Nomenclature cis/trans

This w-ay of naming double bonds is far superior to cis/trans nomenclature because you can use this E/Z system for any double bond, even if all four groups are different. Cis/trans nomenclature requires two groups to be the same. 

The cis-trans nomenclature for geometric isomers sometimes gives an ambiguous name. For example, the isomers of 1-hromo-l-chloropropene are not clearly cis or trans because it is not obvious which substituents are referred to as being cis or trans. 

The cases of polysubstituted cyclic systems are obviously more complex due to the existence of a number of possible configurational isomers. A simple cis-trans nomenclature is obviously not sufficient here, so the IUPAC recommends to designate the various diastereoisomers by choosing a reference substituent (the lowest numbered substituent, designated r) and defining its cis or trans (c or t) relationship to all other substituents (see [60]). Thus, the three diastereoisomers of LIII are c-2,c-5-dimethyl-, f-2, f-5-dimethyl and c-2, t-5-dimethyl-r-1 -cyclopentanol. 

If two substituents are not identical, the same principles of cis/trans nomenclature still apply. However, the degeneracy is removed in the case of cis-1,3- and cw-l,2-disubstituted compounds. For example, cis-3-fluorocyclohexanecarboxylic acid (27), shown in the more populated chair conformation, is now chiral, unlike the meso 15/16. 

This problem is intended to show the difficulty of using cis-trans nomenclature with any but the simplest alkenes. Cis and trans are ambiguous the first alkene in part (a) is cis if the two similar substituents are considered, but trans if the chain is considered. The E-Z nomenclature is unambiguous and is preferred for all four of these isomers. 

The R-/S- notation is valid only for the the absolute configuration of center having single bonds only. In the case of a double bond, the traditional cis/trans nomenclature system is not sufficiently accurate and the E-IZ- is currently preferred. 

For disubstituted alkenes (with two substituents on the double bond), alkenes can be named using the cis-trans nomenclature ... 

Scheme 6.25. Diastereoselective cyclopropanations of vinyl carbenoids [97]. For disubstituted carbenes, cis/trans nomenclature is used to describe relative configuration, referring to Ri relative to the carbonyl moiety, as shown in bold.

Before we continue with other aspects of chirality, let us digress briefly to describe a useful extension of the Cahn-lngold-Prelog system of nomenclature to cis-trans isomers. Although we can easily use cis-trans nomenclature for 1,2-dichloroethene or 2-butene (see Sec. 3.5), that system is sometimes ambiguous, as in the following examples ... 

If the groups to be compared on each carbon of the C=C unit are not the same, the cis/trans nomenclature does not apply. In 2-pentene, for example, both a methyl group and an ethyl group are attached to the C=C unit. Two stereoisomers are possible, 29 and 30. Because the groups are not identical in 29 or 30, cis or trans cannot be used. 

The cis-trans nomenclature is quite useful, but in many cases determining cis versus trans is impossible because no two groups are identical, as shown earlier for 29 and 30. An alternative method has been developed that determines the relative priority of groups attached to the C=C unit and compares those priorities. For the 1-chloro-l-pentenes 31 and 32, one chlorine, one propyl, and two hydrogen atoms are attached to the C=C units of these stereoisomers. No two... 

WORKED PROBLEM 3.10 In Figure 3.23, the structure of 3-methyl-l,3-heptadiene is not completely specified by that name. Even if the cis/trans nomenclature is applied, problems remain. What s the difficulty Can you devise a system for resolving the ambiguity ... 

FIGURE 1.9 cis trans (or Z/ ) stereochemistry. If a = a we use the cis/trans nomenclature, but if a a (which is the case for fatty adds because the terminus of the a group is the CH3 group and the terminus of the a group is carboxylate), we should more correctly use the Z/E system based on Cahn-Ingold-Prelog priority rules. In practice, it must be noted that most people still use the cis/treats system for the stereochemistry of fatty acids. 

Unfortunately, the cis-trans nomenclature is not useful for naming highly substituted alkenes. Consider for example the following stereoisomers of l-bromo-l-chloro-2-fluoroethene ... 

---