Trans isomers definition

Of course, the trans isomer will give the opposite results the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition of Br2), the erythro pair is a meso compound. In addition to triple-bond compounds of the type ACsCA, syn addition results in a cis alkene and anti addition in a trans alkene. By the definition given on page 166 addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective. 

The steric outcome in the above cyclization can be explained on the basis of either a zwitterionic intermediate or a concerted [7t2s -I- 7t2s] process , depending on the nature of the reactants . Definite predictions are practically impossible as yet. The more stable trans-isomers (i.e. 221a) can be obtained by stirring the isomeric mixture with catalytic amounts of potassium t-butoxide in t-butyl alcohol for several days . 

Stereospecificity as applied to olefin metathesis may be considered in two ways (a) How does the cis/trans isomer ratio of a product olefin compare with its equilibrium ratio, or (b) how does this cis/trans value differ from 1.0, which is the statistically expected value in terms of probabilities. In the present discussion, the latter definition applies. 

As we pointed out above, there are two main reasons for the differences in the correlations in these molecules. To analyze the stmctural changes induced in the molecule, we define the cis mole fraction as the mole fraction of molecules in the range -Till < ( ) < Ji/2 and the trans isomer in the range nl2< < 3/2n. With these definitions we find for the model the following mole fractions of the different stages of occupation of the molecule. 

As for the thermal isomerization of cw-Cr(en)2(OH)2+, our values of k are about twice those of Olson and Garner (although for somewhat different substrates), and our activation energy of 35 kcal. is definitely higher than their 32 kcal. However, using parallel runs with cis and trans isomers allowed us to correct for aquation, and made the values of K and k less interdependent. As noted under Experimental, the latitude in K values, stemming from the presence of some aquation (K and the aquation rate constant are again interrelated), was considerable. We can only delimit AH° of isomerization as 0 =fc 2 kcal. 

Carotenoids can be converted into mixtures of geometrical isomers under appropriate conditions, the most common being iodine catalyzed photoisomerization. This produces an equilibrium mixture of isomers, in general the all-trans isomers predominates. These isomers in an isomeric mixture cannot be measured separately by simple spectrophotometric determination. The usual method of subsequent measurement would be chromatographic separation, diode-array detection, and spectral analysis. In the absence of any definitive data on extinction coefficients for cfv-isomcrs, they are quantified against the all-trans isomer. Modem procedures involve the direct synthesis of c/.v-carotcnoids. 

Although the term prochirality is frequently used, especially by biochemists, it suffers from a limitation which arises from a corresponding limitation in the definition of chirality. Molecules may display purely stereochemical differences without being chiral cis-tram isomers of olefins and certain achiral cis-trans isomers of cyclanes are examples. Thus (Fig. 2) (Z)- and ( )-1,2-dichloroethylene (4, 5) are achiral diastereomers, as are cis- and /rtww-1,3-dibromocyclobutanes (6, 7) being devoid of chirality these compounds have no chiral centers (or other chiral elements). Thus it is inappropriate to associate stereoisomerism with the occurrence of chiral... 

Here we are primarily concerned with the fact that this almost exclusively formed ortho adduct may occur in the form of two diastereomers. The diastereomers are formed as a 57 43 cis trans mixture in the absence of A1C13, but a 95 5 cis trans mixture is obtained in the presence of A1C13. In the latter case, thus, one is dealing with a Diels-Alder reaction that exhibits a substantial simple diastereoselectivity (see Section 9.3.2 for a definition of the term). Here, the simple diastereoselectivity is due to kinetic rather than thermodynamic control, since the preferentially formed cw-disubstituted cyclohexene is less stable than its trans isomer. 

A stereocenter is an atom at which the interchange of two groups gives a stereoisomer. Stereocenters include both chirality centers and double-bonded carbons giving rise to cis-trans isomers. For example, the isomers of but-2-ene are achiral and they contain stereocenters (circled), so they would meet this definition. They have no chiral diastereomers, however, so they are not correctly called meso. 

An alternative and more rigorous treatment considers C-1 and C-3 as centres of pseudoasymmetry and assigns them the appropriate symbols r and y (actually and see definitions below). (Note that according to this treatment, the cw-isomer is ls, 3s and the trans-isomer lr ,3r i.e., changing the configuration at one centre changes both descriptors.) For a full explanation, see Eliel and Wilen, p. 667. 

It has been suggested that cis-bis(2,2 -bipyridyl) complexes show greater complexity in the region 700-800 cm" than do iraras-bis(2,2 -bipyridyl) complexes (500). One complex, [Rh(III)(bipy)2Cl2], which is now known to be definitely cis and which gives a rich spectrum between 700 and 800 cm, has also been said to be the trans isomer (287, 452). Since the complex spectra are generally recorded for the solid state, it is clear that spectra-structure correlations of the above type must be used with caution. 

Catalyst selectivity is somewhat meaningless unless the term is defined. There also are selective catalysts that do not meet the technical or practical definition of hydrogen selectivity. Such catalysts are sulfur-poisoned catalyst. Sulfided nickel catalyst produces high trans-isomers, has lower activity than conventional nickel, exhibits longer reaction times, and is used for specialty applications (e.g., coating fats and hard butters). 

Hydrogen is added to oleflns and aromatics by a mechanism that is predominantly, if not exclusively, cis addition. The formation of trans isomers seems definitely to involve olefin intermediates that have desorbed and turned over on the catalyst. We find no evidence for the formation of any trans isomers by the direct trans addition of hydrogen. Such a mechanism, if operative at aU, must be trivial. 

In highly methylated ethylene the (N,V) and 3s bands separate and the Rydberg band is definitely at lower frequencies . The same applies to most fluoroethy-lenes In the chloroethylenes conditions are likely to be similar althou some problems remain with the band assignments Now, in a recent paper Ausubel and Wijnen have shown that the photolysis of both cis and tram 1,2-dichloroethylenes (between 200 and 240 nm) involve two excited states. For the trans isomer they obtained, from the lower excited state, the following primary steps ... 

A simple conceptual illustration of the appheation of RDCs is in the differentiation of axial and equatorial protons in pyranose sugars 103 through the eonsideration of one-bond Dch values [125]. Within the moleeular fiamework, all axial protons share the same parallel orientation (bond veetors in blaek) and thus exhibit RDCs of the same size, whereas the equatorial protons point in different directions (vectors in red) and so display different values, enabling their identifieation. A similar analysis allows the definition of the relative configuration of a dihydropyridone 10.4 [126]. In the trans isomer, the highlighted C-H... 

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