← Organic compounds

4 IUPAC naming

Follow the same procedure every time: it turns “impossible” names into a checklist. Watch: IUPAC naming Opens a YouTube search: pick a video that clicks for you

The 5-step naming procedure

  • 1. Longest chain containing the main functional group = the parent (meth/eth/prop/but/pent/hex…).
  • 2. Number the chain to give the main functional group the lowest locant.
  • 3. Identify substituents (branches, halogens, other groups) and their numbers.
  • 4. Alphabetise substituents; use di/tri/tetra for repeats (ignore these when alphabetising).
  • 5. Assemble: locants-prefixes-parent-suffix, e.g. 3-bromo-2-methylbutan-1-ol.
Worked example A branched alcohol Name: CH₃CH(CH₃)CH₂CH₂OH Longest chain through the –OH: 4 carbons → butan-…-ol (–OH is highest priority, gets the suffix). Number from the –OH end so it gets the lowest locant: OH on C1 → butan-1-ol. Methyl branch on C3. Assemble: 3-methylbutan-1-ol. ✓ 3-methylbutan-1-ol Tricky case Two functional groups: priority matters Name: HOOC–CH₂–CH(OH)–CH₃ Groups present: carboxylic acid (–COOH) and alcohol (–OH). Carboxylic acid is higher priority ⟹ it takes the suffix and C1. Parent chain = 4 carbons → butanoic acid. –COOH carbon is C1. The –OH is now a prefix: hydroxy-, located on C3. Assemble: 3-hydroxybutanoic acid. ✓ 3-hydroxybutanoic acid (not “…-ol”, the acid outranks the alcohol). Tricky case Lowest set of locants Name: CH₃–CH(Cl)–CH(CH₃)–CH₂–CH₃ Longest chain = 5 C → pentane. Substituents: Cl and CH₃. Number from the left: Cl on C2, methyl on C3 → locant set {2,3}. From the right: {3,4}. Choose the lower set {2,3}. Alphabetise: chloro before methyl. Assemble: 2-chloro-3-methylpentane. ✓ 2-chloro-3-methylpentane ✓

Naming esters

Esters are named alcohol part + acid part: the group from the alcohol (–yl) comes first, then the acid chain (–oate). CH₃COOCH₂CH₃ = ethyl ethanoate (ethyl from ethanol, ethanoate from ethanoic acid).

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