Cis-trans terminology

These compounds are clearly not the same. We cannot use cis trans terminology to differentiate them, because we don t have two identical groups to compare. In situations when all four groups on the double bond are different, we have to use another method for naming them. 

Designate the configuration of the confounds whose structures are drawn below using the cis-trans terminology. 

The terms cis and trans are used commonly as prefixes to distinguish between stereoisomers in square planar systems of the form [Ma2b2], where M is the central atom, and a and b are different types of donor atom. Similar donor atoms occupy coordination sites adjacent to one another in the cis isomer, and opposite to one another in the trans isomer. The cis-trans terminology is not adequate to distinguish between the three isomers of a square planar coordination entity [Mabcd], but could be used, in principle, for an [Ma2bc] system (where the terms cis and trans would refer to the relative locations of the similar donor atoms). This latter use is not recommended. 

We could number either chain from the other end and still have the double bond starting at carbon 3 we number from the end that gives the carbon bearing the ethyl group the lowest possible position number, 3. Carbon 3 has two equivalent substituents, so geometric isomerism is not possible, and we do not use the cis-trans terminology. The name is 3-ethyl-3-hexene. 

Carbon 3 has two equivalent substituents, so geometric isomerism is not possible, and we do not use the cis-trans terminology. The name is 3-ethyl-2-pentene. 

The stereodescriptors cis and trans can only be used to indicate the relative arrangement of similar groups. When an alkene possesses nonsimilar groups, usage of cis-trans terminology would be ambiguous. Consider, for example, the following two compounds ... 

When the RelSi terminology (29) was introduced (28), its use was suggested also for describing the faces of double bonds. This was to be accomplished by citing the descriptors of their trigonal atoms when seen from the same side. Although this extension seems an obvious choice, it is subject to a possible misinterpretation. If one characterizes a face of the olefinic bond of 2-butenoic acid as Re-Re, one can deduce that the compound in question must be the Z and not the E isomer. It might be inferred from this that the cis-trans isomerism of these acids can be factored into two elements of prostereoisomerism. This... 

Note that) cis-tran.-i terminology alone is not adequate to distinguish between the three i.somers ofa square planar coordination entity [Mubcd. ... 

NOMENCLATUBE. The relationships in the steroids which find many counterparts in numerous solanidine derivatives suggest a rational nomenclature based on that of the steroids. The oxygen-free saturated compound V serves as the basic skeleton and is termed solanidane. This terminology will be used here, adopting the latest principles of steroid nomenclature (63). Instead of the earlier terms cis-, trans-, and alio-, the letters a, and J will be used to indicate configuration in the same way that they are used in the steroid group. 

The terminology associated with conformational analysis is in many ways akin to the terminology used to discuss stereochemistry—namely, cis, trans, anti, syn, etc. In Chapter 6 there is a glossary of terms used in conformational analysis and stereochemistry, so if a term is used below that you are unfamiliar with, look to the end of Chapter 6 for a definition. 

When using cis-tmns terminology, we look for two identical groups. In this case, there are two ethyl groups that are in the trans configuration ... 

Table 1 displays the properties thatcan be discussed in terms of the tram effect (or cis or metal effect) together with some terminology, in order to allow the reader to connect this article to the literature concerning the simpler metal complexes (9-11,16-19) and to prevent some Tower-of-Babel confusion of language into which an ill-prepared reader might fall. The most general and appropriate terminology is the one used by Hill, Pratt, and Williams (Table 1, Column 2) it correlates with the properties to be discussed. Pratt additionally states that the cis or trans effects cover the three levels of properties, A-C (Table 1), and that these levels show diminishing...

Ruedenberg s terminology peaked, sloped, and intermediate, as shown in Figure 8. Often the chemically relevant conical intersection point is located along a valley on the excited state potential energy surface (i.e., a peaked intersection). Figure 9 illustrates a two-dimensional model example that occurs in the photochemical trans —> cis isomerization of octatetraene.28 Here two potential energy surfaces are connected via a conical intersection. This intersection... 

If the positions of fatty acids on the TAG molecule are known, the sn system is preferred for identifying their locations sn immediately before the word glycerol, asinl-stearoyl-2-oleoyl-3-myristoyl-sn-glycerol, identifies the respective fatty acids in the 1, 2, and 3 positions the term rac (racemic mixture), as in rac-StOM, identifies the middle acid in the 2-glycerol position and the remaining fatty acids are equally divided between the sn-1 and sn-1 positions and the term 3, as in 3-StOM, identifies the middle acid in the 2-glycerol position, but distribution of the other two acids is unknown.33 Despite international efforts to standardize chemical terminology and abbreviations, a variety is still used, especially in cis and trans notations. 

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