The rational nomenclature received its name from the Latin ratio - "reason". This system allowed for the first time to give logical names to organic compounds, based on their belonging to a particular class. She replaced the trivial names, which in no way reflected the structural features of their molecules. And not everyone can remember the structural formulas of numerous organic compounds.
The new nomenclature, also called radical or functional, began to take as its basis the simplest substances in the homologous series of a class. And the remaining compounds began to be considered as substituted derivatives from them. There are clear requirements for the name of the compounds. In the 19th century, these rules were innovative, and their emergence became possible thanks to the theory of radicals, type theory and a number of scientific treatises of that time.
General rules of rational nomenclature
The starting point of the name is the first or second member of the homologous series of the class of organic substances under consideration. The rest of the hydrocarbon skeleton of the molecule is recorded as radicals attached to it.
The names of the substituents themselves are formed, based on the homologous series of alkanes, replacing the suffixes -an with -yl, for example methyl, ethyl, etc. . For example, isopropyl, isobutyl.
Another common way to show the branching of substituents is to add the prefixes of the second and third. This entry indicates that a secondary or tertiary carbon atom has free valency. If the free valency is at the end of the unbranched residue of the molecule, then the letter n is added (from the word "normal"), but in fact it is rarely written.
In order to reflect the position of the radicals relative to the main fragment of the molecule, there are several ways:
- in numbers;
- letters of the Greek alphabet;
- prefixes sym. or asymmetr., characterizing the symmetry of the molecule;
- the addition of the letters o- (ortho), m- (meta) or p- (pair) in the description of cyclic compounds;
- letters corresponding to the name of the heteroatoms (N for nitrogen, O for oxygen, S for sulfur, etc.).
The number of identical substituents is indicated using the prefixes di-, tri-, tetra-, penta- , etc.
Examples of rational nomenclature of organic compounds:
- CΗ 3 -CΗ 2 -CΗ 2 -C (CΗ 3 ) 2 -CΗ (CΗ 3 ) -CH 3 - dimethylpropylisopropylmethane;
- CΗ 3 -CΗ 2 -CΗ (CΗ 3 ) -CΗ 2 -CH 3 - methylethyl-sec-butylmethane;
- C 6 Η 5 -CΗ (CΗ 3 ) -CH 3 - isopropylbenzene;
- H 3 C — CΗ 2 —CΗBr — CΗ 2 —CH 2 —CH 3 — ethylpropyl bromethane;
- HO-C 6 H 4 -C 2 H 5 - o-hydroxyethylbenzene.
Alkanes
According to the rules of rational nomenclature, alkanes are considered as methane or ethane, in which the hydrogen atoms are replaced by all kinds of hydrocarbon radicals. Moreover, in complex molecules, the least hydrogenated carbon atom, that is, one that is associated with a large number of substituents, is considered to be the methane residue.
The names of the radicals are written in the order of precedence, that is, by increasing the complexity of their structure, while the multiplying prefixes do not affect this order.
If the molecule contains non-hydrocarbon substituents, for example, halogens, then their names are mentioned after the remaining radicals.
Homological series of alkanes| Molecular formula | Simplified structural formula | IUPAC name | Rational name | The name of the radical |
| CΗ 4 | CΗ 4 | methane | methane | methyl |
| C 2 Η 6 | CΗ 3 -CH 3 | ethane | methyl methane | ethyl |
| C 3 Η 8 | CH 3 —CΗ 2 —CH 3 | propane | dimethylmethane | drank |
| C 4 Η 10 | CH 3 -CΗ2-CΗ 2 -CΗ 3 | butane | dimethylethane | butyl |
| C 5 Η 12 | CH 3 - (CΗ 2 ) 3 -CΗ 3 | pentane | methylethane | pentyl |
| C 6 Η 14 | CH 3 - (CΗ 2 ) 4 -CΗ 3 | hexane | diethylethane | hexyl |
| C 7 Η 16 | CH 3 - (Η 2 ) 5 -Η 3 | heptane | ethyl propylene | heptyl |
| C 8 Η 18 | CH 3 - (CΗ 2 ) 6 -CΗ 3 | octane | dipropylene | octyl |
| C 9 Η 20 | CH 3 - (CΗ 2 ) 7 -CΗ 3 | nonan | propylbutyl ethane | nonil |
| C 10 Η 22 | CH 3 - (Η 2 ) 8 -Η 3 | dean | dibutylethane | decile |
Alkenes
Hydrocarbons with a double C = C bond are generally considered substituted with ethylene. The location of the radicals is indicated by the words sym. (unbalanced) or in Greek letters. If the substituents of the same type are connected to different carbon atoms at a double bond, then such an alkene is called symmetric, and if with the same, then asymmetric.
In cases where the substituents have a highly branched structure and it is very problematic to formulate the name of an alkene according to rational nomenclature, the compound is considered as methane derivatives. In this case, the double bond is part of one of the substituents, and the least hydrogenated carbon atom is chosen as the basis of the name.
Dienes
The features of compiling the name of diene hydrocarbons according to the rational nomenclature depend on the mutual arrangement of multiple bonds in the molecule. If the carbon-carbon double bonds in the compound are cumulated or conjugated, then the substance is considered a derivative of allen or divinyl, respectively. If double bonds are isolated, that is, if two or more single bonds are located between them, then the substances are referred to as derivatives of methane or ethane. And multiple bonds are in the substituents.
Rational names of diene hydrocarbonsSimplified structural formula | Title |
| CH 2 = C = CH 2 | allen |
| Η 3 -Η = = 2 | methylallen |
| Η 2 = Η-Η = 2 | divinyl |
| Η 3 -Η 2 -Η = = 2 | ethylallen |
| Η 3 -Η = Η- = 2 | α-methyldivinyl |
| Η 2 = Η-Η 2 -Η = 2 | divinylmethane |
| Η 3 -Η = = Η- 3 | sym dimethylallen |
| Η 3 - (Η 3 ) = = Η 2 | unbalanced dimethylallen |
| Η 2 = (Η 3 ) -Η = Η 2 | β-methyldivinyl |
Alkins
Rational nomenclature considers alkynes as substituted acetylene. Unlike ethylene hydrocarbons, in molecules from hydrocarbons with ≡ bond there cannot be two radicals connected to the same carbon atom with it. Therefore, there is no need to indicate their relative position. It is only necessary to list the names of the radicals in order of their complication.
If the molecule has a complex structure, then when compiling its names, they act as with alkynes. The triple bond is then reflected in the composition of the substituent, for example, acetylenyl, and methane or ethane are taken as the basis. In this case, limit radicals are first recorded, and then unsaturated radicals with the same number of carbon atoms. Examples:
- H 3 C-C≡C-CH 3 - dimethylacetylene;
- Η3 C-CΗ 2 -C≡C-CΗ 2 -CΗ 2 -CΗ 3 - ethyl propylacetylene;
- HC≡C-C (CΗ 3 ) (C 2 Η 5 ) -CΗ (CΗ 3 ) -CH 3 - methylethylacetylenylisopropylmethane.
Aromatic compounds
Mononuclear aromatic hydrocarbons are considered rational nomenclature as substituted benzene compounds. The relative position of the radicals is denoted by the letters o, m or n.
In molecules with a complex hydrocarbon skeleton, the presence of a benzene ring is reflected in the form of the name of the corresponding radical. The presence of a hydroxyl group in the phenols is indicated by the prefix hydroxy, for example, oxybenzene.
Alcohols
The rational nomenclature of alcohols has its own characteristics for each type of compounds of this class. Monohydric alcohols are taken as derivatives of carbinol (methyl alcohol). In this case, the names of the radicals are written in the order of increasing carbon atoms in them:
- CH 3 —CH 2 —OH — methylcarbinol;
- Η 3 -Η (Η) -≡Η - methylacetylenylcarbinol.
Diatomic alcohols, in whose molecules the hydroxyl groups -OΗ are located at adjacent carbon atoms, are called ethylene glycols:
- Η 3 -Η (Η) -Η 2 (Η) - methylene glycol.
And if hydroxyls are not connected to adjacent carbon atoms, then the number of methylene groups -CH2- located between them is recorded in the names:
- Η-Η 2 -Η 2 -Η 2 -Η 2 -Η - tetramethylene glycol;
- Η-Η 2 -Η 2 -Η 2 -Η 2 -Η 2 -Η 2 -Η - hexamethylene glycol.
Aldehydes and Ketones
The rational nomenclature names aldehydes based on the structure of ethanal, called in this case acetic aldehyde. For instance:
- CH 3 -CHA — acetic aldehyde;
- Η 3 -Η 2 -Η 2 -Η - ethyl acetic aldehyde;
- CH 3 —CΗ (CΗ 3 ) —CHOH — dimethylacetic aldehyde.
If there are several functional groups in the compound, then the -CON group is indicated by the addition of the prefix aldo-:
- NOS-CH 2 -COOH - aldopropionic acid.
Ketones are called by listing substituents associated with the carbonyl group> C = O and adding the word ketone:
- CH 3 —CO — CH 3 — dimethyl ketone;
- Η 2 = Η--Η 3 - methyl vinyl ketone.
Using the letters of the Greek alphabet, they indicate not only the relative position of several carbonyl groups, but also the substituents in the main hydrocarbon chain:
- H 3 C — CO — CO — CH 3 —α-diketone;
- CΗ 3 —CO — CΗ 2 —CO — CΗ 3 — β-diketone;
- Br-H 2 C — CO — CH 2 —CH 3 - (α-bromomethyl) ethyl ketone.
Carboxylic acids
For monobasic carboxylic acids with an unbranched hydrocarbon chain, trivial names are used (butyric, acetic, etc.). If the molecule has a branched structure, then its name according to rational nomenclature is made up of the corresponding radicals combined with acetic acid or its homologues. Thus, the same acid can have several rational names:
- H 3 C — CH (CH 3 ) —COOH — dimethylacetic or methylpropionic acid;
- Η 3 C-CΗ 2 -CΗ (CΗ 3 ) -COOΗ - methylethylacetic or α-methylbutyric acid.
If the position of the substituents is marked in Greek letters, then the counting starts not with carbonyl carbon, but with the next one. According to the rational nomenclature, the names of unsaturated acids are obtained based on the acrylic acid molecule if the double bond is located between α and β carbon atoms. In other cases, the multiple bond is indicated as a substituent. Examples:
- H 2 C = CH-COOH - acrylic acid;
- H 3 C-HC = CH-COOH - β-methylacrylic acid;
- Η 2 C = CΗ-CΗ (CΗ 3 ) -COOΗ - methyl vinyl acetic acid.
Esters
According to rational nomenclature, the names of esters are built from two residues: the trivial name of the acid and the alcohol radical. For instance:
- CH 3 -COO-CH 3 - acetic methyl ether
- Η 3 -Η 2 -- 3 - propionomethyl ether
- Η 3 -Η 2 -Η 2 --Η (Η 3 ) -Η 3 - oil-isopropyl ether
- CH 3 -COO-C 5 H 11 - acetic isoamyl ether.
Cycloalkanes
Marginal cyclic hydrocarbons are named based on the number of methylene groups —CH 2 -, which form a ring. Their number is indicated by the prefixes tri-, tetra-, penta-, hexa-, hepta- , etc. If the cycle is connected to one or more substituents, then their names are listed first.
In modern chemistry, the rational nomenclature is not as widespread as Geneva. Nevertheless, at one time it was her principles that laid the foundation for the creation of modern naming systems for various compounds.
The main disadvantage of the rational nomenclature is the ambiguity of the requirements for the choice of substance on which the construction of the name is based. This leads to the fact that compounds with a very complex branched structure can have several names. And in especially difficult cases, their construction is completely impossible. However, for some classes of organic substances, for example, alkanes or alcohols, today the names chosen according to its rules are used quite often.