Replacement of Carbon by Other Elements

The replacement of carbon by other elements produces changes in several structural parameters and consequently affects the conformational characteristics of the molecule. In this section, we will first describe some stereochemical features of heterocyclic analogs of cycloalkanes. For the purpose of elaborating conformational principles, the discussion will focus on six-membered rings, so that the properties may be considered in the context of a ring system possessing a limited number of low-energy conformations. 

In Section 23.1, you learned that hydrogen atoms bonded to carhon atoms in hydrocarbons can be replaced by halogen atoms. Many other kinds of atoms or groups of atoms also can bond to carbon in the place of hydrogen atoms. In addition to structural variations, replacement of hydrogen by other elements is a reason that such a wide variety of organic compounds is possible. 

These varieties of carbonated apatite whose formulae may be represented as Cajo ,(P04)6 (C03) j (F,0H)2, where jc = 1, are often designated as Francolite (F OH) or Dahllite (OH F). Up to 25% replacement of PO4 by CO3 is, however, sometimes found, and replacement of up to 10% Ca by Mg can occur. A wide variety of other metals, including uranium are often incorporated in trace amounts. Common major impurities found with phosphorites are iron, alumina, quartz, montmorillonite and organic matter. Almost every element has been found, at least in trace amounts, in phosphorite minerals. Much of this arises from the remarkable nature of the Apatite crystal structure which allows substitution of the Ca ", and F by alternative cations and anions (Chapter 5.3). 

The isotopic contribution of various atoms is additive. For low molecular weight compounds, the isotopic contribution originates mainly from the carbon atom as long as no other element with a second isotope of significant abundance is present. For a molecule of Mr 192 the intensity of the m/z 194 ion represents 12% of the [M+H]+ peak (m/z 193 Fig. 1.3A). Chlorine (Cl) has two intense isotopes Cl and Cl (76% and 24% abundance, respectively). Replacing one H by a Cl atom results in a change of the isotopic distribution of the molecule... 

The modified element name sila indicates replacement in the carbon skeleton, and similar treatment can be applied to other element names. The parent hydride names of Table 5.2 may all be adapted in this way and used in the same fashion as in the oxa-aza nomenclature of organic chemistry. In inorganic chemistry, a major use is in names of cyclic derivatives that have heteroelement atoms replacing carbon atoms in structures. It may be possible to name such species by Hantzsch-Widman procedures (see p. 77), and these should always be used when applicable. 

Two ligands are enantiotopic if replacement of either one of them by a different achiral ligand7 (see also footnote 5 on p. 9) gives rise to enantiomeric products. Examples are shown in Fig. 13. The marked hydrogens (HA, HB) in CH2ClBr (30), meso-tartaric acid (32), cyclobutanone (34) [at C(2) and C(4) but not C(3)] and chloroallene (36) [at C(3)] are enantiotopic, as are the methyl carbons in isopropyl alcohol (55). meso-Tartaric acid, incidentally, exemplifies one of the rare instances of a molecule with heterotopic ligands but no discernible prochiral center or other element of prochirality. 

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