Chapter 4 : Stereochemistry B.sc 1st Year Page : 1 Organic Chemistry
Stereochemistry is the chemistry of compounds in three dimensions. Its found atoms were laid by Jacobus Vant Hoff and Joseph Achille Le Bel in 1874. They proposed the four bonds of carbon that directed towards the center of tetrahedron. Stereochemistry deals with the study of stereoisomers.
Compounds of identical structure but differing in the arrangement of atoms in three dimensional space are called ‘stereoisomers’. Three dimensional spatial arrangement or configuration is different for such compounds although their structure is same. This phenomenon is known as ‘stereoisomerism’. Now stereochemistry may be defined as a branch of chemistry which deals with the study of identical structure but differing in three dimensional spatial arrangements and effect of their structure on physical and chemical properties as well as rate of their reactions. Isomers may be classified as.
Chiral (Greek; ‘Cheir’ means hand) :
Structure that are non superimposable on their mirror image and can therefore exist as two enantiomers are called ‘chiral.
Molecular Chirality :
Every thing has a mirror image but not all things are superimposed on their mirror images.
Enantiomers (Enantiomer in Greek means opposite):
Structures that are not identical but are mirror images of each other are called ‘enantomers’.
(A) and (B) are enantiomers.
Properties of Enantiomers : Each enantiomer of a pair has the same physical and chemical properties with the exceptions of their :
- Optical activity; one enantiomer shows dextro rotation(+) and the other levo rotation(-).
- Interaction with chiral reagents will be different.
- Achiral Molecules : Structures which are superimposable on their mirror images are called ‘achiral’. Therefore, they can not exist as two enantiomers. Since C and D are identical molecules; they are superimposable upon their own mirror images. Therefore can not exist as two enantiomers.
- Stereogenic Centre : A carbon atom carrying four different groups is called as a stereogenic or chiral centre.
Optical activity :
A molecule is said to be optically active if it rotates plane polarised light Polarimetric measurements are carried out in a polarimeter which has a single wavelength (monochromatic) light source, (generally sodium lamp), a plane polarising filter, a cell containing solution of substance under examination and a detector that indicates angle of rotation. The angle of rotation of a compound depends on various factors. The most important ones being path length, concentration, temperature, solvent and wavelength. When l’ is measured in decimeters and ‘C’ in grams per milli litre, optical rotation is called ‘specific rotation [α].
Where ‘D’ denotes the wavelength in nm of sodium lamp when optical rotation measured at temperature t.
Stereoisomers that are not mirror images of one another are called ‘diastereoisomers’
In order to have diastereoisomers correspond to a given structural formula, there must be atleast two asymmetric carbon atoms..
2) Diastereomers have different physical and chemical properties.
3) Interchanging the position of two groups of any stereogenic centre gives new diastereomers.
4) The stereoisomers that are not enantiomers are called ‘diastereomers’.
Thus (l) is diastereomer of (III) and (IV) and (II) is diastereomer of (III) and (IV). ) Diatereomers can be chiral or achiral.
(vi) A compound with ‘n’ stereogenic centres has 2 n-1 diastereoisomers.
(vii) Diastereomers are different compounds.
(viii) They may or may not be optically active.
Threo and Erythro Diastereomers :
A molecule with two adjacent stereocentres and two groups which are common to each carbon, the system of the type Cabx – Caby gives rise to erythro and threo diastereomers. When the similar groups are on the same side, the isomer is called ‘erythro’, if they are on opposite sides the isomer is called ‘threo’ and each erythro and threo responds enantiomers.
Meso compounds :
Compounds that contain stereogenic centres but are themselves achiral are called as ‘meso compounds’. This means that meso compounds a.e achiral due to plane of symmetry. e.g.Tartaric acid can exist as two diastereoisomers, one with two enantiomers and the other achiral (a meso compound).
Optical isomerism of lactic acid CH CH(OH)COOH :
Lactic acid contains one asymmetric arbon atom. It can exist in two optically active forms (+) lactic acid and (-) lactic acid as shown below :
The two forms I and Il are mirror images to each other and are non-superimposable. Such non perimposable mirror image forms are optical isomers and are called “enantiomers”. Besides, these O optically active forms of lactic acid, a third form, a racemic mixture is also possible. This racemic mixture racemic form is equimolar mixture of (+) and (-) lactic acid .This form is optically inactive due to external mpensation. In other words the rotation caused by (+) lactic acid is completally cancelled out by the equal and opposite rotation caused by (-) lactic acid. Thus, there are three forms of lactic acid, two are optically cave and the remaining one is optically inactive. These have the following properties :
- (+) Lactic acid; (m.p. 26°C): It rotates the plane polarized light to the clock-wise direction ( and it is called dextro rotatory. This form of lactic acid is extracted from meat.
- (-) Lactic acid; (m.p.=26°C) :It rotates the plane polarized light to the anticlock-wise direction ( ) and it is called laevo rotatory. It is obtained by fermentation of sucrose by Basillus acidi laevo lacitti.
- (-,+) Lactic acid; (m.p.=18°C) :It does not rotate the plane polarised light to the left or right hand direction. It is obtained from sour milk. It can be resolved into dextro and laevo forms.
Optical isomerism of tartaric acid [HOOCC*H (OH)C*H (OH)COOH ]: The optical activity this compound is due to the presence of two asymmetric carbon atoms in it. It exists in four isomeric oms (+) tartaric acid, (-) tartaric acid, (+) tartaric acid and meso tartaric acid. Out of them, first two forms of this acid are optically active and the remaining two forms are optically inactive. Racemic (+) tartaric acid is nchcally inactive due to the external compensation while the meso tartaric acid is optically inactive due to the internal compensation because of the presence of the plane of symmetry.