Transcript Addition Reactions

Chapter 6
Isomers and
Stereochemistry
Dr. Sujatha Krishnaswamy
Chemistry Faculty
Chandler Gilbert Community College
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Stereochemistry refers to the three-dimensional structure of a
molecule.
Stereoisomers differ only in the way the atoms are oriented in space.
They have identical IUPAC names (except for a prefix like cis or trans).
They always have the same functional group(s).
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• Although everything has a mirror image, mirror images
may or may not be superimposable.
• Some molecules are like hands. Left and right hands are
mirror
images,
superimposable.
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but
they
are
not
identical,
or
Mirror, Mirror on the wall…..
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Fig. 5.13
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We can now consider molecules to determine whether
they are chiral or achiral.
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• A and B are stereoisomers—specifically, they are
enantiomers.
• A carbon atom with four different groups is a tetrahedral
stereogenic center.
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How To Locate Stereogenic Centers?
• Always omit from consideration all C atoms that cannot
be tetrahedral stereogenic centers. These include
CH2 and CH3 groups
Any sp or sp2 hybridized C
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Larger organic molecules can have two, three or even
hundreds of stereogenic centers.
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Stereogenic Centers in Rings
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Locate the stereogenic centers.
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Right or Left Handed Molecule?
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Labeling Stereogenic Centers with R or S
Naming enantiomers with the prefixes R or S is called the
Cahn-Ingold-Prelog system.
1. To designate enantiomers as R or S, priorities must be
assigned to each group bonded to the stereogenic
center. The atom of highest atomic number gets the
highest priority (1).
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2. If two atoms on a stereogenic center are the same,
assign priority based on the atomic number of the
atoms bonded to these atoms. One atom of higher
atomic number determines the higher priority.
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3. If two isotopes are bonded to the stereogenic center,
assign priorities in order of decreasing mass number.
Thus, in comparing the three isotopes of hydrogen, the
order of priorities is:
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4. To assign a priority to an atom that is part of a multiple
bond, treat a multiply bonded atom as an equivalent number
of singly bonded atoms. For example, the C of a C=O is
considered to be bonded to two O atoms (phantom atoms).
• Other common multiple bonds are drawn below:
Page 150 Problem 10
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Assign priorities to the following sets of
substituents.
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Assign priorities to the following sets of
substituents.
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Assign R or S to the following molecules.
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Assign R or S to the following molecules.
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Assign R or S to the following molecules.
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Page 150 Problem 11
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Assign R or S to the following molecules.
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Assign R or S to both stereogenic carbons.
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Assign R or S to the following molecules.
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Page 151 Problem 14
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Identify the relationship between the
molecules in each pair.
The relationships are: same compound (not isomers), different compounds
(different molecular formula), constitutional isomers, enantiomers
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How many stereoisomers can exist for Cholesterol?
2n (n = # of stereogenic carbons)
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How many stereoisomers can exist for
Morphine?
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Optical Activity of a Achiral Compound
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Optical Activity of a Chiral Compound
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• The rotation of polarized light can be clockwise or
anticlockwise.
• If the rotation is clockwise, the compound is called
dextrorotatory. The rotation is labeled d or (+).
• If the rotation is counterclockwise, the compound is
called levorotatory. The rotation is labeled l or (-).
• Two enantiomers rotate plane-polarized light to an equal
extent but in opposite directions. Thus, if enantiomer A
rotates polarized light +5°, the same concentration of
enantiomer B rotates it –5°.
• No relationship exists between R and S prefixes and the
(+) and (-) designations that indicate optical rotation.
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dextro- and levo- Limonene
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An equal amount of two enantiomers is called a racemic
mixture or a racemate. A racemic mixture is optically
inactive. Because two enantiomers rotate plane-polarized
light to an equal extent but in opposite directions, the
rotations cancel, and no rotation is observed.
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Enantiomers can be dangerously different!
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Flipping back to front can turn science
upside down.
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Enzyme Recognition
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Meso Compounds
Have two or more asymmetric carbons and a plane of
Symmetry. They are achiral molecules.
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E,Z Stereochemical Nomenclature
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E or Z?
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E or Z?
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E or Z?
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