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ISOMERISM OF TARTARIC ACID

Isomerism of Tartaric Acid

Let us now proceed to discuss the optical isomerism of tartaric acid which contains two similar asymmetric carbon atoms, in detail.

The two asymmetric carbon atoms in tartaric acid,

Its molecule can be represented by space models of two tetrahedra joined corner to corner but for the sake of convenience we will use the planed formulas. The end groups being identical, in all four arrangements are possible according as one or both H groups and OH groups are on the left or on the right.

Out of these, formula IV when rotated through 180o in the plane of the paper becomes identical with formula III. Therefore, for tartaric acid we can have only three different arrangements, viz. I, II and III.

Now, if the force which rotates the plane of polarised light be directed from H to OH,

1. structure I will rotate the plane of polarised light to the right and will represent (+)-tartaric  acid;
2. structure II will rotate the plane of polarised light to the left and will represent (-)-tartaric acid; and
3. structure III will represent optically inactive tartaric acid, since the rotatory power of the upper half    of the molecules is balanced by that of the lower half.

It may also be noted that formulas I and II are mirror images of each other and hence represent (+)- and (–)-isomers. Formula III, however, has a plane of symmetry (dotted line) and hence represents and inactive isomer of tartaric acid.

In actual practice, four tartaric acids are known :

1. (+)-Tartaric acid ;   
2. (–)-Tartaric acid ;
3. Inactive or i-tartaric acid; this is also known as meso-tartaric acid or m-tartaric acid ;
4. (±) Tartaric acid ; this form of tartaric acid being a mixture of equal amounts of (+)- and  (–)-isomers

Number of optical isomers

As it has been discussed above, a compound containing two dissimilar carbon atoms can exist in four optically active forms. Reasoning in the same fashion, we will find that a compound containing three such asymmetric carbon atoms can exist in eight different configurations which represent optical isomers. Thus in general, the number of stereoisomers for a compound with n distinct (different) asymmetric carbon atoms in 2n.

When an organic compound contains two similar asymmetric carbon atoms in its molecule, abdC-Cabd, the number of optically active isomers would be less than 2n. Thus, tartaric acid [HO2CCH(OH)CH(OH)CO2H] has two similar asymmetric carbon atoms and exists in only three forms, of which two are optically active and one is optically inactive (meso form). Thus, the general guidelines for predicting the number of optical isomers is given as under.

1. When the molecule is unsymmetrical:

Number of d and l isomers (a) = 2n.

Number of meso forms (m) = 0.

Total number of optical isomers (a + m) = 2n.

where n is the number of chiral carbon atom(s). Common example is CH3CH(Br)CH(Br)COOH.

2. When the molecule is symmetrical and has even number of chiral carbon atoms:

Number of d and l isomers (a) = 2n-1.

3. When the molecule is symmetrical and has an odd number of chiral carbon atoms:

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