Dr Seemal Jelani Chem261 1 1112020 STEREOISOME RS

Dr Seemal Jelani Chem-261 1 11/1/2020 STEREOISOME RS

All 11/1/2020 molecules have a mirror image – but for most molecules it is the same molecule. Dr Seemal Jelani Chem-261 2

For 11/1/2020 Dr Seemal Jelani some molecules the mirror image is a different molecule (the mirror image is non-superimposable). Chem-261 3

11/1/2020 Dr Seemal Jelani Chem-261 4

11/1/2020 • Left and right hands are an example of non-superimposable mirror images. Dr Seemal Jelani Chem-261 5

This 11/1/2020 usually happens when a molecule contains a C atom with four different groups attached (chiral / asymmetric C). Such molecules are said to be chiral or optically active. Dr Seemal Jelani Chem-261 6

The optical isomers are called enantiomers. 11/1/2020 These A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate. Dr Seemal Jelani are distinguished by +/-, D/L or more correctly R/S. Chem-261 7

TASK Which of the following molecules are optically active? 11/1/2020 5) 6) 7) 8) butanone 2 -methylbutanoic acid butan-2 -ol 1 -chloro-3 -methylpentane Chem-261 propan-2 -ol 2 -chlorobutane 1 -chlorobutane 3 -methylhexane Dr Seemal Jelani 1) 2) 3) 4) 8

propan-2 -ol 11/1/2020 Dr Seemal Jelani NOT OPTICALLY ACTIVE Chem-261 9

2 -chlorobutane 11/1/2020 Dr Seemal Jelani Chem-261 10 OPTICALLY ACTIVE

1 -chlorobutane 11/1/2020 Dr Seemal Jelani NOT OPTICALLY ACTIVE Chem-261 11

3 -methylhexane 11/1/2020 Dr Seemal Jelani Chem-261 12 OPTICALLY ACTIVE

butanone 11/1/2020 Dr Seemal Jelani NOT OPTICALLY ACTIVE Chem-261 13

propan-2 -ol 11/1/2020 Dr Seemal Jelani NOT OPTICALLY ACTIVE Chem-261 14

2 -methylbutanoic acid 11/1/2020 Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE 15

butan-2 -ol 11/1/2020 Dr Seemal Jelani Chem-261 16 OPTICALLY ACTIVE

1 -chloro-3 -methylpentane 11/1/2020 Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE 17

Molecules Chem-261 • Their effect on plane polarised light • Their reaction with other chiral molecules Dr Seemal Jelani • Enantiomers have identical chemical and physical properties, except: 11/1/2020 that are optical isomers are called enantiomers. 18

2 -CHLOROBUTANE 11/1/2020 Dr Seemal Jelani Chem-261 19

3 -CHLOROCYCLOHEXENE 11/1/2020 Dr Seemal Jelani Chem-261 20

FORMULA TO CALCULATE STEREOISOMERS Dr Seemal Jelani Chem-261 a molecule with 1 chiral center, 2 x 1 = 2 stereoisomers are possible For a molecule with 2 chiral centers, a maximum of 2 x 2 = 4 stereoisomers are possible For a molecule with n chiral centers, a maximum of 2 n stereoisomers are possible 11/1/2020 For 21

STEREOISOMERS WITH TOW CHIRAL CARBON ATOMS ü Dr Seemal Jelani ü Molecules with two similar chiral center gives Enantiomers Diastereomers and Meso compounds 11/1/2020 Chem-261 22

2, 3, 4 -TRIHYDROXYBUTANAL Dr Seemal Jelani chiral centers 22 = 4 stereoisomers exist; two pairs of enantiomers R (a) S (b) R (c) S (d) 11/1/2020 two Chem-261 R S S R 23

Dr Seemal Jelani stereoisomers= 22 =4 (a ) and (b) are non superposable mirror images of each otherefore they are a “pair of enantiomers” 11/1/2020 Maximum Chem-261 24

Dr Seemal Jelani Chem-261 exists in 2 pairs of enantiomer (a, b) and (c, d) (a) = (2 R, 3 R) erythrose (b) = (2 S, 3 S) erythrose (c) = (2 R, 3 S) threose (d) = (2 S, 3 R) threose 11/1/2020 2, 3, 4 -Trihydroxybutanol 25

Dr Seemal Jelani erythrose and threose belong to class carbohydrates Erythrose is found in erythrocytes (RBC) 11/1/2020 Both Chem-261 26

DETERMINE THE RELATIONSHIP Find the relationship between the following Chem-261 and (c) (a) and (d) (b) and (c) (b) and (d) They are non-superposable, but are not mirror images so they are Diastereoisomers. Dr Seemal Jelani (a) 11/1/2020 27

11/1/2020 Dr Seemal Jelani 28 2 -Methylcyclopentanol Chem-261 DIASTEREOISOMER

DIASTEREOMERS: that are not Dr Seemal Jelani Chem-261 mirror images Refers to the relationship among two or more objects 11/1/2020 Stereoisomers 29

11/1/2020 Dr Seemal Jelani Chem-261 30

1, 2 -Cyclopentanediol OH HO H H H cis-1, 2 -Cyclopentanediol (a meso compound) H H HO OH H H HO trans- 1, 2 -Cyclopentanediol (a pair of enantiomers) Chem-261 OH diastereomers Dr Seemal Jelani H 11/1/2020 OH HO 31

PRACTICE Dr Seemal Jelani Chem-261 stereoisomers of 1, 2, 3 -butanetriol Write IUPAC names and show the R and S configuration of each stereocenter Identify enantiomers Identify diasteromers 11/1/2020 Four 32

11/1/2020 Dr Seemal Jelani Chem-261 33

Chem-261 2, 3 -Dihydroxybutanedioic acid (tartaric acid) Dr Seemal Jelani 34 11/1/2020 MESO COMPOUNDS

Dr Seemal Jelani Chem-261 sterocenters Special symmetry leads to reduce number of stereoisomers than expected by the 2 n Rule Example: 2, 3 -Dihydroxybutanedioic acid Commonly called Tartaric acid 11/1/2020 Multiple 35

Dr Seemal Jelani Chem-261 is a colorless, crystalline compound. During the fermentation of grape juice, potassium bitartrate (one carboxyl group is present as a potassium salt, -COO-K+), deposits as a crust on the sides of wine casks 11/1/2020 TA 36

Dr Seemal Jelani is collected and purified and sold commercially as a cream of tartar 11/1/2020 This Chem-261 37

(B) (A) (D) 1800 (C) 11/1/2020 Dr Seemal Jelani Chem-261 38

Chem-261 (a) and (b) are Non superposable mirror images Therefore (a) and (b) are A pair of Enantiomer Dr Seemal Jelani 2 and 3 are stereocenters 2 n 2 x 2=4 stereoisomers or stereorepresentations 11/1/2020 Carbon 39

Dr Seemal Jelani Chem-261 and (d) are mirror images but they are superposable How? Imagine that you first rotate (d) by 1800 in the plane of the paper, cut and place it on (c) In doing so we found that (d) is superposable on (c) therefore we 11/1/2020 (c) 40

11/1/2020 Dr Seemal Jelani Chem-261 Say that (c) and (d) are not different molecules, they are the same molecules just oriented differently Because (c) and its mirror image are superposable so (c) is achiral 41

ANOTHER WAY TO DETERMINE THAT (C) IS ACHIRAL Dr Seemal Jelani Chem-261 can see that it has plane of symmetry that bisect the molecule in such a way that the top half is the reflection of the bottom half Thus even though ( c) has two sterocenters it is achiral 11/1/2020 We 42

CONCLUSION Chem-261 So meso compound is an achiral compound that contains two or more stereocenters Dr Seemal Jelani stereoisomers of tartaric acid are represented ( c) or (d) is called MESO COMPOUND 11/1/2020 The 43

Dr Seemal Jelani chiral centers 2 n = 4, but only three stereoisomers exist 11/1/2020 Two Chem-261 44