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Pharmaceutical ORGANIC CHEMISTRY Stereochemistry.

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Presentation on theme: "Pharmaceutical ORGANIC CHEMISTRY Stereochemistry."— Presentation transcript:

1 Pharmaceutical ORGANIC CHEMISTRY Stereochemistry

2 Stereochemistry is the study of molecules on space is the study of molecules on space i.e. how atoms or groups in a molecule i.e. how atoms or groups in a molecule arranged in space relative to each other arranged in space relative to each other it is that part of the science which deals with structure in three dimensionsit is that part of the science which deals with structure in three dimensions is the field of chemistry that concerned with isomerism in space is the field of chemistry that concerned with isomerism in space ( stereoisomerism ) ( stereoisomerism )

3 Lecture contents  Isomerism  Structural Isomerism 1. Chain isomerism 2. Position isomerism 3. Functional isomerism  Stereoisomerism 1.Optical active isomers. 2.Geometrical isomers. 3.Conformational isomers.

4 Isomerism When two or more organic compounds have the same molecular formula but differ in their physical or chemical properties they are called Isomers and the phenomenon called isomerism. Isomerism (stereoisomerism ) A phenomenon resulting from molecules having the same molecular formula but different arrangement In space A phenomenon resulting from molecules having the same molecular formula but different arrangement In space

5 Stereoisomerism Greek, Stereos=occupying space). The stereoisomers have the same structural formulas but differ in the spatial arrangement of atoms or groups in the molecule. In other words, stereoisomerism is exhibited by such compounds which have identical molecular structure but different configurations.

6 Stereochemistry Type of Isomerism 1- Structural – The resulting isomerism are known as 1- Structural – The resulting isomerism are known as Structural isomers Structural isomers 2- Geometrical – The resulting isomerism are known as Diastereoisomers Diastereoisomers 3- Optical – The resulting isomerism are known as Enantiomers Enantiomers

7 They are compounds that contain the same atoms bonded together in different ways they include:- 1. Constitutional (Structural) isomers. 2. Stereoisomers. They are compounds that contain the same atoms bonded together in different ways they include:- 1. Constitutional (Structural) isomers. 2. Stereoisomers. Isomers

8 Isomerism Constitutional (Structural) isomers Constitutional (Structural) isomers Stereoisomers. Geometrical isomerism Geometrical isomerism Positional isomerism Positional isomerism Optical isomerism Optical isomerism Functional isomerism Functional isomerism Chain isomerism Chain isomerism

9 Constitutional Isomers They differ in the way their atom are connected. – 1. Chain isomerism – 2. Position isomerism – 3. Functional isomerism

10 Chain Isomerism (or Skeletal Isomerism) Chain isomers have the same molecular formula but differ in order in which the carbon atoms are bonded.

11 C 4 H 10 C 5 H 12 Predict

12 Position Isomerism Position isomers have the same molecular formula but differ in the position of functional group on the carbon chain. Examples are:-

13 C6H6O2C6H6O2C6H6O2C6H6O2 C3H8OC3H8OC3H8OC3H8O Predict

14 Functional Isomerism Functional isomers have the same molecular formula but different functional groups. Examples are;

15 Stereochemistry Geometric isomerism An isomerism resulting from rigidity in molecules and accures only in two class of compounds and accures only in two class of compounds alkenes and cyclic compounds alkenes and cyclic compounds Rigidity ≠ Flexibility σ – bond free rotation ( flexibility ) σ – bond free rotation ( flexibility )  - bond C = C no free rotation ( rigidity )  - bond C = C no free rotation ( rigidity ) σ – bond restricted rotation ( semi-fexable ) σ – bond restricted rotation ( semi-fexable )

16 Free rotation' of carbon- carbon single bonds Restricted rotation' of carbon- carbon double bonds

17 Stereochemistry 1- Geometric isomerism in alkenes The requirement for this type of isomerism is that each carbon atom involved in the double bond has different substitution atom involved in the double bond has different substitution D ≠ E A ≠ B On same side cis cis On differ side trans trans

18 Stereochemistry Diastereoisomers are designed cis or trans according to the priority orders or sequence rules the priority orders or sequence rules [ atomic number = atomic weight \ 2 ] [ atomic number = atomic weight \ 2 ] 1- Geometric isomerism in alkenes 126 I > 80 Br > 35 Cl > 32 S > 19 F > 16 O > 14 N > 12 C > 1 H 126 I > 80 Br > 35 Cl > 32 S > 19 F > 16 O > 14 N > 12 C > 1 H

19 Stereochemistry Sequence Rules or Priority Order 1- If the two atoms in question are different, the atom of highest atomic number receives the highest priority highest atomic number receives the highest priority I > Br > Cl > …… I > Br > Cl > ……

20 Stereochemistry Sequence Rules or Priority Order 2- If the two atoms are identical, the atomic numbers of next atoms are used, until the priority is determined at the first atoms are used, until the priority is determined at the first points of difference along the chain points of difference along the chain

21 Stereochemistry Sequence Rules or Priority Order 3- Atoms attached to double or triple bonds are given single-bond equivalencies, each doubly bonded atom is single-bond equivalencies, each doubly bonded atom is duplicated ( or triplicated for triple bonds ) duplicated ( or triplicated for triple bonds ) RCH 2 OH RCN -CH=CR 2 >>>>>>

22 Stereochemistry Sequence Rules or Priority Order 3- Atoms attached to double or triple bonds are given single-bond equivalencies, each doubly bonded atom is single-bond equivalencies, each doubly bonded atom is duplicated ( or triplicated for triple bonds ) duplicated ( or triplicated for triple bonds )

23 Stereochemistry Sequence Rules or Priority Order 3- Atoms attached to double or triple bonds are given single-bond equivalencies, each doubly bonded atom is single-bond equivalencies, each doubly bonded atom is duplicated ( or triplicated for triple bonds ) duplicated ( or triplicated for triple bonds ) Diastereoisomer

24 Stereochemistry E, Z – system of nomenclature Diastereoisomers can also be designted by : Diastereoisomers can also be designted by : 1- E instead of trans 1- E instead of trans 2- Z instead of cis 2- Z instead of cis

25 StereochemistryStereochemistry Compounds which have the same molecular formula Compounds which have the same molecular formula but differ in arrangement of atoms or groups in space but differ in arrangement of atoms or groups in space different configuration different configuration Diastereoisomers have different physical and chemical Diastereoisomers have different physical and chemical properties, as solubility and boiling points properties, as solubility and boiling points thus: thus: - if are liquids……. Fractional distillation - if are liquids……. Fractional distillation - if are solids …….. Fractional crystallization - if are solids …….. Fractional crystallization

26 Stereochemistry 2- Geometric isomerism in cyclic compounds The most illustrative example is cyclohexane Rotation of group ( atoms ) around sigma bonds, result in different conformations ( conformer ) result in different conformations ( conformer )

27 Stereochemistry 2- Geometric isomerism in cyclic compounds All cyclic compounds except cyclopentane would be strained because …… strained because ……

28 Stereochemistry 2- Geometric isomerism in cyclic compounds Conformation is different arrangement …. In ethane Newman projection Ball and stick 3D formula

29 Stereochemistry 2- Geometric isomerism in cyclic compounds Cyclohexane is not flat …. Axial hydrogen ( Ha ) : H parallel to axis Equatorial hydrogen ( He ) : H in plane upon flipping and reflipping between conformers, axial becomes upon flipping and reflipping between conformers, axial becomes equatorial and equatorial becomes axial equatorial and equatorial becomes axial

30 Stereochemistry 2- Geometric isomerism in cyclic compounds Mono-substituted cyclic compounds, do not exhibit geometrical ……… geometrical ……… cis or trans cis or trans

31 Stereochemistry 2- Geometric isomerism in cyclic compounds 1,2-disubstituted : same as 1,4-disubstituted Cis both down = e,a both down = e,a both up = a,e both up = a,e Trans up and down = a,a up and down = a,a down and up = e,e down and up = e,e In summary : a,e e,a a,a e,e a,a e,e 1,4-disubistuted cyclohexane as 1,2

32 Stereochemistry 2- Geometric isomerism in cyclic compounds 1,3-disubstituted : both down = e,e both up = a,a both up = a,a up and down = a,e up and down = a,e down and up = e,a down and up = e,a

33 Label each of the following disubistiuted rings as cis or trans and as a,a ; e,e ; or a,e


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