In chemical nomenclature, a preferred IUPAC name is a unique name, assigned to a chemical substance and preferred among the possible names generated by IUPAC nomenclature. The "preferred IUPAC nomenclature" provides a set of rules for choosing between multiple possibilities in situations where it is important to decide on a unique name. It is intended for use in legal and regulatory situations. Preferred IUPAC names are applicable only for organic compounds, to which the IUPAC has the definition as compounds which contain at least a single carbon atom but no alkali, alkaline earth or transition metals and can be named by the nomenclature of organic compounds.. Rules for the remaining organic and inorganic compounds are still under development. The concept of PINs is defined in the introductory chapter and chapter 5 of the "Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013", which replace two former publications: the "Nomenclature of Organic Chemistry", 1979 and "A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993". The full draft version of the PIN recommendations is also available.
Definitions
A preferred IUPAC name or PIN is a name that is preferred among two or more IUPAC names. An IUPAC name is a systematic name that meets the recommended IUPAC rules. IUPAC names include retained names. A general IUPAC name is any IUPAC name that is not a "preferred IUPAC name". A retained name is a traditional or otherwise often used name, usually a trivial name, that may be used in IUPAC nomenclature. Since systematic names often are not human-readable a PIN may be a retained name. Both "PINs" and "retained names" have to be chosen explicitly, unlike other IUPAC names, which automatically arise from IUPAC nomenclatural rules. Thus, the PIN is sometimes the retained name, while in other cases, the systematic name was chosen over a very common retained name. A preselected name is a preferred name chosen among two or more names for parent hydrides or other parent structures that do not contain carbon. "Preselected names" are used in the nomenclature of organic compounds as the basis for PINs for organic derivatives. They are needed for derivatives of organic compounds that do not contain carbon themselves. A preselected name is not necessarily a PIN in inorganic chemical nomenclature.
Basic principles
The systems of chemical nomenclature developed by the International Union of Pure and Applied Chemistry have traditionally concentrated on ensuring that chemical names are unambiguous, that is that a name can only refer to one substance. However, a single substance can have more than one acceptable name, like toluene, which may also be correctly named as "methylbenzene" or "phenylmethane". Some alternative names remain available as "retained names" for more general contexts. For example, tetrahydrofuran remains an unambiguous and acceptable name for the common organic solvent, even if the preferred IUPAC name is "oxolane". Substitutive nomenclature is used most extensively, for example "ethoxyethane" instead of diethyl ether and "tetrachloromethane" instead of carbon tetrachloride. Functional class nomenclature is used for acid anhydrides, esters, acyl halides and pseudohalides and salts. Also skeletal replacement operations, additive and subtractive operations and conjunctive operations are applied.
Retained IUPAC names
The number of retained non-systematic, trivial names of simple organic compounds has been reduced considerably for preferred IUPAC names, although a larger set of retained names is available for general nomenclature. The traditional names of simple monosaccharides, α-amino acids and many natural products have been retained as preferred IUPAC names; in these cases the systematic names may be very complicated and virtually never used. The name for water itself is a retained IUPAC name.
