Tartaric acid, stereoisomers physical properties

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. 

Some physical properties of the three stereoisomers are listed in Table 9.3. The (+)- and (-j-tartaric acids have identical melting points, solubilities, and densities but differ 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 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. 

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

A molecule that cannot be superimposed on its mirror image is said to be chiral. When a carbon atom is bonded to four different atoms or groups of atoms, it is called a chiral carbon. Two stereoisomers that are nonsuperimposable mirror images of one another are a pair of enantiomers. As mentioned in Section 17.3, the chemical and physical properties of enantiomers are identical, with the exception that they rotate plane-polarized light to the same degree but in opposite directions. This is exactly the phenomenon that Pasteur observed with the mirror-image crystals of tartaric acid salts. 

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. 

Table 5.1 Physical Properties of the Stereoisomers of Tartaric Acid...

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