Tartaric acid meso form

Diglycollic acid, 39B, 4 Acetylenedicarboxylic acid dihydrate, 11, D-Tartaric acid, 13, 452 31B, 5 45B, 5 meso-Tartaric acid (triclinic form), 32B, Crotonamide, 40B, 4 Diacetamide (trans-cis form), 41B, 6 Diacetamide (trans-trans form), 41B, 6... 

Tartaric acid [526-83-0] (2,3-dihydroxybutanedioic acid, 2,3-dihydroxysuccinic acid), C H O, is a dihydroxy dicarboxyhc acid with two chiral centers. It exists as the dextro- and levorotatory acid the meso form (which is inactive owing to internal compensation), and the racemic mixture (which is commonly known as racemic acid). The commercial product in the United States is the natural, dextrorotatory form, (R-R, R )-tartaric acid (L(+)-tartaric acid) [87-69-4]. This enantiomer occurs in grapes as its acid potassium salt (cream of tartar). In the fermentation of wine (qv), this salt forms deposits in the vats free crystallized tartaric acid was first obtained from such fermentation residues by Scheele in 1769. 

The calcium salt of the principal product, d/-tartaric acid, crystallizes with four molecules of water, while the secondary product, meso-tartaric acid, forms a calcium salt which crystallizes with three molecules of water. The amount of sulfuric acid actually required may readily be calculated from the percentage of calcium found on analysis in the regular way or it may be estimated by igniting a sample, and titrating the residue with standard acid. 

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. 

Let s return for a last look at Pasteur s pioneering work. Pasteur took an optically inactive tartaric acid salt and found that he could crystallize from it two optically active forms having what we would now call the 2R,3R and 2S,3S configurations. But what was the optically inactive form he started with It couldn t have been meso-tartaric acid, because meso-tartaric acid is a different chemical compound and can t interconvert with the two chiral enantiomers without breaking and re-forming chemical bonds. 

Although four is the maximum possible number of isomers when the compound has two chiral centers (chiral compounds without a chiral carbon, or with one chiral carbon and another type of chiral center, also follow the rules described here), some compounds have fewer. When the three groups on one chiral atom are the same as those on the other, one of the isomers (called a meso form) has a plane of symmetry, and hence is optically inactive, even though it has two chiral carbons. Tartaric acid is a typical case. There are only three isomers of tartaric acid a pair of enantiomers and an inactive meso form. For compounds that have two chiral atoms, meso forms are found only where the four groups on one of the chiral atoms are the same as those on the other chiral atom. 

In general, the maximum number of optically active isomers is given by 2n where n represents the number of asymmetric carbon atoms. Thus for a compound where n = 1, as in lactic acid, there would be two stereoisomers, one the dextro and the other the laevo. For a compound with two asymmetric carbon atoms, there would be 22 = 4 stereoisomers. But if the two asymmetric carbon atoms carry exactly identical groups, as in tartaric acid, the number would be fewer than four and we know that it exists in three forms, the d the 1 and the meso. 

Now, let us consider another similar molecule, tartaric acid, where there are two chiral carbons. In tartaric acid, four isomeric forms are theoretically expected (2 = 4). However, because one half of the tartaric acid molecule is a mirror image of the other half, we get a meso structure. This means this compound and its mirror image are superimposable, i.e. they are the same compound. Thus, instead of four, we obtain only three stereoisomers for tartaric acid. 

The plane of symmetry bisecting varenicline means that it is meso, rather than chiral. Nevertheless, in combination with a chiral additive such as tartaric acid or camphorsulfonic add (CSA), it may still be possible to achieve asymmetric induction via aminocatalysis. In such a scenario, a nucleophilic enamine formed from varenicline might be desymmetrized by selective protonation of one of the two heteroaromatic nitrogens. Alternatively, the enamine might effect asymmetric induction merely by virtue of the chiral environment resulting from the presence of either one or two chiral camphorsulfonate counterions. However, varenicline... 

If you find yourself confused about the d,l, and meso forms of tartaric acid, a simple analogy may help keep matters straight. Consider three sets of shoes. A right shoe beside a left shoe is a meso combination with a plane of symmetry. A left shoe next to a left shoe is not identical with, but is... 

Tartaric acid can be obtained in four forms dextro-, laevo-, meso- and the mixed-isomer equilibrium, or racemic, form. Commercially, it is usually available as cferfro-tartaric acid. This acid has a sharper flavour than citric and it may therefore be used at a slightly lower level to give equivalent palate acidity. (Palate acidity is a purely subjective evaluation and it is generally agreed that a number of acids can be used at concentrations different from those indicated by their chemical acid equivalent, see Table 5.3.)... 

The tartaric acids incorporate two equivalently substituted stereogenic centers. (+)-Tartaric acid, as noted in the text, is the 2R,3R stereoisomer. There will be two additional stereoisomers, the enantiomeric ( )-tartaric acid (2S,3S) and an optically inactive meso form. 

Compounds with n different stereogenic centers may exist in a maximum of 2" forms. Of these, there will be 272 pairs of enantiomers. Compounds from different enantiomeric pairs are diastereomers. If two (or more) of the stereogenic centers are identical, certain isomers will be achiral. A meso form is an optically inactive, achiral form of a compound with stereogenic centers. Tartaric acid, which has two identical stereogenic centers, exists in three forms the R,R and S,S forms (a pair of enantiomers) and the achiral meso form. 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

With your models, construct a pair of enantiomers. From each of the models, remove the same common element (e.g., the white component) and the connecting links (bonds). Reconnect the two central carbons by a bond. What you have constructed is the meso form of a molecule, such as meso-tartaric acid. How many chiral carbons are there in this compound (5a) ... 

Thus if we let these three models represent different isomers of tartaric acid, we find that there are three stereoisomers for tartaric acid—a meso form and a pair of enantiomers. 

A meso form with any one of the enantiomers of tartaric acid represents a pair of diastereomers. Although it may not be true for this compound because of the meso form, in general, if you have n stereocenters, there are 2n stereoisomers possible (see Post-Lab question no. 3). 

Use the Fischer projection of meso-tartaric acid and carry out even and odd exchanges of the groups follow these exchanges with a model. Does an odd exchange lead to an enantiomer, a diastereomer, or to a system identical to the meso form (15a) Does an even exchange lead to an enantiomer, a diastereomer, or to a system identical to the meso form (15b) ... 

The racemic mixture is different still. Though a mixture of enantiomers, J racemates usually act as though they were pure compounds, different fromi either enantiomer. Thus, the physical properties of racemic tartaric acid differ from those of the two enantiomers and ftrom those of the meso form. 

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). 

The 2R,3S and 2S,3R structures are identical because the molecule has a plane of symmetry and is therefore achiral. The symmetry plane cuts through the C2-C3 bond, making one half of the molecule a mirror image of the other half (Figure 9.11). Because of the [ lane of symmetry, the molecule is achiral, despite the fact that it has two chirality centers. Compounds that are achiral, yet contain chirality centers, are called meso (me-zo) compounds. Thus, tartaric acid exists in three stereoisomeric forms two enantiomers and one meso form. 

It is this fourth unresolvable inactive tartaric acid which gives to tartaric acid its especial interest and importance in connection with the theory of stereo-isomerism. This acid, like the other three, has been fully explained in accordance with the tetra-hedral theory of van t Hoff and LeBel. The explanation rests upon the fact that there is a second asymmetric carbon atom in tartaric acid. We may construct, by models, or, by drawings, space-formulas for tartaric acid. According to the tetra-hedral theory, the dextro, levo and racemic inactive forms will be as follows, analogous to the corresponding formulas for the three lactic acids. The meso-tartaric acid is represented by the third drawing. 

Thus from a study of the crystalline sodium-ammonium salt of racemic acid and of dextro tartaric acid Pasteur showed, conclusively, the relationship of these two acids to each other and also discovered the existence of a third isomer optically active but of opposite direction to the ordinary tartaric acid already known. Racemic acid, therefore, is optically inactive because it consists of equal molecules of the ordinary dextro tartaric acid and the newly discovered levo tartaric acid. Also racemic acid can be resolved into its optically isomeric components by mechanically separating the two forms of crystals of the sodium-ammonium salt. The two active forms of tartaric acid, when mixed in equal molecular amounts, yield the inactive or racemic acid. Later, Pasteur prepared the fourth variety of tartaric acid, viz., meso-tartaric acid, by heating the cinchonine salt of dextro tartaric acid. This new acid proved to be inactive like racemic acid, but, unlike it, was unable to be resolved into optically active components. Its relation to the other three forms of tartaric acid was unexplained by Pasteur. 

German name, Trauben-saure, is derived from the word for grapes. It is probable that it does not exist in grapes as racemic acid but that it is formed from the dextro acid as this transformation can easily be effected by the action of acids or even by water alone. When tartaric acid is prepared synthetically from succinic acid, from glyoxal, or from malic, maleic or fumaric acids either racemic acid or meso-tartaric acid is always formed. That is, synthetic reactions result in the formation of an inactive form. The methods of splitting racemic acid into its optically active components has been fully discussed. The sodium-ammonium racemate is the only salt that is of importance. This has been spoken of in connection with the method of splitting racemic acid into its components.. Like the free acid this salt exists, in dilute solution, as equal molecular parts of the dextro and levo forms. Only in concentrated solution does it exist as the racemate itself. 

This acid, the inactive by intra-molecular compensation and un-resolvable into optically active components, was first obtained by Pasteur by heating the cinchonine salt of dextro tartaric acid, to 170 . It may also be prepared by boiling the dextro tartaric acid with an excess of hydrochloric acid, or with sodium hydroxide. Also by long boiling with water alone or by heating with a small amount of water to 165°. When di-brom succinic acid is treated with silver hydroxide, or when malic acid is oxidized, in the presence of water, both meso-tartaric acid and racemic acid are formed. When meso-tartaric acid is heated to 200° it is partly converted into racemic acid. Meso-tartaric acid crystallizes in rectangular plates with one molecule of water. The water free acid melts at i40°-i45°. 

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