chapter 26 stereoisomerism the mirror image of this
DESCRIPTION
Course Outline 26.1 Review of Isomerism 26.2 Plane-Polarized Light 26.3 Optical Activity 26.4 Fischer Projection Formulas 26.5 Enantiomers 26.6 Racemic Mixtures 26.7 Diastereomers and Meso Compounds Chapter 26 Summary 2 2TRANSCRIPT
Chapter 26 Stereoisomerism The mirror image of this
childrens ballet class is not superimposable on the class.
Introduction to General, Organic, and Biochemistry, 10e John Wiley
& Sons, Inc Morris Hein, Scott Pattison, and Susan Arena 1
Course Outline 26.1 Review of Isomerism 26.2 Plane-Polarized
Light
26.3Optical Activity 26.4Fischer Projection Formulas
26.5Enantiomers 26.6Racemic Mixtures 26.7Diastereomers and Meso
Compounds Chapter 26 Summary 2 2 Review of Isomerism 1. Isomers are
molecules that have the same chemical formula but differ in either:
(a) how the atoms are connected or, (b) how the connected atoms are
arranged in space. 2. Isomers that differ only in their
connectivity are called structural isomers while those that differ
in the spatial arrangement of their atoms are called stereoisomers.
3 Review of Isomerism The difference between structural isomers is
due to different structural arrangements of the atoms that form the
molecules. Examples of structural isomers are shown below. 4 Review
of Isomerism Compounds that have the same structural formulas but
differ in their spatial arrangement are called stereoisomers.There
are two types of stereoisomers. Cistrans or geometric isomers,
which we have already considered. Optical isomers, the subject of
this chapter. One feature of optical isomers is that they have the
ability to rotate the plane of plane-polarized light . . . 5
Plane-Polarized Light
Plane-polarized light is light that is vibrating in only one plane.
Ordinary (unpolarized) light consists of electromagnetic waves
vibrating in all directions (planes) perpendicular to the direction
in which the light is traveling. 6 Plane-Polarized Light
When ordinary light passes through a polarizer, it emerges
vibrating in only one plane and is called plane-polarized light. 7
Plane-Polarized Light
The rotation of plane-polarized light is quantitatively measured
with an instrument called a polarimeter. When the sample tube in
the polarimeter contains a solution of a material that is not
optically active the plane of polarized light has been rotated zero
degrees. When the sample tube in the polarimeter contains a
solution of a material that is optically active the plane of
polarized light has been rotated a specific number of degrees. 8
Plane-Polarized Light
A schematic diagram of a polarimeter measuring an optically active
sample is shown below. 9 Plane-Polarized Light
The specific rotation [] of an optically active sample can be
calculated using the following formula using the observed rotation
in degrees measured by the polarimter. 10 Optical Activity Optical
activity is the ability of a substance to rotate plane-polarized
light to the right or left. A substance that rotates polarized
light to the right (clockwise) is said to be dextrorotatory
(designated (+) in the name). A substance that rotates polarized
light to the left (counterclockwise) is said to be levorotatory
(designated () in the name). 11 Optical Activity A substance that
can rotate plane-polarized light is optically active. A necessary
condition for optical activity is the property chirality. Chirality
is a propertypresent in an object that cannot be superimposed on
its mirror image. Chiral objects or chiral molecules do not have a
plane of symmetry.They are asymmetric. 12 Optical Activity Your
right hand and your left hand are mirror images of each other. Your
left hand and right hand are not superimposable. Therefore your
right hand and your left hand are chiral objects. Superimposable
means that, when we lay one object upon another, all parts of both
objects coincide exactly. 13 Optical Activity Chirality is
typically seen in molecules that have a chiral or asymmetric carbon
atom. Chiral or asymmetric carbon atoms have four different atoms
or four different group attached to it. 14 Optical Activity A
molecule that is not superimposable on its mirror image is said to
be chiral. Chiral molecules relate to each other in the same manner
as the right and left hands.They are not superimposable on their
mirror images. Molecules or objects that are superimposable on each
other are achiral. A molecule is achiral if it has a plane of
symmetry. 15 Your Turn! Draw the mirror-image isomers for the
following compounds that can exist as stereoisomers.
CH3CHOHCH2CH2OH CH3CHBrCH(CH3)2 16 Your Turn! Draw the mirror-image
isomers for the following compounds that can exist as
stereoisomers. CH3CHOHCH2CH2OH This molecule has a chiral atom and
exists as two stereoisomers.The carbon atom in red is connected to
four different groups. 17 Your Turn! Draw the mirror-image isomers
for the following compounds that can exist as enantiomers. 2)
CH3CHBrCH(CH3)2 This molecule has a chiral atom and exists as two
stereoisomers.The carbon atom in red is connected to four different
groups. 18 Your Turn! Draw all the structural formulas for the
butyl alcohols, C4H9OH, and indicate which molecules have optical
activity. 19 Your Turn! Draw all the structural formulas for the
butyl alcohols, C4H9OH, and indicate which molecules have optical
activity. There are the four structural isomers. 20 Your Turn! Draw
all the structural formulas for the butyl alcohols, C4H9OH, and
indicate which molecules have optical activity. Only one of these
has a chiral atom and is optically active.This atom is connected to
four different groups. 21 Fischer Projection Formulas
A Fischer projection is a two-dimensional structural formula used
to represent a three-dimensional structure on paper. Figure I is a
three-dimensional representation of lactic acid.Figures II and III
are two Fischer projections of the molecule. 22 Fischer Projection
Formulas
In the Fischer Projections II and III: The horizontal bonds
represent the bonds that project in front of the paper or toward
the viewer. The vertical bonds represent the bonds that project
behind the paper or away from the viewer. 23 Fischer Projection
Formulas
It is important to be careful when comparing projection formulas.
Two rules apply: Projection formulas must not be turned
90.Projection formulas must not be lifted or flipped out of the
plane of the paper. Projection formulas may be turned 180 in the
plane of the paper without changing the spatial arrangement of the
molecule. 24 Fischer Projection Formulas
Formulas I, II, III, IV, and V represent the same molecule.Formula
IV was obtained by turning formula III 180. Formula V is formula IV
drawn in a three-dimensional representation. 25 Fischer Projection
Formulas
If formula III is turned 90, the other stereoisomer of lactic acid
is represented, as shown in formulas VI and VII. 26 Your Turn! Are
molecules A and B the same molecule? 27 Your Turn! To answer this
question you would perform an in-plane 180 rotation and then
determine if the rotated molecule is superimposable on the other
molecule. 28 Your Turn! A andB are not the same molecules because
they are nonsuperimposable mirror images. 29 Your Turn! Redraw this
Fischer Projection as a three-dimensional formula. 30 Your Turn!
Redraw this Fischer Projection as a three-dimensional formula.
Horizontal lines project toward the viewer, vertical lines project
away from the viewer, and the circle represents the chiral carbon
atom. 31 Enantiomers Enantiomersare optically active,
non-superimposable mirror image molecules that have the property of
chirality. A molecule that has a nonsuperimposable mirror image is
chiral. 32 Enantiomers Most chiral molecules consist of enantiomer
pairs where:
(+) is assigned to the enantiomer that rotates polarized light to
the right like (+)-lactic acid. () is assigned to the enantiomer
that rotates polarized light to the left like ()-lactic acid. 33
Figure 26. 8 These molecules are enantiomers
Figure These molecules are enantiomers.They arenon-superimposable
mirror images of each other.()-Lactic acid rotates plane-polarized
light to the left while (+)-lactic acid rotates plane-polarized
light to the right. 34 Enantiomers The Fisher Projections of
()-lactic acid and (+)-lactic acid are shown on the right. 35 Your
Turn! Draw mirror-image isomers for any of the compounds than can
exist as enantiomers. CH3CH2CH2CHBrCH2CH2CH3 CH2BrCH2CHBrCH2CH3
CH3CH2CBr2CH2CH2CH3 36 Your Turn! Draw mirror-image isomers for any
of the compounds than can exist as enantiomers. First determine if
the molecules have chiral carbon atoms and can exist as
enantiomers. CH3CH2CH2CHBrCH2CH2CH3No chiral atoms
CH2BrCH2CHBrCH2CH3One chiral atom. CH3CH2CBr2CH2CH2CH3No chiral
atoms 37 Your Turn! Draw mirror-image isomers for any of the
compounds than can exist as enantiomers. Draw the mirrror-images of
molecule b).These molecules are enantiomers. b) CH2BrCH2CHBrCH2CH3
38 Enantiomers Enantiomers ordinarily have the same chemical
properties, and other than optical rotation, they also have the
same physical properties. Enantiomers rotate plane-polarized light
the same number of degrees, but in opposite directions. Enantiomers
usually differ in their biochemical properties. In fact, most
living cells are able to use only one enantiomer of an enantiomeric
pair. 39 Enantiomers The key factors of enantiomers and optical
isomerism can be summarized as follows. 1. A carbon atom that has
four different groups bonded to it is called an asymmetric or a
chiral carbon atom. 2. A compound with one chiral carbon atom can
exist in two stereoisomeric forms called enantiomers. 3.
Enantiomers are nonsuperimposable mirror-image isomers. 40
Enantiomers 4. Enantiomers are optically active.They rotate
plane-polarized light. 5. One isomer of an enantiomeric pair
rotates polarized light to the left (counterclockwise).The other
isomer rotates polarized light to the right (clockwise). The degree
of rotation is the same but in opposite directions. 6. Rotation of
polarized light to the right is indicated by (+) placed in front of
the name of the compound, and rotation to the left is indicated by
a () in the name. 41 Racemic Mixtures A mixture containing equal
amounts of a pair of enantiomers is known as a racemic mixture.
These mixtures areoptically inactive.The mixtures have no observed
rotation in a polarimeter because each enantiomer rotates the plane
of polarized light an equal amount but in opposite directions so
that each rotation cancels out. 42 Racemic Mixtures The () symbol
is often used to designate racemic mixtures. For example, a racemic
mixture of lactic acid is written as ()-lactic acid because this
mixture contains equal molar amounts of (+)-lactic acid and
()-lactic acid. 43 Racemic Mixtures Racemic mixtures are usually
obtained in laboratory syntheses of compounds in which a chiral
carbon atom is formed. For example the catalytic reduction of
pyruvic acid (an achiral compound) to lactic acid produces a
racemic mixture containing equal amounts of (+)- and ()-lactic
acid: 44 Racemic Mixtures As a general rule, only one of the
isomers is produced in the biological synthesis of optically active
compounds. For example, only (+)-lactic acid is produced by
reactions occurring in muscle tissue, and only ()-lactic acid is
produced by lactic acid bacteria in the souring of milk. 45 Racemic
Mixtures Many pharmaceuticals are synthesized as racemic mixtures
since organic syntheses are often not stereospecific. Typically,
only one half of these racemic mixtures is medically active.
Examples of this are shown Figure 26.9 on the next slide . . . 46
Racemic Mixtures Figure 26.9 Some examples of common chiral drugs.
47 Diastereomers and Meso Compounds
The enantiomers are stereoisomers that differ only in the spatial
arrangement of the atoms and groups within the molecule. The number
of stereoisomers increases as the number of chiral carbon atoms
increases. The maximum number of stereoisomers for a given compound
is obtained by the formula 2n, where n is the number of chiral
carbon atoms in the molecules. 48 Diastereomers and Meso
Compounds
A substance with two nonidentical chiral carbon atoms, such as
2-bromo-3-chlorobutane, four stereoisomers are possible (22 = 4).
Formulas XVIII and XIX and formulas XX and XXI are enantiomers. All
four compounds are optically active. 49 Diastereomers and Meso
Compounds
Enantiomers XVIII and XIX and enantiomers XX and XXI are not
mirror-image isomers of each other. Stereoisomers that are not
enantiomers (not mirror images of each other) are called
diastereomers. 50 Diastereomers and Meso Compounds
There are four different pairs of diastereomers of
2-bromo-3-chlorobutane: XVIII and XX, XVIII and XXI, XIX and XX,
and XIX and XXI. 51 Diastereomers and Meso Compounds
Look at another example.The 2n formula indicates that four
stereoisomers of tartaric acid are possible. Formulas XXII and
XXIII represent nonsuperimposable mirror-image isomers and are
enantiomers. 52 Diastereomers and Meso Compounds
Formulas XXIV and XXV are also mirror images but they are
superimposable. Formula XXIV and XXV represent the same
compound.Only three stereoisomers of tartaric acid exist. 53
Diastereomers and Meso Compounds
Compound XXIV is achiral and does not rotate polarized light. A
plane of symmetry can be passed between carbons 2 and 3 so that the
top and bottom halves of the molecule are mirror images. 54
Diastereomers and Meso Compounds
Stereoisomers that contain chiral carbon atoms and are
superimposable on their own mirror images are called meso
compounds, or meso structures. All meso compounds are optically
inactive. 55 Diastereomers and Meso Compounds
The three stereoisomers of tartaric acid are represented and
designated in this fashion. 56 Diastereomers and Meso
Compounds
The (+) and (-) isomers are enantiomers.The meso compound is a
diastereomer of the (+) and (-) isomers. The physical properties of
these three isomers are shown on the next slide . . . 57
Diastereomers and Meso Compounds
The enantomers have identical properties except for the specific
rotation.The diastereomers differ in other physical properties. 58
Your Turn! How many stereoisomers exist for the following compound?
59 Your Turn! How many stereoisomers exist for the following
compound?
There are four stereoisomers.None of these structures are meso
structures. 60 Your Turn! How many stereoisomers exist for the
following compound? 61 Your Turn! How many stereoisomers exist for
the following compound?
There are three stereoisomers.The first two structures have a plane
of symmetry and are identical.The last two structures are
enantiomers. 62 Chapter 26 Summary Isomerism is the phenomenon of
two or more compounds having the same number and kind of atoms. In
stereoisomerism the isomers have the same structural formula but
differ in the spatial arrangement of atoms.Stereoisomers have the
same structural formula but differ in their spatial arrangement. 63
63 Chapter 26 Summary A polarimeter uses two polarizers to measure
the rotation of plane-polarized light caused by a solution that
contains an optically active compound. Compounds that are able to
rotate polarized light are said to be optically active.Optical
activity is commonly associated with asymmetric carbon atoms. A
compound with an asymmetric carbon atom is not superimposable on
its mirror image. 64 64 Chapter 26 Summary A molecule that is not
superimposable on its mirror image is said to be chiral. A molecule
with one chiral carbon atom can be in two optically active isomeric
forms. Fischer projection formulas depict a three-dimensional
molecule as a flat, two-dimensional drawing. Chiral molecules that
are mirror images of each other are stereoisomers and are called
enantiomers. 65 65 Chapter 26 Summary A mixture containing equal
amounts of a pair of enantiomers is known as a racemic mixture. The
maximum number of stereoisomers for a given chiral compound is
equal to 2n, where n equals the number of chiral carbon atoms in
the molecule. Stereoisomers that are not mirror images
(enantiomers) are called diastereomers. 66 66 Chapter 26 Summary
Stereoisomers that contain chiral carbon atoms and are
superimposable on their mirror images are called meso compounds or
meso structures. Meso compounds are not optically active and are
achiral compounds. 67 67