JAM Basic Concepts of Organic Chemistry — IUPAC and Isomerism
Before any mechanism question can be answered, the molecule has to be read correctly — which functional group is the "main" one, what the correct name is, and which of two given structures are actually different compounds at all. This article covers IUPAC naming rules and the full family of structural isomerism. Stereoisomerism (R/S, E/Z, meso compounds) gets its own complete treatment, with worked CIP examples, in the companion stereochemistry article linked below — here it is only classified, not re-derived.
IUPAC nomenclature — the method, step by step
- Identify the principal characteristic group — the functional group with the highest seniority present in the molecule becomes the suffix; every other group present is named as a prefix instead.
- Find the longest continuous chain that contains the principal characteristic group. If two chains of equal length both contain it, choose the one with the greater number of substituents.
- Number the chain to give the principal characteristic group the lowest possible locant first. If that alone does not decide the direction (a genuine tie), move to the next rule: lowest locants to the set of substituents considered together, then, if still tied, lowest locant to the substituent that comes first alphabetically.
- Name substituents alphabetically with their locants, using multiplying prefixes (di-, tri-, tetra-) where needed — these prefixes are ignored when alphabetising.
This ordering is why a molecule carrying both an –OH and a –C(=O)– group is always named as a "hydroxy...one" (ketone as suffix, hydroxy as prefix) and never the reverse — the ketone outranks the alcohol.
Worked example 1 — naming HOCH₂–CH₂–CO–CH₂–CH₃ from scratch.
Step 1: the molecule has a ketone (–CO–) and an alcohol (–OH). Ketone outranks alcohol, so the suffix is "-one" and –OH becomes the prefix "hydroxy-".
Step 2: the longest chain containing the ketone carbon has 5 carbons — the parent name is a pentanone.
Step 3 (numbering): the carbonyl carbon sits at position 3 whether you number from the –OH end or the –CH₃ end (it is the middle carbon), so the principal group's locant does not decide the direction — a genuine tie. Applying the next rule, lowest locant to the substituent (hydroxy), numbering from the –OH end gives it locant 1; numbering from the other end would give it locant 5. Locant 1 wins.
Name: 1-hydroxypentan-3-one.
Structural (constitutional) isomerism — same formula, different connectivity
| Type | What differs | Example |
|---|---|---|
| Chain isomerism | Carbon skeleton — straight vs branched | Butane vs 2-methylpropane, both C₄H₁₀ |
| Position isomerism | Location of the same functional group on the same skeleton | Propan-1-ol vs propan-2-ol, both C₃H₈O |
| Functional group isomerism | An entirely different functional group | Ethanol (C₂H₆O, an alcohol) vs dimethyl ether (C₂H₆O, an ether) |
| Metamerism | Different alkyl groups distributed on either side of the same functional group | Diethyl ether vs methyl n-propyl ether, both C₄H₁₀O |
| Tautomerism | Rapid, dynamic equilibrium via proton (and double-bond) migration — not a stable, isolable pair | Keto and enol forms of a carbonyl compound |
Tautomers are genuinely different from the other four rows: chain, position, functional and metameric isomers are separate, isolable compounds that happen to share a molecular formula, while tautomers interconvert continuously in solution and exist together as one equilibrium mixture, not as two distinct bottles on a shelf.
Worked example 2 — every structural isomer of C₄H₁₀O, classified. C₄H₁₀O has zero degrees of unsaturation, so every isomer is either an alcohol or an ether.
4 alcohols: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol,
2-methylpropan-2-ol.
3 ethers: diethyl ether, methyl n-propyl ether, methyl isopropyl
ether.
Total: 7 constitutional isomers.
Classifying a few pairs: butan-1-ol vs butan-2-ol are position isomers (same unbranched skeleton, –OH at a different carbon); butan-1-ol vs 2-methylpropan-1-ol are chain isomers (different skeleton); any alcohol vs any ether is a functional group isomer pair; diethyl ether vs methyl n-propyl ether are metamers of each other (same –O– functional group, different alkyl groups on either side).
Worked example 3 — an unusually large tautomeric equilibrium. Pentane-2,4-dione (acetylacetone), CH₃COCH₂COCH₃, shows a far larger enol content than a simple ketone like acetone, which is almost entirely in the keto form. Why?
The enol tautomer of pentane-2,4-dione is stabilised in two ways at once: the C=C is conjugated with the remaining C=O (extended conjugation lowers energy), and the enol –OH forms a strong intramolecular hydrogen bond with the neighbouring carbonyl oxygen, closing a stable six-membered ring. Between them, these effects are strong enough that well over half — by some measurements the majority — of the pure liquid exists as the enol tautomer, a striking contrast to the negligible enol content of an ordinary ketone.
Stereoisomerism — classified only, not derived here
Stereoisomers share both molecular formula and connectivity, differing only in the spatial arrangement of atoms: geometrical isomerism (cis-trans / E-Z, from restricted rotation about a double bond or within a ring) and optical isomerism (from chirality — enantiomers and diastereomers). The full CIP priority method for assigning R/S and E/Z, worked stereocentre-counting examples, and the meso-compound trap are covered completely in the companion stereochemistry article, linked below.
Common mistakes
- Choosing the alcohol suffix over a present ketone or acid. Always check the full seniority order before deciding the suffix — a lower-ranked group never gets to be the suffix if a higher-ranked one is present.
- Numbering for lowest substituent locants before checking the principal characteristic group's locant. The principal group always gets first claim to the lowest number; substituent locants are the tiebreaker, not the first rule.
- Calling metamerism a form of position isomerism. Metamerism is specific to molecules with a functional group flanked by two alkyl groups (ethers, ketones, esters, secondary amines) and describes how those flanking groups are distributed, not where the functional group itself sits on a single chain.
- Treating tautomers as ordinary, separately isolable structural isomers. They exist in a rapid, continuous equilibrium and are not bottled separately the way, say, two chain isomers are.
- Forgetting that a complex (branched) substituent needs its own internal numbering, starting at its point of attachment to the main chain, when named as a compound substituent in brackets.
Exam relevance
| Question style | What to check first |
|---|---|
| Name a given structure | Which functional group is senior (decides the suffix), then the longest chain containing it |
| "How many isomers does this formula have?" | Degree of unsaturation first, then systematically list chain/position/functional variants |
| Classify a given isomer pair | Whether the skeleton, the group's position, the group itself, or the flanking alkyls differ |
| Explain an unusually high enol content | Conjugation plus intramolecular hydrogen bonding in the enol form |
| Distinguish structural from stereoisomerism | Whether connectivity itself differs (structural) or only spatial arrangement (stereo) |
Confirm a formula before naming or counting isomers. The molar mass tool accepts any formula and shows the element-wise breakdown, a quick way to double-check a molecular formula before working through its isomers.
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