Isomers. Stereoisomers the same molecular formulas but different arrangement of atoms in space. C 5 H 10

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Blaise Hubbard
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1 Isomers The compounds that have identical molecular formulas but different arrangement of atoms and properties. Constitutional isomers (structural isomers). C 5 10 Stereoisomers the same molecular formulas but different arrangement of atoms in space. 1

2 Isomers Constitutional (structural) isomers Stereoisomers Chain isomers Positional isomers Tautomers Configurational isomers Conformers 2

3 I. Constitutional isomers The compounds that have identical molecular formulas but different connectivity of atoms and properties: 1) Chain isomers. 2) Positional isomers. 3) Tautomers. 3

4 1) Chain isomers Differ in various forms of chains (linear and branched) and cycles. 4

5 2) Positional isomers (1) Differ in position of heteroatoms, functional groups or unsaturated bonds in the molecule. 2 C C C 2 C 3 3 C C C C 3 1-butene 2-butene 3 C C 2 C 2 O 3 C C C 3 O 1-propanol 2-propanol 5

6 2) Positional isomers (2) In cyclic compounds, the position isomerism is characterized by different positions of substituents in the cycle. ortho- meta- para-isomer R1 R2 R1 R1 R2 R2 6

7 3) Tautomers Tautomerism - chemical isomerism characterized by relatively easy interconversion of isomeric forms in equilibrium. Tautomers - organic compounds that are interconvertible. 7

8 Forms of tautomerism (1) Prototropic tautomerism - shift of the proton from one atom to the another atom. O O O N N << O N N 8

9 Forms of tautomerism (2) Ring-chain tautomerism - the movement of the proton is accompanied by a change from an open structure to a ring. 9

10 II. Stereoisomers The compounds that have identical molecular formulas and same connectivity of atoms but different arrangement of atoms in space: 1) Conformational isomers (conformers). 2) Configurational isomers: Enantiomers. Diastereomers. 10

11 Conformation and configuration Changing the conformation of a molecule means rotating about bonds, but not breaking them. Changing the configuration of a molecule always means that bonds are broken. 11

12 Conformations 12

13 1) Conformational isomers Linear alkane conformations. Cyclohexane conformations. 13

14 Newman projections C C Užstotoji arba sinperiplanarinė konformacija Emax Sustabdytoji arba antiperiplanarinė konformacija Emin 14

15 Linear conformational isomers (1) Isomers resulting from the free rotation of a single bond. Staggered conformation - this conformation is favoured. Ethane: C 3 -C 3 15

16 Linear conformational isomers (2) Eclipsed conformation. 16

17 Linear conformational isomers (3) O-C 2 -C 2 -N 2 N 2 N 2 O N 2 N 2 O O O 17

18 Stereoisomers Enantiomers and diastereomers They affect the plane of polarised light. Enantiomers are non-superimposable mirror images. Enantiomers occur only with those compounds, which are chiral. Diastereomers are stereoisomers that are not mirror images of each other. 18

19 Chirality (1) The word chiral comes from the Greek word meaning hand. It is used to describe an object that is non-superimposable on its mirror image. 19

20 Chirality (2) 20

21 Chirality (3) The mirror images of chiral compounds can not be superimposed. 21

22 Achiral structures 22

Chiral and achiral molecules The essential difference between the two molecules lies in their symmetry: If there are two groups the same attached to the central carbon

23 Chiral and achiral molecules The essential difference between the two molecules lies in their symmetry: If there are two groups the same attached to the central carbon atom, the molecule has a plane of symmetry achiral molecule. Where there are four different groups attached, there is no symmetry anywhere in the molecule chiral molecule. 23

24 Asymmetric center (Chiral center) The carbon atom with the four different groups attached, which causes the lack of symmetry is a chiral centre (asymmetric carbon atom). 24

25 Locating a stereocenter O C 3 * C C O O no stereocenter 2 N * C C O O Lactic acid C 2 O Serine 25

26 Enantiomers A pair of enantiomers is always possible for molecules that contain one tetrahedral atom with four different groups attached to it. 26

27 Drawing Fischer projections Place the most oxidized group at the top. Use vertical lines in place of dashes for bonds that go back. Use horizontal line in place of wedges for bonds that come forward. 27

28 D- and L- configuration By convention, the letter L is assigned to the structure with the -O (or N 2 ) in the left. The letter D is assigned to the structure with the - O (or N 2 ) on the right. 28

29 D and L compounds: carbohydrates Natural sugars have the D configuration (L sugars are rare in nature). Often drawn as Fischer projections with most oxidized carbon at top. CO O group to the right on bottom stereocenter O O O O C 2 O D-glucose 29

30 D and L compounds: amino acids Natural amino acids have the L configuration. Often drawn as Fischer projections with carboxyl group at the top. Since enzymes are proteins, they are inherently chiral, and can differentiate enantiomeric substrates. CO 2 CO 2 2 N N 2 C 2 C 2 S S L-Cysteine D-Cysteine 30

31 Importance of enantiomerism Enantiomers react in a different way with other compounds: the configuration of an isomer should correspond to a configuration of assymetric atom of another compound in a way how a key fit into a lock. Otherwise interaction is impossible. 31

32 The danger of (S)-thalidomide (R and S configuration) 32

Thalidomide birth defects Thalidomide is an antinausea and sedative drug that was introduced in the late 1950s to be used as a sleeping pill and was quickly discovered to help pregnant

33 Thalidomide birth defects Thalidomide is an antinausea and sedative drug that was introduced in the late 1950s to be used as a sleeping pill and was quickly discovered to help pregnant women with the effects of morning sickness ] It was sold from 1957 until 1962, when it was withdrawn after being found to be a teratogen, which caused many different forms of birth defects 33

34 Optical characteristics of enantiomers Enantiomers are optical isomers. They could rotate polarized light clockwise (+) or counterclockwise (-) 34

35 Polarization of light 35

36 Optical activity Chiral molecules rotate polarised light! Clockwise (+) Counterclockwise (-) An equimolar solution of (+) and (-) enantiomers is optically inactive, and ( ) solution of enantiomers is called racemic mixture or racemate. 36

37 Diastereomers Non-mirror image stereoisomers, and have opposite configurations at one or more of the chiral centers. If two compounds are stereoisomers but they are not enantiomers, then they are diastereomers: have different physical properties and reactivity. can be separated by ordinary physical methods. 37

38 2-amino-3-hydroxybutanoic acid (threonine) O enantiomers COO N 2 2 N COO O COO N 2 O enantiomers 2 N O COO C 3 C 3 C 3 C 3 D(+)-threonine L(-)-threonine D(-)-allo-threonine L(+)-allo-threonine diastereomers 38

39 ow many stereoisomers are possible? Number of stereoisomers = 2 n, where n = number of chiral atoms in the molecule. There are 4 chiral atoms in the molecule of glucose Consequently, there are 8 pairs of enantiomers (16 isomers). CO O group to the right on bottom stereocenter O O O O C 2 O D-glucose 39

40 Mesomers D L Enantiomers meso isomers Formulas of enantiomers represent non superimposable mirror image isomers. Meso isomers are also mirror images. But by turning one of them 180 it is exactly superimposable on another one.therefore these meso forms represent the same compound. A plane of symmetry can be passed between carbons 2 and 3 so that the top and bottom halves of the molecule are mirror images. 40

41 pi Diasteromers (cis/trans) Br Br Br Br cis-2,3-dibromo-but-2-ene trans-2,3-dibromo-but-2-ene They occur when there is a restricted rotation in a molecule. 41

42 Example of isomerisation Maleic (cis-butenedioic) acid and fumaric acid are pi diasteromers. 42

43 Example of stereospecificity 43

Isomers Have same molecular formula, but different structures

Isomers Have same molecular formula, but different structures Isomers ave same molecular formula, but different structures Constitutional Isomers Differ in the order of attachment of atoms (different bond connectivity) Stereoisomers Atoms are connected in the same