CHEM10009 Study Guide - Final Guide: Chemical Formula, Molar Mass, Stereoisomerism
11 Jun 2018
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Isomers: compounds with same molecular formula but different structural formulas or
diff spatial arrangement
Constitutional (structural) isomers: same molecular formula and molar mass - but
atoms linked differently -> diff. physical properties
Stereoisomers (spatial isomers): compounds that share identical molecular formulas,
atom-to-atom linkages, and bonding distances, but differ in their 3D arrangement
Configurational Isomers
Diastereomers: non-superimposable, non-mirror images (requires all diff
groups)
Cis-Trans Isomers: arise from compounds with restricted rotation
(eg. Double bond)
Cis have substituents in same direction□
Trans have substituents in opposing directions□
Diastomers containing a chiral centre
Enantiomers: non-superimposable, mirror-images
Identical physical and chemical properties except when they interact
with chiral systems
§
Differ in the way they interact with plane polarised light
§
Example: lactic acid - one stereogenic centre -> 2 enantiomers exist
§
Conformational isomers:
Free rotation about a sigma-bond
○
Infinite array of conformations is possible through rotation
○
Conformer is one specific conformation of molecule
○
Rotamers
Atropisomers: arise because of hindered rotation about a single bond,
where energy differences due to steric strain or other contributors create a
barrier to rotation that is high enough to allow for isolation of individual
conformers
Ring inversion Isomers
Newman projections
Staggered conformers are more stable (lower in energy)
Reduced steric interactions
○
-
Conformers of butane
-
Conformers of Acetylcholine
Acetylcholine - ester of acetic acid and choline - functions in brain and body
of many types of animals - eg. In humans as neurotransmitter
○
-
Conformation of long-chain alkanes
The most stable conformer of long-chain alkanes is when all anti-staggered
conformations are adopted -> zig-zag structure
-
Cycloalkanes
Cyclic structure
-
Formula CnH2n
-
Cyclopropane
Highly strained
○
Unfavourable bond angles + unfavourable eclipsing interactions
○
-
Cyclobutane
High strain, but less than cyclobutane
○
-
Cyclopentane
-
Cyclohexane
6-membered rings are strain free
○
All C-C-C angles close to optimal 109.5 for tetrahedral + all staggered
○
-
This is possible by adopting a 'chair' conformation
2 types of hydrogen environments; axial and equatorial
§
○
Ring-flipping -ring inversion isomers
2 chair conformations exist which interconvert
§
Ring-flipping interchanges axial
§
○
Substituted cyclohexanes
Equatorial substituents result in a more stable conformation due to
decreased steric interactions
§
○
Disubstituted cyclohexane's
Rings prevent free rotation about single bonds
§
Isomers can exist when two or more substituents are attached
§
Isomers don’t interconvert (would require breaking of C-C bonds)
§
STEREOISOMERS: differ by arrangement of atoms in space
§
Also called cis/trans isomers - diastereomers
§
○
Tetrahedral carbon with 4 diff groups is NOT superimposable on its mirror
image
They are isomers!!
§
If 2 or 3 identical groups -> mirror images
§
○
Cycloalkanes
-
WEDGE BOND (OUT OF PAGE) -> UP
○
DASHED BOND (INTO PAGE) -> DOWN
○
Chirality (handedness)
Molecules that possess non-superimposable mirror images are chiral
-
Tetrahedral carbon with 4 diff groups has no plane of symmetry -stereogenic
centre
-
Enantiomers and plane polarised light
All chiral compounds rotate plane of polarised light to some extent - optically
active
-
Enantiomers rotate in opposite directions
-
R and S descriptors
Rank substituents
-
Determine clockwise or anticlockwise
-
Assing R or S
-
Decreasing order of atomic number 1)
If 2 directly attached atoms are same - compare second atoms 2)
Treat multiple bonds as same as single bonds x23)
View molecule so group 4 is in the back 4)
Examples:
If lowest priority is pointing out of the page
Rotate molecule so it is pointing into page -> normal rules
§
Reverse the anticlockwise/clockwise rule
§
Use thumb
§
○
Racemic mixture = equal mix of enantiomers
Same physical and chemical properties
§
○
More than one asymmetric centre
Each asymmetric centre has 2 possible configurations (R or S)
§
Molecule with n centres has max of 2^n stereoisomers
§
○
These molecules are stereoisomers - diastereomers - non-mirror images
○
Diastereomers
All asymmetric centres of opposite configuration = enantiomers•
At least one asymmetric centre of same configuration, at least one opposite =
diastereomers
•
Separation of Enantiomers
Enantiomers
Identical physical and chemical properties -> cannot be separated easily
○
•
Diastereomers
Diff 3D shape -> diff chemical and physical properties
○
•
To separate enantiomers, convert them into diastereomers, separate them, and
re-transform them into enantiomers
•
-
Alkenes
CnH2n
-
C=C double bond
-
Planar arrangement
-
Trigonal planar
-
Direct overlap of sp2 hybrid orbitals -> sigma bond
-
Sideways indirect overlap of unhybridised p orbitals -> pi bond
Pi is weaker than sigma due to less efficient orbital overlap •
-
Isomers - stereoisomers possible
-
1 and 2 are structural isomers
2 and 4 are stereoisomers - diastereomers
Structural requirements for alkene diastereomers
Each end of C=C bond must have 2 diff groups
○
•
E/Z - nomenclature
Assign priorities to group at each end of double bond1)
High priority same side -> Z OR high priority opposite -> E 2)
Cis/trans isomerism in bio
11-trans-retinal -> 11-cis-retinal
-
Light absorption causes double bond isomerism, hydrolysis and dissociation of 11-
trans-retinal from opsin
-
Organic Chemistry
Monday, 28 May 2018
10:19 pm
Isomers: compounds with same molecular formula but different structural formulas or
diff spatial arrangement
Constitutional (structural) isomers: same molecular formula and molar mass - but
atoms linked differently -> diff. physical properties
Stereoisomers (spatial isomers): compounds that share identical molecular formulas,
atom-to-atom linkages, and bonding distances, but differ in their 3D arrangement
Configurational Isomers
Diastereomers: non-superimposable, non-mirror images (requires all diff
groups)
Cis-Trans Isomers: arise from compounds with restricted rotation
(eg. Double bond)
Cis have substituents in same direction□
Trans have substituents in opposing directions□
Diastomers containing a chiral centre
Enantiomers: non-superimposable, mirror-images
Identical physical and chemical properties except when they interact
with chiral systems
§
Differ in the way they interact with plane polarised light
§
Example: lactic acid - one stereogenic centre -> 2 enantiomers exist
§
Conformational isomers:
Free rotation about a sigma-bond
○
Infinite array of conformations is possible through rotation
○
Conformer is one specific conformation of molecule
○
Rotamers
Atropisomers: arise because of hindered rotation about a single bond,
where energy differences due to steric strain or other contributors create a
barrier to rotation that is high enough to allow for isolation of individual
conformers
Ring inversion Isomers
Newman projections
Staggered conformers are more stable (lower in energy)
Reduced steric interactions
○
-
Conformers of butane
-
Conformers of Acetylcholine
Acetylcholine - ester of acetic acid and choline - functions in brain and body
of many types of animals - eg. In humans as neurotransmitter
○
-
Conformation of long-chain alkanes
The most stable conformer of long-chain alkanes is when all anti-staggered
conformations are adopted -> zig-zag structure
-
Cycloalkanes
Cyclic structure
-
Formula CnH2n
-
Cyclopropane
Highly strained
○
Unfavourable bond angles + unfavourable eclipsing interactions
○
-
Cyclobutane
High strain, but less than cyclobutane
○
-
Cyclopentane
-
Cyclohexane
6-membered rings are strain free
○
All C-C-C angles close to optimal 109.5 for tetrahedral + all staggered
○
-
This is possible by adopting a 'chair' conformation
2 types of hydrogen environments; axial and equatorial
§
○
Ring-flipping -ring inversion isomers
2 chair conformations exist which interconvert
§
Ring-flipping interchanges axial
§
○
Substituted cyclohexanes
Equatorial substituents result in a more stable conformation due to
decreased steric interactions
§
○
Disubstituted cyclohexane's
Rings prevent free rotation about single bonds
§
Isomers can exist when two or more substituents are attached
§
Isomers don’t interconvert (would require breaking of C-C bonds)
§
STEREOISOMERS: differ by arrangement of atoms in space
§
Also called cis/trans isomers - diastereomers
§
○
Tetrahedral carbon with 4 diff groups is NOT superimposable on its mirror
image
They are isomers!!
§
If 2 or 3 identical groups -> mirror images
§
○
Cycloalkanes
-
WEDGE BOND (OUT OF PAGE) -> UP
○
DASHED BOND (INTO PAGE) -> DOWN
○
Chirality (handedness)
Molecules that possess non-superimposable mirror images are chiral
-
Tetrahedral carbon with 4 diff groups has no plane of symmetry -stereogenic
centre
-
Enantiomers and plane polarised light
All chiral compounds rotate plane of polarised light to some extent - optically
active
-
Enantiomers rotate in opposite directions
-
R and S descriptors
Rank substituents
-
Determine clockwise or anticlockwise
-
Assing R or S
-
Decreasing order of atomic number 1)
If 2 directly attached atoms are same - compare second atoms 2)
Treat multiple bonds as same as single bonds x23)
View molecule so group 4 is in the back 4)
Examples:
If lowest priority is pointing out of the page
Rotate molecule so it is pointing into page -> normal rules
§
Reverse the anticlockwise/clockwise rule
§
Use thumb
§
○
Racemic mixture = equal mix of enantiomers
Same physical and chemical properties
§
○
More than one asymmetric centre
Each asymmetric centre has 2 possible configurations (R or S)
§
Molecule with n centres has max of 2^n stereoisomers
§
○
These molecules are stereoisomers - diastereomers - non-mirror images
○
Diastereomers
All asymmetric centres of opposite configuration = enantiomers•
At least one asymmetric centre of same configuration, at least one opposite =
diastereomers
•
Separation of Enantiomers
Enantiomers
Identical physical and chemical properties -> cannot be separated easily
○
•
Diastereomers
Diff 3D shape -> diff chemical and physical properties
○
•
To separate enantiomers, convert them into diastereomers, separate them, and
re-transform them into enantiomers
•
-
Alkenes
CnH2n
-
C=C double bond
-
Planar arrangement
-
Trigonal planar
-
Direct overlap of sp2 hybrid orbitals -> sigma bond
-
Sideways indirect overlap of unhybridised p orbitals -> pi bond
Pi is weaker than sigma due to less efficient orbital overlap •
-
Isomers - stereoisomers possible
-
1 and 2 are structural isomers
2 and 4 are stereoisomers - diastereomers
Structural requirements for alkene diastereomers
Each end of C=C bond must have 2 diff groups
○
•
E/Z - nomenclature
Assign priorities to group at each end of double bond1)
High priority same side -> Z OR high priority opposite -> E 2)
Cis/trans isomerism in bio
11-trans-retinal -> 11-cis-retinal
-
Light absorption causes double bond isomerism, hydrolysis and dissociation of 11-
trans-retinal from opsin
-
Organic Chemistry
Monday, 28 May 2018
10:19 pm
Isomers: compounds with same molecular formula but different structural formulas or
diff spatial arrangement
Constitutional (structural) isomers: same molecular formula and molar mass - but
atoms linked differently -> diff. physical properties
Stereoisomers (spatial isomers): compounds that share identical molecular formulas,
atom-to-atom linkages, and bonding distances, but differ in their 3D arrangement
Configurational Isomers
Diastereomers: non-superimposable, non-mirror images (requires all diff
groups)
Cis-Trans Isomers: arise from compounds with restricted rotation
(eg. Double bond)
Cis have substituents in same direction
□
Trans have substituents in opposing directions
□
Diastomers containing a chiral centre
Enantiomers: non-superimposable, mirror-images
Identical physical and chemical properties except when they interact
with chiral systems
§
Differ in the way they interact with plane polarised light
§
Example: lactic acid - one stereogenic centre -> 2 enantiomers exist
§
Conformational isomers:
Free rotation about a sigma-bond
○
Infinite array of conformations is possible through rotation
○
Conformer is one specific conformation of molecule
○
Rotamers
Atropisomers: arise because of hindered rotation about a single bond,
where energy differences due to steric strain or other contributors create a
barrier to rotation that is high enough to allow for isolation of individual
conformers
Ring inversion Isomers
Newman projections
Staggered conformers are more stable (lower in energy)
Reduced steric interactions
○
-
Conformers of butane
-
Conformers of Acetylcholine
Acetylcholine - ester of acetic acid and choline - functions in brain and body
of many types of animals - eg. In humans as neurotransmitter
○
-
Conformation of long-chain alkanes
The most stable conformer of long-chain alkanes is when all anti-staggered
conformations are adopted -> zig-zag structure
-
Cycloalkanes
Cyclic structure
-
Formula CnH2n
-
Cyclopropane
Highly strained
○
Unfavourable bond angles + unfavourable eclipsing interactions
○
-
Cyclobutane
High strain, but less than cyclobutane
○
-
Cyclopentane
-
Cyclohexane
6-membered rings are strain free
○
All C-C-C angles close to optimal 109.5 for tetrahedral + all staggered
○
-
This is possible by adopting a 'chair' conformation
2 types of hydrogen environments; axial and equatorial
§
○
Ring-flipping -ring inversion isomers
2 chair conformations exist which interconvert
§
Ring-flipping interchanges axial
§
○
Substituted cyclohexanes
Equatorial substituents result in a more stable conformation due to
decreased steric interactions
§
○
Disubstituted cyclohexane's
Rings prevent free rotation about single bonds
§
Isomers can exist when two or more substituents are attached
§
Isomers don’t interconvert (would require breaking of C-C bonds)
§
STEREOISOMERS: differ by arrangement of atoms in space
§
Also called cis/trans isomers - diastereomers
§
○
Tetrahedral carbon with 4 diff groups is NOT superimposable on its mirror
image
They are isomers!!
§
If 2 or 3 identical groups -> mirror images
§
○
Cycloalkanes
-
WEDGE BOND (OUT OF PAGE) -> UP
○
DASHED BOND (INTO PAGE) -> DOWN
○
Chirality (handedness)
Molecules that possess non-superimposable mirror images are chiral
-
Tetrahedral carbon with 4 diff groups has no plane of symmetry -stereogenic
centre
-
Enantiomers and plane polarised light
All chiral compounds rotate plane of polarised light to some extent - optically
active
-
Enantiomers rotate in opposite directions
-
R and S descriptors
Rank substituents
-
Determine clockwise or anticlockwise
-
Assing R or S
-
Decreasing order of atomic number 1)
If 2 directly attached atoms are same - compare second atoms 2)
Treat multiple bonds as same as single bonds x23)
View molecule so group 4 is in the back 4)
Examples:
If lowest priority is pointing out of the page
Rotate molecule so it is pointing into page -> normal rules
§
Reverse the anticlockwise/clockwise rule
§
Use thumb
§
○
Racemic mixture = equal mix of enantiomers
Same physical and chemical properties
§
○
More than one asymmetric centre
Each asymmetric centre has 2 possible configurations (R or S)
§
Molecule with n centres has max of 2^n stereoisomers
§
○
These molecules are stereoisomers - diastereomers - non-mirror images
○
Diastereomers
All asymmetric centres of opposite configuration = enantiomers•
At least one asymmetric centre of same configuration, at least one opposite =
diastereomers
•
Separation of Enantiomers
Enantiomers
Identical physical and chemical properties -> cannot be separated easily
○
•
Diastereomers
Diff 3D shape -> diff chemical and physical properties
○
•
To separate enantiomers, convert them into diastereomers, separate them, and
re-transform them into enantiomers
•
-
Alkenes
CnH2n
-
C=C double bond
-
Planar arrangement
-
Trigonal planar
-
Direct overlap of sp2 hybrid orbitals -> sigma bond
-
Sideways indirect overlap of unhybridised p orbitals -> pi bond
Pi is weaker than sigma due to less efficient orbital overlap •
-
Isomers - stereoisomers possible
-
1 and 2 are structural isomers
2 and 4 are stereoisomers - diastereomers
Structural requirements for alkene diastereomers
Each end of C=C bond must have 2 diff groups
○
•
E/Z - nomenclature
Assign priorities to group at each end of double bond1)
High priority same side -> Z OR high priority opposite -> E 2)
Cis/trans isomerism in bio
11-trans-retinal -> 11-cis-retinal
-
Light absorption causes double bond isomerism, hydrolysis and dissociation of 11-
trans-retinal from opsin
-
Organic Chemistry
Monday, 28 May 2018 10:19 pm
Document Summary
Isomers: compounds with same molecular formula but different structural formulas or diff spatial arrangement. Constitutional (structural) isomers: same molecular formula and molar mass - but atoms linked differently -> diff. physical properties. Stereoisomers (spatial isomers): compounds that share identical molecular formulas, Stereoisomers (spatial isomers): compounds that share identical molecular formulas, atom-to-atom linkages, and bonding distances, but differ in their 3d arrangement. Diastereomers: non-superimposable, non-mirror images (requires all diff groups) Cis-trans isomers: arise from compounds with restricted rotation (eg. double bond) Identical physical and chemical properties except when they interact with chiral systems. Differ in the way they interact with plane polarised light. Example: lactic acid - one stereogenic centre -> 2 enantiomers exist. Infinite array of conformations is possible through rotation. Atropisomers: arise because of hindered rotation about a single bond, where energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to allow for isolation of individual conformers conformers.
