8/2/2018 Stereoisomers Dr Seemal Jelani Chem-261

All molecules have a mirror image – but for most molecules it is the same molecule. 8/2/2018 Dr Seemal Jelani Chem-261

For some molecules the mirror image is a different molecule (the mirror image is non- superimposable). 8/2/2018 Dr Seemal Jelani Chem-261

8/2/2018 Dr Seemal Jelani Chem-261

Left and right hands are an example of non-superimposable mirror images. 8/2/2018 Dr Seemal Jelani Chem-261

Such molecules are said to be chiral or optically active. This 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. 8/2/2018 Dr Seemal Jelani Chem-261

The optical isomers are called enantiomers. These are distinguished by +/-, D/L or more correctly R/S. A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate. 8/2/2018 Dr Seemal Jelani Chem-261

1-chloro-3-methylpentane TASK Which of the following molecules are optically active? 8/2/2018 propan-2-ol 2-chlorobutane 1-chlorobutane 3-methylhexane butanone 2-methylbutanoic acid butan-2-ol 1-chloro-3-methylpentane Dr Seemal Jelani Chem-261

propan-2-ol 8/2/2018 NOT OPTICALLY ACTIVE Dr Seemal Jelani Chem-261

2-chlorobutane 8/2/2018 Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE

1-chlorobutane 8/2/2018 NOT OPTICALLY ACTIVE Dr Seemal Jelani Chem-261

3-methylhexane 8/2/2018 Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE

butanone 8/2/2018 NOT OPTICALLY ACTIVE Dr Seemal Jelani Chem-261

propan-2-ol 8/2/2018 NOT OPTICALLY ACTIVE Dr Seemal Jelani Chem-261

OPTICALLY ACTIVE 2-methylbutanoic acid 8/2/2018 Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE

butan-2-ol 8/2/2018 Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE

OPTICALLY ACTIVE 1-chloro-3- methylpen tane 8/2/2018 Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE

Molecules that are optical isomers are called enantiomers. 8/2/2018 Enantiomers have identical chemical and physical properties, except: Their effect on plane polarised light Their reaction with other chiral molecules Dr Seemal Jelani Chem-261

2-Chlorobutane 8/2/2018 Dr Seemal Jelani Chem-261

3-Chlorocyclohexene 8/2/2018 Dr Seemal Jelani Chem-261

Formula to calculate stereoisomers 8/2/2018 For a molecule with 1 chiral center, 2x1 = 2 stereoisomers are possible For a molecule with 2 chiral centers, a maximum of 2x2 = 4 stereoisomers are possible For a molecule with n chiral centers, a maximum of 2n stereoisomers are possible Dr Seemal Jelani Chem-261

Stereoisomers with tow chiral carbon atoms 8/2/2018 Molecules with two similar chiral center gives Enantiomers Diastereomers and Meso compounds Dr Seemal Jelani Chem-261

2,3,4-Trihydroxybutanal R S S R two chiral centers 8/2/2018 two chiral centers 22 = 4 stereoisomers exist; two pairs of enantiomers R (a) S (b) R (c) S (d) Dr Seemal Jelani Chem-261 Maximum stereoisomers= 22 =4

Maximum stereoisomers= 22 =4 8/2/2018 Maximum stereoisomers= 22 =4 (a ) and (b) are non superposable mirror images of each other therefore they are a “pair of enantiomers” Dr Seemal Jelani Chem-261

8/2/2018 2,3,4-Trihydroxybutanol exists in 2 pairs of enantiomer (a,b) and (c,d) (a) = (2R,3R) erythrose (b) = (2S,3S) erythrose (c) = (2R,3S) threose (d) = (2S,3R) threose Dr Seemal Jelani Chem-261

Both erythrose and threose belong to class carbohydrates 8/2/2018 Both erythrose and threose belong to class carbohydrates Erythrose is found in erythrocytes (RBC) Dr Seemal Jelani Chem-261

Determine the relationship 8/2/2018 Find the relationship between the following (a) 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 Chem-261

2-Methylcyclopentanol 8/2/2018 DIASTEREOISOMER Dr Seemal Jelani Chem-261 2-Methylcyclopentanol

Stereoisomers that are not mirror images Diastereomers: 8/2/2018 Stereoisomers that are not mirror images Refers to the relationship among two or more objects Dr Seemal Jelani Chem-261

8/2/2018 Dr Seemal Jelani Chem-261

1,2-Cyclopentanediol O H H O O H H O H H H H cis- 1,2-Cyclopentanediol 8/2/2018 O H H O O H H O H H H H cis- 1,2-Cyclopentanediol (a meso compound) diastereomers Dr Seemal Jelani Chem-261 O H H H H O H H O O H H trans- 1,2-Cyclopentanediol (a pair of enantiomers)

Practice Four stereoisomers of 1,2,3- butanetriol 8/2/2018 Four stereoisomers of 1,2,3- butanetriol Write IUPAC names and show the R and S configuration of each stereocenter Identify enantiomers Identify diasteromers Dr Seemal Jelani Chem-261

8/2/2018 Dr Seemal Jelani Chem-261

MESO COMPOUNDS 2,3-Dihydroxybutanedioic acid (tartaric acid) 8/2/2018 Dr Seemal Jelani Chem-261 2,3-Dihydroxybutanedioic acid (tartaric acid)

Multiple sterocenters 8/2/2018 Multiple sterocenters Special symmetry leads to reduce number of stereoisomers than expected by the 2n Rule Example: 2,3- Dihydroxybutanedioic acid Commonly called Tartaric acid Dr Seemal Jelani Chem-261

TA is a colorless, crystalline compound. 8/2/2018 TA 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 Dr Seemal Jelani Chem-261

8/2/2018 This is collected and purified and sold commercially as a cream of tartar Dr Seemal Jelani Chem-261

(c) (d) (a) (b) 1800 8/2/2018 Dr Seemal Jelani Chem-261

Carbon 2 and 3 are stereocenters 8/2/2018 Carbon 2 and 3 are stereocenters 2n 2x2=4 stereoisomers or stereorepresentations (a) and (b) are Non superposable mirror images Therefore (a) and (b) are A pair of Enantiomer Dr Seemal Jelani Chem-261

(c) and (d) are mirror images but they are superposable How? 8/2/2018 (c) 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 Dr Seemal Jelani Chem-261

8/2/2018 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 Dr Seemal Jelani Chem-261

Another way to determine that (c) is Achiral 8/2/2018 We 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 Dr Seemal Jelani Chem-261

conclusion 8/2/2018 The stereoisomers of tartaric acid are represented ( c) or (d) is called MESO COMPOUND So meso compound is an achiral compound that contains two or more stereocenters Dr Seemal Jelani Chem-261

2n = 4, but only three stereoisomers exist 8/2/2018 Two chiral centers 2n = 4, but only three stereoisomers exist Dr Seemal Jelani Chem-261