In chemical nomenclature, the IUPAC nomenclature of organic chemistry is a method of naming organic chemical compounds as recommended[1] [2] by the International Union of Pure and Applied Chemistry (IUPAC). It is published in the Nomenclature of Organic Chemistry (informally called the Blue Book). Ideally, every possible organic compound should have a name from which an unambiguous structural formula can be created. There is also an IUPAC nomenclature of inorganic chemistry.
To avoid long and tedious names in normal communication, the official IUPAC naming recommendations are not always followed in practice, except when it is necessary to give an unambiguous and absolute definition to a compound. IUPAC names can sometimes be simpler than older names, as with ethanol, instead of ethyl alcohol. For relatively simple molecules they can be more easily understood than non-systematic names, which must be learnt or looked over. However, the common or trivial name is often substantially shorter and clearer, and so preferred. These non-systematic names are often derived from an original source of the compound. Also, very long names may be less clear than structural formulas.
In chemistry, a number of prefixes, suffixes and infixes are used to describe the type and position of the functional groups in the compound.
The steps for naming an organic compound are:[3]
The numbers for that type of side chain will be grouped in ascending order and written before the name of the side-chain. If there are two side-chains with the same alpha carbon, the number will be written twice. Example: 2,2,3-trimethyl- . If there are both double bonds and triple bonds, "en" (double bond) is written before "yne" (triple bond). When the main functional group is a terminal functional group (a group which can exist only at the end of a chain, like formyl and carboxyl groups), there is no need to number it.
The resulting name appears as:
where each "#" represents a number. The group secondary functional groups and side chains may not look the same as shown here, as the side chains and secondary functional groups are arranged alphabetically. The di- and tri- have been used just to show their usage. (di- after #,#, tri- after #,#,#, etc.)
Here is a sample molecule with the parent carbons numbered:
For simplicity, here is an image of the same molecule, where the hydrogens in the parent chain are removed and the carbons are shown by their numbers:
Now, following the above steps:
The final name is (6E,13E)-18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15-methoxytricosa-6,13-dien-19-yne-3,9-dione.
See main article: Alkane.
Straight-chain alkanes take the suffix "-ane" and are prefixed depending on the number of carbon atoms in the chain, following standard rules. The first few are:
| Number of carbons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Prefix | Meth | Eth | Prop | But | Pent | Hex | Hept | Oct | Non | Dec | Undec | Dodec | Tridec | Tetradec | Pentadec | Hexadec | Heptadec | Octadec | Nonadec | Icos |
For example, the simplest alkane is methane, and the nine-carbon alkane is named nonane. The names of the first four alkanes were derived from methanol, ether, propionic acid and butyric acid, respectively. The rest are named with a Greek numeric prefix, with the exceptions of nonane which has a Latin prefix, and undecane which has mixed-language prefixes.
Cyclic alkanes are simply prefixed with "cyclo-": for example, is cyclobutane (not to be confused with butene) and is cyclohexane (not to be confused with hexene).
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. For example,, 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. However, although the name 2-methylpropane could be used, it is easier and more logical to call it simply methylpropane – the methyl group could not possibly occur on any of the other carbon atoms (that would lengthen the chain and result in butane, not propane) and therefore the use of the number "2" is unnecessary.
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 smaller number is used. For example, (isopentane) is named 2-methylbutane, not 3-methylbutane.
If there are multiple side-branches of the same size alkyl group, their positions are separated by commas and the group prefixed with multiplier prefixes depending on the number of branches. For example, (neopentane) is named 2,2-dimethylpropane. If there are different groups, they are added in alphabetical order, separated by commas or hyphens. 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).
See main article: Alkene.
Alkenes are named for their parent alkane chain with the suffix "-ene" and a numerical root indicating the position of the carbon with the lower number for each double bond in the chain: is but-1-ene.Multiple double bonds take the form -diene, -triene, etc., with the size prefix of the chain taking an extra "a": is buta-1,3-diene. Simple cis and trans isomers may be indicated with a prefixed cis- or trans-: cis-but-2-ene, trans-but-2-ene. However, cis- and trans- are relative descriptors. It is IUPAC convention to describe all alkenes using absolute descriptors of Z- (same side) and E- (opposite) with the Cahn–Ingold–Prelog priority rules (see alse E–Z notation).
See main article: Alkyne.
Alkynes are named using the same system, with the suffix "-yne" indicating a triple bond: ethyne (acetylene), propyne (methylacetylene).
See main article: Haloalkane and Haloarene.
In haloalkanes and haloarenes, Halogen functional groups are prefixed with the bonding position and take the form of fluoro-, chloro-, bromo-, iodo-, etc., depending on the halogen. Multiple groups are dichloro-, trichloro-, etc., and dissimilar groups are ordered alphabetically as before. For example, (chloroform) is trichloromethane. The anesthetic halothane is 2-bromo-2-chloro-1,1,1-trifluoroethane.
See main article: Alcohols.
Alcohols take the suffix "-ol" with a numerical suffix indicating the bonding position: is propan-1-ol. The suffixes,,, etc., are used for multiple groups: Ethylene glycol 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: is 2-hydroxypropanoic acid.
See main article: Ethers.
Ethers 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 becomes the suffix of the name of the ether. Thus, is methoxymethane, and 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 is 2-methoxypropane.
Alternatively, an ether chain can be named as an alkane in which one carbon is replaced by an oxygen, a replacement denoted by the prefix "oxa". For example, could also be called 2-oxabutane, and an epoxide could be called oxacyclopropane. This method is especially useful when both groups attached to the oxygen atom are complex.[4]
See main article: Aldehydes.
Aldehydes take the suffix "-al". If other functional groups are present, the chain is numbered such that the aldehyde carbon is in the "1" position, unless functional groups of higher precedence are present.
If a prefix form is required, "oxo-" is used (as for ketones), with the position number indicating the end of a chain: 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: is cyclohexanecarbaldehyde. If an aldehyde is attached to a benzene and is the main functional group, the suffix becomes benzaldehyde.
See main article: Ketones.
In general ketones take the suffix "-one" (pronounced own, not won) with a suffixed position number: is pentan-2-one. If a higher precedence suffix is in use, the prefix "oxo-" is used: is 3-oxohexanal.
See main article: Carboxylic acids.
In general, carboxylic acids are named with the suffix -oic acid (etymologically a back-formation from benzoic acid). As with aldehydes, the carboxyl functional group must take the "1" position on the main chain and so the locant need not be stated. For example, (lactic acid) is named 2-hydroxypropanoic acid with no "1" stated. Some traditional names for common carboxylic acids (such as acetic acid) are in such widespread use that they are retained in IUPAC nomenclature,[5] though systematic names like ethanoic acid are also used. Carboxylic acids attached to a benzene ring are structural analogs of benzoic acid and are named as one of its derivatives.
If there are multiple carboxyl groups on the same parent chain, multiplying prefixes are used: Malonic acid,, is systematically named propanedioic acid. Alternatively, the suffix can be used in place of "oic acid", combined with a multiplying prefix if necessary - mellitic acid is benzenehexacarboxylic acid, for example. In the latter case, the carbon atoms in the carboxyl groups do not count as being part of the main chain, a rule that also applies to the prefix form "carboxy-". Citric acid serves as an example: it is formally named rather than .
See main article: Carboxylate.
Salts of carboxylic acids are named following the usual cation-then-anion conventions used for ionic compounds in both IUPAC and common nomenclature systems. The name of the carboxylate anion is derived from that of the parent acid by replacing the "–oic acid" ending with "–oate" or "carboxylate." For example,, the sodium salt of benzoic acid, is called sodium benzoate. Where an acid has both a systematic and a common name (like, for example, which is known as both acetic acid and as ethanoic acid), its salts can be named from either parent name. Thus, can be named as potassium acetate or as potassium ethanoate. The prefix form, is "carboxylato-".
See main article: Esters.
Esters are named as alkyl derivatives of carboxylic acids. The alkyl (R') group is named first. The part is then named as a separate word based on the carboxylic acid name, with the ending changed from "-oic acid" to "-oate" or "-carboxylate" For example, is methyl pentanoate, and is ethyl 4-methylpentanoate. For esters such as ethyl acetate, ethyl formate 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 suffixed before "-yl": may be called butan-2-yl propanoate or butan-2-yl propionate.. The prefix form is "oxycarbonyl-" with the (R') group preceding.
See main article: Acyl group.
Acyl groups are named by stripping the "-ic acid" of the corresponding carboxylic acid and replacing it with "-yl." For example, is called ethanoyl-R.
Simply add the name of the attached halide to the end of the acyl group. For example, is ethanoyl chloride. An alternate suffix is "-carbonyl halide" as opposed to "-oyl halide". The prefix form is "halocarbonyl-".
Acid anhydrides have two acyl groups linked by an oxygen atom. If both acyl groups are the same, then the name of the carboxylic acid with the word acid is replaced with the word anhydride and the IUPAC name consists of two words. If the acyl groups are different, then they are named in alphabetical order in the same way, with anhydride replacing acid and IUPAC name consists of three words. For example, is called ethanoic anhydride and is called ethanoic propanoic anhydride.
See main article: Amine.
Amines are named for the attached alkane chain with the suffix "-amine" (e.g., methanamine). If necessary, the bonding position is suffixed: propan-1-amine, propan-2-amine. The prefix form is "amino-".
For secondary amines (of the form), 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: is N-methylethanamine. Tertiary amines are treated similarly: is N-ethyl-N-methylpropanamine. Again, the substituent groups are ordered alphabetically.
See main article: Amide.
Amides take the suffix "-amide", or "-carboxamide" if the carbon in the amide group cannot be included in the main chain. The prefix form is "carbamoyl-". e.g., methanamide, ethanamide.
Amides that have additional substituents on the nitrogen are treated similarly to the case of amines: they are ordered alphabetically with the location prefix N: is N,N-dimethylmethanamide, is N,N-dimethylethanamide.
See main article: Nitrile.
Nitriles are named by adding the suffix "-nitrile" to the longest hydrocarbon chain (including the carbon of the cyano group). It can also be named by replacing the "-oic acid" of their corresponding carboxylic acids with "-carbonitrile." The prefix form is "cyano-." Functional class IUPAC nomenclature may also be used in the form of alkyl cyanides. For example, is called pentanenitrile or butyl cyanide.
Cycloalkanes and aromatic compounds can be treated as the main parent chain of the compound, in which case the positions of substituents are numbered around the ring structure. For example, the three isomers of xylene, 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 being accepted as base names for compounds derived from them.
When compounds contain more than one functional group, the order of precedence determines which groups are named with prefix or suffix forms. The table below shows common groups in decreasing order of precedence. 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., is N,2-dimethylpropanamine.
| Priority | Functional group | Formula | Prefix | Suffix | |
|---|---|---|---|---|---|
| 1 | Cations e.g. Ammonium | -onio- ammonio- | -onium -ammonium | ||
| 2 | Carboxylic acids
Carboselenoic Se-acids | carboxy- sulfanylcarbonyl- selanylcarbonyl- sulfo- sulfino- | -oic acid* -thioic S-acid* -selenoic Se-acid* -sulfonic acid -sulfinic acid | ||
| 3 | Carboxylic acid derivatives | acyloxy- R-oxycarbonyl- halocarbonyl- carbamoyl- -imido- amidino- | -R-oic anhydride -R-oate -oyl halide* -amide* -imide* -amidine* | ||
| 4 | Nitriles | cyano- isocyano- | -nitrile* isocyanide | ||
| 5 | Aldehydes | formyl- thioformyl- | -al* -thial* | ||
| 6 | Ketones
Tellones | oxo- sulfanylidene- selanylidene- tellanylidene- | -one -thione -selone -tellone | ||
| 7 | Alcohols | hydroxy- sulfanyl- selanyl- tellanyl- | -ol -thiol -selenol -tellurol | ||
| 8 | Hydroperoxides
Thioperoxols (Sulfenic acid) Dithioperoxols | hydroperoxy- hydroxysulfanyl- disulfanyl- | -peroxol -SO-thioperoxol -dithioperoxol | ||
| 9 | Amines | amino- imino- hydrazino- | -amine -imine -hydrazine |
The order of remaining functional groups is only needed for substituted benzene and hence is not mentioned here.
Common nomenclature uses the older names for some organic compounds instead of using the prefixes for the carbon skeleton above. The pattern can be seen below.
| Number of carbons | Prefix as in new system | Common name for alcohol | Common name for aldehyde | Common name for acid | Common name for ketone |
|---|---|---|---|---|---|
| 1 | Meth- | Methyl alcohol (wood alcohol) | Formaldehyde | Formic acid | NA |
| 2 | Eth- | Ethyl alcohol (grain alcohol) | Acetaldehyde | Acetic acid (vinegar) | NA |
| 3 | Prop- | Propyl alcohol | Propionaldehyde | Propionic acid | Acetone/dimethyl ketone |
| 4 | But- | Butyl alcohol | Butyraldehyde | Butyric acid | Methyl ethyl ketone |
| 5 | Pent- | Amyl alcohol | Valeraldehyde | Valeric acid | •Methyl propyl ketone •Diethyl ketone |
| 6 | Hex- | Caproyl alcohol | Caproaldehyde | Caproic acid | •Butyl methyl ketone•Ethyl propyl ketone |
| 7 | Hept- | Enanthyl alcohol | Enanthaldehyde | Enanthoic acid | •Methyl pentyl ketone•Butyl ethyl ketone •Dipropyl ketone |
| 8 | Oct- | Capryl alcohol | Caprylaldehyde | Caprylic acid | •Hexyl methyl ketone•Ethyl pentyl ketone •Butyl propyl ketone |
| 9 | Non- | Pelargonic alcohol | Pelargonaldehyde | Pelargonic acid | •Heptyl methyl ketone•Ethyl hexyl ketone •Pentyl propyl ketone •Dibutyl ketone |
| 10 | Dec- | Capric alcohol | Capraldehyde | Capric acid | •Methyl octyl ketone•Ethyl heptyl ketone •Hexyl propyl ketone •Butyl pentyl ketone |
| 11 | Undec- | - | - | - | The same pattern continues (see below) |
| 12 | Dodec- | Lauryl alcohol | Lauraldehyde | Lauric acid | |
| 13 | Tridec- | - | - | - | |
| 14 | Tetradec- | Myristyl alcohol | Myristaldehyde | Myristic acid | |
| 15 | Pentadec- | - | - | - | |
| 16 | Hexadec- | Cetyl alcohol Palmityl alcohol | Palmitaldehyde | Palmitic acid | |
| 17 | Heptadec- | - | - | Margaric acid | |
| 18 | Octadec- | Stearyl alcohol | Stearaldehyde | Stearic acid | |
| 19 | Nonadec- | - | - | - | |
| 20 | Icos- | Arachidyl alcohol | - | Arachidic acid | |
| 21 | Henicos- | - | - | - | |
| 22 | Docos- | Behenyl alcohol | - | Behenic acid | |
| 23 | Tricos- | - | - | - | |
| 24 | Tetracos- | Lignoceryl alcohol | - | Lignoceric acid | |
| 25 | Pentacos- | - | - | - | |
| 26 | Hexacos- | Ceryl alcohol | - | Cerotic acid | |
| 27 | Heptacos- | - | - | - | |
| 28 | Octacos- | Montanyl alcohol | - | Montanic acid | |
| 29 | Nonacos- | - | - | - | |
| 30 | Triacont- | Melissyl alcohol | - | Melissic acid | |
| 31 | Hentriacont- | - | - | - | |
| 32 | Dotriacont- | Lacceryl alcohol | - | Lacceroic acid | |
| 33 | Tritriacont- | Psyllic alcohol | - | Psyllic acid | |
| 34 | Tetratriacont- | Geddyl alcohol | - | Geddic acid | |
| 35 | Pentatriacont- | - | - | Ceroplastic acid | |
| 36 | Hexatriacont- | - | - | - | |
| 37 | Heptatriacont- | - | - | - | |
| 38 | Octatriacont- | - | - | - | |
| 39 | Nonatriacont- | - | - | - | |
| 40 | Tetracont- | - | - | - |
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.
The first three of the names shown above are still considered to be acceptable IUPAC names.
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.
The IUPAC nomenclature also provides rules for naming ions.
Hydron is a generic term for hydrogen cation; protons, deuterons and tritons are all hydrons.The hydrons are not found in heavier isotopes, however.
See also: Onium compounds.
Simple cations formed by adding a hydron to a hydride of a halogen, chalcogen or pnictogen are named by adding the suffix "-onium" to the element's root: is ammonium, is oxonium, and H2F+ is fluoronium. Ammonium was adopted instead of nitronium, which commonly refers to .
If the cationic center of the hydride is not a halogen, chalcogen or pnictogen then the suffix "-ium" is added to the name of the neutral hydride after dropping any final 'e'. is methanium, is dioxidanium (HO-OH is dioxidane), and is diazanium (is diazane).
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 prefixed to the hydride cation name. If many substitutions by the same functional group occur, then the number is indicated by prefixing with "di-", "tri-" as with halogenation. is trimethyloxonium. is trifluoromethylammonium.