University of Nizwa Oman Dr. Ghulam Abbas Assistant Professor University of Nizwa

Stereochemistry Stereochemistry is the study of molecules in three dimensions (3D). To study how the shape of a molecule affects its chemistry. 1.Chain isomerism 2. Functional group Isomerism Position Isomerism 4. Metamerism 5. Tautomerism

Isomers Compounds having same chemical formula but different structures are called isomers. There are two classes of isomers 1. Constitutional or Structural Isomerism Stereo Isomerism.

Constitutional or Structural Isomerism Constitutional Isomers will have the same number and types of atoms, but they are connected differently sequentially (they have a different Constitution). A simple example of Constitutional Isomers are ethanol and dimethyl ether. Ethanol Dimethyl ether Types of Constitutional Isomers. 1. Chain isomerism 2. Functional group Isomerism 3. Position Isomerism 4. Metamerism 5. Tautomerism

Chain Isomerism Compounds having same molecular formula but different carbon chain or skeleton are called chain isomers and this phenomenon is called Chain isomerism. Molecular Formula C5H12 Molecular Formula C4H10O

Functional group Isomers Compounds having same molecular formula but different functional functional groups are called chain isomers.

Position isomerism Compounds having same molecular formula but different position of Functional group or double bond or triple bond are called position isomers and this phenomenon is called Position isomerism.

CH3-CH2-NH-CH2-CH3 and CH3-NH-CH2-CH2-CH3 Metamerism Compounds having same molecular formula same functional group but having different alkyl groups attach to same multivalent element like oxygen, Nitrogen, sulpher etc are called metamers and this phenomenon is called metamerism. Amine CH3-CH2-NH-CH2-CH3 and CH3-NH-CH2-CH2-CH3 Tautomerism Keto enol form of a compound is called Tautomerism.

Chirality The word chirality refers to the property of “handnesss” Your right and left hands are very similar, yet they are not identical. They are related to each other as mirror images and as such they can’t be superimposed on top of each other . Molecule can also be chiral if they contain one or more chiral centers.

Chiral Centre (stereogenic Centre) The carbon atom having four different atoms and can not be subdivided in to two equal halves.

Prochiral A molecule is said to be prochiral if it can be converted from achiral to chiral in a single chemical step. For instance, an unsymmetrical ketone like 2-butanone is prochiral because it can be converted to the chiral alcohol 2-butanol by addition of hydrogen. + H2

STEREO ISOMERISM Stereo isomers are the compounds whose atoms are connected in same order but differ from one another only in (spatial arrangement) the arrangement of atoms in three-dimensional space. It is due to the free rotation around the carbon-carbon single bonds in open chain molecule. This process is called stereo isomerism and the compounds are said to stereo isomers of each other.

STEREO ISOMERS ARE OF TWO TYPES. Configurational Isomerism Conformational Isomerism Configurational Isomerism The isomerism in which two compounds are inter converted only by the breaking and making of bonds such compounds are called Configurational Isomerism.

Conformational Isomerism The stereo isomerism in which two compounds are inter converted to one another only by the rotation of bonds around the single bond. Thus the two compounds are called conformational isomers/Conformers. Example: In ethane due the free rotation of bonds around a single bond different arrangement of ethane is possible as shown. These different arrangement of atoms caused by free rotation around a single bond are called conformation and specific structure is called conformer

Examples of Conformational Isomers

CONFIGURATIONAL ISOMERS ARE IN TURN COMPRISED OF ENANTIOMERS and DIASTERIOISOMER. ENANTIOMERS: The non super imposable mirror images of a compounds is called enantiomers (d or l rotatory) they are are optically active because they cane rotate the plane of polarized either to right or to left. Example: 3, 4 Dibromobutane.

2, 3 Dibromobutane Lactic Acid

Not superimposable isomers

Cahn-Ingold-Prelog System

Cahn-Ingold-Prelog System

R and S Configurations

R and S Configuration Examples S = = S c) R

Tartaric Acid: Used by Pasteur Examples Meso Compound

MESO COMPOUND A compound whose molecules are superimposable on their mirror images, even though they contain chiral centers. It is optically inactive. (They have plane of symmetry).

DIASTERIOISOMER Those stereo isomer which are not mirror images of one another are known as diasterioisomers. Or They are the stereoisomerisms which must have opposite (mirror image) configuration at one or more chiral centers but the same configurations at another chiral canters.

Examples Vants Hoff Rule For a molecule with multiple chiral centers, the number of possible diastereoisomers is given by the equation X = 2n Where X is the number of possible isomers and n is the number of stereogenic centers.

Other Examples of Diastereoisomer is 2-chloro-2 Butanol

The relation between structure (i), (iii), (iv) is known as diastereoisomers, Structure (i), (ii) are enatiomers and they must have identical properties, the same true for structure (iii) and (iv). How ever the properties of Structure (i) and (ii) are not identical with those of structure (iii) and (iv). They have different melting points, Boiling points and solubility reactivity and all other properties.

Optical Isomerism The compounds having the same molecular formulas, structural formulas, physical and chemical properties but differing in their action on plane polarized light are called optical isomer and phenomenon is called optical isomerism. Optical activity The property of a compound to rotate the plane polarized light in either direction is called optical activity. OR The substance rotating the plane of polarized to the left or right is called optical active and this property of rotating planner polarized light is called optical activity.

Dextro rotatory ( or d form) Laevo Rotatory (or l form) The molecule rotating the plane of polarized light to the left is called Laevo rotatory or l form. Dextro rotatory ( or d form) The molecule rotating the plane of polarized light to the right is called dextro rotatory or d form.

Geometrical Isomerism Compounds in which carbon to carbon free rotation is restricted may exhibit geometrical isomerism. These compounds don’t rotate the plane of polarized light and properties of isomer are not identical. They are also called as cis-trans isomer

Geometrical Isomerism

Plane polarized light The light oscillating in single plane is called plane-polarized light. If we observed the beam of light of ordinary light from one end, we will find that oscillation of electric field and magnetic fields are occurring in all possible planes perpendicular to the directions of propagations. When ordinary light is passed through a polarizer, it loses its horizontal components. The resulting ray is oscillating in single plane.

when all the four gps are different the there is rule that which convention may be used in a carbon chain the cis isomer is one in which the gps containing the longest carbon chain on the same side of double are termed as cis.

Allenes (C=C=C) don’t show cis trans isomerism when a molecule contains a double and an asymmetric carbon there are four isomer a cis pair of enantiomer and a trans pair of enantiomer

Stereochemistry/Geometric isomers If the pair of stereoisomers contains a double bond then it is possible to get a cis or trans arrangement of the substituents at each end of the double bond. These are referred to as cis-trans isomers. .

Geometric isomers In a diasteromeric cyclic compound the substituents can give the appearance of being either cis or trans to each other and these are referred to as geometric isomers. An example is the two geometric isomers of 1-hydroxy-2-methyl-cyclobutane (form 1 and form 2).

Methods of Resolution Resolution mean resolving the racemic mixture in to pure enantiomers. It a reverse of racemization. There are many ways for the separation of pure enantiomer. Differential Absorption When a racemic mixture is placed on a chromatographic column if the column consists of Optically active substances then the enantiomer should move down the column at different rate and they can be easily separated. This can also be done with paper chromatography.

Physical Or Mechanical Separation Pasture grew the crystals of ammonium salt of tartaric acid. He separated the racemic mixture with the help of tweezer. Under the microscope and got two pure enantiomer. This was a first physical separation of a racemic mixture. But is a tedious and time-consuming method. Secondly it is only limited to those racemic mixtures which are solid and have fine crystals.

Biological separation In the living system enzymes, which controls all the reaction occurring in the living body, are very specific. For example in the human body there are enzyme which are specific for D. glucose (occur in all fruit) and is able to metabolize it). If a person takes a mixture of D. glucose and L. glucose then enzyme will consume the D. glucose and L. glucose will be secreted out as pure enantiomer. It is very perfect method of separation of racemic mixture but the disadvantage of this method is that we can get only pure enantiomer.

Biochemical process Certain Bacteria’s and mould of chiral nature when grown in dilute solution of racemic mixture destroy one enantiomer then other. For example Penicillium glaucum when grown in racemic mixture of ammonium tartarte the solution slowly become leveo rotatory (-) because the mould destroy the (+) enantiomer. Chemical Method A racemic mixture of two pure enantiomer is not possible to separate because they have same physical properties and chemical properties. However properties of diasteroisomer are quite different from individual isomer and therefore the mixture of diasteroisomer can be separated by fractional distillation.

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