Stereochemistry

steriochemstry, isomerism ,elements of symmetry with important terms.

1. STEREOCHEMISTRY PRESENTED BY: SALMAN KHAN

2. “ The branch of chemistry concerned with the three dimensional arrangement of atoms and molecules and the effect of this on chemical reactions.” “ Stereo- ” means “ three dimensional ” For this reason it is also known as - 3D chemistry

3. “ Configuration ”- arrangement of atoms of a molecules in space Stereoisomerism - It is a phenomenon by which two or more molecules have same structure but different configurations i.e , different orientations of atoms and groups in space.

4. OPTICAL ISOMERISM “Compounds which possess similar chemical and physical properties and differ only in their optical activity are known as optical isomers and the phenomenon as optical isomerism.” Optical activity: The phenomenon of rotating the plane polarized light is known as optical activity. It could be either towards right or left. right- dextrorotatory (d)(+) left-levorotatory(l)(-) The substances exhibiting this property are called optically active compounds.

5. Plane polarized light: A light vibrating in one plane only is known as plane polarized light. NOTE: Optically activity of a compound can both be detected and measured by an instrument called polarimeter .

6. SPECIFIC ROTATION The rotatory power of an optically active substance is measured by a term, called specific rotaion . It is defined as “ the number of degrees of rotation observed when plane polarized light is passed through one decimeter length of its solution having concentration 1 g per milliletre . Specific rotation value is generally expressed with(+) or (-) sign.

7. CHIRAL ATOM Optical activity are shown by molecules having an asymmetric carbon atom or chiral atom. It is a carbon atom to which four different atoms or groups are attached.

8. These models are asymmetric because they do not possess a plane of symmetry , molecules that are non-superimposable on their mirror images are chiral .

9. The number of optical isomers is equal to 2 n , where n is the number of asymmetric carbon atoms. Thus lactic acid having one asymmetric carbon atom, has two optical isomers called dextro and laevo . STEREOGENIC CENTER : just any location in a molecule where the interchange of any two groups gives a new stereoisomer. A chiral center is specifically a stereogenic center.

10. Elements Of Symmetry One determines whether the molecule possesses any form of symmetry via PLANE OF SYMMETRY CENTRE OF SYMMETRY AN ALTERNATING AXIS OF SYMMETRY If the molecule contains at least one of these elements of symmetry ,the molecule is symmetric and optically inactive.

11. PLANE OF SYMMERTY It is a plane that divides the molecules into two identical halves in such a way, one half of the molecules is the mirror image of the other half.

12. Meso -tartaric acid although has two asymmetric carbon atoms is a symmetrical molecule because it can be divided into two identical halves by a plane of symmetry.

13. CENTRE OF SYMMETRY It is a point from which lines, when drawn on one side and produced an equal distance on the other side, will meet identical points in the molecule e.g , 1,3-dicarboxylic acid

14. ALTERNATING AXIS OF SYMMETRY A molecule possesses an n-fold alternating axis of symmetry if when rotated through an angle of 360 ˚ /n about this axis and then reflected in a plane perpendicular to this axis, the molecule is indistinguishable from the original one.e.g,1,2,3,4-tetrametyl cyclobutane .

15. Criteria for optical isomerism The compound must contain an asymmetric carbon atom. Its three-dimensional structure should not have a plane of symmetry. The two possible formulas should be non super imposable mirror images

16. ENANTIOMERS “ The optical isomers which are non superimposable mirror images of each other are known as enantiomers or optical antipodes”

17. They rotate the plane polarized light by equal amounts but in opposite directions, i.e., their value of specific rotation is equal but opposite in sign. Their physical and chemical properties are same, but differ in their rates of reaction with isomers of other optically active compounds.

18. 3-phenyl-2-butanol has two asymmetric carbon atoms and has four optical isomers. Pairs of isomers I-II and III-IV are enantiomers . Enantiomers have identical chemical properties except towards optical active reagents.

19. For example,(+)-glucose is readily metabolized by animals or fermented by yeast, whereas (-)-glucose is neither metabolized by animals nor fermented by yeast. This is because the enzymes which catalyze these biological processes are optically active.