Transcript CHEM 210 Ch05x

Organic Chemistry
PRINCIPLES AND MECHANISMS
Chapter 5: Lecture PowerPoint
Isomerism 2: Chirality, Enantiomers, and Diastereomers
5.1 Defining Configurational Isomers,
Enantiomers, and Diastereomers
• Recall that conformational isomers are related solely by
rotations about single bonds.
• Converting from one configurational isomer to another
requires the breaking of a covalent bond
Flowchart for Determining the
Type of Isomer
5.2 Enantiomers, Mirror Images, and
Superimposability
• Enantiomers are nonsuperimposable mirror images.
• Molecules are nonsuperimposable if there is no orientation in
which all atoms of both molecules can be superimposed.
What does a Superimposable
Image Look Like?
• Every molecule has a mirror image, but not every molecule
has a nonsuperimposable mirror image.
• Superimposable mirror image compounds do not have
enantiomers.
5.3 Strategies for Success:
Drawing Mirror Images
• Drawing a molecule’s mirror image quickly and correctly
is an essential skill.
Steps to Drawing a Mirror Imaged
Compound
Steps to Drawing a Mirror Imaged Compound
continued…
5.4 Chirality
• A molecule is chiral if it has an enantiomer.
• A molecule is achiral if it does not have an enantiomer.
• These objects are chiral:
Chirality and Conformational Isomers
• Rotation about single bonds can determine whether a
molecule is chiral or achiral.
• If a molecule and its mirror image are rapidly
interconverting conformational isomers, then the
molecule is effectively achiral.
• The mirror image of 1,2-dibromoethane in one of its
gauche conformations is its second gauche conformation.
Chirality and Conformational Isomers
continued…
• These two gauche conformations are nonsuperimposable
which suggests that 1,2-dibromoethane is chiral.
• Rapid interconversion between the gauche
conformations causes the molecule to be achiral.
Haworth Projections
• Haworth projections are
helpful in determining the
chirality of all cyclics.
• Taking the chair
conformation of
cyclohexane to a flat
hexagon allows us to
quickly and accurately
determine whether it is
chiral or achiral.
The Plane of Symmetry Test for Chirality
• A molecule has a plane
of symmetry if it can be
bisected in such a way
that one half of the
molecule is the mirror
image of the other half.
• A molecule that has a
plane of symmetry
must be achiral; an
achiral molecule must
not have a plane of
symmetry.
Stereocenters and Stereochemical
Configurations
• A tetrahedral atom bonded to four different groups is
called a stereocenter.
• Tetrahedral stereocenters can be found in cyclics and
acylic compounds.
Stereocenters and Stereochemical
Configurations
continued…
• Each tetrahedral stereocenter has two possible
configurations.
• One configuration is the opposite (or inverse) of the
other.
Stereocenters and Meso Compounds
Stereocenters Other than Carbon
• Nitrogen atoms can be sp3 hybridized, and can therefore
have a tetrahedral electron geometry.
• A nitrogen atom bonded to four different substituents,
such as in an ammonium ion, is a stereocenter.
Nitrogen Inversion
• The nitrogen atom in NRR’R”, such as methylethylamine,
is potentially a stereocenter, so the molecule is
potentially chiral.
Nitrogen Inversion
continued…
• The two methylethylamine species rapidly interconvert
and cannot be isolated, so methylethylamine is achiral
and its nitrogen atom is not a stereocenter.
• The process by which NRR’R” species interconvert is
called nitrogen inversion.
Diastereomers
• Diastereomers are stereoisomers that are not mirror images
of each other.
• Cis-1,2-dichloroethene and trans-1,2-dichloroethene, are
diastereomers.
• These have the same molecular formula and the same
connectivity, but they are different molecules (chlorine atoms
are on opposite sides.
• Also, they are not mirror images of each other.
Determining the Maximum Number
of Stereoisomers
• Because a new configurational isomer can be obtained
for each inversion of a tetrahedral stereocenter’s
configuration, the number of configurational isomers
that exist can double with the addition of each
tetrahedral stereocenter.
• The maximum number of configurational isomers that
can exist for a molecule with n tetrahedral stereocenters
is 2n.
• Fewer than 2n configurational isomers exist when at least
one of the isomers is a meso compound.
5.6 Fischer Projections and
Stereochemistry
• The Fischer projection is a convenient way to depict complex
molecules having more than one stereocenter.
• The intersection of a horizontal line and a vertical line
indicates a carbon stereocenter.
• The substituents on the horizontal bonds are understood to
point toward you (like a bowtie), whereas the substituents on
the vertical bonds are understood to point away from you.
Manipulating a Fischer Projection
5.7 Strategies for Success:
Converting Between Fischer Projections
and Zigzag Conformations
• A molecule represented in a zigzag chain can still be
converted to a Fischer projection.
• C1 is part of the CO2H group and C7 is part of the CH2OH
group, so neither is a stereocenter.
• All that’s left to do is to place an H and an OH on each of the
stereocenters.
Building a Complex Fischer Projection
• Which substituent should be placed on the left of
each stereocenter and which should be placed on
the right?
• This choice dictates each stereocenter’s specific
configuration.
Converting the Zigzag Chain to a Fischer
• A molecular model is helpful when converting a zigzag structure to
a Fischer projection.
Steps
1.
2.
For each stereocenter with the horizontal bonds pointing toward us, simply add
the substituents on the left and right of the Fischer projection as they appear in
the molecular model.
Turn the molecule over so that the remaining stereocenters have their horizontal
bonds pointing toward us, and repeat Step 1.
Converting the Zigzag Chain to a Fischer
continued…
Converting the Fischer to a Zigzag
Steps
1.
2.
For each stereocenter in the model in which the horizontal bonds are pointing
toward us, attach the substituents on the left and right as they appear in the
Fischer projection.
Turn the molecule over so that the remaining stereocenters have their horizontal
bonds pointing toward us and repeat Step 1.
5.8 Physical and Chemical
Properties of Isomers
• Constitutional Isomers
Due to different
connectivities, these
isomers must have
different physical and
chemical properties.
• Enantiomers
Have the same
connectivities and
precisely the same
polarities.
Physical and Chemical Properties of Isomers
continued…
Diastereomers
• Just as with enantiomers, diastereomers have the same
connectivity.
• They are not mirror images of each other, however, so they must
behave differently (chemically).
• Diastereomers must have different physical and chemical
properties.
5.9 Stability of Double Bonds and
Chemical Properties of Isomers
• Alkyl substitution is the number of alkyl groups bonded
to the alkene carbon atoms.
• The greater the degree of alkyl substitution the more
stable the alkene.
• The stability of the alkene is observed in its heat of
combustion (DHc).
Heats of Combustion of
the Isomeric Alkenes of C6H12
• Double bond stability
increases as the amount of
alkyl substitution increases.
• Trans-alkenes are more
stable than cis-alkenes due
to having less steric
repulsion.
5.10 Separating Configurational Isomers
• Since diastereomers have different physical properties
they can often be separated by common laboratory
techniques such as fractional distillation, crystallization,
and simple chromatography.
• Enantiomers cannot be separated by these methods in
an achiral environment.
Louis Pasteur
•
•
•
Louis Pasteur was the first to isolate a pair of enantiomers from each other.
Pasteur noted that the crystals appeared to grow in one of two varieties—lefthanded crystals and right-handed crystals—that are mirror images of each other
Pasteur physically separated the two types of crystals using tweezers.
Separating Enantiomers
• Separating enantiomers the way that Pasteur did is not
feasible in most situations.
• A different way to separate enantiomers takes advantage
of diastereomers having different physical properties.
Option for Separating Enantiomers
1.
2.
3.
Temporarily convert the enantiomers into a pair of diastereomers
(will now have different physical properties).
Separate those diastereomers from each other by exploiting their
different physical and chemical properties.
Regenerate the enantiomers from the separated diastereomers.
5.11 Optical Activity
• Chiral molecules interact with plane-polarized light.
• When all photons from a light source have their electric
fields oscillating in the same plane, then the light is
plane polarized.
The Polarizer
• Most light sources emit light that is unpolarized.
• A polarizer generates plane-polarized light by filtering
out light whose electric field oscillates in any other plane.
• If plane-polarized light passes through a sample of a
compound, the plane in which the light is polarized can
change, depending upon whether the compound is chiral
or achiral.
Enantiomers can Plane Polarized Light
• One enantiomer rotates polarized light in one direction
while the other enantiomer rotates it in the opposite
direction.
• Enantiomers have identical physical and chemical
properties except the direction at which they rotate
polarized light.
Optical Activity
• The angle a chiral compound rotates plane-polarized light, called the
measured angle of rotation ([a]), can be obtained using an analyzer.
• Light enters the analyzer after it exits the sample. Chiral compounds that
rotate light clockwise (in the + direction) are called dextrorotatory, and
those that rotate light counterclockwise (in the - direction) are called
levorotatory.
• The direction of rotation generally
is unknown without performing the
experiment.
Factors That Govern the Angle of Rotation
• As the concentration (c) of a chiral compound increases,
so does the angle of rotation.
• As the length of the sample (l) increases, so does the
angle of rotation.
• The specific rotation, [a]lT, is a constant that is
characteristic of a chiral compound’s propensity to
rotated plane-polarized light.
– By convention, c is in units of g/mL and l is in units of
decimeters (dm) (1 dm = 1/10 meter).
• Enantiomers have equal but opposite [a]lT.
Racemic Mixture and Enantiomeric Excess
• A racemic mixture contains equal amounts of the (+) and (-)
enantiomers of a chiral molecule.
• Light traveling through a racemic mixture encounters an equal
number of molecules of each enantiomer.
• A racemic mixture of enantiomers is optically inactive, despite
being made up of chiral molecules.
• If a mixture of enantiomers is not racemic, then it will be optically
active.
• The excess percentage that favors one enantiomer is the
enantiomeric excess (ee), which is viewed as being composed of
one of the pure enantiomers.
5.13 The Chirality of Biomolecules
• Thalidomide is teratogenic (causes birth defects).
• It is estimated that more than 10,000 children worldwide were born with
deformed or missing limbs.
• Thalidomide is chiral and was sold as a racemic mixture of its two
enantiomers.
• The enantiomer on the left is primarily responsible for reducing nausea,
whereas the one on the right is responsible for the teratogenic properties.
5.14 The D/L System for Classifying
Monosaccharides and Amino Acids
• Each chiral amino acid and monosaccharide has two
enantiomers, specified using the D/L system.
• The basis of the D/L system is the optical rotation of
glyceraldehyde.
• In the Fischer projection of any D sugar, the OH group of
the highest numbered stereocenter is on the right.
5.15 The D Family of
Aldoses
Summary and Conclusions
• A variety of isomers types were examined in this chapter.
• A molecule is chiral if it has an enantiomers. If it does not, it
is achiral.
• Enantiomers and diastereomers are types of configurational
isomers.
• Enantiomers are mirror images of each other, while
diastereomers are not.
• A tetrahedral atom bonded to four different substituents is
called a stereocenter.
• Meso compounds, although having tetrahedral stereocenters,
are overall achiral as a result of an internal plane of symmetry.
Summary and Conclusions
continued…
• Fischer projections are convenient ways of depicting complicated
molecules that have more than one stereocenter.
• Alkyl substitution affects the stability of alkenes (can be observed
experimentally by their heats of combustion).
• Chiral compounds can be detected and their concentrations
measured by using plane-polarized light.
• Enantiomers have equal but opposite specific rotations.
• In Chapter 8 (and will apply throughout the rest of the book), when
a reaction produces a mixture of configurational isomers, we will be
able to determine whether they will be produced in equal or
unequal amounts.