Properties of cis- and trans

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). 

Bhowmick et al. [1986] investigated properties of cis- and trans-polyisoprene blends (cPI/tPI), with a rather cursory look at the effect of gamma irradiation (Table 11.9). The blends consisted of guayule mbber (cPI) and synthetic tPI (Trans-PIP from Polysar). The blends were prepared by mixing the two polymers, along with the typical... 

MLCT Transitions in Geometric Isomers Isomerization is another exciting approach toward tuning the spechal properties of metal complexes (Fig. 5) [44-46]. The electronic properties of cis-and trans-isomers of [RuL2(X)2], (L = 4,4 -dicarboxylic acid-2,2 -bipyridine X = Cl ,... 

TABLE IX Properties of Cis and Trans Isomers of Square [PtAsBg]... 

Figure 1.31. Different s)mimetry properties of cis- and trans-polyacetylene. Only in tran -polyacetylene there is a mirror plane through the conjugational defect. In cw-polyacetylene the defect separates two different structures with different lattice energies. (Reprinted with permission from ref 22)...

Worked example 3.1 Symmetry properties of cis- and trans-ti2f2... 

Lewisite [L or L-1 dichloro(2-chlorovinyl) arsine] is an arsenical vesicant developed early in the twentieth century. Lewisite occurs as cis- and trans-isomers, the t5T>ical ratio being 10 90. Several impurities, including bis(2-chlorovinyl)chloroarsine (L-2) and tris(2-chlorovi-nyl)arsine (L-3), are typically present. The chemical and physical properties of cis- and trans-isomers are similar. 

Singlet and triplet carbenes exhibit different properties and, to great extent, show markedly different chemistry . For example, a singlet carbene will add to a c/ -disubstituted alkene to produce only cis-disubstituted cyclopropane products (and to a rara-disubstituted alkene to produce only /rara-disubstituted cyclopropane products), while a triplet carbene will add non-stereospecifically to produce a mixture of cis and trans products. 

The commerical polybutadiene (a highly 1,4 polymer with about equal amounts of cis and trans content) produced by anionic polymerization of 1,3-butadiene (lithium or organolithium initiation in a hydrocarbon solvent) offers some advantages compared to those manufactured by other polymerization methods (e.g., it is free from metal impurities). In addition, molecular weight distributions and microstructure can easily be modifed by applying appropriate experimental conditions. In contrast with polyisoprene, where high cis content is necessary for suitable mechanical properties, these nonstereoselective but dominantly 1,4-polybutadienes are suitable for practical applications.184,482... 

Many studies have established that the Diels-Alder reaction is strongly accelerated by an increase in pressure and this property has been applied to the reaction between 1-methoxybuta-1,3-diene and various aldehydes, ketones and a-keto esters, when good yields of dihydropyrans are obtained at 50 °C (79S41, 81JOC2230). Endo addition yielding the cis isomer is preferred, whilst differences in the ratio of cis and trans products obtained at atmospheric and at high pressures suggest that endo and exo additions have different activation volumes. 

The absence of electronwithdrawing properties and small steric volume of hydrogen substituents are the reasons for the higher stability of cation 36. For 1-substituted analogs of F-allyl and F-methallyl cations (existing as a mixture of cis- and trans-isomers, except 43, X=H for which only czs-isomer was found in solution), stability correlates well with electronegativity of the substituent [63] ... 

Because compounds representing the mixture of cis- and trans-isomers were used in polyaddition reactions, carbosiloxane polymers should possess the atactic structure. Such conclusion is also pro-ved by the above-mentioned NMR spectra data. In this connection, the absence of ability to crystal-lize in the majority carbosiloxane polymers is quite explainable. However, the absence of mesomor-phous properties in polymer with decamethylcyclohexasiloxane fragments is the unexpected result. The case is that at the polymerization degree P>5, atactic non-crystallizing cyclolinear polyorganosi-loxanes with... 

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