Geometric isomer, definition

On the other hand, selective, usually applied to a synthesis, means that of all the possible isomers only one isomer is obtained. However, if the reaction product was/is a mixture of isomers one could speak then of the "degree of selectivity". Since usually one of the isomers will be the predominant isomer, we may say that the reaction (or the synthesis) is selective with respect to this particular isomer. As in the case of "specificity", we may refer to "regioselectivity" or to "stereoselectivity" (either diastereoselectivity or enantioselectivity) and may say, for instance, that a synthesis is 80% diastereoselective. According to the most updated terminology "diastereomers" are all the "stereoisomers" that are not "enantiomers", so geometrical isomers are also included in such a definition. 

By definition, it is necessary to recognize geometric isomers, isotopomers and stereoisomers as distinct species. Moreover, there is the pragmatic issue that regio-selectivity, isotopic labeling and stereo-chemical investigations are three very important avenues of mechanistic enquiry. 

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. 

The absence of such an effect has been definitely excluded by Rappoport and Apeloig (1970) by the observation that the two geometric isomers 143 and 144 react at virtually the same rate in ethanol-water 4 1 and acetic acid at 120° as well as in formic acid-acetic acid 1 1 at 99-7°. Moreover, the rate of 143 and 144 is intermediate between that of 141 and 142, as expected for the additivity of purely inductive effects. 

Compounds 1 and 2 can be regarded as non-interconvertible and are called geometric isomers. they also fulfil the definition of diastereoiso-mers, though they are only occasionally so called. The cis and trans nomenclature is unambiguous for disubstituted alkenes however, the situation is not clear-cut for tri- and tetrasubstituted alkenes. With these latter compounds, problems arise in the definition of which substituent is cis or trans to which. 

The difference between photophysical and photochemical processes is not always clear cut. For example, the E Z isomerization of alkenes represents a photochemical process by the above definition, yet the same process might be seen as photophysical in terminal alkenes, where geometric isomers are indistinguishable in the absence of isotopic labels. 

Definition A geometrical isomer of borneol Empirical CioHisO Formula C10H17OH... 

Silver ion HPLC appears to be the best method for separation and quantification of positional and geometrical isomers of CLA when a definitive analysis is required of a particular sample. Conmiercial CLA preparations can be analyzed without difficulty. However, natural CLA isomers may require some concentration by appropriate methods to ensure that sufficient material is available for detection of minor components (see below). 

A variety of melting points have been reported for [57]. Although the presence of geometric isomers has been su ested to explain these discrepancies, there is as yet no evidence to support this proposal. At least a partial explanation is to be found in the observation that 57 undergoes photodimeiization slowly in ordinary light and more rapidly when exposed to ultraviolet radiation. The monomer and dimer can be separated by vacuum sublimation. Infrared, ultraviolet, and proton magnetic resonance spectra of the dimer support the cyclobutane structure 58, although the stereochemistry has not been definitely established. 

Cooper [40] studied the TLC behavior of four trans-cis pairs of some geometric isomers, intermediates in the synthesis of some alicyclic analogues of mescaline on silica gel with different mobile phases without founding a definite correlation. Generally, the cii-isomers migrated faster than the trun -isomers. 

In keeping with the principles and definitions in Chapter 9, isomers are compounds with the same number of atoms of each kind per molecule, but they are different substances because of differences in molecular structure. There are three classes of isomers structural, geometric, and optical. 

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