A chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC).
The IUPAC's rules for naming organic and inorganic compounds are contained in two publications, known as the Blue Book[1][2] and the Red Book[3], respectively. A third publication, known as the Green Book,[4] describes the recommendations for the use of symbols for physical quantities (in association with the IUPAP), while a fourth, the Gold Book,[5] contains the definitions of a large number of technical terms used in chemistry. Similar compendia exist for biochemistry[6] (the White Book, in association with the IUBMB), analytical chemistry[7] (the Orange Book), macromolecular chemistry[8] (the Purple Book) and clinical chemistry[9] (the Silver Book). These "color books" are supplemented by shorter recommendations for specific circumstances that are published from time to time in the journal Pure and Applied Chemistry.
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The primary function of chemical nomenclature is to ensure that a spoken or written chemical name leaves no ambiguity as to what chemical compound the name refers: each chemical name should refer to a single substance. A less important aim is to ensure that each substance has a single name, although the number of acceptable names is limited.
Preferably, the name also conveys some information about the structure or chemistry of a compound. CAS numbers form an extreme example of names that do not perform this function: each CAS number refers to a single compound but none contain information about the structure.
The form of nomenclature used depends on the audience to which it is addressed. As such, no single correct form exists, but rather there are different forms that are more or less appropriate in different circumstances.
A common name will often suffice to identify a chemical compound in a particular set of circumstances. To be more generally applicable, the name should indicate at least the chemical formula. To be more specific still, the three-dimensional arrangement of the atoms may need to be specified.
In a few specific circumstances (such as the construction of large indices), it becomes necessary to ensure that each compound has a unique name: This requires the addition of extra rules to the standard IUPAC system (the CAS system is the most commonly used in this context), at the expense of having names that are longer and less familiar to most readers. Another system gaining popularity is the International Chemical Identifier (InChI)—while InChI symbols are not human-readable, they contain complete information about substance structure. That makes them more general than CAS numbers.
The IUPAC system is often criticized for the above failures when they become relevant (for example, in differing reactivity of sulfur allotropes, which IUPAC does not distinguish). While IUPAC has a human-readable advantage over CAS numbering, it would be difficult to claim that the IUPAC names for some larger, relevant molecules (such as rapamycin) are human-readable, and so most researchers simply use the informal names.
It is generally understood that the aims of normal English lexicography versus chemical nomenclature vary and are to an extent at odds. Dictionaries of English words on the web or otherwise collect and report the meanings of words as their uses appear and change over time. Chemical nomenclature on the other hand (with IUPAC nomenclature as the best example) is necessarily more restrictive: It aims to standardize communication and practice so that, when a chemical term is used, it has a fixed meaning relating to chemical structure, thereby giving insights into chemical properties and derived molecular functions. These differing aims can have profound effects on valid understanding in chemistry, especially with regard to chemical classes made popular for perceived health benefits; this is particularly true in the new media age of the Web, where word meanings can rapidly change. A discussion related to the term polyphenol is a clear example (where various web definitions and common uses of the word are at odds with its chemical nomenclature, which connects polyphenol structure and bioactivity).
The nomenclature of alchemy is rich in description, but does not effectively meet the aims outlined above. Opinions differ about whether this was deliberate on the part of the early practitioners of alchemy or whether it was a consequence of the particular (and often esoteric) theoretical framework in which they worked.
While both explanations are probably valid to some extent, it is remarkable that the first "modern" system of chemical nomenclature appeared at the same time as the distinction (by Lavoisier) between elements and compounds, in the late eighteenth century.
The French chemist Louis-Bernard Guyton de Morveau published his recommendations[10] in 1782, hoping that his "constant method of denomination" would "help the intelligence and relieve the memory". The system was refined in collaboration with Berthollet, de Fourcroy and Lavoisier,[11] and promoted by the latter in a textbook that would survive long after his death at the guillotine in 1794.[12] The project was also espoused by Jöns Jakob Berzelius,[13][14] who adapted the ideas for the German-speaking world.
The recommendations of Guyton covered only what would be today known as inorganic compounds. With the massive expansion of organic chemistry in the mid-nineteenth century and the greater understanding of the structure of organic compounds, the need for a less ad hoc system of nomenclature was felt just as the theoretical tools became available to make this possible. An international conference was convened in Geneva in 1892 by the national chemical societies, from which the first widely accepted proposals for standardization arose.[15]
A commission was set up in 1913 by the Council of the International Association of Chemical Societies, but its work was interrupted by World War I. After the war, the task passed to the newly formed International Union of Pure and Applied Chemistry, which first appointed commissions for organic, inorganic, and biochemical nomenclature in 1921 and continues to do so to this day.
Examples of compositional names are:
An alternative method uses the oxidation state on the metal in place of suffices, e.g.:
Generally this system, known as Stock nomenclature or international nomenclature, is preferred over the prefix system for ionic compounds.
Simple compounds include binary compounds type I.ionic, binary compounds type II.ionic, ionic compounds with polyatomic ions, and binary compounds type III.covalent.
Binary compounds type I. ionic-
Binary compounds type II. ionic-
Ionic compounds with polyatomic ions-
Binary compounds type III. covalent-
This naming method generally follows established IUPAC organic nomenclature. Hydrides of the main group elements (groups 13–17) are given -ane base names, e.g. borane (BH3), oxidane (H2O), phosphane (PH3) (the name phosphine is also in common use, but is not recommended by IUPAC). The compound PCl3 would be named substitutively as trichlorophosphane.
This naming method has been developed principally for coordination compounds although it can be more widely applied. An example of its application is:
Note that ligands such as chloride become chlorido- rather than chloro- as in substitutive naming.