Concept 05 / 08Organic Chemistry - Basic Principles and Techniques
IUPAC Nomenclature of Organic Compounds
Systematic naming of alkanes, alkenes, alkynes and compounds with functional groups: choosing the parent chain, numbering by the lowest locant rule, alphabetical order of substituents, prefixes and suffixes, priority order of functional groups, and naming of alicyclic and aromatic compounds.
An IUPAC name is built as: prefixes (substituents, in alphabetical order) + word root (number of carbons in the parent chain) + primary suffix (-ane, -ene, -yne) + secondary suffix (principal functional group).
Word roots: meth (), eth (), prop (), but (), pent (), hex (), hept (), oct (), non (), dec ().
Rules for alkanes.
1. Choose the longest continuous carbon chain as the parent. If two chains are equally long, choose the one with more substituents.
2. Number the chain so that substituents get the lowest set of locants (first point of difference rule, see figure).
3. Name substituents as alkyl prefixes (methyl, ethyl, isopropyl, tert-butyl), arranged alphabetically, ignoring multiplying prefixes (di, tri, tetra) and sec, tert, but counting iso and neo.
4. If the locant sets are identical from both ends, the substituent cited first alphabetically gets the lower number.
Unsaturated chains. The parent chain must contain the maximum number of multiple bonds, and multiple bonds get the lowest locants. When a double and a triple bond tie, the double bond gets the lower number: is but-1-en-3-yne.
Functional groups. The principal group is named as a suffix; others as prefixes. Priority order (highest first):
carboxylic acid (-oic acid) > sulfonic acid > anhydride > ester (-oate) > acid halide (-oyl halide) > amide (-amide) > nitrile (-nitrile) > aldehyde (-al) > ketone (-one) > alcohol (-ol) > amine (-amine) > alkene > alkyne.
Always named as prefixes only: halo (fluoro, chloro, bromo, iodo), nitro, alkoxy, and alkyl.
The chain must include the principal group and it gets the lowest locant. The carbon of , , and is counted in the chain as C1.
Examples: butan-2-ol; prop-2-enal; 3-oxobutanoic acid; 3-hydroxypropanenitrile; ethyl ethanoate; 2-aminopropanoic acid.
When a group cannot be included in the chain (such as COOH on a ring), use suffixes like -carboxylic acid, -carbaldehyde, -carbonitrile: cyclohexanecarboxylic acid.
Cyclic and aromatic compounds. Add cyclo to the ring name; number the ring to give substituents the lowest locants. In benzene derivatives, positions , are ortho, , meta, , para. Common names are retained: toluene (methylbenzene), phenol, aniline, benzaldehyde, benzoic acid, acetophenone (1-phenylethanone).
Key Takeaways:
- Longest chain containing the principal functional group and maximum multiple bonds.
- Lowest locants by the first point of difference; ties go to the alphabetically first substituent, and to C=C over C≡C.
- Suffix priority: acid > ester > acid halide > amide > nitrile > aldehyde > ketone > alcohol > amine.
- Halo, nitro and alkoxy are always prefixes.
- Pitfall: the aldehyde and carboxyl carbons are C1 and are part of the chain; do not give them a locant in the name (ethanal, not ethan-1-al).
Brain-Teaser 1: Name .
Solution: The longest chain has five carbons. Numbering from the end nearer the tert-butyl carbon gives methyls at , , rather than , , (see figure). Name: 2,2,4-trimethylpentane.
Brain-Teaser 2: Name and .
Solution: First: aldehyde outranks ketone; the aldehyde carbon is C1 and the ketone is at C4: 4-oxopentanal. Second: acid is principal; COOH is C1 and OH on C3: 3-hydroxybutanoic acid.
Brain-Teaser 3: Write the structure of 3-ethyl-2-methylpent-1-ene and check that its name is correct.
Solution: . The longest chain that contains the double bond has five carbons, numbered so C=C starts at C1; methyl at C2, ethyl at C3. Substituents alphabetical (ethyl before methyl): 3-ethyl-2-methylpent-1-ene.
Practice this concept
Free · no account required · instant solutions