Isomers, and configuration

Stereoisomers, on the other hand, are compounds with the same molecular formula, and the same sequence of covalently bonded atoms, but with a different spatial orientation. Two major classes of stereoisomers are recognized, conformational isomers and configurational isomers. 

There are two major types of stereoisomer conformational isomers and configurational isomers. Configurational isomers include optical isomers, geometrical isomers, enantiomers and diastereomers. 

Understanding the three-dimensional properties of organic molecules is an essential part of organic chemistry. The experiments in this chapter are designed to provide an introduction to stereoisomers. Such isomers have molecular skeletons that are identical, but they differ in the three-dimensional orientation of their atoms in space. The two broad subclasses of stereoisomers that are of importance in organic chemistry are conformational isomers and configurational isomers. 

In a subsequent activity we will break down the term configurational stereoisomers into finer categories, so it is critical that you are comfortable with the distinction between conformers, constitutional isomers, and configurational stereoisomers before you move on. 

The next sections describe briefly the nomenclature of configurational isomers, and how this stereochemistry can be handled by computer. 

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformationsIf they are not interconvertible, they are called configurations Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and are thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer . A number of methods have been used to determine conformations. These include X-ray and electron diffraction, IR, Raman, UV, NMR, and microwave spectra, photoelectron spectroscopy, supersonic molecular jet spectroscopy, and optical rotatory dispersion (ORD) and CD measurements. Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution. Conformations can be calculated by a method called molecular mechanics (p. 178). 

A survey of transition-metal polyazacycloalkane complexes in general, with data also for Cd and Hg species in particular, has been published.180 Structural, 3H and 13C NMR, UV/vis, and conductivity data of several transition-metal complexes, including Cd and Hg complexes, with derivatives of the 16-membered ligand l,9-dithia-5,13-diazacyclohexadecane have been compiled and compared. In particular, conversion between configurational isomers and exchange processes in solution have been discussed.181... 

The AG values for the rotation around the C3-N bond were obtained by the coalescence method and kinetic measurements of the equilibration of isolated and configurationally established cis-trans isomers of type 232. The barrier of rotation is considerably lower than in ordinary immonium cations, e.g. 233 AG is decreased by electron-withdrawing substituents at the nitrogen atom, whilst at the three-ring the opposite effect is observed. 

Infrared and Raman spectroscopic studies of XNSO, where X=F, Cl, Br, I, indicate that these compounds have a cis configuration in the gas phase89). This result has also been confirmed by electron diffraction study of CINSO90. Once again, an additional important electronic factor is responsible for the greater stability of the cis isomer and this will be discussed in a later section. 

Silver nitrate may be incorporated in the adsorbent slurry (25 g l-1) giving a final concentration of about 5% in the dry plate. The silver ions bind reversibly with the double bonds in the unsaturated compounds, resulting in selective retardation, and the lipids are separated according to the number and configuration (cis or trans) of their double bonds. This technique is extremely useful in fatty acids, mono-, di- and particularly triacylglycerol analyses when even positional isomers may be resolved. Borate ions may also be incorporated in the silica gel and these plates are used to separate compounds with adjacent free hydroxyl groups. 

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