Isomers and Stereochemistry

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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, but they are not
  identical, or superimposable.

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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 stereoisomersspecifically, 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.
• 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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1. 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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• 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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1. 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 CO is considered to be
   bonded to two O atoms (phantom atoms).

• Other common multiple bonds are drawn below

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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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