Isomers nomenclature

The next sections describe briefly the nomenclature of configurational isomers, and how this stereochemistry can be handled by computer. 

Garboranes. The term carborane is widely used in American Hterature as a contraction of the lUPAC approved nomenclature carbaborane. The first carboranes, isomers of C2B2H, and were prepared in the mid-1950s at Olin Mathiesen. These carboranes were obtained as a mixture... 

It is often the case that chemists involved or familiar with ethylene oxide (oxiran) chemistry refer to these cyclic oligomers as EO-4, EO-6 and EO-7 respectively. Such designations are informal if not colloquial but, like any name, are useful if they correctly convey an idea. The difficulty with these informalities, indeed with the crown nomenclature system is that it cannot adequately deal with complicated structures or even isomers of simple ones. 

We can present and illustrate the most important of the lUPAC r-ules for alkane nomenclature by naming the five C6H14 isomers. By definition (Table 2.2), the unbranched C6H14 isomer is hexane. 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

In the 1,3-dipolar cycloaddition reactions of especially allyl anion type 1,3-dipoles with alkenes the formation of diastereomers has to be considered. In reactions of nitrones with a terminal alkene the nitrone can approach the alkene in an endo or an exo fashion giving rise to two different diastereomers. The nomenclature endo and exo is well known from the Diels-Alder reaction [3]. The endo isomer arises from the reaction in which the nitrogen atom of the dipole points in the same direction as the substituent of the alkene as outlined in Scheme 6.7. However, compared with the Diels-Alder reaction in which the endo transition state is stabilized by secondary 7t-orbital interactions, the actual interaction of the N-nitrone p -orbital with a vicinal p -orbital on the alkene, and thus the stabilization, is small [25]. The endojexo selectivity in the 1,3-dipolar cycloaddition reaction is therefore primarily controlled by the structure of the substrates or by a catalyst. 

Two isomeric terpenes are known under the name of phellandrene. Before the distinction between the two bodies was recognised, phellandrene had been discovered and reported in a number of essential oils, so that in many cases it is impossible at present to decide which isomer is that actually present in a ven oil. The two terpenes—still another case of the absurd nomenclature which has been retained for so many of the terpenes—are known as a-phellandrene and /3-phellandrene. The constitutions of the two hydrocarbons are probably as follows —... 

Alkanes are a class of saturated hydrocarbons with the general formula C H2n. -2- They contain no functional groups, are relatively inert, and can be either straight-chain (normal) or branched. Alkanes are named by a series of IUPAC rules of nomenclature. Compounds that have the same chemical formula but different structures are called isomers. More specifically, compounds such as butane and isobutane, which differ in their connections between atoms, are called constitutional isomers. 

Alkane Hydrocarbon containing only single carbon-carbon bonds. The simplest example is methane, CK, 580 cis-trans isomer in, 598 isomers in, 580-582 nomenclature, 582t physical properties, 591t sources, 583-585 structural isomerism, 597 tests for, 602 uses, 583-585... 

Representation of and the nomenclature for stereo-isomers are given in Appendix 1. 

A system of nomenclature has been devised to describe optical isomers conveniently. These isomers differ in configuration and we have to be able to specify the configuration at the asymmetric atom unambiguously. 

Hantzsch s proposal is consistent with characteristic infrared vibration frequencies of syn- and a fr-diazoates, suitably labeled with 15N, measured by Kiibler and Liittke (1963) and by an X-ray structure determination of a sy -diazoate by Alcock et al. (1980b). It is appropriate, therefore, to replace the syn/anti by the (Z/E) nomenclature for isomers (Sec. 7.1)... 

What Do We Need to Know Already This chapter draws on the introduction to organic formulas and nomenclature in Sections C and D, the structure of molecules (Chapters 2 and 3), intermolecular forces (Sections 5.3-5.5), reaction enthalpy (Section 6.13), reaction mechanisms (Sections 13.7-13.9), and isomers (Section 16.7). 

Nomenclature for the carboranes is discussed by R. Adams, Inorg. Chem., 2, 1087 (1963). Briefly, the accepted name for the isomer of Bi0C2Hi2 containing adjacent carbon atoms is... 

In the/flc isomer, the three chloride ions are located on the corners of one of the triangular faces of the octahedron. In the mer isomer, the three chloride ions are located around an edge (meridian) of the octahedron. The IUPAC system of nomenclature does not use this approach. A summary of the IUPAC procedures is presented in the book by Huheey, Keiter, and Keiter that is cited in the references listed at the end of this chapter. 

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