01:160:307 Lecture Notes - Lecture 3: Stereospecificity, Reagent, Enantiomeric Excess
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ORGANIC CHEMISTRY 307
Fall 2017
Lecture Notes III
Chapter 3
R. Boikess
Stereoisomerism:
Already encountered. Two compounds are stereoisomers
when they differ only in the spatial relationship (usually 3
dimensional) of their parts. Their composition is the same, their
connectivity is the same. It’s only when we consider the way in
which different parts of the compound are oriented with respect to
each other that we see the difference. Remember when two
structures are isomers they do not readily interconvert at the
temperatures on the surface of our planet. In most (but not all)
cases, interconversion of stereoisomers requires breaking a bond.
Our approach is that if two structures can be interconverted at or
close to room temperature without breaking a bond, they are not
isomers. (Contrary to what it says in BF).
Two stereoisomers are said to have different configurations.
The only difference in their structures is in the spatial arrangement
of their atoms; but we cannot go from one configuration to another
without breaking a bond. So for example, when we refer to the two
configurations of 1,2-dimethylcyclopentane, we are referring to
two different compounds that are related as stereoisomers.
Stereoisomers have one or more stereocenters, which are
carbon atoms that bear at least two different substituents.
Switching the positions of the two substituents on a stereocenter
creates another stereoisomer.
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Molecules are in constant motion, not only through space but
also internally. There are vibrations of bonds (which we will
discuss in Chapter 12) and rotations or partial rotations around
single bonds. But the energy a molecule needs for these kinds of
internal motions is substantially less than the energy needed to
break a bond. These kinds of internal motions result in many
different three dimensional arrangements of a given molecule,
which are called conformations. Different conformations of a
molecule readily interconvert, without any bonds breaking, simply
by rotations and wiggles (partial rotations) around single bonds.
Therefore we will not consider conformations of a given molecule
as stereoisomers. (Contrary to BF)
A. Restriction of rotation can lead to isomerism.
1. This kind of isomerism is generally called cis-trans or
geometric isomerism. We have already seen examples in
the cycloakanes. We will see more examples when we get
to alkenes.
2. When will we observe geometric isomerism? Three
conditions must be met:
a. When there is a plane through the molecule that
causes it to have a top and a bottom. In a ring the plane is the
plane of the regular polygon corresponding to the ring. In an
alkene (as we will see) the plane is the plane defined by the
two atoms of the double bond and the vertical axes of the two
overlapping p-orbitals that form the π bond. In figure (a) it’s
a plane perpendicular to the plane of the paper; and in figure
(b) it’s the plane of the paper.
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b. When there are two different substituents on at least
two different atoms. If the same substituents are on the same
side of the plane, we have the cis isomer and on opposite
sides, the trans isomer.
c. When there is restricted rotation that prevents a
substituent from moving from one side of the plane to the
other.
3. What can restrict rotation at RT?
a. Rings, tying up the two ends
b. π bonds, which have a stereoelectronic
requirement. If you rotate then the pi overlap weakens because the
p-orbitals are no longer parallel.
In general it is hard to break bonds (either σ bonds or π
bonds) at RT, so isomers that could interconvert by bond breaking
and then free rotation don’t.
B. We have seen that there can be two ways to arrange atoms
in 3D (keeping connections the same) when there is an appropriate
plane. We know from experience, that there are other situations
that permit more than one arrangement of connected objects in 3D
that do not easily convert.
1. The classic is your hands. Discuss.
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Document Summary
Two compounds are stereoisomers when they differ only in the spatial relationship (usually 3 dimensional) of their parts. Their composition is the same, their connectivity is the same. It"s only when we consider the way in which different parts of the compound are oriented with respect to each other that we see the difference. Remember when two structures are isomers they do not readily interconvert at the temperatures on the surface of our planet. In most (but not all) cases, interconversion of stereoisomers requires breaking a bond. Our approach is that if two structures can be interconverted at or close to room temperature without breaking a bond, they are not isomers. (contrary to what it says in bf). Two stereoisomers are said to have different configurations. The only difference in their structures is in the spatial arrangement of their atoms; but we cannot go from one configuration to another without breaking a bond.
