Thia- prefix

Note. The appropriate prefix is thio, not thia the latter is used in systematic organic chemical nomenclature to indicate replacement of CH2 by S. 

Numbering always begins at the heteroatom in the case of morpholine, numbering starts at oxygen, the heteroatom of higher atomic number. Remember that an accepted alternative in nomenclature is to indicate a heteroatom by the prefix aza-, oxa, or thia-in the appropriate carbocycle (see Section 1.4). Thus, we could name piperidine as azacyclohexane, and tetrahydrofuran as oxacyclopentane. 

Since trivial names are so well established, it is now very difficult to abandon them in favour of a logical nomenclature system that provides structural information. Nevertheless, a predictive method of this type is very desirable, especially for molecules where there may be two or more heteroatoms present in the ring. One approach is to use a prefix, which is indicative of the heteroatom [aza (N), oxa (O), thia (S), bora (B), phos-pha (P), sila (Si), etc.], to the name of the corresponding carbocycle. Thus, pyridine becomes azabenzene and piperidine is azacyclohexane. 

The ring Index system combines (1) the prefix oxa- for O, aza- for N or thia- for S and (2) a stem for ring size and saturation or unsaturation. These are summarized in Table 20-1. 

Two or more identical hetero-atoms are indicated by use of the multiplying prefixes di-, tri- or tetra-. When more than one distinct hetero-atom is present, the appropriate prefixes are cited in the name in descending order of group number in the periodic table, e.g. oxa- takes precedence over aza-. If both lie within the same group of the periodic table, then the order is determined by increasing atomic number, e.g. oxa- precedes thia-. 

The types of heteroatom present in a ring are indicated by prefixes oxa , thia and aza denote oxygen, sulfur and nitrogen, respectively (the final a is elided before a vowel). Two or more identical heteroatoms are indicated by dioxa , triaza , etc., and different heteroatoms by combining the above prefixes in order of preference, i.e. O, S and N. 

In replacement nomenclature, the name of a heterocyclic compound is formed by prefixing a terms, such as oxa , thia , aza , etc. to the name of the corresponding cyclic hydrocarbon. As IUPAC Rule B-4 states that the replacement method should be applied to a homocyclic hydrocarbon, we use pentalene for this purpose. The replacement procedure forbids the use of heterocyclic parent structures and for this reason the name thiathiophthene , which is found in the literature, is not in compliance with IUPAC recommendations. Another name which is sometimes found is thiothiophthene , which is even worse because of an incorrect use of the prefix thio . 

The prefixes are the normal replacement prefixes (see Replacement Nomenclature, page 50), although elision of the final a often occurs. The prefixes are cited in the following order fluora-, chlora-, broma-, ioda-, oxa-, thia-, selena-, tellura-, aza-, phospha-, bora-, and mercura-. Chemical Abstracts does not use Hantzsch-Widman names for rings containing silicon. 

A sulfur atom in a chain or as part of a ring is termed thia as a prefix when there is another group of higher priority present. For example,... 

Heterocycles with oxygen and sulfur heteroatoms are named similarly. The prefix for oxygen is oxa and that for sulfur is thia. ... 

The prefix "5-thia" tells us that a carbon is replaced with sulfur at the 5 position of 1,3-cyclopentadiene, therefore the structural formula is ... 

The heteroatom is given a name and is used as a prefix N, aza- O, oxa- S, thia- P, phospha- As, arsa- Si, sila- Se, selena-, B, bora, and so on. The a ending is dropped if the next syllable starts with a vowel. Thus aza-irine is properly written azirine. ... 

The heterocycloalkanes can be named using cycloalkane nomenclature. The prefix aza- for nitrogen, oxa- for oxygen, thia- for sulfur, and so forth, indicates the heteroatom. Other systematic and common names abound in the literature, particularly for the aromatic heterocycles. 

Use the carbocyclic ring name with heteroatom replacement prefixes oxa-, thia-, and so forth. 

In addition, names may be formed from any hydrocarbon by indicating replacement of a ring member by, for instance, oxygen, sulphur, or nitrogen by oxa-, thia-, aza-prefixes, respectively. The numbering of the hydrocarbon is retained by this procedure, and if more than one kind of heteroatom is present they are cited in a prescribed order (e.g.j that just given). An example would be 8ff-7-thia-l,9-diaza-phenanthrene (XXI). This nomenclature must be applied to the hydrocarbon parent , not to a... 

Hyphens, on the other hand, are used less in USA than in British practice (see p. 38). Identical letters belonging to different words are not separated by hyphens in USA (methyllithium, tetraacetate) hyphens are not used in words such as coordinate, unionized, nor to join portions/ of partial names (keto ester, amino acid), nor after voiced vowels ending isolated parts of nomenclature (thia compound, methoxy group, amino derivative). They are also not used between a multiplicative prefix and a parenthesis, e.g.j in USA it is correct to write p-bis(2 -chloroethyl)-benzene in Great Britain the name would start p-di-(2-... or p-bis-(2-... on the grounds that parentheses do not alter pre-existing punctuation. 

Bridged-ring nomenclature uses a prefix, suffix, and bracket term as in bicyclo[4.3.2]undecane (Figure 6.37). The prefix indicates the number of bridging rings (bicycle, tricycle, etc.) and whether those rings include any heteroatoms (oxa. thia, aza, etc.). 

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