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STEREOCHEMISTRY “_
Stereochemistry, a subdiscipline of chemistry,
involves the study of the relative spatial arrangement of atoms within molecules. An important branch
of stereochemistry is the study of chiral
molecules.
Stereochemistry
is also known as 3D chemistry
because the prefix "stereo-" means "three-dimensionality".
The
study of stereochemical problems spans the entire range of organic, inorganic, biological, physical and supramolecular
chemistries. Stereochemistry includes methods for determining and describing
these relationships; the effect on the physical or biological properties these
relationships impart upon the molecules in question, and the manner in which
these relationships influence the reactivity of the molecules in question (dynamic
stereochemistry).
History and Significance
Louis Pasteur could
rightly be described as the first stereochemist, having observed in 1849 that salts of tartaric
acid collected from wine production vessels could
rotate plane polarized light, but
that salts from other sources did not. This property, the only physical property
in which the two types of tartrate salts differed, is due to optical isomerism. In
1874, Jacobus
Henricus van 't Hoff and Joseph Le Bel explained
optical activity in terms of the tetrahedral arrangement of the atoms bound to
carbon.
Cahn-Ingold-Prelog
priority rules are part of a system for describing a molecule's
stereochemistry. They rank the atoms around a stereocenter in a standard way,
allowing the relative position of these atoms in the molecule to be described
unambiguously. A Fischer projection is
a simplified way to depict the stereochemistry around a stereocenter
1. What are we talking about?
The bottom line of this whole chapter is learning
the difference between isomers. There are two types of isomers,
constitutional and stereoisomers. Constitutional isomers are two
compounds that have the same atoms present, but differ in their
connectivity. ie:
These compounds contain the same number of atoms,
but the oxygen has been moved to form an ether instead of an alcohol.
Therefore, these compounds are constitutional
isomers.
Stereoisomers also have the same atoms present,
however the connectivity is the same. This means the same number
of hydrogens will be attached to each carbon and the same number of carbons will
be attached to each carbon. Picture this:
Now, these structures both appear to be the same, but
careful observation will reveal that the amine groups attached are in the cis
conformation on the left and the trans conformation on the right.
Therefore, the same atoms are present, but just in a different spatial
arrangement.
Not to beat this idea into your head, but here is another
example of a stereoisomer, but this time we will use a hydrocarbon chain.
Notice that the chain on the left is in the cis
conformation at the double bond and the chain on the right is trans.
This makes them stereoisomers.
2. I understand that chiral compounds are mirror images of each other that are not superposable, but how do I tell they are superposable?
The easiest way to tell if the mirror image is
superimposable or not and superposable is to find the stereochemistry at the
stereocenter. This entails you to find the stereocenter first and then label the
groups attached to it in order of their priority. This means the atom with the
highest atomic number will be labeled A and the next highest B. The next step is
to rotate the molecule so the D group is facing away from you.
ie.
If the groups go from A to C clockwise, it is in the R
configuration. If the groups are arranged counterclockwise, it is in the S
configuration.
Practice a few
A
B C
A has two stereocenters.
The top stereocenter is an R configuration and the bottom stereocenter is
an S configuration. For B the stereocenter is an S.
C does not have to be considered because there are two of the same
groups attached, and is not chiral.
If the two compounds you are looking at are
mirror images of each other, but the configuration at the stereocenter differs,
they are not superposable. Therefore they are chiral
compounds. If they are superposable, then they are
achiral.
3. How do I tell the difference between an Enantiomer and Diastereomer?
The easiest way to tell apart an enantiomer and a
diastereomer is to look at whether or not the compounds are mirror images of
each other. The best way to learn this is through practice. Here are a few
examples, see if you can determine whether or not the compounds are enantiomers,
the same, or diastereomers.
Hint: first determine if the compounds are mirror
images of each other, and then find the individual stereochemistry around each
chiral carbon. Remember the hand rule or the
clockwise/counterclockwise arrangement discussed in the previous section.
D
If you are
having problems determining the configuration at each stereocenter, I suggest
building a model.
A is a pair of diastereomers, because
the configuration is S, S in the first compound and R,S in the second compound.
B is a tricky one. They are both
in the trans configuration and there is a plane of symmetry. Also,
notice there is no carbon with four different groups. Therefore,
they are not enantiomers and there is no stereochemistry.
C does not have a carbon with four different
groups, so it does not have a stereocenter either.
D is a pair of enatiomers. Notice they
are mirror images of each other.
4. There is an R and there is an S, but I don’t know what to do with them. Help!
If you have read the past few sections you
know what the S and R designations are. They tell what
type of stereochemistry is found at the stereocenter. Finding the
stereochemistry at the stereocenters can help determine whether two compounds
are enantiomers or diastereomers. Also, R and S versions of the
same compound will have different optical activity values.
5. Quick Review of optical activity
Optical activity is
the only physical property that differs from one enantiomer to the
next. Optical activity is measured when plane polarized light is
passed through a compound. When the light passes through the
compound, it is bent either with positive rotation (dextrorotary) or with
negative rotation (levorotary). There is no correlation between
positive or negative rotation with the S or R configuration. S can
be either dextrorotary or levorotary and the R enantiomer will be the opposite
of the S. The value given to optical activity is specific
rotation. The equation to figure out specific rotation can be
found page 203 in your textbook.
6. Okay, I’m getting this stereocenter thing, but somebody had to go and screw everything up and stick two stereocenters together.
When dealing with two or more
stereocenters on the same compound, there are a lot of
possibilities. The first possibility is that the compounds are
enantiomers of each other, the second that they are diastereomers, and finally
that they can be meso compounds. Diastereomers occur when the
compounds have the same chemical formula, but are not mirror images of each
other.
ie.
Now look at these same atoms arranged differently to form
an enatiomer. These compounds are mirror images of each
other. However, they do have different stereochemistries, which
makes them enantiomers.
You should also look at these next compounds and discover
what makes them different from the above.
These compounds appear to be enatiomers, because they
are mirror images of each other. They really are not. The middle two compounds
are the meso compound, since they are the same. The outside two compounds are
enatiomers of each other. Therefore, a meso compound is observed with
stereoisomers where you would expect four different possible structures (two
pairs of enantiomers), but there are only three stereoisomers.
7. Fischer Projections doesn’t mean a weekend out on the lake. How do I interpret them?
Fischer projections are a quick way to show three dimensions
without the hassle of having to draw 3-D. They are very
effective for those of us who lack artistic skills. When you look
at the diagram the horizontal lines represent atoms that are coming out at
you. The vertical lines mean they are going away from you.
Fischer projections can be rotated 180 degrees and still be the same
compound. However, if you flip it vertically or horizontally, it
becomes the enantiomer.
This Fischer projection has been flipped
horizontally. These two are enatiomers of each other.
The first projection has an S, R configuration. The second
projection has an R, S configuration.
Now lets look at a vertically flipped
diagram.
T
hese compounds are enatiomers of each other.
Finally, notice what happens when the diagrams are
rotated 180 degrees in the plane of the paper.
The configuration at each stereocenter remains the
same.
8. Cyclic Compounds
If you are
anything like me, it is very hard for you to determine the stereochemistry
in cyclic compounds the best way is just practice. Hopefully,
this area will help. Do your best to determine the
stereochemistry.
Analysis:
Optical activity is the only physical property that differs from one enantiomer to the next. Optical activity is measured when plane polarized light is passed through a compound. When the light passes through the compound, it is bent either with positive rotation (dextrorotary) or with negative rotation (levorotary). There is no correlation between positive or negative rotation with the S or R configuration. S can be either dextrorotary or levorotary and the R enantiomer will be the opposite of the S. The value given to optical activity is specific rotation
BalasHapuscan you explain about that guys,
Optical activity is the only physical property that differs from one enantiomer to the next. Optical activity is measured when plane polarized light is passed through a compound. When the light passes through the compound, it is bent either with positive rotation (dextrorotary) or with negative rotation (levorotary). There is no correlation between positive or negative rotation with the S or R configuration. S can be either dextrorotary or levorotary and the R enantiomer will be the opposite of the S. The value given to optical activity is specific rotation.
BalasHapuscan you expalin about that guys.
can you explain with detail how is method determine the stereochemistry in cyclic compounds ????
BalasHapusthx before............
why the R and S versions of the same compound will have different optical activity values? thank's
BalasHapuscan you explain The first projection has an S, R configuration. The second projection has an R, S configuration. ??
BalasHapus