Title: Summary Slide
1Summary Slide
- Stereochemistry
- For those students using Fundamentals of Organic
Chem. this presentation refers to Chapter 6.
2Stereochemistry
3The Origins of Stereochemistry
Stereochemistry is the branch of chemistry
concerned with the three dimensional nature of
molecules. This branch of chemistry originated
as an offshoot of the research of the French
physicist Jean Baptiste Biot (1774-1862). Biot
was investigating the nature of "plane -
polarized light" when he accidentally discovered
optical activity. This discovery eventually led
to the development of stereochemistry.
4Plane Polarized Light
- A beam of ordinary (unpolarized) light consists
of waves that oscillate equally in all directions
perpendicular to the line which defines the path
of the light ray. - Certain materials affect ordinary light in a
special way. Polarized films interact with all
the oscillating waves of ordinary light and
filter out all planes of oscillation except one.
The light which emerges from a Polaroid film
consists of waves oscillating in one plane only
and hence is referred to as "plane polarized
light".
5Manipulation of Light Using Polaroid Films
- In order to see an object, you need light.
- Polarized films only transmit light vibrating in
one plane and filter out the rest. If two
polarized films are set up in front of one
another and one is rotated 90 degrees to the
other, then the first will transmit light
vibrating in a plane that is absorbed by the
second and hence no light is transmitted and the
object cannot be seen. If one of the two
Polaroid films is rotated 90 degrees then the
object can be seen again with maximum brightness
6Optical Activity
- In 1815, Biot discovered that when a beam of
plane polarized light is passed through solutions
of certain organic molecules, such as sugar or
camphor, the plane of polarization is rotated.
We call molecules that exhibit this property
optically active. - A. The amount of rotation can be measured with
an instrument known as polarimeter. In a
polarimeter, plane polarized light is passed
through a tube containing a solution of some
optically active molecules and rotation occurs.
The extent of rotation is determined by rotating
a second polarized film until the light passes
through it. The observed rotation is symbolized
by the Greek letter a. In addition to
determining the extent of rotation, the direction
is also given.
7The Direction of Rotation in a Polarimeter
- Some optically active molecules rotate plane
polarized light to the left (counter clockwise)
and are said to be levoratatory. - Others rotate polarized light to the right
(clockwise), and are said to be dextrorotatory - By convention rotation to the left is given a (-)
minus sign, and rotation to the right is given a
() positive sign
8A Simple Polarimeter
- Measures extent of rotation of plane polarized
light - Operator lines up polarizing analyzer and
measures angle between incoming and outgoing light
9The Amount of Rotation
- The amount of rotation obtained from a
polarimeter is dependent upon the number of
optically active molecules the beam encounters
and the nature of the light source.
Consequently, the amount of rotation is dependent
upon - length of sample tube
- concentration of optically active molecules in
solution - the wavelength of the light used
- Because optical rotation is dependent upon these
three variables,we must choose standard
conditions so that comparisons can be made.
10Specific Rotation
- To have a basis for comparison, define specific
rotation, ?D for an optically active compound - aD observed rotation a
. Path length l (dm) X
concentration (g/ml) - Specific rotation is that observed for 1 g/mL in
solution in cell with a 10 cm path using light
from sodium metal vapor (589 nanometers)
11Louis Pasteur and Optical Isomers
- . While recrystallizing sodium ammonium
tartrate, Louis Pasteur noticed two differently
shaped types of crystals. Separating the two
types using tweezers, Louis Pasteur discovered
that solutions of each type had specific
rotations that were equal in degree, but opposite
in sign. Louis Pasteur had discovered optical
isomers. Optical isomers are also called
enantiomers
12Isomers
- You will recall that isomers are molecules having
the same molecular formula but different
structural formulas.
n-
13Optical Isomers
- Optical isomers also fall under the general
definition of isomerism, but the difference in
their structural formulas is much more subtle! A
pair of optical isomers have structural formulas
that are related as "nonsuperimposable mirror
images.They have the same relationship as do
your two hands - In fact, after separating the two types of
crystals, Louis Pasteur noticed that their shapes
had this same relationship
14Mirror Images and Optical Isomers
- Every molecule has a mirror image. Only if the
mirror image of a molecule is nonsuperimposable
on the original do the two structures represent a
pair of optical isomers. Only if the mirror image
of a molecule is nonsuperimposable on the
original do the two rotate plane polarized light
to the same of degrees but in opposite
directions. The criterion of sameness in
chemistry is"superimposability". - If two structures are superimposable, then they
represent the same molecule
15Criteria for Optical Isomerism
- In order for a molecule to exist as a pair of
optical isomers it must meet the following two
criteria - It must contain at least one carbon bonded to
four different groups - It must not contain a plane of symmetry
16Mirror-image Forms of Lactic Acid
- When H andOH substituents match up, COOH and CH3
dont - when COOH and CH3 coincide, H and OH dont
17Optical Isomers (Enantiomers) and Chirality
- Chirality a pair of optical isomers (or
enantiomers) are related as are your two hands.
They are nonsuperimposible mirror images of one
another. Therefore, a pair of enantiomers have
the structural property of "opposite handedness".
The Greek word for hand is cheir" and from this
we get the words "chiral" and "chirality". We
may therefore call a pair of enantiomers "chiral"
or say they posses"chirality". The opposite of
chiral is achiral
18Facts About Optical Isomers
- . Physical Properties of Optical Isomers
Although they are different substances, the
structures of a pair of enantiomers are so
similar that their physical properties are
identical (bp, mp, density, etc.). - Racemic mixtures a mixture that is a 5050 mix
of each member of a pair of optical isomers.
This mixture is optically inactive because the
optical effect of each isomer cancels the other. - If a molecule possesses n chiral centers, then
the max number of optical isomers is 2n - 1 chiral center 21 2 optical isomers
19Reactions of Optical Isomers
- Chemical reactions for a pair of enantiomers are
identical if the enantiomers are reacted with
achiral (non- optically active) reagents - right hand ski pole ski pole in hand
- left hand ski pole ski pole in hand
- Two different reaction products are observed for
each member of an optically active pair if they
are forced to react with a chiral reagent. - right hand right glove R hand R glove
- left hand right glove Left hand in R. glove
20The Biochemistry of Chirality
- Many of the sensory receptors in the human body
are chiral Therefore, the biochemical effect of
each member of an enantiomeric pair is quite
different.
21Bitter-Sweet Story of Asparagine
- Asparagine is an amino acid and furthermore two
enantiomers of asparagine exist.
These two represent a pair of enantiomers. ie.
They are nonsuperimposable mirror images of one
another. Each of these two has a different
biochemistry. One tastes bitter, one is sweet
22Chirality Centers
- As mentioned earlier, the most common cause of
chirality in a molecule is a carbon that is
attached to four different groups. Such a carbon
is referred to as a chiral center - Detecting chiral centers in a complex molecule
can be difficult because it is not always
apparent that 4 different groups are bonded to a
given carbon. The differences do not necessarily
appear right next to the carbon centers .
23Chirality Centers in Cyclic Molecules
- Groups are considered different if there is any
structural variation - In cyclic molecules, we compare by following in
each direction of the ring
24Identifying Specific Enantiomers
- The arrangement of groups around a chiral carbon
is different for each member of an enantiomeric
pair. We need a way of verbally identifying each
member of an enantiomer pair - We need a set of rules for specifying the exact
configuration around a chiral carbon
25Sequence Rules for Specification of Configuration
- These rules allow us to specify the exact
arrangement of atoms about each chiral carbon
without having to draw a perspective structural
formula
26Sequence Rules (IUPAC)
- Assign each group that is attached to the chiral
carbon a priority according to the
Cahn-Ingold-Prelog scheme highest 1 and
lowest 4 - If, when the thumb of your left hand points in
the direction of the lowest priority group(4),
your fingers curl in the direction of descending
priority (1-2-3), then your molecule has an S
Configuration - If your right hand is needed to accomplish the
above, then the configuration is R
27Right Hand R Configuration
28Left Hand S Configuration
29Diastereomers
- If a molecule contains one chiral center, then 2
stereoisomers exist for this molecule. The two
stereoisomers represent a pair of enantiomers. - If a molecule contains more than one chiral
center, then more than 2 stereoisomers exist for
that molecule (actually the max number is 2n
where n number of chiral centers). These
stereoisomers usually exist as pairs of
enantiomers. One member of each enantiomer pair
is the mirror image of the other member and has
the opposite coniguration at each chiral center.
What is the relationship between two members of
different enantiomeric pairs. These molecules
are still stereoisomers of one another but they
are not related as object and its mirror image.
These molecules are called Diastereomers (2R,3R
and 2R,3S or 2S,3S and 2R,3S)
30Meso Compounds
- Compounds having n chiral centers have a max
number of 2n stereoisomers. These stereoisomers
exist as pairs of enantiomers that have opposite
configuration at each chiral center. If a pair
of enantiomers is seen to have a plane of
symmetry then both structures represent the same
molecule (the two structures are superimposable).
Any compound that contains both chiral centers
and a plane of symmetry is called a Meso
compound. - Meso compounds have different physical
properties than their enantiomers. Meso
compounds are achiral .
31Physical Properties of Stereoisomers
- Enantiomeric molecules differ in the direction in
which they rotate plane polarized but their other
common physical properties are the same - Daistereomers have a complete set of different
common physical properties
32A Brief Review of Isomerism
- The flowchart summarizes the types of isomers we
have seen
33Constitutional Isomers
- Different order of connections gives different
carbon backbone and/or different functional groups
34Stereoisomers
- Same connections, different spatial arrangement
of atoms - Enantiomers (nonsuperimposable mirror images)
- Diastereomers (all other stereoisomers)
- Includes cis, trans and configurational