Chapter 9 INTRODUCTION TO STEREOCHEMISTRY

Isomers are compounds with the same molecular formula but not identical structures

Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected

Constitutional Isomers Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected

start by connecting the carbons in a line determine the C skeleton of the other isomers Drawing Constitutional or Structural Isomers of Alkanes Example 8.4 Write all the constitutional isomers having the molecular formula C 6 H 14 Example 8.4 Write all the constitutional isomers having the molecular formula C 6 H 14

fill in the H to give each C 4 bonds Example 8.4 Write all the constitutional isomers having the molecular formula C 6 H 14 Example 8.4 Write all the constitutional isomers having the molecular formula C 6 H 14

convert each to a carbon skeleton formula – each bend and the ends represent C atoms Example 8.4 Write all the constitutional isomers having the molecular formula C 6 H 14 Example 8.4 Write all the constitutional isomers having the molecular formula C 6 H 14

Stereoisomers have the same molecular formula, maintain the same connectivity, but differ in the way their atoms are arranged in space

Conformational isomers (or conformers or rotational isomers or rotamers) are stereoisomers produced by rotation about single bonds, and are often rapidly interconverting at room temperature

Conformations of Alkanes and Cycloalkanes Conformations of Ethane ethane Staggered conformation of ethane Eclipsed conformation of ethane Sawhorse Representation Newman Projection

ethane Staggered conformation of ethane Eclipsed conformation of ethane Sawhorse Representation Newman Projection

Conformations of Alkanes and Cycloalkanes Conformations of Butane butane Staggered conformation of butane Eclipsed conformation of butane

Staggered conformation of butane Eclipsed conformation of butane Conformations of Alkanes and Cycloalkanes

Configurational Isomers are stereoisomers that do not readily interconvert at room temperature and can (in principle at least) be separated.

Geometric isomers are configurational isomers that differ in the spatial position around a bond with restricted rotation (e.g. a double bond):

Geometric (Cis and Trans) Isomers Geometric (Cis and Trans) Isomers result from restriction rotation Compounds with double bonds cis isomer – have same substituents on the same side of the double bond (= Z with more complex molecules having high priority groups on the same side) trans isomer – have the same substituents on the opposite side of the double bond (= E with more complex molecules having high priority groups on opposite sides) Compounds with bonds in a ring: cis isomer – have the same substituents on the same side of the ring trans isomer - have the same substituents on the opposite side of the ring Geometric (Cis and Trans) Isomers result from restriction rotation Compounds with double bonds cis isomer – have same substituents on the same side of the double bond (= Z with more complex molecules having high priority groups on the same side) trans isomer – have the same substituents on the opposite side of the double bond (= E with more complex molecules having high priority groups on opposite sides) Compounds with bonds in a ring: cis isomer – have the same substituents on the same side of the ring trans isomer - have the same substituents on the opposite side of the ring

Cis-trans (Geometric) isomerism in Alkenes cis-1,2-dichloroethenetrans-1,2-dichloroethene (E)-1,2-dichloroethene (Z)-1,2-dichloroethene

Cis-trans (Geometric) isomerism in Alkenes 1,1-dichloroethene *If one of the two carbon atoms of the double bond has two identical substituents, there are no cis-trans isomers for that molecule

Identifying cis and trans isomers of Alkenes Example 11.3 Two isomers of 2-butene are shown below. Which is the cis isomer and which is the trans isomer Example 11.3 Two isomers of 2-butene are shown below. Which is the cis isomer and which is the trans isomer cis-2-butenetrans-2-butene

Naming cis and trans compounds Example 11.4 Name the following geometric isomers. Example 11.4 Name the following geometric isomers. trans-3,4-dichloro-3-heptene cis-3,4-dimethyl-3-octene

Identifying Geometric Isomers Example 11.5 Determine whether each of the following molecules can exist as cis-trans isomers: (1)1-pentene (2)3-ethyl-3-hexene (3)3-methyl-2-pentene Example 11.5 Determine whether each of the following molecules can exist as cis-trans isomers: (1)1-pentene (2)3-ethyl-3-hexene (3)3-methyl-2-pentene 1-pentene3-ethyl-3-hexene cis-3-methyl-2-pentenetrans-3-methyl-2-pentene

cis-9-octadecenoic acid Cis and Trans Fatty Acids trans-9-octadecenoic acid

Cis and Trans Fatty Acids

Cis-trans (Geometric) isomerism in Cycloalkanes Cis-trans isomers are molecules having the same arrangement of atoms but differ in the spatial orientation of their substituents. cis-1,2-dichlorocyclohexanetrans-1,2-dichlorocyclohexane

Naming cis-trans Isomers of Substituted Cycloalkanes Example 10.6 Determine whether the following susbstituted cycloalkanes are cis or trans isomers. Example 10.6 Determine whether the following susbstituted cycloalkanes are cis or trans isomers. trans-1,2-dimethylcyclopentanecis-1,2-dimethylcyclopentane

Conformations of Alkanes and Cycloalkanes Conformations of Cyclohexane cyclohexane Chair conformation of cyclohexane Boat conformation of cyclohexane

Chair conformation of cyclohexane Boat conformation of cyclohexane

Chair-Chair Interconversion

Optical isomers are configurational isomers that differ in the 3D relationship of the substituents about one or more atoms.

Diastereomers are optical isomers (stereoisomers) that are not enantiomers.

Enantiomers are optical isomers that are non-superimposable mirror images.

Chirality Chiral objects are objects with left-handed and right-handed forms Achiral objects - objects that have superimposable mirror images Nonsuperimposable mirror images - a mirror image that is not the same as the image itself - chiral objects have nonsuperimposable mirror images Chiral objects are objects with left-handed and right-handed forms Achiral objects - objects that have superimposable mirror images Nonsuperimposable mirror images - a mirror image that is not the same as the image itself - chiral objects have nonsuperimposable mirror images

Assymetric Center Chirality is not reserved just for objects - molecules can be chiral Chiral molecules - generally molecules containing an asymmetric center Asymmetric (chiral) center - tetrahedral atom bonded to four different groups - indicated with an asterisk (*) Chirality is not reserved just for objects - molecules can be chiral Chiral molecules - generally molecules containing an asymmetric center Asymmetric (chiral) center - tetrahedral atom bonded to four different groups - indicated with an asterisk (*)

Chiral Molecules with One Asymmetric Center Molecules with one chiral center would have 2 enantiomers *Chiral molecules would have 2 n enantiomers (where n is the number of chiral centers) Molecules with one chiral center would have 2 enantiomers *Chiral molecules would have 2 n enantiomers (where n is the number of chiral centers)

Chiral vs Achiral

How to Represent Enantiomers: Perspective Formulas Perspective Formulas - shows two bonds of the asymmetric center as lines in the plane of the paper, another bond as a solid wedge protruding forward out of the paper, and the fourth bond as a hatched wedge extending behind the paper

Fischer Projections Fisher Projection - representation of an asymmetric center as the point of intersection of two perpendicular lines Horizontal lines represent bonds that project out of the plane of the paper Vertical lines represent bonds that extend back from the plane of the paper away from the viewer Fisher Projection - representation of an asymmetric center as the point of intersection of two perpendicular lines Horizontal lines represent bonds that project out of the plane of the paper Vertical lines represent bonds that extend back from the plane of the paper away from the viewer

Fischer Projections

Naming Enantiomers -R,S System STEP 1. Rank the groups/atoms bonded to the asymmetric center in order of priority. - rank in terms of atomic mass. Higher atomic mass, higher priority. - let’s have 1-chloro-1-ethanol as an example. 1.Chlorine = 36 amu 2.Oxygen = 16 amu 3.Carbon = 12 amu 4.Hydrogen = 1 amu STEP 2. Rotate the molecule so that the lowest priority group is pointing away from the reader. STEP 1. Rank the groups/atoms bonded to the asymmetric center in order of priority. - rank in terms of atomic mass. Higher atomic mass, higher priority. - let’s have 1-chloro-1-ethanol as an example. 1.Chlorine = 36 amu 2.Oxygen = 16 amu 3.Carbon = 12 amu 4.Hydrogen = 1 amu STEP 2. Rotate the molecule so that the lowest priority group is pointing away from the reader.

Naming Enantiomers -R,S System Step 3. Trace your finger around the three highest priority groups in order of the priority. - If the circle is moving counterclockwise the CIP designation is “S”. If the circle is moving clockwise the CIP designation is “R”. Step 4. Name the enantiomer in terms of its R or S configuration. - the name for this isomer is (S)-1-chloro-1-ethanol Step 3. Trace your finger around the three highest priority groups in order of the priority. - If the circle is moving counterclockwise the CIP designation is “S”. If the circle is moving clockwise the CIP designation is “R”. Step 4. Name the enantiomer in terms of its R or S configuration. - the name for this isomer is (S)-1-chloro-1-ethanol

Naming Enantiomers -R,S System Assigning Priorities to Groups 1.Oxygen (from CH 2 OH) = 16 2.Carbon (from CH 2 CH 3 ) = attached to carbon 3.Carbon (from CH 3 ) = attached to hydrogen 4.Hydrogen = 1 Assigning Priorities to Groups 1.Oxygen (from CH 2 OH) = 16 2.Carbon (from CH 2 CH 3 ) = attached to carbon 3.Carbon (from CH 3 ) = attached to hydrogen 4.Hydrogen = 1

Naming Enantiomers -R,S System