Organic Pulse notes · Stereochemistry
Isomerism
Dr. Sandeep G. Sontakke · M.Sc., Ph.D., CSIR-NET (JRF), SET
- At a glance
- Structural isomers
- How many isomers?
- Geometrical (cis–trans) isomers
- Optical isomers
- How to classify any pair
- Do isomers behave differently?
- One-minute revision
At a glance
Isomers: same molecular formula, different arrangement
- Isomers are different compounds with the same molecular formula.
- Structural (constitutional) isomers differ in which atoms are joined to which.
- Stereoisomers have the same connections but a different arrangement in 3D.
- Because isomers are different compounds, their properties differ. Only enantiomers share almost all physical properties.
An early famous pair: in 1828 Wöhler heated ammonium cyanate and obtained urea. Both have the formula CH₄N₂O, yet they are completely different compounds.
Structural isomers
Structural isomers have the same atoms joined in a different order. There are four main types, plus two special cases.
Pairs, left to right: chain isomers, position isomers, functional group isomers, metamers.
| Type | What changes | Example pair (same formula) |
|---|---|---|
| Chain | the carbon skeleton: straight or branched | butane and 2-methylpropane (C₄H₁₀) |
| Position | the position of a group or multiple bond on the same chain | propan-1-ol and propan-2-ol (C₃H₈O); but-1-ene and but-2-ene (C₄H₈) |
| Functional group | the functional group itself | ethanol and methoxymethane (C₂H₆O); propanal and propanone (C₃H₆O) |
| Metamerism | the alkyl groups on either side of –O–, –S–, –NH– or –CO– | ethoxyethane and 1-methoxypropane (C₄H₁₀O) |
| Ring–chain | an open chain or a ring | propene and cyclopropane (C₃H₆) |
| Tautomerism | an H atom and a double bond move, in rapid equilibrium | propanone and prop-1-en-2-ol (keto and enol forms) |
How many isomers?
The number of structural isomers grows explosively as the carbon chain gets longer. For the alkanes:
| Alkane formula | Structural isomers | |
|---|---|---|
| C₄H₁₀ | 2 | |
| C₅H₁₂ | 3 | |
| C₆H₁₄ | 5 | |
| C₇H₁₆ | 9 | |
| C₈H₁₈ | 18 | |
| C₉H₂₀ | 35 | |
| C₁₀H₂₂ | 75 | |
| C₂₀H₄₂ | 366,319 |
Data: OpenStax Organic Chemistry, section 3.2 (Table 3.2). The bar is on a log scale.
Geometrical (cis–trans) isomers
When rotation about a bond is blocked (a C=C double bond, or a ring), two groups can sit on the same side (cis) or on opposite sides (trans).
Melting and boiling points: Wikipedia, '2-Butene'. The more symmetrical trans isomer packs better in the solid, so it melts higher.
- Two conditions: restricted rotation, and two different groups on each carbon of the double bond.
- But-1-ene, CH₂=CH–CH₂CH₃, has no cis/trans isomers: C1 carries two identical H atoms.
- Rings work too: cis- and trans-1,2-dimethylcyclohexane cannot interconvert without breaking a bond.
Optical isomers: enantiomers and diastereomers
A carbon carrying four different groups is a stereocentre (chiral carbon, C*). The molecule and its mirror image cannot be laid on top of each other: they are enantiomers.
- Enantiomers are non-superimposable mirror images. They have identical melting points, boiling points and solubilities, and rotate plane-polarised light by equal amounts in opposite directions.
- Diastereomers are stereoisomers that are not mirror images: for example cis- and trans-but-2-ene, or two forms of a molecule with two stereocentres that differ at only one of them. Their physical properties differ.
- A 50:50 mixture of two enantiomers is a racemic mixture. It is optically inactive, because the two rotations cancel.
How to classify any pair
Ask three questions, in this order:
| Question | If yes | If no |
|---|---|---|
| 1. Same molecular formula? | go to question 2 | not isomers |
| 2. Atoms joined in the same order? | stereoisomers: go to question 3 | structural isomers |
| 3. Mirror images of each other? | enantiomers | diastereomers (this includes cis/trans isomers) |
Do isomers behave differently?
| Pair | Physical properties | Chemical properties |
|---|---|---|
| Structural isomers | different: ethanol boils at 78 °C, dimethyl ether at −24 °C | usually different: ethanol reacts with sodium to give H₂; dimethyl ether does not |
| Diastereomers | different melting points, boiling points and solubilities | similar reactions, but at different rates |
| Enantiomers | identical, except the direction of optical rotation | identical with non-chiral reagents; different with chiral ones such as enzymes |
One-minute revision
- Isomers: same molecular formula, different compounds.
- Structural: chain, position, functional group, metamerism (plus ring–chain and tautomerism).
- Stereoisomers: same connections, different 3D arrangement.
- Cis/trans needs restricted rotation and two different groups on each carbon; E/Z is the general system.
- Enantiomers are mirror images with identical physical properties; diastereomers are not mirror images and differ.
Organic Pulse · free notes by Dr. Sandeep G. Sontakke. Main sources: NCERT Chemistry Class 11, Unit 8 (section 8.6); OpenStax Organic Chemistry, sections 3.2 and 5.9; Clayden et al., Organic Chemistry (2nd ed.), ch. 14. Watch the shorts: youtube.com/@OrganicPulse-y9q