Nomenclature of cis-trans isomers

Trans cycloalkenes are unstable unless the ring is large enough (at least eight carbon atoms) to accommodate the trans double bond (Section 7-7D). Therefore, all cycloalkenes are assumed to be cis unless they are specifically named trans. The cis name is rarely used with cycloalkenes, except to distinguish a large cycloalkene from its trans isomer. 

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

To deal with this problem, we use the E-Z system of nomenclature for cis-trans isomers, which is patterned after the Cahn-Ingold-Prelog convention for asymmetric carbon atoms (Section 5-3). It assigns a unique configuration of either or Z to any double bond capable of geometric isomerism. 

If the alkene has more than one double bond, the stereochemistry about each double bond should be specified. The following compound is properly named 3-bromo-(3Z,5 )-octadiene  

The use of E-Z names (rather than cis and trans) is always an option, but it is required whenever a double bond is not clearly cis or trans. Most trisubstituted and tetrasubsti-tuted double bonds are more clearly named or Z rather than cis or trans. 

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The Orbital Description of the Alkene Double Bond 286 7-3 Elements of Unsaturation 287 7-4 Nomenclature of Alkenes 289 7-5 Nomenclature of Cis-Trans Isomers 291 Summary Rules for Naming Alkenes 293 7-6 Commercial Importance of Alkenes 294 7-7 Stability of Alkenes 296 7-8 Physical Properties of Alkenes 302... 

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 ... 

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. 

The three isomers are cis/trans isomers with respect to the substituents -phenyl and -0-0 at C-4, and -OC2H5, = 0 at the P atom. For convenience, we use the E/Z nomenclature recommended for ethylene diastereoisomer assignment. If we replace the plane in ethylenes through the sp and sp atoms by a plane vertical to the phosphacyclohexadiene (2,4) ring (thought of in first approximation as planar) through C—4 and the phosphorus atom, we have to denote the isomer with -0—0 at C—4 and = O at P on the same side of this plane as Z , and that with -0-0 at C—4 and -OC2H5 at P on different sides as E diasteroisomer. 

These ere also sometimes called cis-trans isomers, but the foc-mer nomenclature is unambiguous. However, the HJPAC recommendations are moving away from the use of terms like facial and meridional. 

The first three chapters constitute a review of bonding and an introduction to organic compounds. Functional groups are introduced. Resonance is covered extensively, and numerous examples are provided. Acid-base chemistry is discussed in Chapter 4, and this reaction is used to introduce many of the general features of reactions, including the effect of structure on reactivity. Nomenclature of all of the functional groups is covered in Chapters 5 and 12. In this edition, stereochemistry is covered in two chapters to break up the material Chapter 6 discusses cis trans isomers and conformations, and Chapter 7 addresses chiral molecules. 

According to lUPAC the term geometric isomerism" is an obsolete synonym of cis-trans isomerism and its use is strongly discouraged. Sometimes the term geometric isomerism has been used as a synonym of stereoisomerism, i.e. optical isomers being considered to be geometric isomers. This, however, is not consistent with current standard chemical nomenclature. The exact term for stereoisomers that are not optical isomers is diastereomers. 

Some nomenclature note that in the trans isomer, the two methyl groups are on opposite sides of the double bond, or across from each other. In the cis isomer, they are on the same side of the double bond. Like constitutional isomers, diastereomers have different physical and chemical properties. 

It is easy to remember which is which. People who devise nomenclature are maliciously foolish and, just as Emeans trans and Z means cis each letter has the shape of the wrong isomer), so a means below and p means above—each word begins with the wrong letter. 

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. 

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. 

Octahydro- (we have used this nomenclature throughout) or perhydrobenzo[c]thiophene (35) exists as the cis and trans isomers and is more commonly named 2-thiahydrindane or 8-thiabicyclo-[4.3.0]nonane. Each stereoisomer may be prepared by treating the corresponding isomer of l,2-di(bromomethyl)cyclohexane with sodium sulfide (details of the products are given in Table IV).45 Cyclization of the cis isomer may be effected partially with thiourea via the formation of cis-hexahydro-o-xylylenebis(isothiouronium bromide) with sodium disulfide it gives a mixture of cis-octahydro-benzo[c]thiophene and cis-2,3-dithiadecalin.46 Optically pure ( —)-(8 R,9I )-lran -octahydrobenzo[c]thiophene has been prepared from (+ )-dicarboxylic acid via reduction of the diacid to the diol, tosylation, and ring closure of the bistosylate with sodium sulfide.47... 

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 first proposed nomenclature suggested that isomers should be called cis when W and Y are on the same side of the double bond and trans when they are on the opposite side (Fig. 1.2), provided W X and Y Z. However, this nomenclature was limited to the particular case where W and Y are identical. The more recent nomenclature of Cahn-Ingold-Prelog, based on the German Zusammen (Z) and Entgegen (E) notation, was extended to systems where W and Y are different substituents (Fig. 1.2). There is no direct relation between the two nomenclatures since they depend on the nature of substituents and so the Z isomer is not necessarily cis. Moreover, the order of priority is determined by the atomic number of each atom connected to the C=C double bond [1]. Although the E/Z nomenclature may also be applied to compounds B/B and C/C, these are considered as conformational isomers, whereas compounds A/A are configurational isomers. 

Metal complexes display a wide variety of coordination geometries that permit the existence of several geometric isomers. The situation is rather more complicated thanwith organic molecules since the three-dimensional arrangement of coordinates around the metal core leads to the multiplication of possible diastereo-mers. The cis/trans notation is usually employed but this nomenclature is based on a spatial reference cif means adjacenf while tranf means opposite (Fig. 1.5)... 

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