IUPAC Rules for Nomenclature of Organic Compounds

Turning the three-piece prefix + root + suffix pattern into an actual working procedure needs two more pieces of machinery: the suffix system, and a five-step naming method.

Suffixes come in two layers. The PRIMARY suffix, attached directly to the root word, records only the chain's degree of saturation: -ane for an all-single-bonded (saturated) chain, and -ene/-diene/-triene for a chain carrying one, two or three C=C double bonds, or -yne/-diyne for one or two C≡C triple bonds (Table 11.3). The SECONDARY suffix, layered on top of the primary suffix by dropping its terminal 'e', then records the compound's actual functional group -- alcohols take -ol, aldehydes -al, ketones -one, carboxylic acids -oic acid, esters -oate, acid chlorides -oyl chloride, amides -amide, amines -amine, nitriles -nitrile, sulphonic acids -sulphonic acid, and thiols -thiol (Table 11.4). Table 11.4 also gives the PREFIX form of each of these same functional groups (hydroxy- for -OH, oxo- for a ketone C=O, carboxy- for -COOH, and so on) -- used whenever that functional group is not the compound's principal group and so must instead be cited as an ordinary substituent. Table 11.5 extends this substituent-prefix list to the common non-functional-group substituents: the four halogens (fluoro, chloro, bromo, iodo), the nitro and nitroso groups, alkoxy groups (methoxy, ethoxy), and the everyday branched alkyl groups -- methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, and neopentyl among them.

The five-step naming procedure: (1) Identify the longest continuous carbon chain and treat everything else attached to it as a substituent -- crucially, if the compound carries a recognised functional group (one of -OH, -CHO, -COOH, -CN, -COOR, -CONH2, -NH2, -COR, -COX, -SO3H, -SH, -SR), the chosen chain MUST include that functional group's carbon, even if a longer chain excluding it exists; if more than one functional group is present, the chain must include whichever one sits highest in the precedence order -COOH > -SO3H > -COOR/-COX > -CONH2 > -CN > -CHO > -CO- > -OH > -SH/-NH2 > -C≡C- > -C=C- > -C-C- > -O- > -X > -NO2; and if the SAME functional group appears more than once, the chain must be chosen to include the maximum possible number of occurrences of it. When two candidate chains tie in length, the one carrying more substituents is preferred. (2) Number the chosen chain from whichever end gives the lowest locant, in this strict order of priority: first to the principal (suffix) functional group, then to the chain's unsaturation (double/triple bonds), and only then to the substituents as a set; if a genuine tie remains even after that, the lower number goes to whichever substituent is cited first in alphabetical order. (3) Name every substituent using the prefix tables. (4) List multiple substituents in ALPHABETICAL order before the root word (ignoring multiplying prefixes such as di-, tri-, tetra- for this alphabetising, even though those prefixes remain part of the written name and are not themselves separated by a hyphen). (5) Assemble the final name as prefix + root word + primary suffix + secondary suffix.

A handful of punctuation and formatting conventions complete the system: an IUPAC name is written as one unbroken word (organic salts, acids and acid derivatives are the recognised exception); commas separate adjacent numeral or letter locants while hyphens separate a numeral from an adjacent letter (2,2-Dimethyl-3-hexene; N,N-Dimethylmethanamide); italicised structural descriptors such as cis-, trans- and meso- are hyphen-joined to the name and ignored both when alphabetising substituents and when deciding what letter starts a sentence (trans-2-Butene); and the multiplying prefixes di-, tri-, tetra- are written plainly (no italics, no extra hyphen) and are likewise skipped over when alphabetising (4-Ethyl-2,2-dimethylhexane). …