Stereoisomerism and its kinds

STEREOISOMERISM IN DETAIL

1. STEREOISOMERISM

2. This Photo by Unknown Author is licensed under CC BY-NC CONTENT: Introduction to Stereoisomerism Classification of stereoisomers Enantiomers Optical activity and Polarimeter Dextrorotatory and Levorotatory Diastereomers Types of diastereomers Geometric Isomers i.Cis isomers ii.Trans isomers Conformational isomers i . Eclipsed conformer ii. Staggered conformers Interesting facts

3. Introduction to Stereoisomerism: Stereoisomerism is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. The molecules that differ in three dimensions of their space having same molecular formula and sequence of bonded atoms are called stereoisomers.

4. Classification: This Photo by Unknown Author is licensed under CC BY-SA

5. Enantiomers: Enantiomers are chiral molecules(isomers) that are mirror images of one another and the molecules are non-superimposable on one another. Non-superimposable means that the molecules cannot be placed on top of one another and give the same molecule. They have identical physical properties but differ in direction of rotation of plane polarized. The mixture of equal quantities of enantiomers forms the optically inactive compound called as racemic mixture. Examples:

6. Optical activity and Polarimeter: Optical activity is the ability of a chiral molecule to rotate a plane of plane polarized light. Polarimeter is scientific instrument used to measure the optical activity specifically the specific angle of rotation. Polarimeter consists of a light source, polarizing lens, sample tube and analyzing lens.

7. Dextrorotatory and Leuvorotatory : On the basis of rotation of plane polarized, pair of enantiomers consists i . dextrorotatory(+) ii. levorotatory(-) If a compound rotates plane polarized light in the clockwise (+) direction, it is said to be dextrorotatory. If compound rotates light in the counterclockwise (-) direction it is levorotatory. It is determined by only use of polarimeters .

8. Diastereomers: Stereoisomers of the substance that are not mirror of each other are termed as diastereomers. They have different physical properties like m.p , b.p , solubility,density etc. Diastereomers have different specific rotation but they may have same or opposite signs.

9. Types of diastereomers: Generally,diastereomers have two types; i . Geometric Isomers ii. Conformational isomers Diastereomerism can occur at a double bond(geometric isomerism), where the cis vs trans relative positions of substituents give two non-superposable isomers. Many conformational isomers are diastereomers as well.

10. Geometric Isomers: Stereoisomers , that is, pairs of molecules which have the same formula but whose functional groups are  rotated  into a different orientation in three-dimensional space are called geometric isomers. Geometric isomers occurs in alkenes and cycloalkanes. Geometrical isomerism is due to the restricted or hindered rotation around the carbon-carbon double bond. Due to the hindered rotation around carbon-carbon double bond, the relative positions of atoms or group attached to the doubly bonded carbon atoms get fixed .

11. Cis isomer- have same substituents on the same side of the double bond[Z- zusammen (together) with more complex molecules having high priority groups on the same side). Trans isomer- have same substituents on the opposite side of the double bond[E- entgegen (opposite)with more complex molecules having high priority groups on opposite sides].

12. Conformational isomers: Conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds. Conformation -number of possible spatial arrangements of atoms in a molecule that result from rotation of its constituent groups of atoms about single bonds. Conformer- a specific rotation i . rotation about single bonds ii. Amine inversion

13. Eclipsed conformer- in which atoms or groups are as close as possible to the atoms or groups on an adjacent carbon in a single bond. Staggered conformer- in which atoms or groups are as faraway as possible to the atoms or groups on an adjacent carbon in a single bond. Since eclipsed conformation has a greater potential energy due to torsional strain, it is less stable than the staggered conformation.

14. Interesting facts: Stereoisomers are critically important in biochemistry and medicine because nearly every biological molecule - amino acids, sugars, fats, enzymes, etc - has one or more stereoisomer. Our ability to taste and smell is regulated by chiral molecules in our mouths and noses that act as receptors to "sense" foreign substances. The two enantiomers of the amino acid, leucine, for example, have different tastes - one is bitter, whereas the other is sweet. Enantiomers also can smell different, as is known from the odors of the two carvones. One has the odor of caraway and the other of spearmint.

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