IUPAC nomenclature is a system of naming chemical compounds and of describing the science of chemistry in general. It is developed and kept up to date under the auspices of the International Union of Pure and Applied Chemistry (IUPAC). The rules for naming organic and inorganic compounds are contained in two publications, known as the Blue Book and the Red Book respectively. A third publication, known as the Green Book, describes the recommendations for the use of symbols for physical quantities (in association with the IUPAP), while a fourth, the Gold Book, contains the definitions of a large number of technical terms used in chemistry. Similar compendium exist for biochemistry (the White Book, in association with the IUBMB), analytical chemistry (the Orange Book), macromolecular chemistry (the Purple Book) and clinical chemistry (the Silver Book). These "colour books" are supplemented by shorter recommendations for specific circumstances which are published from time to time in the journal Pure and Applied Chemistry.
The primary function of chemical nomenclature is to ensure that the person who hears or reads a chemical name is under no ambiguity as to which chemical compound it refers to: each name should refer to a single substance. It is considered less important to ensure that each substance should have a single name, although the number of acceptable names is limited.
It is also preferable that the name convey some information about the structure or chemistry of a compound. CAS numbers form an extreme example of names which do not perform this function: each refers to a single compound but none contain information about the structure.
The form of nomenclature which should be used depends on the public to which it is addressed: as such there is no single correct form, but rather different forms which 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 which are longer and less familiar to most readers. Another system gaining popularity is the International Chemical Identifier—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 doesn't 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.
The nomenclature of alchemy is rich in description, but does not effectively meet the aims outlined above. Opinions differ 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 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, and promoted by the latter in a textbook which would survive long after his death at the guillotine in 1794. The project was also espoused by Jöns Jakob Berzelius, 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.
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.
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, 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.