Two approaches to naming in organic chemistry
Historically, two parallel types of nomenclature have formed in chemical practice:
- Trivial (traditional) nomenclature. These are names assigned to substances as they were accidentally discovered. They reflect appearance, taste, odor, method of synthesis, or source of isolation, but do not explain the molecular structure. Despite the lack of strict logic, trivial names continue to be actively used in modern practice.
- Systematic nomenclature (IUPAC). Developed specifically for unification and convenience. It obeys strict algorithms and allows deriving the name of any molecule based solely on its structural formula.
What a systematic name consists of
The name of any organic molecule is constructed like a puzzle and includes five basic elements:
- Root — indicates the parent compound (the main carbon chain). The names of saturated hydrocarbons (unbranched alkanes) are always taken as the basis.
- Suffix — reflects the presence of multiple bonds (double or triple) and indicates the most important (senior) functional group.
- Prefix — used to list hydrocarbon radicals and all junior functional groups.
- Multiplying prefixes (numerals) — show the number of identical substituents (e.g., di-, tri-, tetra-).
- Locants (numbers) — fix the exact position of all substituents, groups, and multiple bonds on the carbon skeleton.
Homologous series of alkanes (basis for roots)
| Name | Formula | Name | Formula |
|---|---|---|---|
| Methane | $CH_4$ | Hexane | $C_6H_{14}$ |
| Ethane | $C_2H_6$ | Heptane | $C_7H_{16}$ |
| Propane | $C_3H_8$ | Octane | $C_8H_{18}$ |
| Butane | $C_4H_{10}$ | Nonane | $C_9H_{20}$ |
| Pentane | $C_5H_{12}$ | Decane | $C_{10}H_{22}$ |
Choice of the main chain and radicals
To correctly name a molecule, you need to find its foundation — the parent chain, and then number it correctly. The main rule states: you must choose the longest sequence of carbon atoms. All branches from this chain are considered radicals (hydrogen atom substituents).
Numbering begins from the end of the chain closest to a radical or multiple bond. However, there is a critically important condition here: a multiple bond (double or triple) must be part of the parent chain, even if this results in the chosen chain not being the longest of all possible ones.
Among hydrocarbon radicals, the most common are:
- Linear: methyl ($-CH_3$), ethyl ($-C_2H_5$), propyl, butyl, pentyl.
- Branched: isopropyl (branching at the carbon atom bonded to the chain), isobutyl (branching at the end of the chain), sec-butyl (bond at a secondary carbon), tert-butyl (bond at a tertiary carbon).
- Unsaturated: vinyl ($-CH=CH_2$).
Functional groups and the seniority rule
The main chain must necessarily include the functional group, because of which it, again, may not turn out to be the longest. If there are several different functional groups in a molecule, the conditional seniority rule comes into force.
The senior group determines the class of the compound and is indicated in the name by a suffix. All other groups are recognized as junior and are placed in the prefix.
Order of decreasing seniority:
- Carboxyl (–COOH)
- Aldehyde (–CHO)
- Ketone (>C=O)
- Alcohol (–OH)
- Amino group (–NH2)
- Nitro group (–NO2)
- Alkoxyl (–OR)
- Halogens (–F, –Cl, –Br, –I)
For example, if a molecule contains both an alcohol and a ketone group, the ketone group will be senior (will go into the suffix), and the alcohol group will become a prefix (designated as "hydroxy").
Briefly: answers to topical questions
What suffixes are used to denote double and triple bonds in a carbon chain?
To denote multiple (double and triple) bonds in a carbon chain, specific suffixes are used, which are added to the root of the parent compound's name. Depending on the type of bond, the following options are used:
- Double bond — denoted by the suffix "-ene" (characteristic of alkenes).
- Triple bond — denoted by the suffix "-yne" (characteristic of alkynes).
- Two double bonds — denoted by the suffix "-diene" (characteristic of dienes).
What suffixes and prefixes are used to designate the alcohol (hydroxyl) group?
To designate the alcohol (hydroxyl) group in systematic nomenclature, the following are used:
- Suffix "-ol" — designates the alcohol group.
- Prefix "hydroxy-" — used to designate the hydroxyl group as a junior functional group.
How are carboxyl and aldehyde groups designated in the suffix of the senior group?
In IUPAC systematic nomenclature, carboxyl and aldehyde groups, acting as senior functional groups, are designated in the compound's name using specific suffixes.
- Carboxyl group ($-\text{COOH}$) — is the highest in the series of functional groups and is denoted by the suffix -oic acid.
- Aldehyde group ($-\text{CHO}$) — ranks second in seniority and is denoted by the suffix -al.
Which functional groups are always indicated only as prefixes according to IUPAC rules?
The provided materials do not contain an exhaustive list of groups that are always indicated only as prefixes. The groups for which prefix designations are specified in the sources include:
- Nitro group ($-\text{NO}_2$) — prefix nitro-.
- Alkoxy group ($\text{R–O–R}$) — prefixes methoxy-, ethoxy-, and others.
- Halogens (–F, –Cl, –Br, –I) — designated as prefixes.
To memorize the beginning of the functional group priority order (the most important ones), use the mnemonic CAKS: Carboxyl, Aldehyde, Ketone (carbonyl), Secondary alcohol.
Class and exam questions
What to do if the longest carbon chain does not contain a double bond?
According to nomenclature rules, a multiple bond must be included in the parent chain. Therefore, the chain that includes this bond is chosen, even if it is shorter than an alternative option without multiple bonds.
Where are lower-priority functional groups placed in the name?
Lower-priority functional groups become regular substituents. They are indicated by prefixes at the beginning of the name, just like hydrocarbon radicals.
How are multiple identical functional groups designated?
The number of identical groups is indicated by multiplying prefixes: di-, tri-, tetra-, penta-, and so on. The exact position of each is indicated by numerals (locants).
Why are trivial names needed if there is an international system?
Trivial names arose historically as substances were discovered. Despite being associated with arbitrary features (smell, source) and not reflecting the structure, they have become firmly established in practice and are still widely used.
What else to study on this topic
- Detailed analysis of numbering examples in non-cyclic branched chains
- Specifics of naming complex compounds with multiple different functional groups
- Differences in radical structures: isopropyl, isobutyl, sec-butyl, and tert-butyl
- Peculiarities of naming when the keto group goes into the prefix ("oxo-" prefix)
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