Naming organic compounds (IUPAC system)
An IUPAC name is built as prefix + word root + primary suffix + secondary suffix, for example eth + ane + ol gives ethanol.
Word roots for the longest chain
| Carbon atoms | Word root | Carbon atoms | Word root |
|---|---|---|---|
| 1 | Meth | 6 | Hex |
| 2 | Eth | 7 | Hept |
| 3 | Prop | 8 | Oct |
| 4 | But | 9 | Non |
| 5 | Pent | 10 | Dec |
The four parts of a name
| Part | Tells you | Examples |
|---|---|---|
| Prefix | A side group (substituent) and its position | chloro (-Cl), bromo (-Br), iodo (-I), methyl (-CH₃) |
| Word root | The number of carbons in the longest chain | meth, eth, prop, but |
| Primary suffix | Single, double or triple bond | -ane, -ene, -yne |
| Secondary suffix | The functional group | alcohol -ol, aldehyde -al, ketone -one, carboxylic acid -oic acid |
Rules for naming
- 1Find the longest continuous chain of carbon atoms. This gives the word root.
- 2Find the functional group, any double or triple bond, and any side group.
- 3★Number the chain from the end nearer to the functional group (or the multiple bond, or the side group), so that it gets the lowest number.
- 4Write prefix + word root + primary suffix + secondary suffix, with position numbers. Join numbers to words with a hyphen.
- 5If the secondary suffix begins with a vowel, drop the final 'e' of the primary suffix: ethane + ol → ethanol.
★ key step
From structure to name
Name: (i) CH₃-CH₂-CH₂-OH (ii) CH₃-CH₂-CHO (iii) CH₃-CO-CH₃ (iv) CH₂=CH-CH₃ (v) CH₃-CH₂-Cl
- (i) 3 carbons → prop; single bonds → ane; -OH on carbon 1 → ol: propan-1-ol (1-propanol)
- (ii) 3 carbons, counting the -CHO carbon → prop + ane + al: propanal
- (iii) 3 carbons, C=O on the middle carbon → prop + ane + one: propanone (acetone)
- (iv) 3 carbons with a double bond from carbon 1 → prop + ene: propene (prop-1-ene)
- (v) 2 carbons with Cl as a prefix → chloro + eth + ane: chloroethane
Propan-1-ol, propanal, propanone, propene and chloroethane.
From name to structure
Write the structural formula of (i) butanoic acid (ii) but-1-yne (iii) 2-chloropropane.
- (i) but = 4 carbons; oic acid = -COOH, whose carbon is carbon 1: CH₃-CH₂-CH₂-COOH
- (ii) but = 4 carbons; 1-yne = triple bond starting at carbon 1: CH≡C-CH₂-CH₃
- (iii) prop = 3 carbons; 2-chloro = Cl on the middle carbon: CH₃-CHCl-CH₃
CH₃CH₂CH₂COOH; CH≡C-CH₂-CH₃; CH₃-CHCl-CH₃.
The acetone in nail-polish remover is propanone, and the vinegar in pickles contains ethanoic acid. IUPAC names mean that a chemist in Chennai and one in Tokyo picture exactly the same compound.
1 marks: “What is the secondary suffix for an aldehyde?” -al, as in ethanal (CH₃CHO).
2 marks: “Give the IUPAC name of CH₃-CH₂-CH₂-CH₂-OH.” 4 carbons (but), single bonds (ane), -OH on carbon 1 (ol): butan-1-ol.
4 marks: “Give the IUPAC names and classes of: (i) CH₃CHO (ii) CH₃COCH₃ (iii) CH₃COOH (iv) CH₃CH₂OH” (i) Ethanal, aldehyde (ii) propanone, ketone (iii) ethanoic acid, carboxylic acid (iv) ethanol, alcohol.
Parts in order: Please Read Properly Slowly: Prefix, Root, Primary suffix, Secondary suffix. Roots: 'Monkeys Eat Peeled Bananas' for meth, eth, prop, but.