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ORGANIC CHEMISTRY COURSE LowerSixth

CHAPTER III: ISOMERISM

B- STEREOISOMERISM

Stereoisomer has the same molecular and structural formulae but they differ in the special arrangement of atoms in space. Stereoisomerism can be geometric and optical isomerism:

  1. Geometric isomerism (CIS-TRANS ISOMERISM): occurs in compounds in which free rotation is restricted due to the present of multiple bonds (double or triple bonds). Rings structure or steric factors. The isomers differs in the geometric arrangement of the atom in the cis isomer identical groups of the same side of the double bond while the trans isomer they identical group are the opposite side of the bond.

 

  1. Optical isomerism: the isomers have the same molecular formulae but different optical properties. It results in compound with an asymmetric (CHIRAL) carbon atom. An asymmetrical carbon atom is one which is bonded to four different atoms of the group. The isomers have the capacity to rotate the plain of polarized light. Plane polarized light is light in which all the vibration is all in one plane.

 A chiral molecule is a molecule that has neither a plane nor a central of symmetry. The two isomers which are mirrors images of each other and which are not super impossible are called enantiomers. Enantiomers have the same physical properties like melting and boiling points but differ by their ability to rotate the plane of polarized light in opposite direction. They usually have center of chirality with four different substituent attach to it (center of asymmetry or stereogenic center) the enantiomers which rotated the plane of polarized light to the right is said to be dextrorotatory (R) and also has the plus form mean while the rotation to the left is called laevorotatory (L) and has the minus form e.g. 2-hydroxyl propanioic acid or lactic acid.

 

SEPARARTION OR DISTINGUISH BETWEEN CIS AND TRANS

The isomer of dibromo-ethene and but-2-ene has very distinctive physical properties. A good example of the different physical and geometric isomers is provided by the isomers of butane dioic acid. The CIS isomers (maleic acid) melt at 139 to 140 degree. When heated to 160 degree under reduces pressure, it loses water to form an anhydride. The TRANS isomers (fomaric acid) when heated to 200 degree is sublimes. On further heating to 300 degree it rearranges to form a CIS isomer and a small amount of the anhydride. This shows that rotation about the double bond is possible at high temperature.

 

 

  1. RACEMIC MIXTURES 

This is the equimolar mixture of two enantiomers OR it consists of dextrorotatory and laevorotatory isomers in equal proportion.

RESOLUTION OF RACEMIC MIXTURE

The separation of the racemic mixture into two optically active components is called racemic resolution. The following method can be use in the separation of the racemic mixture.

  1. Biochemical method

When certain bacteria are added to a solution of a racemic mixture, they may bring about the decomposition of one of the optically active form more rapidly than the other. For example the penicillium glaucium decomposes +-2, 3 dihydroxyl butene-dioic acid more readily than the –isomer so that the latter can be obtain from the residue remaining after treatment with the bacteria.

  1. Compound formation

The racemic mixture is reacted with an optically active compound leading to the formation of two compounds which are optically inactive. They have different physical properties and can be separated by fractional crystallization or chromatography. The product are then hydrolyzed to obtain the pure compound.

  1. Crystal picking

It is a solution of racemic mixture. The crystal of racemic mixture form can be obtained. The crystal can be separated by hand picking using the lense because the crystals will be enantiomorphic.

par Claude Foumtum


ORGANIC CHEMISTRY COURSE LowerSixth