Double bond cis/trans

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. 

Double bond cis-trans isomerization occurs during hydrogenation with a relative rate dependent on structure. The less stable double bond isomerizes to the more stable one, but, of course, kinetics and thermodynamics control the extent of isomerization. In a linear carbon chain, one can expect the cis alkene to isomerize to trans and vice versa if the thermodynamics are favorable. However, in a strained cyclic system, trans will isomerize to cis (Fig. 2.13).117... 

The monomeric units formed via 1,4-polymerisation of monomers of the CH2=CH-CH=CHR type display stereoisomerism at the double bond (cis trans) and at the tertiary carbon atom which can assume two opposite configurations. Hence, the obtained 1,4-polymers can appear as cis- 1,4-isotactic, cis-... 

The heating of petroleum products in the presence of a catalyst (usually an aluminosilicate mineral), causing bond cleavage to form alkenes and alkanes of lower molecular weight, (p. 315) (geometric isomers) Isomers that differ in their cis-trans arrangement on a ring or double bond. Cis-trans isomers are a subclass of diastereomers. (p. 291)... 

The skeletal isomerization of C4 and C5 n-olefins is an acid-catalyzed reaction requiring relatively strong acid sites that proceeds via carbenium ion intermediates formed upon protonation of the double bond (17). Double bond cis-trans isomerization usually occurs on the acid sites before skeletal isomerization. The general reaction mechanism for branching isomerization is depicted in Fig. 2 2. Protonation of the double bond leads to a secondary carbenium ion, which then rearranges into a protonated cyclopropane (PCP) structure. In the case of n-butenes,... 

Unfortunately, none of the analogs 34, 35, 37, or (A-unsubstituted secondary amide) 36, which would mimic a rranj -olefin geometry) showed any appreciable tubulin-polymerizing or antiproliferative activity, despite the fact that preliminary NMR studies with compound (34) in DMSO/water indicate that the preferred conformation about the 12/13 A-methyl amide bond is indeed cis, that is, the methyl group and the carbonyl oxygen are located on the same side of the partial C-N double bond (cis/trans-mtio 4/l 34 may thus be considered a direct structural mimetic of Epo D). The underlying reasons for the lack of biological activity of... 

Monomolecular photochemistry of butadiene is rather complex. Direct irradiation in dilute solution causes double-bond cis-trans isomerization and rearrangements to cyclobutene, bicyclo[l.l.01butane, and l-methylcyclo-propene (Srinivasan, 1968 Squillacote and Semple, 1990). Matrix-isolation studies have established another efficient pathway, s-cis-s-trans isomerization (Squillacote et al., 1979 Arnold et al., 1990, 1991). 

While double-bond cis-trans isomerization in / -substituted a,/3-enones occurs at both 313 and 254 nm, oxetene forms only when short wavelengths are used (Friedrich and Schuster, 1%9, 1972). This differentiation has been explained on the basis of calculations that revealed a S,-So conical intersection lying 15 and 10 kcal/mol above the (n. ) minimum of s-trans- and s-c/s-acrolein, respectively (Reguero et al., 1994). Oxabicyclobutane, however, has not been detected. This behavior of o,/3-enones is in contrast with that of butadiene, where cyclobutene and bicyclobutane are formed simultaneously. (Cf. Sections 6.2.1 and 7.5.1.)... 

Double bond cis-trans configuration The saturated carbon chain is... 

Papers (4, 47, 48) demonstrate that, while the character of the carrier (silica gel, active carbon) of the active component has no pronounced influence on the process of hydrogenation, there are distinct differences in the effect of the active components themselves. Side reactions occurred on rhodium and palladium catalysts, while on platinum catalysts they could not be observed in most cases (migration of the double bond, cis-trans isomerization). These reactions occurred only if a sufficient amount of hydrogen was present in the reaction mixture (part of hydrogen is irreversibly consumed by hydrogenation). Neither the carrier alone nor the catalyst in an inert atmosphere provoked any side reactions, which shows that hydrogen in one of its forms participates directly in the isomerization process. 

Mixtures of saturated and unsaturated lipids of a class are fractionated in accordance with the chain length and number of double bonds cis-trans isomers and unsaturated positional isomers are generally inseparable (see [160], however). Saturated and unsaturated members of a lipid class which differ from one another by two methylene groups- and a double bond, are usually not or only incompletely separated (see p. 410, however). 

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