Organic Chemistry – Some Basic Principles and Techniques · Lesson 6 of 15
Isomerism: One Formula, Different Compounds
“Distinguish different connectivities and spatial arrangements without confusing them with different drawings of one molecule.”
• Explain why a molecular formula does not uniquely identify a compound. • Distinguish chain, position, functional-group, and metameric isomerism. • Recognise the basic meaning of stereoisomerism. • Check whether two drawings represent the same compound or different isomers.
A shared formula can hide different structures
A molecular formula counts atoms but leaves their arrangement unspecified. The same set of atoms can therefore make different compounds. Their boiling points, reactions, or other properties can differ because their connectivity or three-dimensional arrangement differs. Isomerism is the existence of such compounds, and the compounds are called isomers.
Different compounds with the same molecular formula but different structures or spatial arrangements.
Always check the molecular formula first. If the formulas differ, the compounds cannot be isomers. If the formulas agree, inspect the connectivity. Different connectivities indicate structural isomerism. If the connectivity agrees but the relative spatial arrangement differs in a way that produces distinct compounds, the relationship is stereoisomerism. Simply rotating a drawing on paper does not create a new isomer.
Different carbon skeletons and positions
Chain isomers differ in their carbon skeletons. Moving from an unbranched arrangement to a branched one can produce a new compound without changing the total numbers of carbon and hydrogen atoms. To find such isomers systematically, start with the longest chain and then shorten the parent while placing branches. Check for duplicates by renaming or tracing each skeleton.
Problem
Explain why pentane, 2-methylbutane, and 2,2-dimethylpropane are chain isomers.
- 1.Each structure contains five carbon atoms. Completing carbon valencies gives for every one.
- 2.Their carbon skeletons differ: a five-carbon chain, a four-carbon chain with one branch, and a central carbon attached to four methyl groups.
- 3.Rotating one drawing cannot change which carbons are bonded. The compounds are therefore distinct chain isomers, not alternative orientations of one molecule.
Position isomers retain the same carbon skeleton and functional type but differ in the position of a group, substituent, or multiple bond. Propan-1-ol and propan-2-ol have the same three-carbon chain, yet OH is attached to a terminal carbon in one and the middle carbon in the other. Their formulas agree, but their structures differ.
Problem
Are propan-1-ol and a drawing labelled “propan-3-ol” two position isomers?
- 1.A three-carbon chain can be numbered from either end. An OH group on an end carbon receives locant 1 when the chain is numbered correctly.
- 2.The proposed “propan-3-ol” drawing becomes propan-1-ol when read from its other end. It is the same connectivity and not a second compound.
- 3.Propan-2-ol is genuinely different because OH is bonded to the middle carbon. Renumbering cannot move it to an end carbon.
Different functional groups or different distributions around one group
Functional-group isomers contain different functional groups while sharing a molecular formula. Propanal and propanone both have three carbons, six hydrogens, and one oxygen. In propanal, the carbonyl is at an end and its carbon has a hydrogen. In propanone, the carbonyl carbon lies between two methyl groups. This change makes an aldehyde and a ketone, with different characteristic chemistry.
Metamerism describes different distributions of alkyl groups on either side of a functional group or polyvalent linking atom. In the ether examples, oxygen remains a single-bonded link between two carbon groups. The total carbon count stays the same, but the groups on either side change. This is not a change from ether to alcohol; that would instead be functional-group isomerism.
Problem
Compare with , and compare propanal with propanone.
- 1.The two ethers both have formula . One distributes carbons as 1 and 3 around oxygen; the other as 2 and 2. They illustrate metamerism.
- 2.Propanal and propanone both have formula but belong to different functional families. They illustrate functional-group isomerism.
- 3.In both comparisons, the molecular formula is checked before identifying the particular structural difference.
| Type | Feature that changes | Source example |
|---|---|---|
| Chain | Carbon skeleton | Pentane and 2-methylbutane |
| Position | Position on the same skeleton | Propan-1-ol and propan-2-ol |
| Functional group | Functional family | Propanal and propanone |
| Metamerism | Alkyl distribution around a linking group | Methoxypropane and ethoxyethane |
The beginning of stereoisomerism
Stereoisomers have the same atoms connected in the same sequence, but their relative positions in space differ. The chapter introduces geometrical and optical isomerism as the two categories without developing a full set of rules. Restricted rotation at a double bond helps explain why spatial arrangements may persist. For example, substituents can occupy different relative sides of a suitable double bond even when the connectivity is unchanged.
Optical isomerism also concerns three-dimensional arrangement, often involving structures related as non-superimposable mirror images. At this stage, the key distinction is the level at which the structures differ: connections for structural isomers, spatial arrangement for stereoisomers. A wedge-and-dash representation supplies information that a simple condensed formula can conceal.
Different-looking drawings are not automatically different compounds. First count atoms, then compare connectivity, and only then compare spatial arrangement. Resonance contributors, introduced later, are electron-distribution drawings of one species and are not isomers.
Quiz
What must be identical for two compounds to be isomers?
Propan-1-ol and propan-2-ol illustrate which relationship?
Propanal and propanone are examples of which type?
What stays unchanged in stereoisomers?
Which comparison illustrates metamerism?
Practice Problems
- Draw all three chain isomers of and verify the formula of each.
- Draw propan-1-ol and propan-2-ol and explain the structural difference.
- Draw an aldehyde and a ketone with formula .
- Draw two ethers of formula that illustrate metamerism.
- Explain why rotating a pentane drawing or reversing its numbering does not produce an isomer.
- Describe the distinction between structural and stereoisomerism using connectivity and spatial arrangement.
Key Takeaways
• Isomers share a molecular formula but are distinct compounds. • Structural isomerism concerns connectivity; stereoisomerism concerns spatial arrangement. • Classify structural differences by skeleton, position, group, or distribution. • Check for rotated and renumbered duplicates before counting isomers.