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An in-depth exploration of stereochemistry, focusing on constitutional isomers and stereoisomers, with a particular emphasis on chiral molecules and their properties. It covers topics such as enantiomers, stereogenic centers, and the labeling of stereogenic centers using the Cahn-Ingold-Prelog system.
What you will learn
Typology: Study notes
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Constitutional Isomers = same molecular formula, different
connectivity Stereoisomers = same molecular formula, same connectivity
of atoms but different arrangement of atoms in space
Examples of Constitutional Isomers
formula
constitutional isomers
C
H 3
O 8
CH
CH 3
CH 2
OH 2
CH
CHCH 3
3
C
H 4
10
CH
CH 3
CH 2
CH 2
3
CH
CHCH 3
3
OH CH
3
Same molecular formula – different bond connectivitiesAlways different propertiesVery different properties if different functional groups
Although everything has a mirror image, mirror imagesmay or may not be superimposable.
Some molecules are like hands. Left and right hands aremirror images, but they are not identical, orsuperimposable.
Other molecules are likesocks. Two socks from a pairare mirror images that aresuperimposable. A sock andits mirror image are identical.
-^
A molecule or object that issuperimposable on its mirrorimage is said to be achiral.
We can now consider several molecules to determinewhether or not they are chiral.
The molecule labeled A and its mirror image labeled Bare not superimposable. No matter how you rotate A andB, all the atoms never align. Thus, CHBrClF is a chiralmolecule, and A and B are different compounds.
A and B are stereoisomers—specifically, they areenantiomers. - A carbon atom with four different groups is a tetrahedralstereogenic center.
Larger organic molecules can have two, three or evenhundreds of stereogenic centers.
Stereogenic centers may also occur at carbon atomsthat are part of a ring.
To find stereogenic centers on ring carbons, alwaysdraw the rings as flat polygons, and look for tetrahedralcarbons that are bonded to four different groups.
In 3-methylcyclohexene, the CH
3
and H substituents that
are above and below the plane of the ring are drawn withwedges and dashes as usual.
Manybiologicallyactivemoleculescontainstereogeniccenters onring carbons.
Enantiomers: stereoisomers whose molecules arenonsuperposable mirror images
Diastereomers: stereoisomers whose molecules are notmirror images of each other
Î
Example: cis and trans double bond isomers Î
Example: cis and trans cycloalkane isomers
Chiral molecule - has the property of handedness^ z
Not superimposable on its mirror image z
Can exist as a pair of enantiomers
Pair of enantiomers^ z
A chiral molecule and its mirror image
Achiral molecule^ z
Superimposable on its mirror image
A molecule with a single tetrahedral carbon bonded to fourdifferent groups will always be chiral
Switching two groups at the tetrahedral center leads to theenantiomeric molecule
A molecule with more than one tetrahedral carbon bonded tofour different groups is not always chiral]- LATERStereogenic Center (also called a “stereocenter”)^ Î
In general it is “an atom bearing groups of such nature that aninterchange of any two groups will produce a stereoisomer” Î
sp
3 carbon is the most common example of a tetrahedral stereogenic center. They are usually designated with an asterisk (*)
z
Example: 2-butanol
Since enantiomers are two different compounds, theyneed to be distinguished by name. This is done byadding the prefix
or
to the IUPAC name of the
enantiomer.
Naming enantiomers with the prefixes
or
is called
the Cahn-Ingold-Prelog system.
To designate enantiomers as
or
, priorities must be
assigned to each group bonded to the stereogeniccenter, in order of decreasing atomic number. The atomof highest atomic number gets the highest priority (1).
If two atoms on a stereogenic center are the same,assign priority based on the atomic number of the atomsbonded to these atoms. One atom of higher prioritydetermines the higher priority.
To assign a priority to an atom that is part of a multiple bond,treat a multiply bonded atom as an equivalent number ofsingly bonded atoms. For example, the C of a C=O isconsidered to be bonded to two O atoms. Labeling Stereogenic Centers with
or
Other common multiple bonds are drawn below:
Plane polarized light oscillates in a single plane
Like a rope through a picket
fence
The plane-polarized light cannot get throughTwo filters that are 90
˚^ to one another.
With achiral compounds, the light that exits the sampletube remains unchanged. A compound that does notchange the plane of polarized light is said to be opticallyinactive.
With chiral compounds, the plane of the polarized light isrotated through an angle
α
. The angle
α
is measured in
degrees (
0 ), and is called the observed rotation. A
compound that rotates polarized light is said to be opticallyactive.
The rotation of polarized light can be clockwise oranticlockwise.
If the rotation is clockwise (to the right of the noonposition), the compound is called dextrorotatory. Therotation is labeled
d
or (+).
If the rotation is counterclockwise, (to the left of noon),the compound is called levorotatory. The rotation islabeled
l
or (-).
Two enantiomers rotate plane-polarized light to an equalextent but in opposite directions. Thus, if enantiomer Arotates polarized light +
0 , the same concentration of
enantiomer B rotates it –
0
No relationship exists between
and
prefixes and the
(+) and (-) designations that indicate optical rotation.
An equal amount of two enantiomers is called a racemicmixture or a racemate. A racemic mixture is opticallyinactive. Because two enantiomers rotate plane-polarized light to an equal extent but in oppositedirections, the rotations cancel, and no rotation isobserved.
No net optical rotation
Often designated as (
Racemic mixture = racemate
Equal amountsof each
Specific rotation is a standardized physical constant forthe amount that a chiral compound rotates plane-polarized light. Specific rotation is denoted by thesymbol [
α
] and defined using a specific sample tube
length (
l , in dm), concentration (
c
in g/mL), temperature
(^0) C) and wavelength (589 nm).
Typically only one enantiomer of a drug is biologically active
Ibuprofen (active)
(inactive)
(S)
(R)
HO
CH
3
H O
HO
H
CH
(^3) O
Stereochemistry
Chemical Properties of Enantiomers
Each new stereogenic center may generate a potential pair ofstereoisomers, so the theoretical number of possible stereoisomersis 2
n
How many stereoisomers?
Relationship of 1 and 2 = enantiomersRelationship of 3 and 4 = enantiomers
(Enantiomers = same properties, cannot be separated)
Relationship of (1 and 3), (2 and 3), or (1 and 4) = diastereomers
z
Diastereomers: stereoisomers that are not mirror images ofeach other.They have
different physical properties
and can be separated
Let’s figure out the configurations of all 4 stereoisomers
CH
3 C
H
Br
C CH
CH 2
3
H
Br
CH
3 C
H
Br
C CH
CH 2
H^3
Br
CH
3 C
Br
H
C CH
CH 2
3
H
Br
CH
3 C
H
Br
C CH
CH 2
3
Br
H
S R Use the same rules, and assign eachstereogenic center separately
When a compound has more than one stereogeniccenter, the
and
configuration must be assigned to
each of them.
One stereoisomer of 2,3-dibromopentane
The complete name is (
S ,
R
)-2,3-dibromopentane
Sometimes molecules with 2 or more stereogenic centerswill have fewer than 2
n^
stereoisomers
Because superposable on its mirror image
Despite the presence of stereogenic centers
Not optically active
Has a plane of symmetry
Definition: a meso compound is a compound that is
achiral despite having stereogenic centers
To draw the other two stereoisomers if they exist, draw mirrorimages of each compound and determine whether thecompound and its mirror image are superimposable.
The
cis
isomer is superimposable on its mirror image, making
the images identical. Thus, A is an achiral meso compound.
The trans isomer is not superimposable on its mirror image,labeled C, making B and C different compounds. B and C areenantiomers.
Because one stereoisomer of 1,3-dibromocyclopentane issuperimposable on its mirror image, there are only threestereoisomers, not four.
z
1,4-dimethylcyclohexane
Î
Neither the cis not trans isomers is optically active Î
1,3-dimethylcyclohexane z
The trans and cis compounds each have twostereogenic centers z
The cis compound has a plane of symmetry and is
meso
z
The trans compound exists as a pair of enantiomers Ring flip of (
a)
produces another
(a),
not the mirror image (
b)
No plane of symmetry
Me Me H
H
plane
of sym
metry
Racemic Form (identical properties)
R S
reaction
R
+
Diastereomers (different properties)
R
R S
R
separate
R
R S
R
- -
R R
pure forms
R S
Cannot be separated directly Why not?Can be separated by a chiral reagent which creates diastereomeric relationship
is a resolving agent. It is
a single enantiomer
of a chiral compound
that can react with
both enantiomers of the racemic mixture toform a pair of diastereomers.
quinine
(primary alkaloid from variousspecies of
Cinchona
)
N
HO H
N
H
CH
O 3
Often used are organic acids or bases which are found optically pure innatureThey form acid-base salts which, as diastereomers, have differentsolubilities in water and can be separated by selective crystallizationOne can then easily regenerate starting acid or base
(+,-)-2-phenylpropanoic acid
(racemic form)
C
H 6
CCOOH 5 CH
3 H
+
(-)-alkaloid
(+)(-)-Salt(-)(-)-Salt diastereomers
(basic)
separate byfractionalcrystallization
(+)(-)-Salt
(-)(-)-Salt
organic phase
water phase
(+)-
(-)-alkaloid asammonium salt
organic phase
water phase
(-)-
(-)-alkaloid asammonium salt
H
O 3
+
C
H 6
CHCCOOH 5
3 H
H
O 3
+
C
H 6
CCOOH 5 CH
3 H
Resolution of a Carboxylic Acid