Chemical nomenclature is a set of rules to generatesystematic names forchemical compounds. The nomenclature used most frequently worldwide is the one created and developed by theInternational Union of Pure and Applied Chemistry (IUPAC).
IUPAC Nomenclature ensures that each compound (and its variousisomers) have only one formally accepted name known as thesystematic IUPAC name. However, some compounds may have alternative names that are also accepted, known as thepreferred IUPAC name which is generally taken from thecommon name of that compound. Preferably, the name should also represent the structure or chemistry of a compound.
For example, the main constituent ofwhite vinegar isCH
3COOH, which is commonly calledacetic acid and is also its recommended IUPAC name, but its formal, systematic IUPAC name is ethanoic acid.
The IUPAC's rules for namingorganic andinorganic compounds are contained in two publications, known as theBlue Book[1][2] and theRed Book,[3] respectively. A third publication, known as theGreen Book,[4] recommends the use ofsymbols forphysical quantities (in association with theIUPAP), while a fourth, theGold Book,[5] defines many technical terms used in chemistry. Similarcompendia exist forbiochemistry[6] (theWhite Book, in association with theIUBMB),analytical chemistry[7] (theOrange Book),macromolecular chemistry[8] (thePurple Book), andclinical chemistry[9] (theSilver Book). These "color books" are supplemented by specific recommendations published periodically in the journalPure and Applied Chemistry.
The main purpose of chemical nomenclature is to disambiguate the spoken or written names of chemical compounds: each name should refer to one compound. Secondarily, each compound should have only one name, although in some cases some alternative names are accepted.
Preferably, the name should also represent the structure or chemistry of a compound. This is achieved by theInternational Chemical Identifier (InChI) nomenclature. However, theAmerican Chemical Society'sCAS numbers nomenclature does not represent a compound's structure.
The nomenclature used depends on the needs of the user, so no single correct nomenclature exists. Rather, different nomenclatures are appropriate for different circumstances.
Acommon name will successfully identify a chemical compound, given context. Without context, the name should indicate at least thechemical composition. To be more specific, the name may need to represent the three-dimensional arrangement of the atoms. This requires adding more rules to the standard IUPAC system (theChemical Abstracts Service system (CAS system) is the one used most commonly in this context), at the expense of having names which are longer and less familiar.
The IUPAC system is often criticized for failing to distinguish relevant compounds (for example, for differing reactivity ofsulfur allotropes, which IUPAC does not distinguish). While IUPAC has a human-readable advantage over CAS numbering, IUPAC names for some larger, relevant molecules (such asrapamycin) are barely human-readable, so common names are used instead.
It is generally understood that the purposes oflexicography versus chemical nomenclature vary and are to an extent at odds. Dictionaries of words, whether in traditional print or on the internet, collect and report the meanings of words as their uses appear and change over time. For internet dictionaries with limited or no formal editorial process, definitions —in this case, definitions of chemical names and terms— can change rapidly without concern for the formal or historical meanings. Chemical nomenclature however (withIUPAC nomenclature as the best example) is necessarily more restrictive: Its purpose is 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 purposes can affect understanding, especially with regard to chemical classes that have achieved popular attention. Examples of the effect of these are as follows:
The rapid pace at which meanings can change on the internet, in particular for chemical compounds with perceived health benefits, ascribed rightly or wrongly, complicate the monosemy of nomenclature (and so access to SAR understanding). Specific examples appear in thePolyphenol article, where varying internet and common-use definitions conflict with any accepted chemical nomenclature connecting polyphenol structure andbioactivity.
Thenomenclature of alchemy is descriptive, but does not effectively represent the functions mentioned above. Opinions differ about whether this was deliberate on the part of the early practitioners ofalchemy or whether it was a consequence of the particular (and often esoteric) theories according to 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 French chemistAntoine Lavoisier) betweenelements andcompounds, during the late eighteenth century.
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The French chemistLouis-Bernard Guyton de Morveau published his recommendations in 1782,[10] hoping that his "constant method of denomination" would "help the intelligence and relieve the memory". The system was refined inMéthode de nomenclature chimique [fr],[11] published in 1787 in collaboration with Lavoisier,Claude Louis Berthollet, andAntoine-François de Fourcroy, and translated into English asMethod of Chymical Nomenclature by James St. John in 1788.[12]Méthode de nomenclature chimique contained handy dictionaries[13] in which older chemical names were listed with their new counterparts[14] and vice versa.[15] New names were provided in both French and Latin for the benefit of an international readership. For a modern reader these dictionaries are still useful, but now to discover and understand older names, rather than the new. In the English version,[16] the new names had been adapted to English, though they did not always align with current conventions. St. John used "acetat" instead of "acetate" for example. For gases, the word "gas" ("gaz") was being popularized by its consistent use in the new names, whereas the old names used the affix "air".
The new system was presented to a wider audience in Lavoisier's 1789 textbookTraité élémentaire de chimie,[17] translated into English asElements of Chemistry byRobert Kerr in 1790,[18] and it would be of great influence long after his death at theguillotine in 1794. The project was also endorsed by Swedish chemistJöns Jakob Berzelius,[19][20] who adapted the ideas for the German-speaking world.
Traité élémentaire de chimie included the first modern list of elements ("simple substances"). Also here were older names provided to explain their new counterparts.[21] Some element names were new and received English versions similar to the French names.[22] For the new "element"caloric, both the new and some of the "old" names (igneous fluid andmatter of fire and of heat) were coined by Lavoisier, their discoverer. Most element names, however, were not new, so they retained their existing English versions. But their status as elements was new—a product of thechemical revolution.
The recommendations of Guyton were only for what would later be known asinorganic compounds. With the massive expansion oforganic chemistry during the mid-nineteenth century and the greater understanding of the structure oforganic compounds, the need for a lessad hoc system of nomenclature was felt just as the theoretical basis 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 acceptedproposals for standardization developed.[23]
A commission was established 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 formedInternational Union of Pure and Applied Chemistry, which first appointed commissions for organic, inorganic, andbiochemical nomenclature in 1921 and continues to do so to this day.
Nomenclature has been developed for both organic and inorganic chemistry. There are also designations having to do with structure – seeDescriptor (chemistry).
For type-Iionicbinary compounds, thecation (ametal in most cases) is named first, and theanion (usually anonmetal) is named second. The cation retains its elemental name (e.g.,iron orzinc), but the suffix of the nonmetal changes to-ide. For example, the compoundLiBr is made ofLi+ cations andBr− anions; thus, it is calledlithium bromide. The compoundBaO, which is composed ofBa2+ cations andO2− anions, is referred to asbarium oxide.
Theoxidation state of each element is unambiguous. When these ions combine into a type-I binary compound, their equal-but-opposite charges are neutralized, so the compound's net charge is zero.
Type-II ionic binary compounds are those in which the cation does not have just one oxidation state. This is common amongtransition metals. To name these compounds, one must determine the charge of the cation and then render the name as would be done with Type-I ionic compounds, except that aRoman numeral (indicating the charge of the cation) is written in parentheses next to the cation name (this is sometimes referred to asStock nomenclature). For example, for the compoundFeCl3, the cation,iron, can occur asFe2+ andFe3+. In order for the compound to have a net charge of zero, the cation must beFe3+ so that the threeCl− anions can be balanced (3+ and 3− balance to 0). Thus, this compound is termediron(III) chloride. Another example could be the compoundPbS2. Because theS2− anion has a subscript of 2 in the formula (giving a 4− charge), the compound must be balanced with a 4+ charge on thePb cation (lead can form cations with a 4+ or a 2+ charge). Thus, the compound is made of onePb4+ cation to every twoS2− anions, the compound is balanced, and its name is written aslead(IV) sulfide.
An older system – relying on Latin names for the elements – is also sometimes used to name Type-II ionic binary compounds. In this system, the metal (instead of a Roman numeral next to it) has a suffix "-ic" or "-ous" added to it to indicate its oxidation state ("-ous" for lower, "-ic" for higher). For example, the compoundFeO contains theFe2+ cation (which balances out with theO2− anion). Since this oxidation state is lower than the other possibility (Fe3+), this compound is sometimes calledferrous oxide. For the compound,SnO2, the tin ion isSn4+ (balancing out the 4− charge on the twoO2− anions), and because this is a higher oxidation state than the alternative (Sn2+), this compound is termedstannic oxide.
Some ionic compounds containpolyatomic ions, which are charged entities containing two or more covalently bonded types of atoms. It is important to know the names of common polyatomic ions; these include:
The formulaNa2SO3 denotes that the cation issodium, orNa+, and that the anion is the sulfite ion (SO2−3). Therefore, this compound is namedsodium sulfite. If the given formula isCa(OH)2, it can be seen thatOH− is the hydroxide ion. Since the charge on the calcium ion is 2+, it makes sense there must be twoOH− ions to balance the charge. Therefore, the name of the compound iscalcium hydroxide. If one is asked to write the formula for copper(I) chromate, the Roman numeral indicates that copper ion isCu+ and one can identify that the compound contains the chromate ion (CrO2−4). Two of the 1+ copper ions are needed to balance the charge of one 2− chromate ion, so the formula isCu2CrO4.
Type-III binary compounds arebonded covalently. Covalent bonding occurs between nonmetal elements. Compounds bonded covalently are also known asmolecules. For the compound, the first element is named first and with its full elemental name. The second element is named as if it were an anion (base name of the element +-ide suffix). Then, prefixes are used to indicate the numbers of each atom present: these prefixes aremono- (one),di- (two),tri- (three),tetra- (four),penta- (five),hexa- (six),hepta- (seven),octa- (eight),nona- (nine), anddeca- (ten). The prefixmono- is never used with the first element. Thus,NCl3 is termednitrogen trichloride,BF3 is termedboron trifluoride, andP2O5 is termeddiphosphorus pentoxide (although thea of the prefixpenta- should actually not be omitted before a vowel: the IUPAC Red Book 2005 page 69 states, "The final vowels of multiplicative prefixes should not be elided (although "monoxide", rather than "monooxide", is an allowed exception because of general usage).").
Carbon dioxide is writtenCO2;sulfur tetrafluoride is writtenSF4. A few compounds, however, have common names that prevail.H2O, for example, is usually termedwater rather thandihydrogen monoxide, andNH3 is preferentially termedammonia rather thannitrogen trihydride.
This naming method generally follows established IUPAC organic nomenclature.Hydrides of the main group elements (groups 13–17) are given the base name ending with-ane, e.g.borane (BH3),oxidane (H2O),phosphane (PH3) (Although the namephosphine is also in common use, it is not recommended by IUPAC). The compoundPCl3 would thus be named substitutively as trichlorophosphane (with chlorine "substituting"). However, not all such names (or stems) are derived from the element name. For example,NH3 is termed "azane".
This method of naming has been developed principally for coordination compounds although it can be applied more widely. An example of its application is[CoCl(NH3)5]Cl2, pentaamminechloridocobalt(III) chloride.
Ligands, too, have a special naming convention. Whereaschloride becomes the prefixchloro- in substitutive naming, for a ligand it becomeschlorido-.
