Parent hydrides

R designates a substituent group derived from a parent hydride by loss of a hydrogen atom. 

Figure 4.20 A schematic illustration comparing parent hydride and symmetry (above cf. Figs. 4.2(b), 4.3(c), and 4.5(b)) with the derivative alkylidene (below) in the idealized banana-bonded limit for Ir(CFl2)F[ (left), Os(CH2)2 (center), and W(CH2)3 (right).

Cations can also be obtained by the formal addition of a hydron (hydron is the recommended name for the normal isotopic mixture of protons, deuterons and tritons, see p. 7) to a binary hydride. In such cases, a formalism of substitutive nomenclature is used the suffix -ium is added to the name, slightly modified, of the parent hydride. The selection of permitted hydride names and their usage are discussed in Section 4.5 on substitutive nomenclature. 

Some variants are also allowed for mononuclear cations of Groups 15, 16 and 17. These are based on the usage of substitutive nomenclature, where the formal addition of a hydron to a parent hydride to give a cation is represented by the suffix -onium. 

Anions obtained formally by loss of a hydron from a parent hydride (see Table 5.2 p. 99 for a list of parent hydride names) are conveniently named by the methods of substitutive nomenclature. 

A substitutive name consists of the name of a parent hydride to which prefixes and suffixes are attached as necessary following the general pattern ... 

A given organic molecule is generally composed of a carbon skeleton and functional groups. A name matches a structure when the name of the parent hydride corresponds to the skeleton, while prefixes and sufiixes represent the functional groups and other structural characteristics, such as geometry. 

The names of unbranched alkanes are of the utmost importance because these alkanes are the parent hydrides used to name all aliphatic molecules, i.e. molecules having a carbon-chain skeleton. 

Selection of the principal chain, which will serve as the parent hydride. 

The groups attached to the principal chain are called substituents, and these may be simple or complex. Simple substituents are formed directly from parent hydrides when a simple substituent is itself substituted, it becomes complex as a consequence. Normal alkyl groups are simple substituents branched alkyl groups are complex substituents. However, as exceptions the names isopropyl, isobutyl, jec-butyl, tert-butyl, isopentyl, tert-pentyl and neopentyl are taken to refer to simple substituents. 

Alphabetical order for citation of detachable prefixes. Prefixes are used to name substituents, as discussed above. Such prefixes are called detachable prefixes. There is a further class of prefix described as non-detachable. An example is cyclo-, as in cyclohexyl, which is different in stoichiometry from the unmodified hexyl. Non-detachable prefixes are used to modify permanently the name of a parent hydride and thus to create a new parent hydride (see also section 4.5.3.4, p. 78). 

When constructing a name, detachable prefixes are cited in front of the name of the parent hydride in alphabetical order. The names are alphabetised by considering the first letter of each name m in methyl, b in butyl, d in 1,2-dimethylpropyl. In retained names, the first non-italicised letter is considered i in isobutyl, n in neopentyl, but b in tert-huiy. ... 

The assembly of the components to construct a full name starts by attaching the names of the detachable prefixes in alphabetical order to the name of the parent hydride. Then, and only then, necessary multiplicative prefixes are introduced, without changing the alphabetical order obtained previously. Finally, locants are inserted. 

General. Continuous-chain alkanes are the sole parent hydrides for all compounds, the skeleton of which is composed of chains. Cyclic parent hydrides are more diverse. In nomenclature, they are classified according to their structure as carbo-cycles (composed of carbon atoms only) and heterocycles (composed of carbon atoms and other atoms, such as N, O and Si). They are also classified as saturated and unsaturated. Saturated cycles have the maximum number of hydrogen atoms attached to every skeletal atom, as judged by a prespecified valence unsaturated cycles have fewer hydrogen atoms and multiple bonds between pairs of atoms. 

Various degrees and kinds of unsaturation are possible. Unsaturation may be cumulative (which means that there are at least three contiguous carbon atoms joined by double bonds, C=C=C) or non-cumulative (which is another arrangement of two or more double bonds, as in -C=C-C=C-). In nomenclature, unsaturated cyclic parent hydrides have, by convention, the maximum number of non-cumulative double bonds. They are generically referred to as mancudes — derived from the acronym MANCUD, the MAximum Number of non-CUmulative Double bonds. Four classes of cyclic parent hydride are therefore recognised ... 

For comparison, only a few dozen alkanes are designated as parent hydrides. Over 73 000 rings and ring systems are mancudes and about 10 000 are carbocycles. 

Heterocyclic parent hydrides. These compounds form a large and diverse group. The names of these parent hydrides are usually formed systematically. However, some 50 trivial names are retained and used in preference to their systematic counterparts. 

The two most important methods for naming heterocyclic parent hydrides are the extended Hantzsch-Widman system and replacement nomenclature. 

Polycyclic parent hydrides. These are classified as bridged polyalkanes (also known as von Baeyer bridged systems, from the nomenclature system developed to name them), spiro compounds, fused polycyclic systems and assemblies of identical rings. The four systems may be either carbocyclic or heterocyclic. In developing their names, the following principles are used. 

When a system does not have a retained name or a name that can be composed systematically as above, and when ortho- and ortho-peri- x% on are possible, it is named using fusion nomenclature, i.e. by combining the names of the two or more systems that are fused. One system is adjudged to be the senior according to criteria described elsewhere and is taken as a parent hydride, and the other is denoted in the name by a non-detachable prefix. The junction of the two systems is indicated in a specific manner. Instead of numerical locants, italic letters a , 6 , c , etc. are used to identify bonds in the parent hydride. The final letter o and normal locants are characteristic of the prefix. The examples below illustrate the fusion operation and the resulting fusion name. 

General. Two methods are used to indicate the degree of saturation in a compound, depending on the nature of the parent hydride. 

In the other method, the ending -ane is changed to -ene or -yne to indicate the presence of a double or triple bond. This is used for alkanes and mono- and polycyclic alkane parent hydrides. In alkanes and cycloalkanes, the change of the -ane ending to -ene or -yne indicates the presence of one double or triple bond. Multiplicative prefixes di-, tri-, tetra-, etc. are used to signal the multiplicity of unsaturated bonds. Locants placed immediately in front of the endings -ene and -yne are used as needed. 

Assemblies of different rings are given substitutive names in which one ring is chosen as the parent hydride, and the other is denoted by a prefix. 

Compounds composed of rings and chains are also named using substitutive nomenclature. The ring, whether carbocyclic or heterocyclic, is always selected to be the parent hydride. 

The carbon atoms in parentheses in some groups belong to the parent hydride, normally a chain. If there are no parentheses in the formula, the name of the group as a whole is added to the name of the parent hydride. 

When the numbering is predetermined by the nature of the parent hydride, as in polycyclic hydrocarbons and in heterocyclic compounds, lowest locants are still the rule. 

Normally, the names of all substituents are cited as prefixes in front of the name of the parent hydride, but there are three exceptions. The names of three substituted benzenes — toluene, styrene and stilbene — are retained and can be used to name substituted derivatives, as long as the substitution is only on the ring. 

In the names of amines, the general use of suffixes and prefixes is not always observed. Normally, the suffix -amine would be added to the name of the parent hydride and engender names such as methanamine (CH3-NH2). Further substitution on the nitrogen atom would then be indicated by prefixes, leading to names that appear very cumbersome, such as A -methylmethanamine for (CH3)2NH and N,N-dimethylmethanamine for (CH3)3N. The traditional names of methylamine, di-methylamine and trimethylamine are much simpler. In these names, the term amine is not a suffix. It is, in fact, the name of the parent hydride, NH3, which now serves as the basis of substitutive names. Names such as diethylamine and tributylamine are representative of the preferred nomenclature. Diamines are named accordingly, as with ethane-1,2-diamine for H2N-CH2-CH2-NH2 and propane-1,3-diamine for H2N-[CH2]3-NH2. There are allowed alternatives for these last two compounds ethylenediamine and propane-1,3-diyldiamine. 

Retained names of carboxylic acids may also be modified to name amides, nitriles and aldehydes, by changing the -ic acid ending to -amide, -onitrile or -aldehyde. Names such as formaldehyde, acetonitrile and propionamide result. Of these, only acetonitrile may be treated as a functional parent hydride. 

A) Names of functional parent hydrides, usable with unlimited or limited substitution, as indicated... 

The terms radical cation or radical ion( +) may be added to the name of the parent hydride when the positions of free valencies and/or charges are not known, or when it is not desirable to specify them. 

---