Designating the Configuration of Cis-Trans Isomers

Build handheld models of both cis-trans isomers of this compound and compare the amount of steric crowding in each. 

The three-dimensional arrangement of groups about a stereocenter in a molecule is termed its configuration. One way to designate the configurations of cis-trans isomers is to use the terms cis and tram. However, these terms are ambiguous in many instances. For example, is the following isomer of 1-chloro-l-fluoro-l-propene the cis- or the trans-stereoisomer  

Priorities are assigned to groups by a series of rules known as the Cahn-Ingold-Prelog sequence rules, named after the three chemists who developed them. These rules use the atomic numbers of the atoms attached to the carbons of the double bond. 

Of the two atoms attached to one carbon of the double bond, the one with the higher atomic number has the higher priority. 

If the two atoms attached to the carbon are the same, compare the atoms attached to them in order of decreasing priority.The decision is made at the first point of difference. 

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Exercise 12-15 Write structural formulas for all of the possible cis-trans isomers of the following compounds and designate the configuration of each by name (see Section 5-1) ... 

Several systems of nomenclature have been developed to designate the configuration of geometric isomers. Historically, the cis-trans system of nomenclature has been applied most frequently. It was developed to assign the configuration of geometric isomers when each isomer contains a like group or atom on each carbon atom of the double bond. 

The pair of isomers designated as and trans 2 butene have the same constitution both have an unbranched carbon chain with a double bond connecting C 2 and C 3 They differ from each other however m that the cis isomer has both of its methyl groups on the same side of the double bond but the methyl groups m the trans isomer are on oppo site sides of the double bond Recall from Section 3 11 that isomers that have the same constitution but differ m the arrangement of their atoms m space are classified as stereoisomers as 2 Butene and trans 2 butene are stereoisomers and the terms as and trans specify the configuration of the double bond... 

Cyclic alkenes with fewer than seven carbons in the ring, such as cyclopentene and cyclohexene, can exist only in the cis configuration because they do not have enough carbons to form a trans double bond. Therefore, it is not necessary to use the cis designation with their names. If the ring has seven or more carbons, however, then both cis and trans isomers are possible, so the configuration of the compound must be specified in its name. 

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. 

The relationship between configuration and boiling point or melting point is only a rule of thumb, to which there are many exceptions (for example, the boiling points of the diiodoethenes). Measurement of dipole moment, on the other hand, frequently enables us positively to designate a particular isomer as cis or trans. 

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