Tartaric acid nomenclature

Glucose on oxidation gives saccharic acid and on the basis of D-glucose structure, XIV would be D and XV, L on the account of the bottom asymmetric carbon atom. But if XV is rotated in the plane of paper through 180° if becomes exactly identical with XIV. Dispute like this and tartaric acid have raised great interest in improving the system of nomenclature. 

RGURE 1 Pasteur separated crystals of two stereoisomers of tartaric acid and showed that solutions of the separated forms rotated polarized light to the same extent but in opposite directions. These dextrorotatory and levorotatory forms were later shown to be the (R,R) and (S,S) isomers represented here. The RS system of nomenclature is explained in the text. 

The nomenclature of optically active organic molecules has been revised since their absolute configuration has been determinable via X-ray diffraction (Chapter 6). The equivalent terms are (d)-tartaric acid = (-l-)-tartaric acid = (2R,3R)-tartaric acid (/)-tartaric acid = (—)-tartaric acid = (2S,3S)-tartaric acid mesotartaric acid = (2R,3S) tartaric acid = (2S,3R)-tartaric acid. 

DL-Nomenclature, although frequently used, is ambiguous when applied to Tartaric acid. The coml. product is normally the natural (2i ,3i )-isomer. Some metal salts have separate entries. Log P -3.27 (calc). 

In general, compounds which contain asymmetric carbon atoms rotate the plane of polarization of plane-polarized light. For this reason they are said to be optically active. When the molecular symmetry is such that the optical activity of one portion of the molecule is cancelled by that of the second portion of the molecule, the compounds are said to be internally compensated and are called meso compounds. The tartaric acid with the formula (X) is such a compound and has been known as the meso-tartaric acid. The tartaric acids identified as (VIII) and (IX) have been known as d-tartaric acid and Z-tartaric acid because of the sign of their optical rotations (dextro and levo, respectively). (The nomenclature of these acids is discussed later in this chapter.) The compounds (VIII) and (IX) are non-superimposable mirror images, called enantiomorphs. The existence of such pairs of asymmetric isomers is the fundamental basis of optical activity. The asymmetry may be in either the molecular structure or the crystal structure. Asymmetric carbon atoms are not always present in optically active molecules. 

There are two naturally occurring aldotetroses, D-erythrose and o-threose (16.49 and 16.50), and one ketotetrose, o-erythrulose (16.51). Erythrose and threose can be distinguished by nitric acid oxidation (notice the less usual but water-soluble oxidant, selective for the aldehyde and primary alcohol over the secondary alcohols) threose leads to a chiral diacid, while the diacid from erythrose is a meso-compound and achiral. Although, in a formal nomenclature sense, these diacids would be threaric acid and erythraric acid, they always go by their more common name of tartaric acid. 

In the days of alchemy and the phlogiston theory, no system of nomenclature that would be considered logical ia the 1990s was possible. Names were not based on composition, but on historical association, eg, Glauber s salt for sodium sulfate decahydrate and Epsom salt for magnesium sulfate physical characteristics, eg, spirit of wiae for ethanol, oil of vitriol for sulfuric acid, butter of antimony for antimony trichloride, Hver of sulfur for potassium sulfide, and cream of tartar for potassium hydrogen tartrate or physiological behavior, eg, caustic soda for sodium hydroxide. Some of these common or trivial names persist, especially ia the nonchemical Hterature. Such names were a necessity at the time they were iatroduced because the concept of molecular stmcture had not been developed, and even elemental composition was incomplete or iadeterminate for many substances. 

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