IUPAC nomenclature of organic chemistry

The IUPAC nomenclature of organic chemistry is a systematic method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). Ideally, every organic compound should have a name from which an unambiguous structural formula can be drawn. There is also an IUPAC nomenclature of inorganic chemistry. See also phanes nomenclature of highly complex cyclic molecules.

For ordinary communication, to spare a tedious description, the official IUPAC naming recommendations are not always followed in practice except when it is necessary to give a concise definition to a compound, or when the IUPAC name is simpler (viz. ethanol against ethyl alcohol). Otherwise the common or trivial name may be used, often derived from the source of the compound (See Sec 14. below)

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Basic principles

In IUPAC nomenclature, a number of prefixes, suffixes and infixes are used to describe the type and position of functional groups in the compound.

For many compounds, naming can begin by determining the name of the parent hydrocarbon and by identifying any functional groups in the molecule that distinguish it from the parent hydrocarbon. The numbering of the parent alkane is used, as modified, if necessary, by application of the Cahn Ingold Prelog priority rules ("CIP") in the case that ambiguity remains after consideration of the structure of the parent hydrocarbon alone. The name of the parent hydrocarbon is modified by the application of the highest-priority functional group suffix, with the remaining functional groups indicated by numbered prefixes, appearing in the name in alphabetical order from first to last.

In many cases, lack of rigor in applying all such nomenclature rules still yields a name that is intelligible — the aim, of course, being to avoid any ambiguity in terms of what substance is being discussed.

For instance, strict application of CIP priority to the naming of the compound

NH₂CH₂CH₂OH

would render the name as 2-aminoethanol, which is preferred. However, the name 2-hydroxyethanamine unambiguously refers to the same compound.

How the name was constructed:

1. There are two carbons in the main chain; this gives the root name "eth".
2. Since the carbons are singly-bonded, the suffix begins with "an".
3. The two functional groups are an alcohol (OH) and an amine (NH₂). The alcohol has the higher atomic number, and takes priority over the amine. The suffix for an alcohol ends in "ol", so that the suffix is "anol".
4. The amine group is not on the carbon with the OH (the #1 carbon), but one carbon over (the #2 carbon); therefore we indicate its presence with the prefix "2-amino".
5. Putting together the prefix, the root and the suffix, we get "2-aminoethanol".

There is also an older naming system for organic compounds known as common nomenclature, which is often used for simple, well-known compounds, and also for complex compounds whose IUPAC names are too complex for everyday use.

Simplified molecular input line entry specification (SMILES) strings are commonly used to describe organic compounds, and as such are a form of 'naming' them.

Alkanes

Main article: Alkanes

Straight-chain alkanes take the suffix "-ane" and are prefixed depending on the number of carbon atoms in the chain, following standard IUPAC numerical multiplier rules. The first few are:

For example, the simplest alkane is CH₄ methane, and the nine-carbon alkane CH₃(CH₂)₇CH₃ is named nonane. If one was to name the 157-carbon alkane CH₃(CH₂)₁₅₅CH₃ the name would be heptapentahectane.

Cyclic alkanes are simply prefixed with "cyclo-", for example C₄H₈ is cyclobutane and C₆H₁₂ is cyclohexane.

Branched alkanes are named as a straight-chain alkane with attached alkyl groups. They are prefixed with a number indicating the carbon the group is attached to, counting from the end of the alkane chain. Infixed is the name of the substituent, as for alkanes in the table above, plus "-yl". For example, (CH₃)₂CHCH₃, commonly known as isobutane, is treated as a propane chain with a methyl group bonded to the middle (2) carbon, and given the systematic name 2-methylpropane. Numbers may be dropped when there is no ambiguity, so 2-methylpropane is just methylpropane. (1-methylpropane would be identical to butane.)

If there is ambiguity in the position of the substituent, depending on which end of the alkane chain is counted as "1", then numbering is chosen so that the smallest number is used. For example, (CH₃)₂CHCH₂CH₃ (isopentane) is named 2-methylbutane, not 3-methylbutane. Since this resolves the ambiguity, the number is again dropped in this case.

If there are multiple side-branches of the same size alkyl group, their positions are separated by commas and the group prefixed with di-, tri-, tetra-, etc., depending on the number of branches (e.g. C(CH₃)₄ 2,2-dimethylpropane). If there are different groups, they are added in alphabetical order, separated by commas or hyphens: 3-ethyl-4-methylhexane. The longest possible main alkane chain is used; therefore 3-ethyl-4-methylhexane instead of 2,3-diethylpentane, even though these describe equivalent structures. The di-, tri- etc. prefixes are ignored for the purpose of alphabetical ordering of side chains (e.g. 3-ethyl-2,4-dimethylpentane, not 2,4-dimethyl-3-ethylpentane). If multiple chains of the longest possible length exist, the chain that has a larger number of branch points is the chain that is used.

Subsidiary branches off a side-chain are prefixed according to a secondary numbering system specific to that side branch, numbering from the point of attachment to the main chain, and then the whole side-branch is parenthesised and treated as a single substituent. For example, 4-(1-methylethyl)octane is an octane chain with a side chain bonded to the 4th carbon, the side chain consisting of an ethyl group with a methyl group attached to the carbon closest to the main chain.

Alkenes and Alkynes

Main articles: Alkenes and Alkynes

Alkenes are named for their parent alkane chain with the suffix "-ene" and an infixed number indicating the position of the double-bonded carbon in the chain: CH₂=CHCH₂CH₃ is but-1-ene. Ethene (ethylene) and propene (propylene) do not require infixed numbers, since there is no ambiguity in the structures. As before, the lowest number is used.

Multiple double bonds take the form -diene, -triene, etc., with the size prefix of the chain taking an extra "a": CH₂=CHCH=CH₂ is buta-1,3-diene. Simple cis and trans isomers are indicated with a prefixed cis- or trans-: cis-but-2-ene, trans-but-2-ene. More complex geometric isomerisations are described using the Cahn Ingold Prelog priority rules.

Alkynes are named using the same system, with the suffix "-yne" indicating a triple bond: ethyne (acetylene), propyne (methylacetylene).

Alcohols

Main article: Alcohols

Alcohols (R-OH) drop the terminal "e" from the name of the parent alkane, and take the suffix "-ol" with an infix numerical bonding position: CH₃CH₂CH₂OH is propan-1-ol. (Methanol and ethanol are unambiguous and do not require position numbers). The suffixes -diol, -triol, -tetraol, etc., are used for multiple -OH groups: Ethylene glycol CH₂OHCH₂OH is ethane-1,2-diol.

If higher precedence functional groups are present (see order of precedence, below), the prefix "hydroxy" is used with the bonding position: CH₃CHOHCOOH is 2-hydroxypropanoic acid.

Halogens (Alkyl Halides)

Main article: Halogens

Halogen functional groups are prefixed with the bonding position and take the form fluoro-, chloro-, bromo-, iodo-, etc., depending on the halogen. Multiple groups are dichloro-, trichloro-, etc, and dissimilar groups are orded alphabetically as before. For example, CHCl₃ (chloroform) is trichloromethane. The anesthetic Halothane (CF₃CHBrCl) is 2-bromo-2-chloro-1,1,1-trifluoroethane.

Ketones

Main article: Ketones

In general ketones (R-CO-R) take the suffix "-one" (pronounced own, not won) with an infix position number: CH₃CH₂CH₂COCH₃ is pentan-2-one. For common ketones some traditional names such as acetone and benzophenone predominate, and these are acceptable IUPAC names, although some introductory chemistry texts use alternative names for acetone such as propan-2-one or propanone (see diagram). Additionally, in such unambiguous cases as propanone, the infixed number can be dropped. If a higher precedence suffix is in use, the prefix "oxo-" is used: CH₃CH₂CH₂COCH₂CHO is 3-oxohexanal.

Aldehydes

Main article: Aldehydes

Aldehydes (R-CHO) take the suffix "-al". Since they are always at the end of an alkane chain, they do not need a position number: HCHO (formaldehyde) is methanal, CH₃CHO (acetaldehyde) is ethanal. If other functional groups are present, the chain is numbered such that the aldehyde carbon is in the "1" position.

If a prefix form is required, "oxo-" is used (as for ketones), with the position number indicating the end of a chain: CHOCH₂COOH is 3-oxopropanoic acid. If the carbon in the carbonyl group cannot be included in the attached chain (for instance in the case of cyclic aldehydes), the prefix "formyl-" or the suffix "-carbaldehyde" is used: C₆H₁₁CHO is cyclohexanecarbaldehyde.

Carboxylic acids

Main article: Carboxylic acids

In general carboxylic acids are named with the suffix -oic acid (etymologically a back-formation from benzoic acid). As for aldehydes, they take the "1" position on the parent chain, but do not have their position number indicated. For example, CH₃CH₂CH₂CH₂COOH (valeric acid) is named pentanoic acid. For common carboxylic acids some traditional names such as acetic acid are in such widespread use they are considered retained IUPAC names, although "systematic" names such as ethanoic acid are also acceptable. For carboxylic acids attached to a benzene ring such as Ph-COOH, these are named as benzoic acid or its derivatives.

If there are multiple carboxyl groups on the same parent chain, the suffix "-carboxylic acid" can be used (as -dicarboxylic acid, -tricarboxylic acid, etc.). In these cases, the carbon in the carboxyl group does not count as being part of the main alkane chain. The same is true for the prefix form, "carboxyl-". Citric acid is one example; it is named 2-hydroxypropane- 1,2,3-tricarboxylic acid, rather than 2-carboxy, 2-hydroxypentanedioic acid.

Ethers

Main article: Ethers

Ethers (R-O-R) consist of an oxygen atom between the two attached carbon chains. The shorter of the two chains becomes the first part of the name with the -ane suffix changed to -oxy, and the longer alkane chain become the suffix of the name of the ether. Thus CH₃OCH₃ is methoxymethane, and CH₃OCH₂CH₃ is methoxyethane (not ethoxymethane). If the oxygen is not attached to the end of the main alkane chain, then the whole shorter alkyl-plus-ether group is treated as a side-chain and prefixed with its bonding position on the main chain. Thus CH₃OCH(CH₃)₂ is 2-methoxypropane.

Esters

Main article: Esters

Esters (R-CO-O-R') are named as alkyl derivatives of carboxylic acids. The alkyl (R') group is named first. The R-CO-O part is then named as a separate word based on the carboxylic acid name, with the ending changed from -oic acid to -oate. For example, CH₃CH₂CH₂CH₂COOCH₃ is methyl pentanoate, and (CH₃)₂CHCH₂CH₂COOCH₂CH₃ is ethyl 4-methylpentanoate. For esters such as ethyl acetate (CH₃COOCH₂CH₃), ethyl formate (HCOOCH₂CH₃) or dimethyl phthalate that are based on common acids, IUPAC recommends use of these established names, called retained names. The -oate changes to -ate. Some simple examples, named both ways, are shown in the figure above.

If the alkyl group is not attached at the end of the chain, the bond position to the ester group is infixed before "-yl": CH₃CH₂CH(CH₃)OOCCH₂CH₃ may be called but-2-yl propanoate or but-2-yl propionate.

Amines and Amides

Main articles: Amines and Amides

Amines (R-NH₂) are named for the attached alkane chain with the suffix "-amine" (e.g. CH₃NH₂ methanamine). If necessary, the bonding position is infixed: CH₃CH₂CH₂NH₂ propan-1-amine, CH₃CHNH₂CH₃ propan-2-amine. The prefix form is "amino-".

For secondary amines (of the form R-NH-R), the longest carbon chain attached to the nitrogen atom becomes the primary name of the amine; the other chain is prefixed as an alkyl group with location prefix given as an italic N: CH₃NHCH₂CH₃ is N-methylethanamine. Tertiary amines (R-NR-R) are treated similarly: CH₃CH₂N(CH₃)CH₂CH₂CH₃ is N-methyl-N-ethylpropanamine.

Amides (R-CO-NH₂) take the suffix "-amide". There is no prefix form, and no location number is required since they always terminate a carbon chain, e.g. CH₃CONH₂ (acetamide) is named ethanamide.

Secondary and tertiary amides are treated similarly to the case of amines: alkane chains bonded to the nitrogen atom are treated as substituents with the location prefix N: HCON(CH₃)₂ is N,N-dimethylmethanamide.

Cyclic compounds

Cycloalkanes and aromatic compounds can be treated as the main parent chain of the compound, in which case the position of substituents are numbered around the ring structure. For example, the three isomers of xylene CH₃C₆H₄CH₃, commonly the ortho-, meta-, and para- forms, are 1,2-dimethylbenzene, 1,3-dimethylbenzene, and 1,4-dimethylbenzene. The cyclic structures can also be treated as functional groups themselves, in which case they take the prefix "cycloalkyl-" (e.g. "cyclohexyl-") or for benzene, "phenyl-".

The IUPAC nomenclature scheme becomes rapidly more elaborate for more complex cyclic structures, with notation for compounds containing conjoined rings, and many common names such as phenol, furan, indole, etc. being accepted as base names for compounds derived from them.

Order of precedence of groups

When compounds contain more than one functional group, the order of precedence determines which groups are named with prefix or suffix forms. The highest precedence group takes the suffix, with all others taking the prefix form. However, double and triple bonds only take suffix form (-en and -yn) and are used with other suffixes.

Prefixed substituents are ordered alphabetically (excluding any modifiers such as di-, tri-, etc.), e.g. chlorofluoromethane, not fluorochloromethane. If there are multiple functional groups of the same type, either prefixed or suffixed, the position numbers are ordered numerically (thus ethane-1,2-diol, not ethane-2,1-diol.) The N position indicator for amines and amides comes before "1", e.g. CH₃CH(CH₃)CH₂NH(CH₃) is N,2-dimethylpropanamine.

*Note: These suffixes, in which the carbon atom is counted as part of the preceding chain, are the most commonly used. See individual functional group articles for more details.

Common nomenclature - trivial names

Common nomenclature is an older system of naming organic compounds. Instead of using the prefixes for the carbon skeleton above, another system is used. The pattern can be seen below.

Ketones

Common names for ketones can be derived by naming the two alkyl or aryl groups bonded to the carbonyl group as separate words followed by the word ketone.

• Acetone
• Acetophenone
• Benzophenone
• Ethyl isopropyl ketone
• Diethyl ketone

The first three of the names shown above are still considered to be acceptable IUPAC names.

Aldehydes

The common name for an aldehyde is derived from the common name of the corresponding carboxylic acid by dropping the word acid and changing the suffix from -ic or -oic to -aldehyde.

• Formaldehyde
• Acetaldehyde

Ions

The IUPAC nomenclature also provides rules for naming ions.

Hydron

Hydron is a generic term for hydrogen cation; protons, deuterons and tritons are all hydrons.

Parent hydride cations

Simple cations formed by adding a hydron to a hydride of a halogen, chalcogen or nitrogen-family element are named by adding the suffix "-onium" to the element's root: H₄N⁺ is ammonium, H₃O⁺ is oxonium, and H₂F⁺ is fluoronium. Ammonium was adopted instead of nitronium, which commonly refers to NO₂⁺.

If the cationic center of the hydride is not a halogen, chalcogen or nitrogen-family element then the suffix "-ium" is added to the name of the neutral hydride after dropping any final 'e'. H₅C⁺ is methanium, HO-O⁺H₂ is dioxidanium (HO-OH is dioxidane), and H₂N-N⁺H₃ is diazanium (H₂N-NH₂ is diazane).

Cations and substitution

The above cations except for methanium are not, strictly speaking, organic, since they do not contain carbon. However, many organic cations are obtained by substituting another element or some functional group for a hydrogen.

The name of each substitution is prepended to the hydride cation name. If many substitutions by the same functional group occur, then the number is indicated by prepending "di-", "tri-" as with halogenation. (CH₃)₃O⁺ is trimethyloxonium. CH₃F₃N⁺ is trifluoromethylammonium.

See also

• IUPAC nomenclature of inorganic chemistry
• Cahn Ingold Prelog priority rules
• Hantzsch-Widman nomenclature

References

1. ^  Nomenclature of Organic Chemistry, Oxford: Pergamon Press, 1979; A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993, Oxford: Blackwell Scientific Publications, 1993.