College of Dentistry
Organic Chemistry
Assistant Lecture
Aayad Ammar
Stereochemistry and Stereoisomers Revisited
Introduction to Isomers
Compounds with identical molecular formulas but different structures are
called isomers. Many types of isomers exist, and several of them are
discussed throughout this text. Constitutional isomers differ from one
another in configuration; that is,they differ in terms of which atoms are
bonded to one another. Constitutional or structural isomers can be
interconverted only by breaking bonds within the molecule and forming
new bonds. Functional group isomers are molecules having the same
molecular formula but different functional groups. For instance, alcohols
and ethers having the same number of carbon atoms are functional group
isomers, such as:
CH3CH2CH2OH
CH3CH2OCH3
1-Propanol (C3H8O) Ethylmethyl ether (C3H8O)
Similarly, carboxylic acids and esters having the same number of carbon
atoms are also functional group isomers, as are aldehydes and ketones.
Geometric isomers also called cis-trans isomers, differ from one another
in the placement of substituents on a double bond or ring.
Stereoisomers are the major focus of this appendix. By definition,
stereoisomers are molecules that have the same structural formulas but
differ in the arrangement of the atoms in space. Stereoisomers may be
distinguished from one another by their different optical properties. They
rotate plane-polarized light in different directions.
Rotation of Plane-Polarized Light
White light is a form of electromagnetic (EM) radiation and thus consists
of waves in motion. In fact, white light is made up of many different
wavelengths (colors) of light. The light waves vibrate in all directions, or
planes, but are always perpendicular to the direction of the light beam
Special light sources, such as sodium or mercury lamps, and filters can be
used to produce monochromatic light, light consisting of only a single
wavelength.When monochromatic light is passed through a polarizing
material, such as a polaroid lens, only light waves in one plane can pass
through; all others are filtered out. The light that emerges from the lens is
called plane-polarized light. Polaroid lenses, like those found in polaroid
sunglasses, consist of parallel arrays of crystals that can be imagined to
look like the slats of Venetian blinds. When light interacts with this
material, the emerging light beam is plane-polarized by the regular
crystalline structure.
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Organic Chemistry
Applying these principles, scientists have developed an instrument called
a polarimeter that is used to measure the optical activity of molecules.
Specifically, the polarimeter measures the ability of a compound to
change the angle of the plane of plane-polarized light The monochromatic
light source of a polarimeter is generally a sodium lamp. The light waves
are directed through a polarizer, and the emerging plane-polarized light
passes through the sample. Finally, the light passes through an analyzer.
If the plane of the light is not altered by the sample, the compound is
optically inactive.
However, if the plane of light is rotated either clockwise or
counterclockwise, the sample is optically active.The angle and direction
of rotation are determined by rotating the analyzer,which is attached to a
round dial graduated in degrees. First, the zero point is determined by
passing light through the polarimeter without the sample present.
The position that allows the maximum amount of light to pass through is
the zero point. Next, the sample is placed in the polarimeter, and the
analyzer is again rotated to allow the maximum amount of light to pass
through. The angle of rotation,or optical rotation, is the difference
between the zero point and the new angle obtained with the sample in
place.The observed angles of rotation are proportional to the number of
optically active molecules in the sample that interact with light. Thus
optical rotation is proportional to the concentration of the sample and to
the length of the sample tube, because both affect the total number of
molecules in the light path. To compare values from different laboratories
that use different concentrations and apparatus, a standard reference, the
specific rotation, was developed. Chemists have defined specific rotation
as the amount of rotation produced by 1.00 g of substance in 1.00 mL of
solution and in a sample tube 1.00 decimeter (dm) in length. Because
rotation is also a function of the temperature, the wavelength of
monochromatic light, and the solvent used (if any), these experimental
variables must also be reported.The following equation is used to
calculate and express specific rotation:
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Organic Chemistry
The Relationship between Molecular
Structure and Optical Activity
some optically active compounds rotate planepolarized light clockwise.
These are said to be dextrorotatory and are designated by a plus sign
before the specific rotation value. Substances that rotate planepolarized
light counterclockwise are called levorotatory and are designated by a
minus sign before the specific rotation value.
It was the experimental work of Louis Pasteur that first revealed a
relationship between structure and optical activity. However, it was not
until 1874 that the Dutch chemist van t Hoff and the French chemist
LeBel independently came up with a basis for the observed optical
activity: tetrahedral carbon atoms bonded to four different atoms or
groups of atoms.
we saw that a carbon atom involved in four single bonds has tetrahedral
geometry. If the carbon atom is bonded to two identical substituents and
two nonidentical substituents, the resulting molecule is symmetrical
(Figure D.1). In other words, a plane of symmetry can be drawn through
this molecule. Furthermore,this molecule is superimposable on its mirror
image. (Prove this to yourself by building the molecules with molecular
models or toothpicks and gumdrops.)
Compare the structure in Figure D.1 with that shown in Figure 17.3 of the
text.In that molecule the tetrahedral carbon is bonded to four nonidentical
groups. The resulting molecule is asymmetric. No plane of symmetry can
be drawn through the molecule, nor can the molecule be superimposed on
its mirror image. (Build the molecules to demonstrate these
characteristics.) As discussed in Section 17.3, the analogy can be made
between these mirrorimage molecules and your left and right hands. Your
hands are, indeed, mirror images of one another; you cannot draw a plane
of symmetry through your hand, nor can you superimpose your left and
right hands on one another. A molecule that cannot be superimposed on
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Organic Chemistry
its mirror image is said to be chiral. When a carbon atom is bonded to
four different atoms or groups of atoms, it is called a chiral carbon. Two
stereoisomers that are nonsuperimposable mirror images of one another
are a pair of enantiomers. the chemical and physical properties of
enantiomers are identical, with the exception that they rotate planepolarized light to the same degree but in opposite directions.This is
exactly the phenomenon that Pasteur observed with the mirror-image
crystals of tartaric acid salts.Refer to Figure 17.4 in the text for the
structures of the enantiomers of glyceraldehyde. Note that when you are
comparing two structures to determine whether two molecules are
enantiomers, you may rotate the structures as much as 180°, but you may
never flip the structure out of the plane of the page. Always remember:
If you are in doubt about the three-dimensional structure of a molecule,
build it with a molecular model kit. This is particularly useful as you
begin your study of organic chemistry, and it will help you in your future
study of biochemistry.When Louis Pasteur measured the specific rotation
of the mixture of left- and right-handed tartaric acid salt crystals, he
observed that it was optically inactive. The reason was that the mixture
contained equal amounts of the (+) enantiomer
and the (-) enantiomer. A mixture of equal amounts of a pair of
enantiomers is called a racemic mixture, or simply a racemate. The prefix
is used to designate a racemic mixture. Consider the following
situation:
In this situation the specific rotation is zero because the rotation caused
by one enantiomer is canceled by the opposite rotation caused by the
mirror-image enantiomer.
Diastereomers :
So far, we have looked only at molecules containing a single carbon. In
this case only two enantiomers are possible. However, it is quite common
to find molecules with two or more chiral carbons. For a molecule of n
chiral carbons the maximum possible number of different configurations
is 2n. Note that this formula predicts the maximum number of
configurations. As we will see, there may actually be fewer.
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Organic Chemistry
In Example D.1, structures (a) and (b) are clearly enantiomers, as are (c)
and (d). But how do we describe the relationship between structure (a)
and (c) or any of the pairs of stereoisomers that are not enantiomers? The
term diastereomers is used to describe a pair of stereoisomers that are not
enantiomers.Although enantiomers differ from one another only in the
direction of rotation of plane-polarized light, diastereomers are different
in their chemical and physical properties.
Meso Compounds
As mentioned previously, the maximum number of configurations for a
molecule with two chiral carbons is 22, or 4. However, if each of the two
chiral carbons is bonded to the same four nonidentical groups, fewer than
four stereoisomers exist.The example of tartaric acid, studied by Pasteur,
helps to explain this phenomenon.
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Organic Chemistry
Note that if you draw a line between chiral carbon-2 and chiral carbon-3
of structure (a) or (b) in Example D.2, the top half of the molecule is the
mirror image of the bottom half. There is a plane of symmetry within the
molecule:
As a result, structure (a) is optically inactive. Even though there are two
chiral carbons, the rotation of plane-polarized light by chiral carbon-2 is
canceled by the opposite rotation of plane-polarized light caused by chiral
carbon-3. This molecule is achiral and is termed meso-tartaric acid. Any
compound with an internal plane of symmetry (i.e., that can be
superimposed on its mirror image) is optically inactive and is termed a
meso-compound.
Assignment of Absolute Configuration:
The (R) and (S) System
Absolute configuration is the actual arrangement of the four groups
around a chiral carbon atom. The (R) and (S) System indicates the
absolute configuration for any chiral carbon. In this system, (R) stands for
a right-handed configuration (Latin rectus), and (S) stands for a left-
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Organic Chemistry
handed configuration (Latin, sinister).To assign an (R) or (S)
configuration to a chiral carbon, the following set of rules is used:
1. Priority rank the atoms or groups of atoms attached to the chiral carbon
according to the sequence rules listed in Table D.1.
2. Draw the molecule with the lowest priority group projecting to the rear.
3. Draw a circular arrow from the group of highest priority to the group
with the next highest priority.
4. If the arrow points in a clockwise direction (right), the configuration of
the chiral carbon is (R); if the arrow points counterclockwise (left), it is
(S).
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