Stereoisomers of tartaric acid

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Since chirality is a property of a molecule as a whole, the specific juxtaposition of two or more stereogenic centers in a molecule may result in an achiral molecule. For example, there are three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral and optically active but the third is not. 

Table 9,3 Some Properties of the Stereoisomers of Tartaric Acid...

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 stereochemistry of a solute is also important, as is shown by the differences between the partial molar volumes and compressibilities of stereoisomers of tartaric acid and its potassium salts in water (Mathieson and Conway, 1975). 

The relative configurations of the stereoisomers of tartaric acid were established by the following syntheses ... 

This stereoisomer of tartaric acid is found in grapes, and its salt, potassium hydrogen tartrate, can precipitate out as crystals at the bottom of bottles of wine. It has two stereogenic centres, so you d expect 22 - 4 stereoisomers two diastereoisomers, each a pair of enantiomers. 

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. 

The physical properties of the three stereoisomers of tartaric acid... 

FIGURE 14.2. The stereoisomers of tartaric acid molecules (a) and (b) are enantiomers, related by miror symmetry. Molecules (c) and (d) are equivalent, termed mesocompounds, because they have two identical chiral centers. Thus there are onl three stereoisomers [(a), (b), and (c) = (d)]. 

Some physical properties of the three stereoisomers of tartaric acid and of the racemic mixture are listed in Table 9.3. As indicated, the ( + )- and (-)-tartaric acids have identical melting points, solubilities, and densities. They differ only in the sign of their rotation of plane-polarized light. The meso isomer, by contrast, is diastereomeric with the (+) and (-) forms. As such, it has no mirror-image relationship to ( + )- and (-)-tartaric acids, is a different compound altogether, and has different physical properties. 

The meso stereoisomer of tartaric acid is achiral, and possesses two self-cancelling stereogenic centres of opposite configuration. 

Draw all the possible stereoisomers of tartaric acid, H(X)C—CHOH— CHOH—COOH. 

The physical properties of the three stereoisomers of tartaric acid are listed in Table 5.1. The meso compound and either one of the enantiomers are diastereomers. Notice that the physical properties of the enantiomers are the same, whereas the physical properties of the diastereomers are different. Also notice that the physical properties of the racemic mixture differ from the physical properties of the enantiomers. 

The figure given below shows a stereoisomer of tartaric acid. Notice that this compound has two chiral (stereogenic) centers. But, there is a plane of symmetry and thus the molecule itself is achiral and optically inactive. Such compounds that contain one or more stereogenic centers, but are achiral, are called meso compounds. Hence, having a stereogenic center or chiral carbon does not always lead to chirality of the entire molecule. 

Figure 17 The stereoisomers of tartaric acid, (a) Tartaric acid structural isomer with atom labels 1 through 8 (only atoms attached to stereocenters are labeled), (b) Application of the configuration group (1)(2), (12) on the four possible stereoisomers. The second and third stereoisomers are identical, (c) The three resulting stereoisomers, a meso form and a dl pair.

The relative configurations of the stereoisomers of tartaric acid were established by the following syntheses D-(-t-)-glyceraldehyde--------- diastereomers A andB (separated)... 

When a molecule has two chiral centers that are identically substituted, the number of stereoisomers is reduced from four to three, as is well known for the case of tartaric acid. The three stereoisomers are the d and l forms (enantiomers) and the diastereomeric meso form. The meso form is superimposable on its mirror image, since it has a plane of symmetry and is achiral and optically inactive. The three possible stereoisomers of tartaric acid are shown below ... 

Stereoisomers of tartaric acid. One pair of enantiomers and one meso compound. The presence of an internal plane of symmetry indicates that the molecule is achiral. 

Another way to verify that (c) is achiral is to see that it has a plane of symmetry that bisects the molecule in such a way that the top half is the reflection of the bottom half. Thus, even though (c) has two stereocenters, it is achiral. The stereoisomer of tartaric acid represented by (c) or (d) is called a meso compound, defined as an achiral compound that contains two or more stereocenters. 

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