chapter 9 introduction to stereochemistry
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 9
INTRODUCTION TO STEREOCHEMISTRY
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Isomers are compounds with the same molecular formula but not identical structures
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Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected
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Constitutional Isomers
H3CCH2
H2C
CH3
butaneChemical Formula: C4H10
CH3
CHCH3H3C
2-methylpropaneChemical Formula: C4H10
Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected
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start by connecting the carbons in a line
determine the C skeleton of the other isomers
C C C C C CC C C C C
C
C C C C C
CC C C C
C
CC C C C
C C
Drawing Constitutional or Structural Isomers of Alkanes
Example 8.4Write all the constitutional isomers having the molecular formula C6H14
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fill in the H to give each C 4 bonds
C C C C
C
CH
H
H
H H H
H
H
H
HHHH
H
C C C C
C
C
H
H
H H H
HHHH
H
H
H
H
H
C C C C C CH
H
H
H
H
H
H
H
H
H H
H
HH
C C C C
C
H
H
H H
HHHH
H
HH
C H
H
H
C C C C C C
C C C C C
C
C C C C C
C
C C C C
C
CC C C C
C C
C C C C
C
H
H
H H
HCHH
H
HH
H
HH
H
Example 8.4Write all the constitutional isomers having the molecular formula C6H14
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convert each to a carbon skeleton formula – each bend and the ends represent C atoms
C C C C
C
CH
H
H
H H H
H
H
H
HHHH
H
C C C C
C
C
H
H
H H H
HHHH
H
H
H
H
H
C C C C C CH
H
H
H
H
H
H
H
H
H H
H
HH
C C C C
C
H
H
H H
HHHH
H
HH
C H
H
H
C C C C
C
H
H
H H
HCHH
H
HH
H
HH
H
Example 8.4Write all the constitutional isomers having the molecular formula C6H14
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Stereoisomers have the same molecular formula, maintain the same connectivity, but differ in the way their atoms are arranged in space
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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
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Conformations of Alkanes and Cycloalkanes
Conformations of Ethane
H3C CH3ethane
Staggered conformation of
ethane
Eclipsed conformation of
ethane
H
H HH
HH
H
H
H
H
HHH
H H
H
H H
H
H H
H
HH
Sawhorse Representation
Sawhorse Representation
Newman Projection
Newman Projection
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ethane
Staggered conformation of
ethane
Eclipsed conformation of
ethane
H
H HH
HH
H
H
H
H
HHH
H H
H
H H
H
H H
H
HH
Sawhorse Representation
Sawhorse Representation
Newman Projection
Newman Projection
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Conformations of Alkanes and Cycloalkanes
Conformations of Butane
butane
Staggered conformation of
butane
Eclipsed conformation of
butane
H3C
H2C
CH2
CH3
CH3
H HCH3
HHH
H3C
H
CH3
HH
CH3
H H
CH3
HH
CH3
H H
CH3
H H
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Staggered conformation of
butane
Eclipsed conformation of
butane
CH3
H HCH3
HHH
H3C
H
CH3
HH
CH3
H H
CH3
HH
CH3
H H
CH3
H H
Conformations of Alkanes and Cycloalkanes
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Configurational Isomers are stereoisomers that do not readily interconvert at room temperature and can (in principle at least) be separated.
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Geometric isomers are configurational isomers that differ in the spatial position around a bond with restricted rotation (e.g. a double bond):
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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
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Cis-trans (Geometric) isomerism in Alkenes
C C
ClCl
H H
cis-1,2-dichloroethene
C C
ClH
Cl H
trans-1,2-dichloroethene
(E)-1,2-dichloroethene(Z)-1,2-dichloroethene
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Cis-trans (Geometric) isomerism in Alkenes
C C
ClH
H Cl
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
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Identifying cis and trans isomers of Alkenes
Example 11.3Two isomers of 2-butene are shown below. Which is the cis isomer and which is the trans isomer
C C
H
H3C
H
CH3
C C
H
H3C
CH3
H
cis-2-butene trans-2-butene
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Naming cis and trans compounds
Example 11.4Name the following geometric isomers.
trans-3,4-dichloro-3-heptene cis-3,4-dimethyl-3-octene
H3C
H2C
CH2
CC
Cl
Cl
H2C
CH3
H3CCH2
CH3C C
H2C
CH2
H2C
CH3
CH3
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Identifying Geometric Isomers
Example 11.5Determine 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-pentene 3-ethyl-3-hexene
CH2CH3
CC
H3CH2C CH2CH3
H
C C
CH2CH2CH3H
H H
cis-3-methyl-2-pentene
C C
CH3H3C
H3CH2C H
C C
HH3C
H3CH2C CH3
trans-3-methyl-2-pentene
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cis-9-octadecenoic acid
Cis and Trans Fatty Acids
O
OH
H H
O
OH
H
H
trans-9-octadecenoic acid
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Cis and Trans Fatty Acids
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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.
H
H
H
H
H
ClCl
H
HH
H H
cis-1,2-dichlorocyclohexane
H
H
H
H
H
ClH
Cl
HH
H H
trans-1,2-dichlorocyclohexane
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Naming cis-trans Isomers of Substituted Cycloalkanes
Example 10.6Determine whether the following susbstituted cycloalkanes are cis or trans isomers.
trans-1,2-dimethylcyclopentane cis-1,2-dimethylcyclopentane
H
H
H
H
H CH3
HCH3 H
H
CH3
CH3
H
H
H
H
CH3CH3
HH H
H
CH3 CH3
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Conformations of Alkanes and Cycloalkanes
Conformations of Cyclohexane
cyclohexaneChair
conformation of cyclohexane
Boat conformation of
cyclohexane
H
H
H
H
H
HH
H
HH
H H
A E
A
E
A
A
E
E
A
E
A
E
A
E
A
E
A
A
E
A
E
A
E
E
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Chair conformation of
cyclohexane
Boat conformation of
cyclohexane
A E
A
E
A
A
E
E
A
E
A
E
A
E
A
E
A
A
E
A
E
A
E
E
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Chair-Chair Interconversion
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Optical isomers are configurational isomers that differ in the 3D relationship of the substituents about one or more atoms.
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Diastereomers are optical isomers (stereoisomers) that are not enantiomers.
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Enantiomers are optical isomers that are non-superimposable mirror images.
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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
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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 (*)
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Chiral Molecules with One Asymmetric Center
Molecules with one chiral center would have 2 enantiomers*Chiral molecules would have 2n enantiomers (where n is the number of chiral centers)
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Chiral vs Achiral
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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
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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
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Fischer Projections
Br
H
CH3H3CH2C
Br
H
CH2CH3H3C
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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 amu4. Hydrogen = 1 amu
STEP 2. Rotate the molecule so that the lowest priority group is pointing away from thereader.
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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
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Naming Enantiomers -R,S System
Assigning Priorities to Groups
1. Oxygen (from CH2OH) = 162. Carbon (from CH2CH3) = attached to carbon3. Carbon (from CH3) = attached to hydrogen4. Hydrogen = 1
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Naming Enantiomers -R,S System
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