Double bonds cis- and trans

As with double bonds, cis and trans isomers are possible, but the restrictions are that W may equal Y and X may equal Z, but W may not equal X and Y may not equal Z. There is an important difference from the double-bond case The... 

Because of the energy barrier to rotation about a double bond, cis and trans isomers caimot interconvert (except under conditions extreme enough to overcome the barrier and break the tt bond). This means that they can be separated from each other. In other words, the two isomers are different compounds with different physical properties, such as different boiling points and different dipole moments. Notice that trans-2- miem... 

Phytol is a fatty alcohol with 20 carbon atoms (3, 7, 11, 15-tetramethyl-2-hexadecene-l-ol, C20H40O) from the group ofterpenes. Phytol is used in the synthesis of vitamins E and K]. In addition, it is a lipophilic component of chlorophyll. Because of a double bond cis- and trans-isomers exist. The trans-isomer is used as a fixener in the perfume industry. 

The interesting isomers are the ones that arise by replacement of H s on the different carbon atoms. One isomer is called cis ( on the same side, in Latin), where the two CTs are on the same side of the double bond. The other is called trans ( beyond one another, in Latin), where the two CTs are on opposite sides of the double bond. Since there is no free rotation around the double bond, cis and trans isomers coexist and are stable molecules having different properties (refer to Section 7.1 about van t Hoff and the isomerism of maleic and fumaric acids). 

Constitutional isomers are molecules in which the same atoms are connected difleiratly. Stereoisomers are molecules in which the same atoms are cormected by the same bonds but the bonds ate oriented differently. Geometrical isomers arise due to restricted rotation about a carbon-carbon double bond. Cis and trans isomers ate geometrical isomers. 

The term configuration refers to the permanent stereostructure of a polymer. The configuration is defined by the polymerization method, and a polymer preserves its configuration until it reacts chemically. A change in configuration requires the rupture of chemical bonds. Different configurations exist in polymers with stereocentres (tacticity) and double bonds cis and trans forms). A polymer with the constitutional repeating unit -CH2-CHX- exhibits... 

When an alkene has identical groups on the same carbon atom of the double bond, cis and trans isomers cannot be drawn. For example, 1,1-dichloropropene has just one condensed structural formula without cis and trans isomers. 

The first stereochemical analysis of the polymerization mechanism of unsaturated monomers was proposed by Arcus 30 years ago (58, 269). The diaste-reomeric relationships within a single chain may be connected to monomer configuration by two factors the type of addition to the double bond (cis or trans) and the way in which successive monomer molecules approach the growing chain. For a CHA=CHB olefin the relationship between the stmcture of the monomer and that of the adding monomeric unit is illustrated in Scheme 22 (31), where cis or trans addition indicates that the new bonds are formed in the same half-space or in the two opposite half-spaces, provided there is no rotation aroimd the central C—C single bond. 

Know the meaning of trigonal carbon, sp2 hybridization, restricted rotation, a and n bonds, cis and trans double-bond isomers. 

In this chapter, molecular factors affecting structural behavior of fat polymorphism are discussed in terms of internal influences of the TAG molecules. In particular, the influences of fatty acid compositions and their positions connected to glycerol carbons on the polymorphism of fat crystals are of primary concern. It has been known that the fats with simple and symmetric fatty acid compositions tend to exhibit typical oc, P, and P forms, whereas those with asymmetric mixed-acid moieties often make the P form more stable (1,9). In the mixed-acid TAG containing unsaturated fatty acid moieties, the number and conformation of the double bond, cis or trans, give rise to remarkable influences on the polymorphic structures (10-12). The TAG containing different saturated fatty acids with different chain-lengths also revealed quite diversified polymorphism (13-15). Therefore, it may be worthwhile now to discuss the molecular aspects of the polymorphism of fats. This consideration may also be a prerequisite for molecular design of structured fats, in combination with nutritional and metabolic properties. 

Figure 2 illustrates diversity in the fatty acid types and compositions of the TAG. The mono-acid and mixed-acid TAG are defined, respectively, on whether the fatty acid chains are of the same fatty acid molecules or not. Even for the monoacid TAG, the diversity maintains in chainlength and parity (odd or even numbers of carbon atoms) of the fatty acids, the number and position of the double bond and the conformation of the double bond, cis or trans, etc. As for the mixed-acid TAG, polymorphic diversity is superimposed over that of the mono-acid TAG in the form of sn (stereo-specific numbered) position. 

When light strikes the rod cells, isomerization of the C11-C12 double bond occnrs and trans-rhodopsin, called metarhodopsin 11, is produced. In the absence of light, this cis-trans isomerization takes approximately 1100 years, hut in the presence of light, it occurs within 200 femtoseconds, or 2 X 10 seconds Isomerization of rhodopsin is accompanied by a change in molecular geometry, which in turn causes a nerve impulse to he sent through the optic nerve to the brain, where it is perceived as vision. 

The terms cis and trans signify, respectively, that the two hydrogen atoms are on the same side or on the opposite side with respect to the perpendicular plane of the double bond. 

Exclusive of compounds with double bonds four hydrocarbons are constitutional isomers of cis and trans 1 2 dimethylcyclopropane Identify these compounds... 

Dehydrohalogenation of cycloalkyl halides lead exclusively to cis cycloaUcenes when the ring has fewer than ten carbons As the ring becomes larger it can accommo date either a cis or a trans double bond and large nng cycloalkyl halides give mixtures of CIS and trans cycloalkenes... 

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. 

Configurations around any double bond give rise to cis and trans stereoisomerism. 

Hydrogenation of polybutadiene converts both cis and trans isomers to the same linear structure and vinyl groups to ethyl branches. A polybutadiene sample of molecular weight 168,000 was found by infrared spectroscopy to contain double bonds consisting of 47.2% cis, 44.9% trans, and 7.9% vinyl. After hydrogenation, what is the average number of backbone carbon atoms between ethyl side chains ... 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

Since the olefmic CC double bond is trisubstituted, the relative configuration cannot be determined on the basis of the cis and trans couplings of vicinal alkene protons in the H NMR spectrum. What is the relative configuration given the C NMR spectra 19 ... 

Hence the compound is nona-2,6-dienal. The relative configuration of both CC double bonds follows from the HH coupling constants of the alkene protons in the H NMR spectrum. The protons of the polarised 2,3-double bond are in trans positions Jhh 5.5 Hz) and those on the 6,7-double bond are in cis positions Jhh = 10.5 Hz). The structure is therefore nom.-2-tmns-6-cis-dienal, D. 

The monomer, norbomene (or bicyclo[2.2.l]hept-2-ene), is produced by the Diels-Alder addition of ethylene to cyclopentadiene. The monomer is polymerised by a ring-opening mechanism to give a linear polymer with a repeat unit containing both an in-chain five-membered ring and a double bond. Both cis-and trans- structures are obtainable according to the choice of catalyst used ... 

The reductive elimination of halohydrins provides a means of introduction of double bonds in specific locations, particularly as the halohydrin may be obtained from the corresponding a-halo ketone. This route is one way of converting a ketone into an olefin. (The elimination of alcohols obtainable by reduction has been covered above, and other routes will be discussed in sections IX and X.) An advantage of this method is that it is unnecessary to separate the epimeric alcohols obtained on reduction of the a-bromo ketone, since both cis- and tran -bromohydrins give olefins (ref. 185, p. 251, 271 cf. ref. 272). Many examples of this approach have been recorded. (For recent examples, see ref. 176, 227, 228, 242, 273.) The preparation of an-drost-16-ene (123) is illustrative, although there are better routes to this compound. 

Polyunsaturated fatty acids pose a slightly more complicated situation for the cell. Consider, for example, the case of linoleic acid shown in Figure 24.24. As with oleic acid, /3-oxidation proceeds through three cycles, and enoyl-CoA isomerase converts the cA-A double bond to a trans-b double bond to permit one more round of /3-oxidation. What results this time, however, is a cA-A enoyl-CoA, which is converted normally by acyl-CoA dehydrogenase to a trans-b, cis-b species. This, however, is a poor substrate for the enoyl-CoA hydratase. This problem is solved by 2,4-dienoyl-CoA reductase, the product of which depends on the organism. The mammalian form of this enzyme produces a trans-b enoyl product, as shown in Figure 24.24, which can be converted by an enoyl-CoA isomerase to the trans-b enoyl-CoA, which can then proceed normally through the /3-oxidation pathway. Escherichia coli possesses a... 

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