Systematic nomenclature Numbering

The practice of assigning ad hoc names to organic compounds was neither avoidable, nor burdensome when only a small number of compounds were recognized. Such ad hoc names are termed "trivial" or "traditional," to indicate that they contain no encoded stmctural information. They are useful for common compounds, and many of them are retained to this day, but they are not helpful in understanding chemical relationships. As they proliferated, the number and variety of them became unmanageable. The development of systematic nomenclature was driven by this circumstance, and was made possible by advances in understanding and determining the stmcture of molecules. 

Implicit ia the base names are the absolute configurations at carbons 8 and 12 and the iadicated numbering systems. Derivatives of these parent stmctures are named according to terpene and steroid nomenclature rules (see Steroids Terpenoids). The lengthy and awkward nature of the chemical abstract systematic nomenclature (12) for these compounds has resulted ia the development (13) and use of simplified nomenclature based on common names. 

Table 1 Hsts a number of chelating agents, grouped according to recognized stmctural classes. Because systematic nomenclature of chelating agents is frequently cumbersome, chelants are commonly referred to by common names and abbreviations. For the macrocyclic complexing agents, special systems of abbreviated nomenclature have been devised and are widely used. Some of the donor atoms involved ia chelation and the many forms ia which they can occur have been reviewed (5).

Pedersen prepared a series of macrocyclic polyethers, cyclic compounds containing four or more oxygens in a ring of 12 or more atoms. He called these compounds crown ethers, because their molecular- models resemble crowns. Systematic nomenclature of crown ethers is somewhat cumbersome, and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms in the ring and is followed by the number of oxygen atoms. 

The systematic nomenclature used originally the term imidazo-1,2,3-triazine. The Chemical Abstracts indexes use the more accurate name imidazo[4,5-d]-Z -triazine (141). The numbering of the substituents is different in the two systems of nomenclature as may be seen in the formulas. 

According to the systematic nomenclature these substances were first named l-f-triazolo[d] pyrimidines in compliance with the general principles of the Ring Index/ More recent papers and Chemical Abstracts indexes use the term i -triazolo[4,5-d]pyrimidine (147) in accord with the lUPAC nomenclature. The numbering of substituents when using the last-mentioned name is different from that of the 8-aza analogs. For the formulas of oxygen and sulfur derivatives names derived from the lactim or thiolactim form are almost exclusively in use (in common with the purine derivatives). These derivatives are thus described as hydroxy and mercapto derivatives, respectively. The name 1,2,3,4,6-pentaazaindene is used only rarely for this system. 

Part I contains entries referring to the names of compounds according to the Chemical Abstracts Systematic Nomenclature (see Index Guide, Chemical Abstracts vol 76, 1972) The systematic name is followed by Chemical Abstracts Registry Number m brackets (see Chemical Abstracts Registry Handbook 1965-71 and Supplements) and page number... 

The names of the individual compounds of this type are formed by replacing (a) the -ose of the systematic or trivial name of the aldose by -uronic acid , (b) the -oside of the name of the glycoside by -osiduronic acid or (c) the -osyl of the name of the glycosyl group by -osy luronic acid . The carbon atom of the (potential) aldehydic carbonyl group (not that of the carboxy group as in normal systematic nomenclature [13,14]) is numbered 1 (see 2-Carb-2.1, note 1). 

The most frequently used systematic nomenclature names the fatty acid after the hydrocarbon with the same number and arrangement of carbon atoms, with -oic being substituted for the final -e (Genevan system). Thus, saturated acids end in -anoic, eg, octanoic acid, and unsaturated acids with double bonds end in -enoic, eg, octadecenoic acid (oleic acid). 

A systematic nomenclature has been developed for the chemokine receptors (see Table 1). Thus, receptors for CC chemokines are referred to as CCR, receptors for CXC chemokines as CXCR, and the receptors for the XC and CX3C chemokines as XCR and CX3CR, respectively. To date, there are 10 CCRs (CCRs 1 to 10), 7 CXCRs, a single XCR, and a single CX3CR. The numbering is based on the date of deposition of the chemokine receptor sequence within the nucleic acid databases. For orphan receptors, this date refers to the point of identification of the orphan receptors as chemokine receptors and not to the date of initial deposition in the cDNA databases. 

Quite often, we find nonsystematic nomenclature used in the literature dealing with organophosphorus compounds. This results in unnecessary confusion, as systematic nomenclature is easy to use and understand. Nomenclature based on the oxidation state of the phosphorus center eliminates the confusion and helps to promote understanding of the chemistry as well as to facilitate communication. Table 1.1 shows structures for tricoordinate and tetracoordinate phosphorus compounds related to oxyacids with their English general names. Also noted are the names for simple esters of the parent acids. They are organized based on oxidation state and the number of bonds of the carbon-phosphorus type. 

Systematic nomenclature was introduced at a relatively late stage in the history of chemistry, and thus common names had already been coined for a wide range of chemicals. Because these names were in everyday usage, and familiar to most chemists, a number have been adopted by lUPAC as the approved name, even though they are not systematic. These are thus names that chemists still use, that are used for labelling reagent bottles, and are those under which the chemical is purchased. Some of these are given in Table 1.4, and it may come as a shock to realize that the systematic names school chemistry courses have provided will probably have to be relearned . 

Obviously, the system of adding a prefix to the name of the compound to denote the structural configuration will become unwieldy as the number of different structural configurations increases. Consider decane, which has 75 isomers. A more systematic nomenclature is desirable. 

The 1,3,5-triazines are amongst the oldest known organic molecules. Originally they were called the symmetric triazines, usually abbreviated to s- or sym- triazines. The numbering follows the usual convention of beginning at the heteroatom as shown for the parent compound 1,3,5-triazine (1). Rather non-systematic nomenclature is prevalent even in the current literature, because some of the compounds have been known for at least 150 years. The non-systematic names of some of the more important 1,3,5-triazines are listed in Table 1. The terms melamine, cyanuric acid and cyan uric chloride will be used throughout this chapter, and the term triazine will refer to 1,3,5-triazines only. In addition to the above names, 2,4,6-trialkoxy-l,3,5-triazines (2) are called cyanurates. Similarly, 1,3,5-trialkyl-1,3,5-triazines (3) are called isocyanurates. 

The representation is unambiguous since it corresponds to one and only one substance, but it is not unique because alternative numberings of the connection table would result in different representations for the same chemical substance (the connection table representation is discussed in more detail below). In addition to being categorized according to their uniqueness and ambiguity, chemical substance representations commonly used within computer-based systems can be further classified as systematic nomenclature, fragment codes, linear notations, connection tables, and coordinate representations. 

The systematic nomenclature for the cycloproparenes is confused because the fusion rule (IUPAC Rule A 21.3) requires that at least two rings of five or more members be present before the prefix cyclopropa may be used. Thus while l//-cyclopropa[a]- and -[/ naphthalene are correct for 10 and 11, respectively, 1 //-cyclopropabenzene is incorrect for 1. The Chemical Abstracts service and IUPAC are unanimous in naming 1 as bicyclo-[4.1. Ojhepta-1,3,5-triene la. Thus if the parent member is strictly named, not only does it differ from that of its higher homologues, but also it could be taken to imply a bond localized structure. Throughout this chapter parent 1 and its derivatives 5-9 are referred to as cyclopropabenzenes and numbered as shown for structure 1. 

Trivial names convey little or no structural information. However they are still widely used in the chemical literature, and it seems unlikely that they will ever be eliminated completely from it. Many such names are now very well established (indeed present-day systematic nomenclature is based to a considerable extent on a residue of trivial names), and they are usually brief and well suited to the spoken language. However, the extensive use of trivial nomenclature in the literature places a substantial burden on the memory of the reader, thus impairing an author s ability to communicate, and is not to be encouraged. The introduction of a new trivial name is rarely justified nowadays, and in writing about chemistry it is best to employ only the relatively small number of trivial names generally accepted at the international level. 

The terms ortho, meta, and para (abbreviated o, m, and p) refer to the location of substituents on the benzene nng and are equivalent to 1,2-, 1,3-, and 1,4-substitution in systematic nomenclature, respectively. The lowest numbers possible are given to substituents. 

Hydrocarbons, esters, etc., which have one or more hydrogen atoms replaced by a halogen atom are named so that the substituents have the lowest possible numbers. When multiple functions are present, they are numbered according to precedences established by IUPAC or CAS nomenclature rules. Both trivial and systematic nomenclatures are used. 

TRIVIAL NAME. The name applied by early chemists to a number of simple organic compounds, usually based on their sources or properties, e.g.. acetone and acetic acid, from Latin acetum (vinegar), urea from urine, glucose and glycerol from Greek g/vc+sweet). Such names remained in common use regardless of the systematic nomenclature later developed. 

If you have access to the 1967-71 Eighth Collective Subject Index of Chemical Abstracts, locate the page number in the index where each of the compounds shown in Exercise 2-8 occurs and give the name used. Notice that past Chemical Abstracts indexes do not use completely systematic nomenclature, especially for compounds with only a few carbons, but these indexes will be made completely systematic in the future. 

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