts from carbon to oxygen as the double bond moves. For most simple carbonyls the keto form dominates, but the equilibrium is real and central to carbonyl chemistry.
Geometrical (cis–trans) Isomerism
Geometrical isomerism arises from restricted rotation — about a \(\ce{C=C}\) double bond or within a ring. It requires each doubly bonded carbon to carry two different groups. When the like groups are on the same side it is cis; on opposite sides, trans.
E–Z Nomenclature
The cis–trans labels fail when each carbon carries two different groups. The unambiguous E–Z system uses Cahn–Ingold–Prelog (CIP) priority: rank the two groups on each carbon by atomic number, then compare sides.
Priority is set by atomic number at the first point of difference: \(\ce{Br} > \ce{Cl} > \ce{O} > \ce{N} > \ce{C} > \ce{H}\). Z often (but not always) coincides with cis.
Optical Isomerism & Chirality
A molecule is chiral if it cannot be superimposed on its mirror image — exactly like your left and right hands. The commonest cause is a chiral (asymmetric) carbon: one bonded to four different groups. Chiral molecules are optically active — they rotate the plane of plane-polarised light.
Enantiomers, Diastereomers & Meso Compounds
With more than one chiral centre, the relationships multiply. A molecule with \(n\) chiral centres has at most \(2^n\) stereoisomers — fewer if symmetry creates a meso form.
| Term | Definition |
|---|---|
| Enantiomers | non-superimposable mirror images; identical properties except rotation & chiral reactions |
| Diastereomers | stereoisomers that are not mirror images; different physical properties |
| Racemic mixture (\(\pm\)) | 50:50 enantiomers; optically inactive (external compensation) |
| Meso compound | has chiral centres but an internal mirror plane → optically inactive |
R–S Configuration
The absolute arrangement at a chiral centre is named by the R–S system, again using CIP priority. It gives each enantiomer an unambiguous label, independent of the measured direction of rotation.
Rank the four groups by atomic number, orient the lowest-priority group behind the page, and read the sense of the remaining three. R/S describes configuration; \(+/-\) describes rotation — the two are independent.
Conformational Isomerism
Conformers are the arrangements a molecule takes by rotating about single bonds. They interconvert freely and cannot be isolated, but they differ in energy. Newman projections — looking straight down a \(\ce{C-C}\) bond — show them best.
Putting It to Work
Problem. Ethanol and dimethyl ether share \(\ce{C2H6O}\). What kind of isomers are they?
Solution. They have different functional groups (\(\ce{-OH}\) vs \(\ce{-O-}\)):
Problem. How many chain isomers does pentane (\(\ce{C5H12}\)) have?
Solution. The carbon skeleton can be arranged three ways:
Problem. Does 1,1-dichloroethene show geometrical isomerism?
Solution. One carbon bears two identical \(\ce{Cl}\) atoms — condition fails:
Problem. How many optical isomers does a molecule with three different chiral centres have?
Solution. Apply the \(2^n\) rule with \(n=3\):
Problem. Tartaric acid has two chiral carbons. Why does it have three, not four, stereoisomers?
Solution. One arrangement has an internal mirror plane (meso):
Problem. With the lowest-priority group pointing away, the order 1→2→3 runs clockwise. What is the configuration?
Solution. Clockwise with lowest priority behind is rectus:
Chapter Summary
Structural (different connectivity) vs stereo (same connectivity, different space).
Chain, position, functional, metamerism, ring-chain; tautomerism is dynamic.
cis–trans from restricted rotation; E–Z by CIP priority (Z = same side).
Chiral carbon (4 different groups); enantiomers rotate light oppositely.
Racemic (inactive mix), meso (internal compensation), \(2^n\) rule, R–S labels.
Free rotation: ethane staggered > eclipsed; butane anti > gauche > eclipsed.
Problems
For each item, first decide whether it is structural, geometrical, optical or conformational — then apply the relevant rule. Difficulty rises down the list.
- Distinguish structural isomerism from stereoisomerism.
- Draw and name the three chain isomers of \(\ce{C5H12}\).
- Give an example each of position, functional and metamerism isomerism.
- What is tautomerism? Illustrate with keto–enol forms.
- State the condition for geometrical isomerism and draw cis- and trans-but-2-ene.
- Explain the E–Z system and assign E or Z to a given alkene using CIP priority.
- Define chirality and a chiral carbon; how is optical activity detected?
- Distinguish enantiomers from diastereomers.
- What is a racemic mixture, and why is it optically inactive?
- Explain why meso-tartaric acid is optically inactive despite having chiral centres.
- How many stereoisomers are possible for a compound with two different chiral centres?
- Draw the staggered and eclipsed Newman projections of ethane and state which is more stable.