In chemical nomenclature, the IUPAC nomenclature of organic chemistry is a method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in the Nomenclature of Organic Chemistry (informally called the Blue Book) and summarized in a IUPAC technical report. 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.
Basic principles 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:
Identification of the most senior group. If more than one functional group, if any, is present, the one with highest group precedence should be used. Identification of the ring or chain with the maximum number of senior groups. Identification of the ring or chain with the most senior elements (In order: N, P, Si, B, O, S, C). Identification of the parent compound. Rings are senior to chains if composed of the same elements. For cyclic systems: Identification of the parent cyclic ring. The cyclic system must obey these rules, in order of precedence: It should have the most senior heteroatom (in order: N, O, S, P, Si, B). It should have the maximum number of rings. It should have the maximum number of atoms. It should have the maximum number of heteroatoms. It should have the maximum number of senior heteroatoms (in order: O, S, N, P, Si, B). For chains: Identification of the parent hydrocarbon chain. This chain must obey the following rules, in order of precedence: It should have the maximum length. It should have the maximum number of heteroatoms. It should have the maximum number of senior heteroatoms (in order: O, S, N, P, Si, B). For cyclic systems and chains after previous rules: It should have the maximum number of multiple, then double bonds. It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest group precedence should be used. Identification of the side-chains. Side chains are the carbon chains that are not in the parent chain, but are branched off from it. Identification of the remaining functional groups, if any, and naming them by their ionic prefixes (such as hydroxy for , oxy for , oxyalkane for , etc.).Different side-chains and functional groups will be grouped together in alphabetical order. (The multiplier prefixes di-, tri-, etc. are not taken into consideration for grouping alphabetically. For example, ethyl comes before dihydroxy or dimethyl, as the "e" in "ethyl" precedes the "h" in "dihydroxy" and the "m" in "dimethyl" alphabetically. The "di" is not considered in either case). When both side chains and secondary functional groups are present, they should be written mixed together in one group rather than in two separate groups. Identification of double/triple bonds. Numbering of the chain. This is done by first numbering the chain in both directions (left to right and right to left), and then choosing the numbering which follows these rules, in order of precedence. Not every rule will apply to every compound, rules can be skipped if they do not apply. Has the lowest-numbered locant (or locants) for heteroatoms. Locants are the numbers on the carbons to which the substituent is directly attached. Has the lowest-numbered locants for the indicated hydrogen. The indicated hydrogen is for some unsaturated heterocyclic compounds. It refers to the hydrogen atoms not attached to atoms with double bonds in the ring system. Has the lowest-numbered locants for the suffix functional group. Has the lowest-numbered locants for multiple bonds ('ene', 'yne'), and hydro prefixes. (The locant of a multiple bond is the number of the adjacent carbon with a lower number). Has the lowest-numbered locants for all substituents cited by prefixes. Has the lowest-numbered locants for substituents in order of citation (for example: in a cyclic ring with only bromine and chlorine functional groups, alphabetically bromo- is cited before chloro- and would receive the lower locant). Numbering of the various substituents and bonds with their locants. If there is more than one of the same type of substituent/double bond, a prefix is added showing how many there are (di – 2, tri – 3, tetra – 4, then as for the number of carbons below with 'a' added at the end)
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.
Arrangement in this form: Group of side chains and secondary functional groups with numbers made in step 6 + prefix of parent hydrocarbon chain (eth, meth) + double/triple bonds with numbers (or "ane") + primary functional group suffix with numbers.Wherever it says "with numbers", it is understood that between the word and the numbers, the prefix (di-, tri-) is used. Adding of punctuation: Commas are put between numbers (2 5 5 becomes 2,5,5) Hyphens are put between a number and a letter (2 5 5 trimethylheptane becomes 2,5,5-trimethylheptane) Successive words are merged into one word (trimethyl heptane becomes trimethylheptane) Note: IUPAC uses one-word names throughout. This is why all parts are connected.
The resulting name appears as: #,#-di<side chain>-#-<secondary functional group>-#-<side chain>-#,#,#-tri<secondary functional group><parent chain prefix><If all bonds are single bonds, use "ane">-#,#-di<double bonds>-#-<triple bonds>-#-<primary functional group>
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.)
Example 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 parent hydrocarbon chain has 23 carbons. It is called tricosa-. The functional groups with the highest precedence are the two ketone groups. The groups are on carbon atoms 3 and 9. As there are two, we write 3,9-dione. The numbering of the molecule is based on the ketone groups. When numbering from left to right, the ketone groups are numbered 3 and 9. When numbering from right to left, the ketone groups are numbered 15 and 21. 3 is less than 15, therefore the ketones are numbered 3 and 9. The smaller number is always used, not the sum of the constituents numbers. The side chains are: an ethyl- at carbon 4, an ethyl- at carbon 8, and a butyl- at carbon 12. Note: the at carbon atom 15 is not a side chain, but it is a methoxy functional group. There are two ethyl- groups. They are combined to create, 4,8-diethyl. The side chains are grouped like this: 12-butyl-4,8-diethyl. (But this is not necessarily the final grouping, as functional groups may be added in between to ensure all groups are listed alphabetically.) The secondary functional groups are: a hydroxy- at carbon 5, a chloro- at carbon 11, a methoxy- at carbon 15, and a bromo- at carbon 18. Grouped with the side chains, this gives 18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15-methoxy. There are two double bonds: one between carbons 6 and 7, and one between carbons 13 and 14. They would be called "6,13-diene", but the presence of alkynes switches it to 6,13-dien. There is one triple bond between carbon atoms 19 and 20. It will be called 19-yne. The arrangement (with punctuation) is: 18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15-methoxytricosa-6,13-dien-19-yne-3,9-dione Finally, due to cis–trans isomerism, we have to specify the relative orientation of functional groups around each double bond. For this example, both double bonds are trans isomers, so we have (6E,13E)
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.
Hydrocarbons
Alkanes
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 carbons1234567891011121314151617181920 PrefixMethEthPropButPentHexHeptOctNonDecUndecDodecTridecTetradecPentadecHexadecHeptadecOctadecNonadecIcos
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).
Alkenes