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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Optical Isomerism

Optical Isomers Definition

Optical isomerism arises due to chirality or asymmetry of the molecule. Optical isomers resemble one another in chemical properties and most of their physical properties but differ in their behavior towards polarized light. The isomer, which rotate the plane of polarized light clockwise is called dextro rotatory isomer (d - isomer) and the one which rotate the plane of polarized light anticlockwise is called laevo rotatory isomer (l - Isomer).

The necessary condition for a molecule to be optically active is asymmetry or chirality of the molecule. Chirality is not just the presence of the asymmetric carbon atom but asymmetry of the molecule as a whole. Most of the chiral molecule contains at least one asymmetric carbon atom (Chiral Carbon atom). Still, there are some organic molecules which exhibit optical isomerism with out having chiral carbon (example: Substituted biphenyls). Some of the organic molecules are optically inactive even though they contain chiral carbon. This is due to internal compensation.

Examples of Optical Isomerism

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

Optical Isomers of lactic acid

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.

Optical isomer 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.

Preparation of Sulphuric Acid (H2SO4)

Production of Sulphuric Acid (H2SO4)

Sulphuric acid is an important chemical used in industry. It is also known as 'King of chemicals'. Sulfuric acid is manufactured by contact process. Contact process Involves the following steps.

Preparation of Sulphurdioxide (SO2)

SO2 is prepared by burning sulphur or sulphide ore in excess of air. It is done in a sulphur burner.

S + O2  ----------->  So2

So2 produced is purified by passing it through

1. Dust precipitator (which removes dust from gas)

2. Water scrubber (which removes soluble impurities)

3. Drying tower (which removes moisture)

4. Arsenic purifier (which removes arsenic impurities)

Oxidation of SO2 to SO3

Purified SO2 gas coming out from arsenic purifier is preheated and admitted to catalytic converter filled with catalyst V2O5. In catalytic chamber the following reversible reaction take place and is known as Contact process.

2SO2 + O2  <========>  2SO3  

According to Le-chatelier's principle, to obtain maximum yield of SO3, low temperature, high pressure and excess of oxygen are required. The optimum condition for the above reaction are
1. a temperature of 720 K
2. a pressure of 2 atm
3. high concentration of reactant
4. Vanadium Pentoxide (V2O5) as catalyst

Conversion of SO3 into Sulfuric acid H2SO4

The SO3 from catalytic converter is absorbed in about 98% H2SO4 resulting in oleum (H2S2O7).

SO3 + H2SO4  ----------->  H2S2O7

SO3 is not directly absorbed in water because it results in mist formation and further absorption become difficult. Oleum is then diluted with water to get Sulphuric acid (H2SO4) of desired concentration.

H2S2O7  +  H2O  ------------>  2H2SO4

Halides of Sulphur

Sulphur hexaflouride (SF6) and Sulfur tetrafluoride (SF4)

Sulphur forms a number of halides in which the oxidation state of sulphur are +1, +2, +4 and +6. The well known halides are

S2X2 (X=F,Cl,Br or I) = oxidation state of S is +1

SX2 (X=F or Cl) = oxidation state of S is +2

Sulphur hexaflouride (SF6)

The oxidation state of Sulphur (s) in Sulphur hexaflouride (SF6) is +6.
Sulphur hexaflouride (SF6) is prepared by direct combination of Sulphur (S) and Flourine (F).

1/8 S8  +  3F2  ----------->  SF6

Sulphur hexaflouride (SF6) has octahedral shape. Sulphur atom is Sp3d2 hybridised.

Sulfur tetrafluoride (SF4)

Sulfur tetrafluoride (SF4) is prepared by fluorination of SCl2 with NaF

3SCl2  +  4 NaF  ------------>  S2Cl2  +  SF4  +  4 NaCl

Sulphur atom is  Sp3d hybridized in SF4 and possess triagonal bipyramidal geometry with one corner is occupied by a lone pair of electrons.

Sulfur dichloride (SCl2)

Sulfur dichloride (SCl2) is formed by saturating S2l2 with chlorine at ordinary temperature.

S2Cl2  +  Cl2  ------------->  2SCl2

In Sulfur dichloride (SCl2), sulphur atom is Sp3 hybridised and has angular structure.

Sulphur monochloride (S2Cl2)

Sulphur monochloride (S2Cl2) is also prepared by direct combination of liquid sulphur and chlorine.

1/4 S8  +  Cl2  ---------->  S2Cl2

The electron diffraction studies have shown a non planer structure for S2Cl2 which is similar to that of H2O2.

Extraction of aluminium

Aluminum is the most abundant metal of earth's crust (8.3%) and exist as oxide and fluoride ores. The metal is extracted from bauxite ore (Al2O3.2H2O). The extraction process involve three stages or steps

step 1 Purification of bauxite
step 2 Electrolytic reduction of Al2O3
step 3 Electrolytic purification of aluminium

1. Purification of bauxite

The bauxite ore contain iron oxide and silicon dioxide as impurities. It is purified by leaching method using concentrated sodium hydroxide solution in which bauxite dissolve forming sodium meta aluminate. On agitating this solution with freshly precipitated Al(OH)3 for several hours Al(OH)3 is precipitated.

NaAlO2  +  2H2O  ------------>  NaOH  +  Al(OH)3

 The precipitate is filtered and dried and NaOH is concentrated and used again for leaching. The aluminium hydroxide precipitate is calcined at 1500oC to obtain pure alumina.

2. Electrolytic reduction of pure alumina

Pure Al2O3 is a bad conductor of electricity and has a very high fusion temperature of 2000oC. Any aluminium formed will vapourise at this temperature as boiling point of aluminum is only 1800oC. Hence alumina mixed with cryolite  Na3AlF6 and CaF2 with a fusion temperature of 900oC is used as the electrolyte. The electrolysis is carried out in an iron tank lined inside with carbon acting as cathode and graphite rods dipped into the electrolyte as anode. Molten aluminium liberated at the cathode gets collected at the bottom of the tank and oxygen liberated at the anode burns away the anode as CO2. The overall reaction may be represented as

2 Al2O3  ------------>  4Al (at cathode) +  3O2 (at anode)

 C(cathode)  + O2  --------------->  CO2

 The anode is replaced from time to time and by adding alumina into the cell and tapping out molten aluminium from the tank the process can be continuously carried out.

Extraction of aluminum

3. Refining of aluminium

 In Hoope's electrolytic method of refining, three liquid layers with differing densities are used. The bottom layer is molten impure aluminium into which Cu and Si has been added to increase density. This molten layer taken is an iron tank lined with carbon is the anode. The electrolyte is a middle layer containing molten mixture of AlF3, BaF2 and NaF. The cathode is the upper layer containing pure molten aluminium. On electrolysis aluminium dissolve from the anode and deposit at the cathode.

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