hi?andrew, thanks for the other answered question. Pls can you help answer this organic questions?
Models and Stereochemistry
One of the most interesting aspects of carbon chemistry is a very subtle form of isomerism
that was predicted by the brilliant chemist Louis Pasteur in 1849 as he was studying the tetravalent
nature of carbon (that is, ability to form four covalent bonds). Pasteur correctly reasoned that if a
carbon atom bonds to four different types of atoms to create a tetrahedral geometry about itself,
mirror-image isomers that are not identical or superimposable, should be possible. Isomers of this
type, called stereoisomers, are based on sp3 hybridized carbon atom. Mirror-image isomers of this
type are called enantiomers. In order for this type of isomer to exist, it must be based on what is
known as an asymmetric or ?chiral carbon.? The word chiral comes from a Latin root meaning
?handedness.? This description is apt, since these molecules form mirror images of one another, just
as your left hand is a mirror-image of your right.
The factor which makes a chiral carbon special is a lack of symmetry. If a plane of
symmetry can be passed through a carbon, then it is not be chiral and enantiomers will not be
possible. A plane of symmetry splits the molecule (including the carbon of interest) into two mirrorimage halves. If a plane of symmetry cannot be found for the carbon and its attached groups, then it
will be chiral. An easier method for recognizing a chiral carbon is to inspect the groups or atoms that
are attached directly to the carbon. If the carbon has atoms of four different elements or groups
attached to it, it will be chiral. Note that it will only be possible to attach four different groups if
carbon is sp3 hybridized.
One of the interesting characteristics of enantiomers is that they have identical physical
properties with one exception?the rotation of plane-polarized light. Although a discussion of planepolarized light is beyond the scope of this exercise, you should be aware that this property can be
measured and used to identify and characterize these molecules. Your Textbook describes plane
polarized light. When placed in a polarimeter, one of the isomers will rotate the plane of polarized
light a characteristic number of degrees in a clockwise direction. The mirror-image isomer will
rotate the plane of polarized light an equal amount in the opposite or counterclockwise direction.
When such a compound exhibits the ability to rotate the plane of polarized light, it is said to be
optically active. Isomers that rotate plane polarized light in the clockwise (dextrorotatory)
direction are given the designation (+). Conversely, isomers that rotate plane polarized light in the
counterclockwise (levorotatory) direction are given the designation (-). The terms dextrorotatory
and levorotatory were the basis of an older designation, where the letters D and L were used to
represent carbohydrate isomers. When both isomers are present in equal concentrations (a racemic
mixture), they will optically counteract one another and the mixture will not be optically active and
will have no effect on plane-polarized light.
Research has shown that compounds produced and used in biological systems are usually
optically active. Enzymes, which catalyze all biological reactions, function by recognizing the three
dimensional characteristics of molecules, and are therefore capable of recognizing and/or producing
only one of the two possible enantiomers. Compounds produced synthetically (in the laboratory),
however, are formed in random fashion, and even if they contain chiral carbons, they will be formed
as racemic mixtures and will be optically inactive. The importance of optical isomers to living
systems cannot be over-emphasized. Biological systems are extremely sensitive to the differences in
these isomers. For example, biological systems are capable of utilizing only the L-amino acids and
D-glucose. Many pharmaceuticals and nutritional supplements exist as enantiomers, where one
isomer may be much more potent than the other. The sensory organs are also capable of
distinguishing the differences between enantiomers. For example, the enantiomers (+)-carvone and
(-)-carvone produce the aromas associated with spearmint and caraway seeds respectively. Models and Stereochemistry, Page 1 Although it is possible to measure the optical rotation that an enantiomer exhibits, until 1949
nobody had determined the arrangement of atoms or groups about a central atom. When this
information about a compound has been established, its absolute configuration is said to be known.
Although the absolute configuration of a compound may not be known, it is still important to
be able to identify a particular configuration. In order for organic chemists to communicate with one
another about the precise configuration around a chiral carbon, a method has been devised to specify
a configuration. The R/S designation refers to the actual configuration about a chiral carbon, but it
tells the chemist nothing about the molecule's effect on polarized light. The R/S designation is just a
convenience used to communicate a specific configuration. In order to designate a molecule's
configuration, perform the following steps:
1. Draw a representation of a chiral carbon that shows the three dimensional arrangement of the
molecule. Several methods are shown below. Their meaning is described in more detail in
A E E A A c B B
D Ball and Stick
Model Line and Wedge E B D Dash and Wedge
Projection Fischer Projection Each of these representations has advantages and disadvantages. However, you should
become familiar with the three dimensional meaning of each of them. The wedge-bonds are
used to create a perspective of depth where necessary, and the line-bonds are considered to
be in the plane of the paper. The third representation utilizes wedges and dashes. The dash
bonds represent bonds that angle away from the reader (below the plane of the paper). The
wedges represent bonds that angle forward, toward the reader (up out of the plane of the
paper). The Fischer projection on the right has the same orientation of bonds as the dash and
wedge representation. Although it appears to show less detail, if you understand its meaning,
it is probably the simplest and most convenient representation to use.
2. Once you have correctly represented the molecule using one of the models or representations
above, you will assign a priority to each group or atom attached to carbon. Priority is based
on the atomic number of each atom directly attached to carbon. If two identical atoms are
directly attached to carbon, the priority of those atoms will be based on what is attached to
them. Whichever of the atoms is bonded to atoms of greater atomic number will be given a
higher priority. More complete details for prioritization, including rules for multiple bonds,
can be found in your textbook.
3. Once priorities are established, you must re-orient the molecule in space to point the lowest
priority group away from you. When this done properly, the low priority group should be
behind the central carbon atom from your perspective (see Figure 1 below). Once this
perspective has been created, you will examine groups with priorities 1, 2 and 3. These
groups should be directed slightly toward you and will appear to be 120? apart. If, in
proceeding from group 1 to group 2 to group 3, you trace out a clockwise arc, the enantiomer
is designated R. If, on the other hand, in going from group 1 to group 2 to group3, you trace
out a counter- clockwise arc, the enantiomer is designated S. An example of the procedure
Models and Stereochemistry, Page 2 described above is shown in Figures 1 and 2 below. You will probably find that this
procedure is most easily performed with models at first. However, you should be able to
designate R or S configurations with any of the representations shown previously. Be sure
that in thinking of these molecules that you do not confuse R and S designations with the
actual optical rotations that these molecules can exhibit. The atom with f ourth
priority has been moved
to the back.
3 C 3
2 Reorientation 4C 2 1 1 Molecule with priorities assigned
Figure 1: Re-orientation of molecule for configuration assignment 3 4C 2 1 Moving from the atom with the first priority to the second and on to the third is
counterclockwise movement. Therefore, the molecule has the S configuration
Figure 2: Assignment of "S" configuration to a molecule
Biological systems, as discussed previously, tend to produce optically active compounds.
These compounds often contain more than one chiral carbon. In general, n chiral carbons will exhibit
2n stereoisomers. Ordinarily, however, only one of those stereoisomers will be recognized or utilized
by an organism. For example, open-chain glucose contains four chiral carbons. There are therefore,
sixteen different stereoisomers in this carbohydrate family. However, the enzymes which process
glucose will only process glucose. Among that family of sixteen isomers, one of them will be a
mirror-image isomer (enantiomer) of glucose. The others, although similar, will not be enantiomers
of glucose. They will be stereoisomers called diastereomers. Glucose will then be a diastereomer of
these other fourteen isomers in the family.
Occasionally, when a compound contains multiple chiral carbons, it will be possible to draw
a plane of symmetry through the molecule. If that is the case, the compound will be known as a
meso compound. The meso compound will be identical to its own enantiomer, and it will be
optically inactive. You should suspect the possibility of a meso compound if a molecule exhibits a
form of symmetry (i.e. it looks the same on both ends). Models and Stereochemistry, Page 3 Name ____________________________________ Problems; Chiral Carbon
In this laboratory exercise you will use models to help you understand this type of subtle
isomerism. You will also learn what type of structural features make this type of isomerism possible.
You will also discover that it is possible to have more than one chiral carbon in a single molecule.
You will also learn how to use some of the more common designations and representations for these
compounds. Some of the exercises involve model construction, and some of the exercises will
require you to represent these isomers on paper.
1. Construct a model with a black carbon atom at the center, and to it attach red, green, grey and
white atoms. The different colored atoms will be used to represent four different groups
attached to the carbon. After you have constructed the model, place it on the bench in front of
you and construct another model that is the mirror image of the first. These two models
represent enantiomers. Have your instructor inspect and approve your models.
Instructor initials: ______________
2. Examine your enantiomer models closely. Is it possible for you, by rotating and turning them
in space, to superimpose them?
Ans: ________________________ 3. Is it possible for a plane of symmetry to bisect either of your models?
Ans: ________________________ 4. Now replace the red atom on each model with another white atom. possible now to imagine a
plane of symmetry that would bisect the molecules into mirror-image halves?
Ans: ________________________ Ans: ________________________ 5. Is it possible now to superimpose the two models? 6. Is it possible for an sp3 carbon to be bonded to two identical atoms or groups and be chiral?
Ans: _________________________ Models and Stereochemistry, Page 4 1. Mark with an asterisk all chiral carbons in the structures below: H 3C OH OH CH3
OH Br Cl H 3C CH CH3 H 3C CH CH2 CH3 Br Problems; R and S Designations
1. Restore the models you constructed in the previous exercise to their original forms. Use the
models to help assign R or S configurations to the following structures. Note: c and d are
Br C I H
Ans: b) H H3C C OH CH2 CH3 Ans: Models and Stereochemistry, Page 5 COOH
H CH3 c) OH Ans:
d) H C
H CH CH 3 CH3
CH2 CH3 Ans:
2. Use the Fischer projection diagrams to correctly draw the following compounds. If you don't
know the structure by name, look it up in one of the references.
a) (S)-2-bromobutane b) (R)-3-methylhexane c) (R)-alanine (an amino acid) Models and Stereochemistry, Page 6 d) (S)-lactic acid Problems; Multiple Chiral Carbons
1. Using the enantiomer models you created originally, remove the white atoms, and connect the
two carbon atoms together. You will note that you now have two chiral carbons in your
molecule. Now construct a mirror image molecule of the two-carbon molecule you just created.
Have your instructor inspect and initial your two enantiomers.
Instructor initials: ______________ 2. Are the two molecules superimposable?
Ans: ________________________ 3. . Is it possible to pass a plane of symmetry through either of these isomers?
Ans: ________________________ 4. Are these two molecules enantiomers or are they really identical?
Ans: ________________________ Ans: ________________________ 5. What is the name associated with this type of molecule? 6. Now construct two identical (not enantiomers) models like the ones you originally made (i.e.
attach red, green, grey and white atoms to a central black carbon atom). Now remove the white
atoms and connect the carbon atoms together as you did before. Do you still have two chiral
carbons in the molecule?
Ans: ________________________ 7. What is the relationship of this two-carbon molecule to the two carbon molecule that you
Ans: ________________________ 8. Is it possible to pass a plane of symmetry through this molecule?
Ans: ________________________ Models and Stereochemistry, Page 7 9. Now construct a mirror image molecule of the two-carbon molecule you just created. Does this
molecule exhibit a plane of symmetry?
Ans: ________________________ Ans: ________________________ 10. Is this isomer superimposable with its mirror image? 11. Are these molecules identical or are they enantiomers?
12. Assign R or S configurations to each of the chiral carbons in open chain glucose below.
H H OH H OH
CH 2OH 13. Draw Fischer projections of all possible stereoisomers of tartaric acid and label the chiral
carbons R or S. Be sure not to draw the same structure twice. Models and Stereochemistry, Page 8
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