Importance of Stereochemistry

Stereo chemistry Importance 

Stereochemistry is an important aspect of carbon compounds. It is prevalent in the whole universe. The human body is structurally chiral with the heart lying to the left and the liver to the right in the body. Many plants show chirality which help them to wind around supporting structures. Most of the molecules found in animals and plants are chiral and usually only one form of chiral molecules occur in a species. All the naturally occurring amino acids have L configuration. The synthesized D-proteins made from D aminoacids are some what resistant to break down by protein digesting enzymes.All naturally occurring sugars are of D-configuration. The enzyme, yeast can specifically ferment D-glucose and not its L-form.

Stereo chemistry also plays an important role in deciding the physiological properties of compounds. (-) Nicotine is much more toxic than (+) Nicotine. (+) Adrenaline is very active in constriction of blood vessels than (-) Adrenaline.

Chirality is crucial for the effect of drugs. In many cases only one enantiomer is found to have the desired effect while the other isomer may be totally inactive or has an opposite effect. (-) Thyroxine, an amino acid of thyroid gland speeds up metabolic processes and causes nervousness and loss of weight. But (+) Thyroxin has none of these effects but is used to lower the cholesterol levels.

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Resolution of racemic mixture (dextro and laevo)

Separation of Dextro and Laevo components


The synthesis of optically active compounds in the laboratory usually results in racemic mixture. The d and l forms can be separated from the racemic mixture. The separation of a racemic mixture in to dextro and laevo components is termed resolution. Due to identical physical properties of optical isomers their separation cannot be effected by simple physical methods. Usual methods which have been used for resolving racemic compounds are Mechanical Separation, Biochemical separation and by means of salt formation.


Methods of separation of a racemic mixture in to dextro and laevo components


1. Mechanical Separation

When the two varieties of isomers form well defined crystals they can be separated by hand picking. The crystals of Sodium ammonium racemate can be separated by this method.

2.Biochemical Separation

In this method certain micro organism such as mould, bacteria or fungi when allowed grow in a solution of racemic mixture destroy one of the optical isomers at a much quicker rate than the other due to selective assimilation. When penicillium glaucum is allowed to grow in a solution of ammonium tartrate, it destroys the dextro isomer leaving the laevo isomer.

3. By means of Salt Formation

This is an effective method for resolution. Here the isomers of racemic mixture are converted to their salts with an optically active acid or base. The two salts obtained often differ in their solubilities  and can be sseparated by fractional crystallisation. The salts on treatment with acid or base regenerate the optically active reagent.


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Example of optical Isomer : Tartaric acid

Optical Isomers of Tartaric acid (HOOC-CHOH-CHOH-COOH)


Two chiral carbon atoms are present in tartaric acid. The difference in spatial arrangements of various groups in tartaric acid results in d-tartaric acid, l-tartaric acid and an active form known as meso form. In addition to these, racemic modification, another inactive form also exist.

Dextro tartaric acid rotates the plane of polarization of light to right. The rotation due to upper half is strengthened by the rotation of lower half. Laevo tartaric acid is a mirror image of d-form, which rotate the palne of polarization to left.

Racemic tartaric acid is an equimolar mixture of d and l -isomers. It is optically inactive due to external compensation, it can be resolved into d and l forms.

Meso tartaric acid is an inactive variety and the rotation of upper half is compensated by the rotation due to lower half. It cannot be resolved into active constituents. It is therefore inactive due to internal compensation. Mesotartaric acid possess a plane of symmetry.


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Optical Isomers Example: lactic acid

Optical Isomers of lactic acid


In lactic acid CH₃ - CHOH - COOH, second carbon is chiral.

There are two optically active isomers of Lactic acid: d-lactic acid and l-lactic acid. In addition to these optically active varieties there is an optically inactive form which results when dextro and laevo (levo) varieties are present in equal quantities. It is called racemic mixture or (+-) lactic acid.

The racemic mixture is 50:50 mixture of d and l -isomers and hence have zero optical rotation as the rotation due to one enantiomer cancels the rotation due to the other. That is racemic mixture is optically inactive due to external compensation. The process of conversion of an enantiomer in to a racemic mixture is known as racemisation. Racemisation can be brought about by the action of heat, light and chemical reagent.

Dextro rotatory lactic acid may be obtained from meat extract and is known as sarcolactic acid. With muscular activity glycogen present in muscles break down to sarcolactic acid. During rest sarcolactic acid is converted back to glycogen.

Leavo rotatory lactic acid may be obtained by the fermentation of sucroseby Bacillus Acidi laevolactiti. Ordinary lactic acid in sour milk or manufactured by fermentation or by synthetic method is racemic mixture.