We will learn the rules of the International Union of Pure and Applied Chemistry(IUPAC), which established the systematic nomenclature of organic compounds. This topic is directly connected to CEO Tsuruta's AO Act 1. Learn to use these names alongside common names.
Basic Knowledge
Four steps of IUPAC nomenclature:
① Select the parent chain: Choose the longest carbon chain. If there are multiple candidates, choose the chain with the greatest number of substituents.
② Numbering: Number from one end of the parent chain. Choose the direction that gives the smallest sum of the locants of substituents and functional groups(the lowest-locant rule).
③ Naming substituents: Arrange them in alphabetical order and assign each a locant. If the same substituent appears multiple times, use multiplicative prefixes such as di-, tri-, and tetra-, but do not count these prefixes in alphabetical ordering.
④ Parent name+suffix: Alkane-ane, alkene-ene, alkyne-yne, alcohol-ol, aldehyde-al, ketone-one, carboxylic acid-oic acid, ester-oate.
Number of carbons 1–10: 1 methane methane, 2 ethane ethane, 3 propane propane, 4 butane butane, 5 pentane pentane, 6 hexane hexane, 7 heptane heptane, 8 octane octane, 9 nonane nonane, 10 decane decane(memorize as Me-Et-Prop-But-Pent-Hex-Hep-Oct-Non-Dec).
Main substituents: -CH3 methyl methyl, -C2H5 ethyl ethyl, -C3H7 propyl propyl(n-/iso-), -F fluoro, -Cl chloro, -Br bromo, -NO2 nitro, -NH2 amino.
① Parent-chain selection(longest chain containing the principal functional group)
② Numbering(lowest-locant rule, functional-group priority)
③ Substituent naming(alphabetical order, multiplicative prefixes)
④ Parent+suffix(-ane/-ene/-yne/-ol/-al/-one/-oic acid)
Functional-group priority(carboxylic acid>ester>amide>nitrile>aldehyde>ketone>alcohol>amine>ether>alkene>alkyne>halogen)
E/Z notation(notation of geometric isomers according to the CIP rules)
Further Exploration (Principles and Applications)
Examples of common and systematic names:
・Acetic acid acetic acid → ethanoic acid ethanoic acid
・Formic acid formic acid → methanoic acid methanoic acid
・Glycerin glycerin → 1,2,3-propanetriol propane-1,2,3-triol
・Ethylene ethylene → ethene ethene
・Acetylene acetylene → ethyne ethyne
・Toluene toluene → methylbenzene methylbenzene
Example 1: CH3-CH(CH3)-CH2-CH3 → parent chain of 4 carbons(butane)+methyl at position 2 → 2-methylbutane(2-methylbutane).
Example 2: CH3-CH2-CH(OH)-CH3 → parent chain of 4 carbons+OH at position 2 → butan-2-ol(butan-2-ol). Numbering begins from the right rather than the left to minimize the locant of OH.
Example 3: CH3-CH=CH-COOH → but-2-enoic acid(but-2-enoic acid). When a carboxylic acid is present, the carbon of -COOH is always position 1.
Stereochemical symbols: (E)-/(Z)-(CIP rules), (R)-/(S)-(absolute configuration), cis/trans(common usage), D/L(common usage for sugars and amino acids), (+)/(-)(sign of optical rotation).
- Four steps: parent chain→numbering→substituents→parent+suffix
- Check functional-group priority first
- The sum of locants follows the lowest-locant rule
- List substituents in alphabetical order
- Multiplicative prefixes(di-, tri-) are excluded from alphabetical ordering
- Stereochemical symbols(E)/(Z), (R)/(S) are the modern standard
- Common names(acetic acid, toluene, etc.) are also retained as IUPAC preferred names
Points to Note(Common Confusions and Frequent Mistakes)
① When there are multiple equally long choices for the parent chain, choose the one with more substituents. ② For carboxylic acids and aldehydes, the carbon of -COOH/-CHO must be position 1(there is no freedom in numbering). ③ Older common substituent names such as isopropyl/sec-butyl/tert-butyl are recommended as “propan-2-yl,” “butan-2-yl,” and “(2-methyl)propan-2-yl” in the 2013 IUPAC recommendations, but the older names are accepted in high school. ④ For cyclic compounds, add “cyclo”(cyclohexane). ⑤ Locant notation: The modern recommended form is “butan-2-ol”(the number is enclosed by hyphens), while the older form is “2-butanol”(both are accepted in entrance examinations).
Practice
- Give the IUPAC name of the following structure:
CH3-CH(CH3)-CH(C2H5)-CH2-CH3. - Write the structural formula of 3-methylpentan-2-ol.
- Write the structural formula of(E)-but-2-enoic acid, clearly showing the stereochemical arrangement.