Isomerism, geometrical

Geometrical isomers are those in which the distinction lies in the relative placement of substituents across a double bond or ring. The original nomenclature for describing such isomers used the prefix cis- to refer to an isomer in which two substituents lie on the same side of a double bond and trans- for those in which the substituents are on opposite sides. Therefore, geraniol is considered to be a trans-isomer because the main terpenoid skeleton runs across the double bond next to the alcohol function. Nerol, in which both of the chain residues are on the same side of the double bond is, correspondingly, the m-isomer. Exactly the same 

Some simple examples of cis-tram isomerism have already been noted. More complex examples include those of cyclic molecules such as chair-shaped 6-rings, (a), where a distinction must be made between equatorial e and axial a bonds, 

Compounds in which rotation is restricted can exhibit geometric isomerism. These compounds do not rotate the plane of polarised light (unless they also happen to be chiral for other reasons). Furthermore, the physical and chemical properties of the various isomers are different. 

The simplest example involves a molecule that contains a carbon/carbon double bond that has an identical group attached to each end of the double bond. In this case, there are two possible isomers, namely  

The idea may be extended to other rigid double bond systems such as an imine, R2C=NR, or azo, RN=NR, in which case the terms syn and anti are used instead of cis or trans. 

Hindered rotation may occur where there is no formal double bond. In such a case, it is sometimes possible to draw at least one canonical structure that has a double bond along the axis about which there is restricted rotation. 

A further example of hindered rotation about a single bond occurs in conjugated dienes. The simple case of buta-1,3-diene exists in two forms, which are described as transoid and cisoid. The transoid form is the thermodynamically more stable form as there is less steric interaction between the terminal methyl groups. 

This form of isomer has already been met when discussing nomenclature cis and trans isomers are examples of geometrical isomers. Interconversion between two geometric isomers is often an important step in mechanisms postulated as those by which coordination compounds catalyse reactions, particularly those involving unsaturated organic molecules. 

In this form of isomerism the distribution of ligands between two coordination centres differs an example is shown below. 

Note that each of these two cations exists in a number of isomeric forms. The reader may find it a useful exercise to draw pictures of all of the forms and to enquire into the isomeric relationship between pairs. 

Since rotation cannot take place about a double bond between two carbon atoms, two nonsuperimposable configurations (geometrical isomers) are possible if the two substituents on each carbon differ from each other. For example, the two monomers maleic acid (IX) and fumaric acid (X) are geometrical isomers, designated cis and trnns resnectivpilv 

In solid, the molecules of trans isomers pack more closely and crystallize more readily than those of cis isomers. This is reflected in the fact that the melting point of fumaric acid is about 160°C higher than that of of maleic acid. Similarly, the differences in the properties of cis and trans isomers of polymers are also significant, as shown below with the example of poly(isoprene). 

Natural rubber is 1,4-polyisoprene and the polymer configuration is cis at each double bond in the chain, as shown , in (XI). Consequently, the polymer molecule has a bent and less symmetrical structure. Natural rubber does not crystallize at room temperature and is amorphous and elastomeric. Balata (guttapercha) is also 

4-polyisoprene, but the polymer configuration is trans at the double bond (XII). The molecule is more extended and has symmetrical structure. The trans isomer is thus a nonelastic, hard, and crystalline polymer. It is used as a thermoplastic. 

According to the new lUPAC nomenclature for carotenoids (98) the cis-trans convention is still used to denote geometrical isomerism of the polyene chain. However, the EjZ designations may also be used, especially when the prefixes cis and tram might lead to ambiquity (98). 

Geometrical isomerism in the carotenoid series is well covered in the earlier literature (174, 181, 182) through 1970. It has been known 

During the last decade additional information about HPLC, NMR and CD properties of cw-carotenoids has been obtained, as already discussed in Part III. 

The cross-conjugated carotenals of the rhodopin-20-al (5) series occur exclusively in the 13-ds configuration 1, 49, 54) and the dW-trans isomer is not formed upon stereomutation 1, 109). When the cross-conjugated aldehyde is reduced, the ail-trans isomers of the corresponding alcohol and acetate may be isolated 49). The preference for the 13-cw configuration in the cross-conjugated 20-al series is ascribed to a combination of steric and electronic factors 49). 

In the 7,7 -diacetylenic series it has recently been shown that no all-trans isomer can be detected in the iodine catalyzed stereomutation mixtures examined by HPLC of alloxanthin (31) and 7,8,7, 8 -tetra-dehydroastaxanthin (34) diacetate, in which the 9,9 -di-cis isomers are dominant (76). However, in the absence of iodine aW-trans (31) and all-trans (34) diacetate are rather stable. In the monoacetylenic series exemplified by diatoxanthin (41) and the diacetate of 7,8-didehydroasta-xanthin (80), appreciable amounts of the aW-trans isomers are present in the iodine catalyzed stereomutation mixture 76,89). Again the preference for cw-configuration is probably caused by both steric and electronic factors. 

This is of most importance in square-planar and octahedral compounds where ligands, or more specifically donor atoms, can occupy positions next to one another (cw) or opposite each other (trans) (Fig. 19.11). 

In the case of an octahedral complex having the general formula ML6, only one compound exists owing to the fact that all bonding positions in the coordination sphere of the metal are equivalent. For a complex having the formula ML5X, there is still only one isomer possible for the same reason. 

For an octahedral complex having the general formula ML4X2 (for example, [Co(NH3)4C12]+), there are two possible isomers that have cis and trans structures  

If the complex has the formula ML3X3, there are two possible isomers. For [Co(NH3)3C13], the two isomers have structures shown as follows. 

In (a), the three chloride ions are on one face of the octahedron. In (b), the three chloride ions are occupying positions around an edge (a meridian) of the octahedron. Therefore, the names include fac and mer to indicate the structures. 

As the number of different groups in the formula increases, the number of possible isomers increases rapidly. For example, if a complex has the general formula MABCDEF (where M represents a metal and A, B. represent different ligands), a large number of isomers are possible. 

A similar type of isomerism occurs for [Masbs] octahedral complexes since each trio of donor atoms can occupy either adjacent positions at the comers of an octahedral face (facial) or positions around the meridian of the octahedron (meridional). (Fig. 19.12.) Geometrical isomers differ in a variety of physical properties, amongst which dipole moment and visible/ultraviolet spectra are often diagnostically important. 

The separation of cis- and trans- isomers is easily effected by crystallization or chromatography. There is no universal method for interconverting members of such a pair, but often heat produces the more stable and light the less stable isomer. Human vision depends on the conversion by light of the 11-of-isomer of retinal to the W-trans-form. As soon as the excitatory beam is shut off, this carotenoid pigment reverts to the m-form, thus terminating the impulses relayed to the brain (Gilardi etal., 1971). 

The ring of cyclopentane is almost flat and gives rise to cis- and trans- isomers, as though it were a big double bond. The ring of cyclohexane, although not flat, is flat enough to give this result. Thus both cis- 12.25) and trans- 12.26) forms of 

Sometimes it has been difficult to decide which two substituents out of the four at the ends of a double bond should be selected for determining whether the configuration is cis or tram. The sequence rule prescribes that the two heaviest atoms should be selected and writes Z (from the German zusammen) for cis and E (from the German entgegen) for tram. In formulae with many possibilities for geometrical isomerism, many authors write r after the numeral of the lowest-numbered substituent, and then c- and t- before the numeral for each position known to be cis or tram, respectively. 

In all work on auxin analogues, the carboxy group can be replaced by other electron-attracting groups (—CN, —NO2, —SO3H) with only moderate loss of biological action. For a summary of connexions between structure and action in this series, see Koepfli, Thimann and Went, 1938 Veldstra, 1963). The action of auxins requires sequential receptors (p. 41). 

The meaning of 5a is that the hydrogen atom in position 5 lies below the general plane of the rings. All substituents which lie below this plane in other positions are designated a, and those that lie above this plane are called jS. The a-substituents are represented by dotted lines, and the jS-substituents by thick lines. These symbols, a- and )3-, are used, with similar meaning, for other polycyclic structures, also, such as triterpenes and alkaloids. The complexity of such structures makes the application of (/ ) and (5) nomenclature too difficult. 

Comparison of the cis- and trans-isomers of the isoprene repeating unit. 

For the cardioactive glycosides, see Section 14. i. For further reading on the mode of action of steroids, see Smellie, 1971 for steroid stereochemistry, see Shoppee (1964), and for steroid biochemistry and pharmacology, see Briggs and Brotherton (1970). 

C/s-4-aminocrotonic acid, an analogue of the central neurotransmitter GABA, has the folded conformation (13.26) and is biologically inactive, whereas the trans isomer (13.23) acts as efficiently as GABA in the mammalian central nervous system. This shows that the extended, rather than the folded, conformation of GABA is the important one in neurotransmission (Johnston et al.y 1975). 

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Although a compound such as (II) could theoretically exist in a number of geometrically isomeric forms, only one form is produced in this synthesis it is almost certainly the trans form throughout the chain. 

Both acids 3deld succinic acid, m.p. 185°, upon catalytic reduction (see Section 111,150), thus establishing their structures. Maleic and fumaric acids are examples of compounds exhibiting cis-trans isomerism (or geometric isomerism). Maleic acid has the cm structure since inter alia it readily 3delds the anhydride (compare Section 111,93). Fumaric acid possesses the trans structure it does not form an anhydride, but when heated to a high temperature gives maleic anhydride. 

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

When two different substituents are attached to each carbon atom of the double bond, cis-trans isomers can exist. In the case of c T-2-butene (Fig. 1.11a), both methyl groups are on the same side of the double bond. The other isomer has the methyl groups on opposite sides and is designated as rran5--2-butene (Fig. l.llb). Their physical properties are quite different. Geometric isomerism can also exist in ring systems examples were cited in the previous discussion on conformational isomers. 

In this section we shall consider three types of isomerism which are encountered in polymers. These are positional isomerism, stereo isomerism, and geometrical isomerism. We shall focus attention on synthetic polymers and shall, for the most part, be concerned with these types of isomerism occurring singly, rather than in combination. The synthetic and analytical aspects of stereo isomerism will be considered in Chap. 7. Our present concern is merely to introduce the possibilities of these isomers and some of the vocabulary associated with them. 

In spite of the assortment of things discussed in this chapter, there are also a variety of topics that could be included but which are not owing to space limitations. We do not discuss copolymers formed by the step-growth mechanism, for example, or the use of Ziegler-Natta catalysts to regulate geometrical isomerism in, say, butadiene polymerization. Some other important omissions are noted in passing in the body of the chapter. 

Complications arising from other types of isomerism. Positional and geometrical isomerism, also described in Sec. 1.6, will be excluded for simplicity. In actual polymers these are not always so easily ignored. Polymerization of 1,2-disubstituted ethylenes. Since these introduce two different asymmetric carbons into the polymer backbone (second substituent Y), they have the potential to display ditacticity. Our attention to these is limited to the illustration of some terminology which is derived from carbohydrate nomenclature (structures [IX]-[XII]) ... 

It is not the purpose of this book to discuss in detail the contributions of NMR spectroscopy to the determination of molecular structure. This is a specialized field in itself and a great deal has been written on the subject. In this section we shall consider only the application of NMR to the elucidation of stereoregularity in polymers. Numerous other applications of this powerful technique have also been made in polymer chemistry, including the study of positional and geometrical isomerism (Sec. 1.6), copolymers (Sec. 7.7), and helix-coil transitions (Sec. 1.11). We shall also make no attempt to compare the NMR spectra of various different polymers instead, we shall examine only the NMR spectra of different poly (methyl methacrylate) preparations to illustrate the capabilities of the method, using the first system that was investigated by this technique as the example. 

Photochemistry. The most important photochemical processes that proceed from the excited state are geometrical isomerization and photochromic reactions. 

Photochromism Based on Geometric Isomerism. The simplest examples of a photochromic reaction involving reversible cis-trans isomerization is the photoisomerization of azobenzene [103-33-3] C22H2QN2 (16). 

Photochromism Based on Tautomerism. Several substituted anils of saHcylaldehydes are photochromic but only in the crystalline state. The photochromic mechanism involves a proton transfer and geometric isomerization (21). An example of a photochromic anil is /V-sa1icylidene-2-ch1oToani1ine [3172-42-7] C H qCINO. 

In contrast, chromium (ITT) and cobalt(III) form 2 1 dye metal complexes that have nonplanar stmctures. Geometrical isomerism exists. The (9,(9 -dihydroxyazo dyes (22) form the Drew-Pfitzner or y rtype (23) (A = C = O) whereas o-hydroxy—o -carboxyazo dyes (24) form the Pfeiffer-Schetty or fac type (25), where A = CO 2 and C = O. 

Oxazol-5(2H)-one, 2-benzylidene-4-methyl-tautomerism, 6, 186 Oxazol-5(2ff)-one, 2-methylene-isomerization, 6, 226 Oxazol-5(2H)-one, 2-trifluoromethyl-acylation, 6, 201 Oxazol-5(4ff)-one, 4-allyl-thermal rearrangements, 6, 199 Oxazol-5(4H)-one, 4(arylmethylene)-Friedel-Crafts reactions, 6, 205 geometrical isomerism, 6, 185 Oxazol-5(4ff)-one, 4-benzylidene-2-phenyl-configuration, 6, 185 photorearrangement, 6, 201 Oxazol-5(4ff)-one, 4-benzyl-2-methyl-Friedel-Crafts reactions, 6, 205 Oxazol-5(4ff)-one, 4-methylene-in amino acid synthesis, 6, 203 Oxazol-5(4ff) -one. 2-trifluoromethyl-hydrolysis, 6, 206 Oxazolones... 

The same mixture of H and I was obtained starting with either of the geometrically isomeric radical precursors E or F. A possible explanation is based on the assumption of a common radical conformer G, stabilized in the geometry shown by electron delocalization involving the radicaloid p-orbital, the p-peroxy oxygen and Jt of the diene unit. The structure of the compounds H and I were determined by H NMR spectra and the conversion of H to diol J, a known intermediate for the synthesis of prostaglandins. 

Asano and co-workers have reported die kinetic effects of pressure, solvent, and substituent on geometric isomerization about die carbon-nitrogen double bond for pyrazol-3-one azomethines 406 (R = H), 406 (R = NO2) and 407, (Scheme 93). The results demonstrate the versatility of die inversion mechanism. The rotation mechanism has been invalidated. First-wder rate constants and activating volumes for diermal E-Z isomerization for 406 (R = H) and 406 (R = NO2) are given at 25°C in benzene and methanol (89JOC379). 

The physical and chemical properties of complex ions and of the coordination compounds they form depend on the spatial orientation of ligands around the central metal atom. Here we consider the geometries associated with the coordination numbers 2,4, and 6. With that background, we then examine the phenomenon of geometric isomerism, in which two or more complex ions have the same chemical formula but different properties because of their different geometries. 

Click Coached Problems for a self-study module on geometric isomerism in transition metel complexes. 

Two or more species with different physical and chemical properties but the same formula are said to be isomers of one another. Complex ions can show many different kinds of isomerism, only one of which we will consider. Geometric isomers are ones that differ only in the spatial orientation of ligands around the central metal atom. Geometric isomerism is found in square planar and octahedral complexes. It cannot occur in tetrahedral complexes where all four positions are equivalent... 

Geometric isomerism can occur with any square complex of the type Mabcd, Ma2bc, or Ma2b2, where M refers to the central metal and a, b, c, and d are different ligands. Conversely, geometric isomerism cannot occur with a square complex of the type Ma4 or Ma3b. Thus there are two different square complexes with the formula Pt(NH3)2ClBr but only one with the formula Pt(NH3)3Cl+. 

Octahedral To understand how geometric isomerism can arise in octahedral complexes, refer back to Figure 15.4. Notice that for any given position of a ligand, four other positions are at the same distance from that ligand, and a fifth is at a greater distance. 

Geometric isomerism can also occur in chelated octahedral complexes (Figure 15.7, p. 416). Notice that an ethylenediamine molecule, here and indeed in all complexes, can only bridge cis positions. It is not long enough to connect to trans positions. 

Which of the following octahedral complexes show geometric isomerism If geometric isomers are possible, draw their structures. 

As we saw earlier, there are three structural isomers of the alkene C4H8. You may be surprised to learn that there are actually/owr different alkenes with this molecular formula. The extra compound arises because of a phenomenon called geometric isomerism. There are two different geometric isomers of the structure shown on the left, on page 597, under (1). 

Of the compounds in Problem 35, which ones show geometric isomerism Draw the cis- and trans- isomers. 

Geometric isomerism A type of isomerism that arises when two species have the same molecular formulas but (Efferent geometric structures, 413 octahedral planar, 415 square planar 414 trans isomer, 414... 

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