Basics Structural isomers
Do Represents coming out of paper plane * Compounds which have same molecular
formula but atoms are joined diff
Represents going into plane of paper * Diff physical and chem props
-
Newman projections
Lowest energy Highest energy
Everything staggered Everything eclipsed
Alkanes
A General molecular formula: CnHan + 2
Sources
* Petrochemicals derived from crude oil
A Crude oil on distillation gives:
Gases C-Cu
↳ Petrol
Diesel
(5 C12
-
[12-C25
Cyclohexanes
Chair conformation
In order to relieve torsional and angle strain, it adopts lowest energy chair confirmation in which two of the
carbon atoms move out of the plane of the ring
AMost favoured, and virtually strain free,
conformation of cyclohexane and most others. No
torsional strain (staggered), and no bond angle
strain (tetrahedral carbon 109.5 deg)
A Ring substituents occupy either axial or
equatorial positions
,Guide to drawing
1) draw 2 off-set horizontal, parallel lines
2) Connect ends of two lines at one side with triangle pointing down
3) Connect ends at other side w/ triangle pointing up
4) ALL axial bonds drawn vertically
5) THREE pairs of equatorial bond are each drawn in parallel
Equatorial vs Axial Substituent Preferences
A At RT, undergoes a rapid ring inversion (flipping) process, during which axial and equatorial groups
change position.
D In general, substituted cyclohexanes prefer to adopt a conformation in which a substituent appears in the
eq position (or orientation)
* Relative proportion of eq/ax molecules roughly depends on size of substituent
Substituent Competition and Reinforcement
A In general, disubstituted cyclohexanes will adopt a conformation in which the largest substituent is in the
equatorial position
Optical Activity and Stereoisomersim
Enantiomers - pair of molecules which are non superimposable mirror images of each other. Contain no mirror
plane. Stereoisomers.
Optically active - molecules which rotate the plane of polarised light. Single enantiomers
A Achiral molecules contain a mirror plane & superimposable with their mirror image
* Chiral molecules contain mirror plane and non-superimposable with their mirror image. Mirror images
called enantiomers. Enantiomers are stereoisomers as only different from where groups pointing in space
* To interconvert enantiomers the position of two bonds must be reversed
, Meso compounds contained an internal mirror plane and are superimposable w/ mirror image. Optically inactive
Chiral axis
* Ortho disubstituted biphenyls, cannot be flat as the groups occupy each others space. When the 2 ortho
groups are different the molecule does not contain a mirror plane. These are therefore chiral and can exist
as enantiomers
Stereoisomeric relationships
*
A molecule with n stereogenic atoms may exist and 2n max diff stereoisomers
Therefore, a molecule w/ stereogenic atoms can exist as a max 4 diff stereoisomers
Diastereoisomers
A Stereoisomers that are not mirror images of each other are called diastereoisomers. Diastereoisomers have
diff physical properties.
A Configurational isomers that possess two or more stereogenic centres of which at least one, and at most
all but one, are different.
Optically active
* Property of non-racemic mix of chiral compounds to cause rotation of plane polarised light, recorded
as the specific rotation
Racemate / racemic mixture
* A mixture that contains equal amounts of both enantiomers. A racemic mixture has no optical activity, or
is optically inactive, since it contains equal amounts of the and isomers. Thus, net specific rotation
is zero.
Sequence rules for designating absolute configurations of chiral centres
1) assign priorities to groups based on atomic no. of individual atoms within groups, with a ↑ and * d
2) Place group of
*, d, pointing away from you so that the other 3 groups are in plane of paper
"
3) If going from priority to priority is clockwise then config is R
If going from ↑ priority to priority is anti-clockwise then config is S
Do Represents coming out of paper plane * Compounds which have same molecular
formula but atoms are joined diff
Represents going into plane of paper * Diff physical and chem props
-
Newman projections
Lowest energy Highest energy
Everything staggered Everything eclipsed
Alkanes
A General molecular formula: CnHan + 2
Sources
* Petrochemicals derived from crude oil
A Crude oil on distillation gives:
Gases C-Cu
↳ Petrol
Diesel
(5 C12
-
[12-C25
Cyclohexanes
Chair conformation
In order to relieve torsional and angle strain, it adopts lowest energy chair confirmation in which two of the
carbon atoms move out of the plane of the ring
AMost favoured, and virtually strain free,
conformation of cyclohexane and most others. No
torsional strain (staggered), and no bond angle
strain (tetrahedral carbon 109.5 deg)
A Ring substituents occupy either axial or
equatorial positions
,Guide to drawing
1) draw 2 off-set horizontal, parallel lines
2) Connect ends of two lines at one side with triangle pointing down
3) Connect ends at other side w/ triangle pointing up
4) ALL axial bonds drawn vertically
5) THREE pairs of equatorial bond are each drawn in parallel
Equatorial vs Axial Substituent Preferences
A At RT, undergoes a rapid ring inversion (flipping) process, during which axial and equatorial groups
change position.
D In general, substituted cyclohexanes prefer to adopt a conformation in which a substituent appears in the
eq position (or orientation)
* Relative proportion of eq/ax molecules roughly depends on size of substituent
Substituent Competition and Reinforcement
A In general, disubstituted cyclohexanes will adopt a conformation in which the largest substituent is in the
equatorial position
Optical Activity and Stereoisomersim
Enantiomers - pair of molecules which are non superimposable mirror images of each other. Contain no mirror
plane. Stereoisomers.
Optically active - molecules which rotate the plane of polarised light. Single enantiomers
A Achiral molecules contain a mirror plane & superimposable with their mirror image
* Chiral molecules contain mirror plane and non-superimposable with their mirror image. Mirror images
called enantiomers. Enantiomers are stereoisomers as only different from where groups pointing in space
* To interconvert enantiomers the position of two bonds must be reversed
, Meso compounds contained an internal mirror plane and are superimposable w/ mirror image. Optically inactive
Chiral axis
* Ortho disubstituted biphenyls, cannot be flat as the groups occupy each others space. When the 2 ortho
groups are different the molecule does not contain a mirror plane. These are therefore chiral and can exist
as enantiomers
Stereoisomeric relationships
*
A molecule with n stereogenic atoms may exist and 2n max diff stereoisomers
Therefore, a molecule w/ stereogenic atoms can exist as a max 4 diff stereoisomers
Diastereoisomers
A Stereoisomers that are not mirror images of each other are called diastereoisomers. Diastereoisomers have
diff physical properties.
A Configurational isomers that possess two or more stereogenic centres of which at least one, and at most
all but one, are different.
Optically active
* Property of non-racemic mix of chiral compounds to cause rotation of plane polarised light, recorded
as the specific rotation
Racemate / racemic mixture
* A mixture that contains equal amounts of both enantiomers. A racemic mixture has no optical activity, or
is optically inactive, since it contains equal amounts of the and isomers. Thus, net specific rotation
is zero.
Sequence rules for designating absolute configurations of chiral centres
1) assign priorities to groups based on atomic no. of individual atoms within groups, with a ↑ and * d
2) Place group of
*, d, pointing away from you so that the other 3 groups are in plane of paper
"
3) If going from priority to priority is clockwise then config is R
If going from ↑ priority to priority is anti-clockwise then config is S
