Dextrorotatory, definition

Comparison of these results indicates identity of the two substances isolated independently by Orekhov and Ehrenstein, but Spiith and Kesztler have suggested that Pictet s nicoteine and Ehrenstein s base, consisted of impure Z-anatabine (p. 46). In this connection it may be noted that Ehrenstein s base was laevorotatory in acid solution, whereas Salts of anabasine are dextrorotatory. These authors have themselves isolated Z-anabasine from tobacco. The identity of synthetic 2-(3 -Pyridyl)piperidine with dZ-anabasine seems to have been definitely established. Anabasis aphylla is the source of the anabasine raanufac-... 

The term meso (Greek, middle ) was used to describe an achiral member of a set of diastereomers, some of which are chiral. The optically inactive isomer seemed to be in the middle between the dextrorotatory and levorotatory isomers. The definition just given ( an achiral compound with chirality centers ) is nearly as complete, and more easily applied, especially when you remember that chirality centers are usually asymmetric carbon atoms. 

The optical purity is usually, but not always, equal to enantiomer excess. In order for the two to be equal, it is necessary that there be no aggregation. It is possible, for example, that a homochiral or heterochiral dimer (see Glossary, Section 1.6, for definitions) would refract the circularly polarized light differently than the monomer (or each other). In 1968 [19] Krow and Hill showed that the specific rotation of (S)-2-ethyl-2-methylsuccinic acid (85% ee) varies markedly with concentration, and even changes from levorotatory to dextrorotatory upon dilution. In 1969 [20], Horeau followed up on Krow and Hill s observation, and showed that the optical purity (at constant concentration) and enantiomer excess of (5)-2-ethyl-2-methylsuccinic acid were unequal except when enantiomerically pure or completely racemic. This deviation from linearity is known as the Horeau effect, and its possible occurence should be remembered when determining enantiomeric purity by polarimetry. 

Definition Mixt. of calcium salts of the dextrorotatory and levorotatory isomers of pantothenic acid Empiricai ... 

While D-glyceraldehyde is dextrorotatory (by definition), other d sugars are not necessarily dextrorotatory. For example, D-erythrose and D-threose are actually levorotatory. 

The assignment of the rotational isomers to the two formulas which can be written for them has been a difficult stereochemical problem. At first, in all sugars of the D-series the more strongly dextrorotatory form was called the a-form. Later it was noticed that the ot-form in the projection formula invariably has the hydroxyl group on the same side as the hydroxyl group determining the D-series (i.e. to the right in the D-series to the left in the L-series). The reason for this is that each C atom contributes a definite amount to the optical rotation (Hudson s rule of superimposition). In the Haworth formulas, this means that, if the OH at C-1 is down, we have the a-form if the OH is up, the /S-form. 

---