15.4 Nomenclature of Aliphatic Organic Compounds

This section outlines the systematic method for naming aliphatic organic compounds as established by the International Union of Pure and Applied Chemistry (IUPAC). Aliphatic compounds include straight-chain, branched-chain, and non-aromatic cyclic molecules, which can be saturated or unsaturated.

For naming to work properly, you must first understand what functional groups are. A Functional Group→ is a specific atom or group of atoms that determines the chemical properties of a compound.

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Key Information

• Systematic Names: The systematic names for most homologous series (except carboxylic acids) are written as a single word.
• Trivial Names: Non-systematic or common names (e.g., acetone, formaldehyde) are still used in many contexts.
• IUPAC: The International Union of Pure and Applied Chemistry provides internationally accepted rules for chemical nomenclature.

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15.4.1 Nomenclature of Alkanes

Alkanes form the foundation for naming all other organic compounds. Their nomenclature rules are the starting point.

Common (Trivial) Names of Alkanes

Simple alkanes may have common names, such as neopentane.

Systematic (IUPAC) Names of Alkanes

The following rules are applied to assign systematic names to alkanes:

1. Identify the Parent Chain:

   • Find the longest continuous chain of carbon atoms. This chain determines the root name of the alkane (e.g., 7 carbons = heptane).
   • The suffix for all alkanes is -ane.
   • Example: The structure below has a 7-carbon parent chain, so its root name is heptane. $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3$

2. Identify and Number Substituents:

   • Groups attached to the parent chain are called substituents or side chains (e.g., alkyl groups like methyl, ethyl).
   • Number the parent chain from the end that gives the substituents the lowest possible locant numbers.
   • The locant is the number of the carbon atom on the parent chain to which the substituent is attached.

3. Handle Multiple Identical Substituents:

   • Use prefixes like di-, tri-, tetra- to indicate how many identical substituents are present.
   • Each substituent must have its own locant number, even if they are on the same carbon. Separate locants with commas.
   • Examples:
     • 3,3-dimethylheptane: Two methyl groups on carbon 3.
     • 3,4-dimethylheptane: One methyl group on carbon 3 and another on carbon 4.
     • 3,3,5-trimethyloctane: Three methyl groups, two on carbon 3 and one on carbon 5.

4. Handle Multiple Different Substituents:

   • List the substituents in alphabetical order (ignoring prefixes like di-, tri-).
   • Each substituent is preceded by its locant number.
   • Example:
     • 5-ethyl-3-methyloctane: 'ethyl' comes before 'methyl' alphabetically.

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15.4.2 Nomenclature of Cycloalkanes

1. Base Name: The name of the corresponding open-chain alkane is prefixed with cyclo-.

   • Examples: cyclopropane, cyclopentane, cyclohexane.

2. Substituted Cycloalkanes:

   • Substituents are named as prefixes.
   • If there is one substituent, no number is needed.
   • If there are multiple substituents, number the ring to give the locants the lowest possible values at the first point of difference. List them alphabetically.
   • Examples:
     • 1-ethyl-2-methylcyclopropane
     • 1,1,3-trimethylcyclohexane
     • 2-ethyl-1,1-dimethylcyclopentane

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15.4.3 Nomenclature of Alkenes and Alkynes

Alkenes (double bonds) and alkynes (triple bonds) have specific rules for their functional groups.

1. Parent Chain: The parent chain must contain the multiple bond(s). The suffix is -ene for alkenes and -yne for alkynes.

2. Numbering: Number the chain from the end closer to the multiple bond to give it the lowest possible locant. The locant is placed just before the suffix.

   • Examples:
     • $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2$ is ethene.
     • $\mathrm{C}{\mathrm{H}}_3-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ is propene.
     • 4-methylpent-1-ene
     • 5-methylhex-1-yne

3. Multiple Double/Triple Bonds:

   • If more than one multiple bond is present, add an "a" to the root name (e.g., butane).
   • Use prefixes di-, tri- before the -ene or -yne suffix.
   • Examples:
     • 2-methylbuta-1,3-diene
     • 3-methylhexa-1,5-diyne

4. Compounds with Both Double and Triple Bonds:

   • The ending is -en-yne.
   • Number from the end closer to the first multiple bond, regardless of type.
   • If there is a tie, the double bond gets the lower number.
   • Examples:
     • 4-methyloct-1-en-7-yne
     • 4-ethylhex-1-en-5-yne

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15.4.4 Nomenclature of Halogenoalkanes

Halogenoalkanes (or alkyl halides) treat the halogen atom as a substituent (halo group). For more on halogens, see Halogens→.

• Prefixes: fluoro-, chloro-, bromo-, iodo-.
• Numbering: Number the parent chain to give the halogen atom the lowest possible locant.
• Common Names: Often named as alkyl halides.
• Examples:
  • $\mathrm{C}{\mathrm{H}}_3-\mathrm{I}$: iodomethane [methyl iodide]
  • $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{Cl}$: chloroethane [ethyl chloride]
  • $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{Cl})\mathrm{C}{\mathrm{H}}_3$: 2-chloropropane [isopropyl chloride]
  • $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{C}{\mathrm{H}}_3)\mathrm{C}{\mathrm{H}}_2\mathrm{Br}$: 1-bromo-2-methylpropane [isobutyl bromide]
  • $\mathrm{C}{\mathrm{H}}_3\mathrm{CHClC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$: 2-chlorobutane [sec-butyl chloride]

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15.4.5 Nomenclature of Alcohols

Alcohols contain the hydroxyl group $(-OH)$ and are named using the suffix -ol.

1. Parent Chain: Select the longest chain containing the $-OH$ group.
2. Numbering: Number from the end closer to the $-OH$ group to give it the lowest locant.
3. Suffix: Replace the terminal -e of the alkane name with -ol.

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15.4.6 Nomenclature of Aldehydes and Ketones

Aldehydes

Aldehydes contain the aldehyde group $(-CHO)$ and are named using the suffix -al.

• The $-CHO$ carbon is always carbon 1, so no locant is needed.
• The terminal -e of the alkane is replaced by -al.

Ketones

Ketones contain the carbonyl group $(\mathrm{C}=\mathrm{O})$ flanked by two carbon groups and are named using the suffix -one.

• Number the chain to give the carbonyl carbon the lowest possible locant.

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15.4.7 Nomenclature of Carboxylic Acids

Carboxylic acids contain the carboxyl group $(-COOH)$ and are named using the suffix -oic acid (written as two words: e.g., ethanoic acid).

• The $-COOH$ carbon is always carbon 1.
• The terminal -e of the alkane is replaced by -oic acid.

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15.4.8 Nomenclature of Amines

Amines contain the amino group $(-N{H}_2)$ and are named using the suffix -amine or the prefix amino-.

• Primary amines ($R-N{H}_2$): named as alkylamines.
• Number the chain to give the $-N{H}_2$ group the lowest locant.

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Summary Table: IUPAC Suffixes for Common Functional Groups