IUPAC nomenclature of organic chemistry

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:

For example, the simplest alkane is CH₄ methane, and the nine-carbon alkane CH₃(CH₂)₇CH₃ 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 and tridecane which have mixed-language prefixes.

Cyclic alkanes are simply prefixed with "cyclo-": for example, C₄H₈ is cyclobutane (not to be confused with butene) and C₆H₁₂ 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, (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. 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, (CH₃)₂CHCH₂CH₃ (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 di-, tri-, tetra-, etc., depending on the number of branches. For example, C(CH₃)₄ (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 are named for their parent alkane chain with the suffix "-ene" and an infixed number indicating the position of the carbon with the lower number for each double bond in the chain: CH₂=CHCH₂CH₃ is but-1-ene. 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 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.

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

In Haloalkanes and Haloarenes (R-X), 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, CHCl₃ (chloroform) is trichloromethane. The anesthetic Halothane (CF₃CHBrCl) is 2-bromo-2-chloro-1,1,1-trifluoroethane.

Alcohols (R-OH) take the suffix "-ol" with an infix numerical bonding position: CH₃CH₂CH₂OH is propan-1-ol. 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.

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 becomes 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.

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, CH₃OCH₂CH₃ 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.[2]

Aldehydes (R-CHO) 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: 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. If an aldehyde is attached to a benzene and is the main functional group, the suffix becomes benzaldehyde.

In general ketones (R-CO-R) take the suffix "-one" (pronounced own, not won) with an infix position number: CH₃CH₂CH₂COCH₃ is pentan-2-one. If a higher precedence suffix is in use, the prefix "oxo-" is used: CH₃CH₂CH₂COCH₂CHO is 3-oxohexanal.

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, CH₃-CH(OH)-COOH (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,[3] though systematic names like ethanoic acid are also used. Carboxylic acids attached to a benzene ring are structural analogs of benzoic acid (Ph-COOH) and are named as one of its derivatives.

Citric acid

If there are multiple carboxyl groups on the same parent chain, multiplying prefixes are used: Malonic acid, CH
₂(COOH)
₂, is systematically named propanedioic acid. Alternatively, the suffix "-carboxylic acid" can be used, combined with a multiplying prefix if necessary – mellitic acid is benzenehexacarboxylic acid, for example. In the latter case, the carbon atom(s) in the carboxyl group(s) 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 2-hydroxypropane-1,2,3-tricarboxylic acid rather than 3-carboxy-3-hydroxypentanedioic acid.

formula of a carboxylate ion

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." For example, C
₆H
₅CO
₂Na, the sodium salt of benzoic acid (C
₆H
₅COOH), is called sodium benzoate. Where an acid has both a systematic and a common name (like CH
₃COOH, for example, which is known as both acetic acid and as ethanoic acid), its salts can be named from either parent name. Thus, CH
₃CO
₂K can be named as potassium acetate or as potassium ethanoate.

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.

Acyl groups are named by stripping the -ic acid of the corresponding carboxylic acid and replacing it with -yl. For Example, CH3CO-R is called Ethanoyl-R.

Simply add the name of the attached halide to the end of the acyl group. For example, CH3COCl is Ethanoyl Chloride.

If both acyl groups are the same, then the name of the carboxylic acid with the word acid replaced with anhydride. If the acyl groups are different, then they are named in alphabetical order in the same way, with anhydride replacing acid. For example, CH3CO-O-OCCH3 is called Ethanoic Anhydride.

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-ethyl-N-methylpropanamine. Again, the substituent groups are ordered alphabetically.

Amides (R-CO-NH₂) take the suffix "-amide", or "-carboxamide" if the carbon in the amide group cannot be included in the main chain. The prefix form is both "carbamoyl-" and "amido-".

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: HCON(CH₃)₂ is N,N-dimethylmethanamide.

Nitriles (RCN) 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 -onitrile. Functional class IUPAC nomenclature may also be used in the form of alkyl cyanides. For example, CH3CH2CH2CH2CN 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 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 being accepted as base names for compounds derived from them.

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.

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

Hydron is a generic term for hydrogen cation; protons, deuterons and tritons are all hydrons. The Hydrons are not found in heavier isotopes, however.

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: 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 pnictogen 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).

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. (CH₃)₃O⁺ is trimethyloxonium. CH₃F₃N⁺ is trifluoromethylammonium.

• Favre, Henri A.; Powell, Warren H. (2013). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. Royal Society of Chemistry. ISBN 978-0-85404-182-4.