IUPAC nomenclature

IUPAC nomenclature is a systematic way of naming organic chemical compounds. Ideally, every organic compound should have a name from which an unambiguous structural formula can be drawn. There is also a IUPAC system for naming inorganic compounds, see the systematic name page for details.
In IUPAC nomenclature, a number of prefixes, suffixes and infixes are used to describe the type and position of functional groups in the compound.

Contents [hide]

1 Alkanes 2 Alkenes and Alkynes 3 Alcohols 4 Halogenated compounds 5 Ketones 6 Aldehydes 7 Carboxylic acids 8 Ethers 9 Esters 10 Amines and Amides 11 Cyclic compounds 12 Order of precedence of groups 13 Common nomenclature 13.1 Ketones 13.2 Aldehydes 14 Ions 14.1 Hydron 14.2 Parent hydride cations 14.3 Cations and substitution 15 External link

Alkanes

Straight-chain alkanes take the suffix "-ane" and are prefixed depending on the number of carbon atoms in the chain, as given by the following table:

Number of carbons	1	2	3	4	5	6	7	8	9	10	11	12
Prefix	Meth	Eth	Prop	But	Pent	Hex	Hept	Oct	Non	Dec	Undec	Dodec

For example, the simplest alkane is CH₄ methane, and the nine-carbon alkane CH₃(CH₂)₇CH₃ is named nonane.
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 hypens: 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 (eg 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 a 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

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 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

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.

Halogenated compounds

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 disimilar 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

Ketones (R-CO-R) take the suffix "-one" (pronounced own, not won) with an infix position number: CH₃COCH₃ (acetone) is propan-2-one. If a higher precedence suffix is in use, the prefix "oxo-" is used: CH₃CH₂CH₂COCH₃CHO is 3-oxohexanal.

Aldehydes

Aldehydes (R-CHO) take the suffix "-al". Since they are always at the end of a 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: CHOCH₃COOH 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

Carboxylic acids are named with the suffix "-anoic acid". As for aldehydes, they take the "1" position on the parent chain, but do not have their position number indicated. For example, HCOOH (formic acid) is named methanoic acid, CH₃COOH (acetic acid) named ethanoic acid.
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-hydroxy-1,2,3-propanetricarboxylic acid, rather than 2-carboxy, 2-hydroxypentanedioic acid.

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

Esters (R-CO-O-R) are named with the suffix "-oate" added the alkene bonded to the carbonyl side of the ester. They are prefixed with the name of the alkyl group bonded on the other side of the ester. Thus HCOOCH₃ (formate) is methyl methanoate, CH₃COOCH₃ (acetate) is methyl ethanoate, and HCOOCH₃ (ethyl formate ) is ethyl methaonate.

If the alkyl group is not attached at the end of the chain, the bond position to the ester group is infixed before "-yl": CH₃CH₂C(CH₃)OOCH₂CH₂CH₃ is but-2-yl propanoate.

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: CH₃CH2₂N(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 (including 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.

Precedence	Group	Formula	Suffix	Prefix
1	Carboxylic acid	R-COOH	-oic acid	carboxy
2	Ester	R-CO-O-R	-oate	none
3	Amide	R-CONH2	-amide	none
4	Cyanide	R-CN	-nitrile	cyano
5	Aldehyde	R-CHO	-al	oxo
6	Ketone	R-CO-R	-one	oxo
7	Alcohol	R-OH	-ol	hydroxy
8	Sulfhydryl	R-SH	-thiol	mercapto
9	Amine	R-NH2	-amine	amino

Common nomenclature

Common nomenclature is an older system of naming organic compounds.

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

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.

External link

• IUPAC Nomenclature of Organic Chemistry