Why IUPAC names?
Old common (trivial) names came from sources: formic acid from ants (Latin formica), acetic acid from vinegar (acetum). They tell nothing about the structure. The IUPAC (International Union of Pure and Applied Chemistry) system gives one systematic name from which you can draw the structure.
Parts of an IUPAC name
Prefix(es) + Word root + Primary suffix + Secondary suffix
- Word root: carbons in the main chain. 1 meth, 2 eth, 3 prop, 4 but, 5 pent, 6 hex, 7 hept, 8 oct, 9 non, 10 dec.
- Primary suffix: saturation. All single = -ane, one C=C = -ene, one C≡C = -yne.
- Secondary suffix: main functional group: -ol (–OH), -al (–CHO), -one (C=O), -oic acid (–COOH), -amine (–NH₂), -nitrile (–CN), -oate (ester).
- Prefixes: side groups and some groups: methyl, ethyl, chloro, bromo, nitro (–NO₂), alkoxy (–OR); ring = cyclo.
Example: propan-1-ol = prop (3 C) + an (single bonds) + 1-ol.
IUPAC rules step by step
- Longest chain: choose the longest carbon chain. If there is a functional group or multiple bond, the chain must contain it (even if another chain is longer).
- Numbering: number from the end that gives the lowest number (locant) to, in order: the main functional group → the double/triple bond → the side groups.
- Lowest set of locants: if there are several branches, compare locant sets term by term; the first point of difference decides (2,2,4 beats 2,4,4).
- Same groups: use di, tri, tetra (2,3-dimethyl). Commas between numbers, hyphens between numbers and words.
- Alphabetical order of different prefixes (ethyl before methyl). Ignore di/tri when sorting.
- Two double bonds: -diene (buta-1,3-diene). The 'a' is added to the root.
Priority of functional groups
If a compound has more than one group, the highest one becomes the suffix and the others become prefixes.
–COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂ > C=C > C≡C. Groups like –X, –NO₂ and –OR are always prefixes.
Example: HO–CH₂–CH₂–COOH is 3-hydroxypropanoic acid (the acid wins; –OH becomes 'hydroxy').
Carbon of the group counted in the chain
For –CHO, –COOH and –CN, the group carbon is C1 of the chain: CH₃CH₂CHO = propanal, CH₃CN = ethanenitrile.
Naming rings and benzene compounds
Rings get the prefix cyclo: cyclopentane, 3-methylcyclohexene. For benzene with two groups we number the ring or use old prefixes: 1,2 = ortho (o-), 1,3 = meta (m-), 1,4 = para (p-). Example: 1,4-dimethylbenzene = p-xylene. Many benzene compounds keep accepted names: phenol, toluene (methylbenzene), aniline (benzenamine).
Isomerism: same formula, different structure
Isomers have the same molecular formula but different structures, so different properties.
Structural isomerism
- Chain isomerism: different carbon skeletons. Pentane, 2-methylbutane, 2,2-dimethylpropane (C₅H₁₂).
- Position isomerism: same skeleton, group at a different place. Propan-1-ol and propan-2-ol.
- Functional group isomerism: different functional groups. Ethanol (C₂H₅OH) and methoxymethane (CH₃OCH₃); propanal and propanone.
- Metamerism: different alkyl groups on each side of the same group (–O–, –S–, –CO–). Methoxypropane and ethoxyethane (C₄H₁₀O).
- Tautomerism (extra): two forms that change into each other by moving an H, like keto and enol forms.
Stereoisomerism
Same bonding order but different arrangement in space.
- Geometrical (cis-trans): needs restricted rotation (C=C or ring) and two different groups on each double-bond carbon. cis = same groups on the same side; trans = opposite sides. cis- and trans-but-2-ene differ in boiling point and dipole moment.
- Optical isomerism: a carbon with 4 different groups is chiral (asymmetric). Its two forms are non-superimposable mirror images called enantiomers. They rotate plane-polarised light in opposite directions (d/+ and l/−). Example: butan-2-ol, lactic acid.
Try it
Matchstick chains for C₄H₁₀ (2 isomers) and C₅H₁₂ (3 isomers). Hold your left and right hands palm-down one above the other: they never match. That is chirality. In the 3D free play, check each isomer pair and name both members before reading the readout.
Key formulas and definitions
- Name = prefixes + word root + primary suffix + secondary suffix
- Roots: 1 meth, 2 eth, 3 prop, 4 but, 5 pent, 6 hex, 7 hept, 8 oct, 9 non, 10 dec
- -ane (single), -ene (C=C), -yne (C≡C)
- Priority: –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > C=O > –OH > –NH₂ > C=C > C≡C
- Structural: chain, position, functional, metamerism; Stereo: geometrical, optical
- Chiral carbon = 4 different groups
Worked examples
1. Name CH₃–CH(CH₃)–CH₂–CH₃.
Line 1: Longest chain = 4 C → but, all single → butane. Line 2: Number from the left: CH₃ branch on C2 (from the right it would be C3). Line 3: Name: 2-methylbutane.
2. Name CH₃–CH(CH₃)–CH(CH₃)–CH₂–CH₃.
Line 1: Longest chain = 5 C → pentane. Line 2: Left numbering gives methyls at 2,3; right gives 3,4. Choose 2,3. Line 3: Two methyls → dimethyl: 2,3-dimethylpentane.
3. Name CH₂=CH–CH₂–CH₃ and CH₃–CH=CH–CH₃.
Line 1: 4 C with one C=C → butene. Line 2: First: C=C starts at C1 → but-1-ene. Line 3: Second: C=C starts at C2 → but-2-ene. These two are position isomers.
4. Name CH₃–CH(OH)–CH₂–CH(CH₃)–CH₃.
Line 1: Chain 5 C with –OH → pentanol. Line 2: Number so –OH gets the lowest number: from the left, OH on C2, methyl on C4. Line 3: Name: 4-methylpentan-2-ol.
5. Name Cl–CH₂–CH₂–CH(C₂H₅)–CH₂–CH₃.
Line 1: Longest chain = 5 C (the ethyl branch gives no longer chain) → pentane; Cl and ethyl are prefixes. Line 2: Numbering from the Cl end gives 1 and 3; from the other end 3 and 5. Choose 1,3. Line 3: Alphabetical: chloro before ethyl → 1-chloro-3-ethylpentane.
6. How many structural isomers does C₅H₁₂ have? Name them.
Line 1: Straight chain of 5: pentane. Line 2: Chain of 4 + one methyl on C2: 2-methylbutane. Line 3: Chain of 3 + two methyls on C2: 2,2-dimethylpropane. Answer: 3 chain isomers.
7. Does but-1-ene show geometrical isomerism? Does but-2-ene?
Line 1: but-1-ene: C1 is CH₂= with two identical H → no cis/trans. Line 2: but-2-ene: each double-bond C has H and CH₃ (two different groups). Line 3: So only but-2-ene shows cis and trans forms.
Common mistakes
- Choosing the longest chain but leaving out the functional group. The main chain must contain the main group or multiple bond.
- Numbering to give branches low numbers when a functional group is present. The main group gets the lowest number first.
- Sorting prefixes with di/tri: 'dimethyl' is sorted under m, so 'ethyl' still comes first.
- Saying every alkene shows cis-trans. Each double-bond carbon needs two different groups.