Tartaric acid, optical rotation

Chemists have known about racemates since Pasteur, at 26 years of age, told the Paris Academy of Sciences how he used tweezers to separate two types of crystals of salts of tartaric acid, which rotate polarized light clockwise (d, dextro) or counterclockwise (l, levo). Unfortunately, this correspondence does not always hold true. In fact, the magnitude and even the direction of optical rotation are complicated functions of the electronic structure surrounding the chiral center. For example, the common enantiomer of the sugar fructose is termed d because of the stereochemical orientation about the chiral atom. But this enantiomer actually rotates the plane of polarization to the left, and its mirror image, L-fructose, rotates the plane of polarization to the right. 

Occasionally an optically inactive sample of tartaric acid was obtained Pasteur noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two mirror image crystal forms With microscope and tweezers Pasteur carefully sep arated the two He found that one kind of crystal (m aqueous solution) was dextrorota tory whereas the mirror image crystals rotated the plane of polarized light an equal amount but were levorotatory... 

Physical Properties. When crystaUized from aqueous solutions above 5°C, natural (R-R, R )-tartaric acid is obtained in the anhydrous form. Below 5°C, tartaric acid forms a monohydrate which is unstable at room temperature. The optical rotation of an aqueous solution varies with concentration. It is stable in air and racemizes with great ease on heating. Some of the physical properties of (R-R, R )-tartaric acid are Hsted in Table 7. 

The answer is that Pasteur started with a 50 50 mixture of the two chiral tartaric acid enantiomers. Such a mixture is called a racemic (ray-see-mi c) mixture, or racemate, and is denoted either by the symbol ( ) or the prefix cl,I to indicate an equal mixture of dextrorotatory and levorotatory forms. Racemic mixtures show no optical rotation because the (+) rotation from one enantiomer exactly cancels the (-) rotation from the other. Through luck, Pasteur was able to separate, or resolve, racemic tartaric acid into its (-f) and (-) enantiomers. Unfortunately, the fractional crystallization technique he used doesn t work for most racemic mixtures, so other methods are needed. 

Let us now apply the technique in some specific cases. The existence of optical activity of several compounds in solution has been explained due to the presence of several active forms of the compound in equilibrium with each other and various assumptions about the forms were also put forward. The equilibrium between the different forms depended on external conditions. But a definite explanation was put forward in 1930 about tartaric acid and it was said that the molecule exists in the following three conformations and each of which makes a certain contribution to the rotation observed. 

Although pure compounds are always optically active if they are composed of chiral molecules, mixtures of equal amounts of enantiomers are optically inactive since the equal and opposite rotations cancel. Such mixtures are called racemic mixtures6 or racematesP Their properties are not always the same as those of the individual enantiomers. The properties in the gaseous or liquid state or in solution usually are the same, since such a mixture is nearly ideal, but properties involving the solid state,8 such as melting points, solubilities, and heats of fusion, are often different. Thus racemic tartaric acid has a melting point of 204-206°C and a solubility in water at 20°C of 206 g/liter, while for the ( + ) or the ( —)... 

Section II, 4a. Optical activity and O.K.D. of (+)-tartaric acid salts of poly-2-vinyl pyridine having different stereoregularity have been determined the optical rotation between 578 m/r and 365 mfi of the salt of atactic poly-2-vinyl-pyridine is lower than that of the salt of the isotactic poly-vinyl-pyridine [R. C. Schulz and J. Schwaab Makromol. Chem. 85, 297 (1965)]. 

Computational efforts to describe the conformational preferences of (R,R)-tartaric acid and its derivatives - mainly for isolated molecules - were made recently [18-25]. The conformations of these molecules also attracted attention from experimental chemists [22-40]. (/ ,/ [-tartaric acid and its dimethyl diester were observed in crystals, in conformations with extended carbon chain and planar a-hydroxy-carboxylic moieties (T.v.v and Tas for the acid and the ester, respectively) [25-28] (see Figure 2). The predominance ofthe T-structure was also shown by studies of optical rotation [31], vibrational circular dichroism (VCD) [23], Raman optical activity [32, 35], and nuclear magnetic resonance (NMR) [22, 33, 34]. The results of ab-initio and semiempirical calculations indicated that for the isolated molecules the Tsv and T as conformers were those of lowest energy [22, 21, 23, 25]. It should be noted, however, that early interpretations of NMR and VCD studies indicated that for the dimethyl diester of (/ ,/ [-tartaric acid the G+ conformation is favored [36-38]. 

An understanding of the three-dimensional structures of molecules has played an important part in the development of organic chemistry. The first experiments of importance to this area were reported in 1815 by the French physicist J. B. Biot, who discovered that certain organic compounds, such as turpentine, sugar, camphor, and tartaric acid, were optically active that is, solutions of these compounds rotated the plane of polarisation of plane-polarized light. Of course, the chemists of this period had no idea of what caused a compound to be optically active because atomic theory was just being developed and the concepts of valence and stereochemistry would not be discovered until far in the future. 

M] or [rh] Molecular rotation, defined as [a] x MW/100. Specific rotation corrected for differences in MW. The symbol [M] and the term molecular rotation are now deemed incorrect, and the term molar rotation denoted by [d ] is preferred. meso- Denotes an internally compensated diastereoisomer of a chiral compound having an even number of chiral centres, e.g., me o-tartaric acid. Formally defined as an achiral member of a set of diastereomers that also contains chiral members, mutarotation Phenomenon shown by some substances, especially sugars, in which the optical activity changes with time. A correct presentation is, e.g., [a]n ° + 20.3 -101.2 (2h)(c, 1.2 in HjO). 

The two glyceraldehyde isomers of 4-13 are identical in all physical properties except that they rotate the plane of polarized light in opposite directions and form enantiomorphous crystals. When more than one asymmetric center is present in a low-molecular-weight species, however, stereoisomers are formed which are not mirror images of each other and which may differ in many physical properties. An example of a compound with two asymmetric carbons (a diastereomer) is tartaric acid, 4-16, which can exist in two optically active forms (d and L, mp 170 C), an optically inactive form (meso, mp 140 C), and as an optically inactive mixture (dl racemic, mp 206°C). 

Diastereomers are not miiror images of each other, and as such, their physical properties are different, including optical rotation. Figure 5.12 compares the physical properties of the three stereoisomers of tartaric acid, consisting of a meso compound that is a diastereomer of a pair of enantiomers. 

In 1820 Sir John Herschel, in considering the question of the different optical rotation of crystalline substances, suggested that it might be connected with an unsymmetrical form of crystallization. Later, Pasteur in 1848 while studying the salts of tartaric acid recalled this suggestion of Herschel and also a statement by Mitscherlich to the effect that the crystalline form of ordinary tartaric acid which is dextro rotatory is identical with that of racemic acid which is inactive. At that time the tw o tartaric acids just mentioned were the only ones known. 

Levo tartaric acid, the optical isomer of dextro tartaric acid, was discovered, as already stated, by Pasteur in 1848. It has the same solubility and melting point as the dextro acid. It crystallizes without water of crystallization in the form enantiomorphic to the dextro acid. Its optical rotation is the same in amount but opposite in direction to the dextro acid. When mixed in equal molecular amount with dextro tartaric acid it yields racemic acid. Its synthetic reactions have been considered. It has no common uses. 

The other example of note is the optically active tartaric acids (Fig. 11). Tartaric add contains two asymmetric carbon atoms. The dextro- and levo-tartaric adds are enantiomers. However, a third isomer is possible in which the two rotations due to the two asymmetric carbon atoms compensate and the molecule is optically inactive as a whole. That is, the molecule contains a plane of symmetry. This form, meso-tartaric acid, was also discovered by Pasteur, differs from the two optically active tartaric adds in being internally compensated, and is not resolvable. Thus, the tetrahedral model for carbon and the asymmetric carbon atom proposed by van t Hoff were able to completely explain the observations of Pasteur relating to the three isomers of tartaric add. 

It is interesting that Nef assigned the correct configurations to the D-galactometasaccharinic acids, as well as to the D-glucometasaccharinic acids, on the basis of analogies between the optical rotations of n-tartaric acid, the 2,4-dihydroxyglutaric acids (obtained by oxidation of the five-carbon metasaccharinic acids), and the 2,3,5-trihydroxyadipic acids (obtained by oxidation of the six-carbon metasaccharinic acids). 

---