Geometrical isomerism

According to PCI

1. PHARMACEUTICAL ORGANIC CHEMISTRY –III TOPIC: Geometrical Isomerism& its nomenclature Represented by: Ishika Choudhary B.Pharma; 2 nd year Enrollment no.-AJU\190857 Roll no.-51 ARKA JAIN UNIVERSITY, Jamshedpur, Jharkhand. 1

2. Geometrical Isomerism (GI) The geometrical isomerism arises when atoms or groups are arranged differently in space due to restricted rotation of a bond or bonds in a molecule . 2

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4. Nomenclature of GI :- 4

5. Isomer in which similar groups or atoms lie on the same side of double bond are called cis -isomer where as isomers in which similar groups lie on the opposite side of double bond are called trans-isomer. Cis-Trans Nomenclature :- 5

6. Factors affecting cis-trans isomers:- 1) The molecule must have a C - C double bond. 2 ) Two atoms or groups attached to doubly bonded carbon atom must be different . 6

7. 3 ) Boiling point : Cis isomer have higher boiling point due to dipole-dipole interaction. Trans isomer have comparatively low boiling point 4 ) Melting point : Cis-isomer has lower melting point because the structure is not symmetrical Due to symmetry, trans isomer fits better in crystal lattice, thus has higher lattice energy and hence higher melting point. 7

8. 5) Dipole moment : In cis isomer, dipole moment of polar groups have additive effect thus have higher dipole moment than corresponding trans isomer. In trans isomer, dipole moment of polar groups have opposing effect, thus tends to cancel each other 8

9. Note :- Ring have less than 8 members , double bond will be always cis-isomer. In 8,9,10 members ring, cis isomer will be more stable. 9

10. 3) In 11 or more member ring, trans-isomer will be more stable. 4) If even number of double bond or ring is present, it will NOT show Geometrical Isomerism (GI). Eg ,.a) b) 10

11. 5) If odd number of double bonds or rings are present, it may show GI. Eg ,.a) or b) 11

12. E-Z nomenclature :- In this system we assign a priority to the groups attached to double bonded carbon atom . If groups of similar priority lie on the similar side of double bond, the isomer is designated as Z ( Zusammen , means together). If groups of similar priority lie on the opposite side of double bond, the isomer is designated as E ( Entgegen , means opposite ) 12

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14. Sequence rules :- The following rules are followed for deciding the precedence order of the atoms or groups: (i) Higher atomic number atoms get higher priority. ( ii) Among the isotopes of same element, isotope of higher mass is given higher priority. Eg ,.The Deuterium isotope (H2 or D) has more priority than protium (H1 or H ). 14

15. (iii) In the groups, the order of precedence is also decided on the basis of atomic number of first atom of the group. ( iv) A double or triple bonded atom is considered equivalent to two or three such atoms, For example;the group >C=O is equal to and the group. 15

16. (v) If one of the position is occupied by lone pair, it is given least priority over the bonded group. 16

17. The oximes are formed when carbonyl compounds are treated with hydroxyl amine. These are represented as : Where R & R1 are hydrogens; or alkyl or aryl groups . The oximes are of two types: 1 ) Aldoximes : These are derived from aldehydes. In this case, at least either R or R1 is hydrogen. 2 ) Ketoximes : These are derived from ketones. In this case, both R or R1are alkyl or aryl groups only. 17 Syn- anti Configuration

18. The oximes show geometrical isomerism due to restricted rotation of C=N bond. Two geometrical forms are possible for the oximes as shown below. Eg ,.syn and anti oximes In case of aldoximes , the syn form is the one in which both the hydrogen and the hydroxyl (-OH) group are on the same side of the C=N. Whereas in the anti form, they are on the opposite side. E.g . The syn and anti forms of acetaldoxime are shown below. 18

19. Ketoximes :- ketoxime of butanone can be named as either syn methyl ethyl ketoxime or anti ethyl methyl ketoxime :- a) With respect to methyl and hydroxyl group ,as both are on the side . Therefore called syn - methyl ethyl ketoximes . 19

20. b) With respect to ethyl and hydroxyl group , as both are in opposite side. Therefore, called anti-ethyl methyl ketoximes . But, 20

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