Chapter 4 Stereochemistry Introduction To Stereochemistry Consider two of the compounds we produced while finding all the isomers of C7H16 CH3 CH3 2 methylhexane Me 3 methylhexane Me Me C Me H 2 methylhexane Bu Bu Me Me H H mirror Me rotate Bu Me H 2 methylhexame is superimposable with its mirror image Introduction To Stereochemistry Consider two of the compounds we produced while finding all the isomers of C7H16 CH3 CH3 2 methylhexane 3 methylhexane H C Et Et Me 3 methylhexane Pr Pr Me Me H H mirror Et rotate H Me Pr 2 methylhexame is superimposable with its mirror image Introduction To Stereochemistry Consider two of the compounds we produced while finding all the isomers of C7H16 CH3 CH3 2 methylhexane 3 methylhexane Compounds that are not superimposable with their mirror image are called chiral in Greek chiral means handed 3 methylhexane is a chiral molecule Compounds that are superimposable with their mirror image are called achiral 2 methylhexane is an achiral molecule An atom usually carbon with 4 different substituents is called a stereogenic center or stereocenter Enantiomers Et Et Pr Pr Me CH3 3 methylhexane Me H H mirror enantiomers Et Et Pr Pr Me Me H H Me Me H H Two compounds that are non superimposable mirror images the two hands are called enantiomers Introduction To Stereochemistry Structural constitutional Isomers Compounds of the same molecular formula with different connectivity structure constitution 2 methylpentane 3 methylpentane Conformational Isomers Compounds of the same structure that differ in rotation around one or more single bonds Me H H H H H H Me Me Me H H Configurational Isomers or Stereoisomers Compounds of the same structure that differ in one or more aspects of stereochemistry how groups are oriented in space enantiomers or diastereomers We need a a way to describe the stereochemistry Me H 3 methylhexane H Me 3 methylhexane The CIP System Revisited 1 Rank each substituent attached to the stereocenter according to the CIP priority system 1 highest 4 lowest 2 1 Me 3 2 H 4 1 H 4 Me 3 2 View the compound with the lowest priority substituent pointing away from you 2 Me 3 2 Et 1 1 Pr 2 Et 1 Pr H4 H 4 2 H4 Me 3 1 H 4 Me 3 Me 3 3 Draw a circular arrow from connecting substituents 1 3 from highest to lowest priority If the arrow moves clockwise the sterocenter is labeled R this stands for rectus If the arrow moves counterclockwise then the stereocenter is labeled S this stands for sinister 2 Me H S 3 methylhexane Et Pr 1 1 2 Pr Et H4 H4 Me 3 Me 3 counterclockwise clockwise H Me R 3 methylhexane The CIP System Revisited H3C CH3 CH3 H F H CH3 H3C O Br H H H3C CH3 CH3 HO CH3 The CIP System Revisited 1 F H3C 3 4 H F CH3 2 H3C CH3 CH3 4 H 1 H H3C 2 2 3 CH3 R 2 fluorobutane S 3 tert butylcyclohexene 1 Br O 4 H 3 H H3C 2 CH3 3 S 4 bromo 2 methyl 1 pentene 2 HO 1 H3C CH3 H3C H 4 1 CH3 CH3 4 R 2 hydroxy 2 methyl butanal 3 Properties of Chiral Molecules Chiral objects can only be recognized as chiral by another chiral object O O Carvone H H Which is S and which is R Your nose can tell O O 3 2 Carvone 3 2 H 4 H 4 1 R 1 S Properties of Chiral Molecules Chiral objects can only be recognized as chiral by another chiral object O O Carvone H H Which is S and which is R Your nose can tell O O 3 2 Carvone 3 2 H 4 H 4 1 R 1 S Properties of Chiral Molecules Chiral objects can only be recognized as chiral by another chiral object O O H R carvone odor of spearmint H S carvone odor of caraway seed Multiple Stereocenters 1 stereocenter 2 stereoisomers Me H H Me R 3 methylhexane S 3 methylhexane Cl 2 stereocenter 4 stereoisomers Cl Me Me Cl H H 3 S 4 S 3 chloro 4 methylhexane Me H H 3 R 4 S 3 chloro 4 methylhexane Cl Me H H 3 S 4 R 3 chloro 4 methylhexane Cl Me H H 3 R 4 R 3 chloro 4 methylhexane Multiple Stereocenters enantiomers Cl Cl H H 3 S 4 S 3 chloro 4 methylhexane 3 R 4 R 3 chloro 4 methylhexane Me Me H H What about these relationships Cl H Cl 3 R 4 S 3 chloro 4 methylhexane H 3 S 4 R 3 chloro 4 methylhexane Me H Me H enantiomers Stereoisomers that are not enantiomers non superimposable mirror images are called diastereomers Multiple Stereocenters Br CH3 Cl 3 stereocenters R R R S S S S R R R S S R S R S R S R R S S S R 8 stereoisomers For any compound the maximum number of stereoisomers is 2n where n is the number of stereocenters Multiple Stereocenters Me How many stereoisomers for 3 4 dimethylhexane Me H Me Me Me H H 3 4 dimethylhexane Me Me H H Me H Me Me The same compound superimposable Me 3 4 dimethylhexane Me H H H H H H 3S 4S 3 4 dimethylhexane Me Me 3R 4R 3 4 dimethylhexane H H H Me Me Me H Me enantiomers Meso Compounds Me 3 4 dimethylhexane has 3 stereoisomers Me H Me H H meso 3 4 dimethylhexane Me Me H Me Me H H Me Me meso 3 4 dimethylhexane H H Me Me A Meso Compound achiral Me H H H H 3S 4S 3 4 dimethylhexane Me H H Me Me 3R 4R 3 4 dimethylhexane H Me Me H Me Enantiomers chiral Meso Compounds The best way to identify a meso compound is to prove that it is superimposable with its mirror image However a quick test is to see if it contains a plane of symmetry plane of symmetry Me Me H H meso 3 4 dimethylhexane Compounds containing a plane of symmetry are achiral Me H H Me 3S 4S 3 4 dimethylhexane H Me Me H 3R 4R 3 4 dimethylhexane No plane of symmetry The Isomers of C3H4Cl2 Revisited Cl Cl Cl Cl Cl CH3 Cl Cl Cl CH3 Cl Cl Cl Cl CH2 Cl Cl Cl Cl Cl Cl Cl Cl The Isomers of C3H4Cl2 Revisited Cl Cl Cl Cl Cl CH3 Cl Cl Cl CH3 Cl Cl Cl Cl Cl CH2 Cl Cl Cl Cl Cl Cl Cl Cl Cl Racemic Mixtures H H H HBr H3C CH3 H3C Br addition reaction CH3 But do we get R or S H H E 2 butene 2 bromobutane achiral chiral We get both CH3 H3C H 50 Br S 2 bromobutane CH3 H3C Br H 50 R 2 bromobutane A 50 50 mixture of 2 enantiomers is called a racemic mixture or a racemate Racemic Mixtures In Medicine CH3 HO CH3 HO CH3 O CH3 …