Chiral Molecules Timberlake-5th-ed (1)

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14.5 Chiral Molecules
An object whose mirror
image is identical to the
original and can be
superimposed on it is
achiral.
Objects such as hands that
have nonsuperimposable
mirror images are chiral.
Learning Goal Identify chiral and achiral carbon atoms in
an organic molecule.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Stereoisomers
Molecules are structural isomers when they have the same
molecular formula but different bonding arrangements.
In stereoisomers, the atoms are bonded in the same
sequence but differ in the way they are arranged in space.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chirality
• Left and right hands are chiral because they have mirror
images that cannot be superimposed on each other.
• When the mirror images cannot be completely matched,
they are nonsuperimposable.
Core Chemistry Skill Identifying Chiral Molecules
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chiral or Achiral
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chiral Carbon Atoms
• Carbon atoms are chiral if they have four
different atoms or groups.
• When stereoisomers cannot be superimposed,
they are called enantiomers.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Achiral Carbon Atoms
• If a molecule with two or more identical atoms
bonded to the same atom is rotated, the atoms
can be superimposed and the mirror images
represent the same structure.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
For each of the following molecules, indicate whether the
carbon in red is chiral or achiral.
A.
B.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
For each of the following molecules, indicate whether the
carbon in red is chiral or achiral.
A.
achiral
B.
chiral
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
• Worked Examples and how to solve
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 14.9 Chiral Carbons
For each of the following, indicate whether the carbon in red is chiral or achiral:
a. Glycerol, which is used to sweeten and preserve foods and as a lubricant in soaps, creams, and hair care products.
b. Monosodium glutamate (MSG), which is the salt of the amino acid glutamic acid used as a flavor enhancer in foods.
c. Ibuprofen, which is a nonsteroidal anti-inflammatory drug used to relieve fever and pain.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 14.9 Chiral Carbons
Continued
Solution
a. Achiral. Two of the substituents on the carbon in red are the same:
b. Chiral. The carbon in red is bonded to four different groups:
c. Chiral. The carbon in red is bonded to four different groups:
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 14.9 Chiral Carbons
Continued
Study Check 14.9
Circle the chiral carbon of penicillamine, which is used in the treatment of rheumatoid arthritis.
Answer
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Fischer Projections
A Fischer projection
• is a two-dimensional representation of a molecule.
• places the most oxidized group at the top.
• uses vertical lines for bonds that go back.
• uses horizontal lines for bonds that go forward.
The intersections of vertical and horizontal lines represent a
carbon atom that is usually chiral.
Core Chemistry Skill Identifying D- and L- Fischer Projections
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Drawing Fischer Projections
For glyceraldehyde, the only chiral carbon is the middle
carbon. In the Fischer projection, the carbonyl group, which
is the most highly oxidized group, is drawn at the top.
In a Fischer
projection, the
chiral carbon atom
is at the center, with
horizontal lines for
bonds that project
forward and vertical
lines for bonds that
point away.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Drawing Fischer Projections
Fischer projections can also be drawn for compounds that
have two or more chiral carbons:
• In the mirror images of erythrose, both of the carbon
atoms at the intersections are chiral.
• The mirror image for L-erythrose is drawn by reversing the
positions of all the — H and — OH groups on the
horizontal lines of D-erythrose
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Drawing Fischer Projections
The designation as a D or an L stereoisomer of a
Fischer projection is determined by the position of
the — OH group attached to the chiral carbon
farthest from the carbonyl group.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Enantiomers in Biological Systems
Molecules in nature also have mirror images, and often
one stereoisomer has a different biological effect than
the other one.
• One enantiomer of carvone, produced by the
spearmint plant, smells and tastes like spearmint,
whereas its mirror image, produced by the caraway
plant, has the odor and taste of caraway in rye bread.
• Our senses of smell and taste are responsive to the
chirality of molecules.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Enantiomers in Biological Systems
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Enantiomers in Biological Systems
Molecules in nature also have mirror images, and often
one stereoisomer has a different biological effect than
the other one.
• One enantiomer of carvone, produced by the
spearmint plant, smells and tastes like spearmint,
whereas its mirror image, produced by the caraway
plant, has the odor and taste of caraway in rye bread.
• Our senses of smell and taste are responsive to the
chirality of molecules.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Enantiomers in Biological Systems
• A substance used to treat Parkinson’s disease is
L-dopa, which is converted to dopamine in the
brain, where it raises the serotonin level.
• The D-dopa enantiomer is not effective for the
treatment of Parkinson’s disease.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Indicate whether each of the following pairs of
Fischer projections can or cannot be superimposed:
A.
and
B.
and
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Indicate whether each of the following pairs of
Fischer projections can or cannot be superimposed:
A.
B.
and
cannot be
superimposed
and
can be
superimposed
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Identify the following Fischer Projection as a D or
an L enantiomer:
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Identify the following Fischer Projection as a D or
an L enantiomer:
The –OH group is
on the right,
it is a D- enantiomer.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Draw the Fischer projection for the following
dash-wedge structure:
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Draw the Fischer projection for the following
dash-wedge structure:
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 14.10 Fischer Projections
Identify each of the following as the D or L stereoisomer:
a.
b.
c.
Solution
a. When the —OH group is drawn to the left of the chiral carbon, it is the L stereoisomer.
b. When the —OH group is drawn to the right of the chiral carbon, it is the D stereoisomer.
c. When the —OH group is drawn to the right of the chiral carbon farthest from the top of the Fischer projection, it is
the D stereoisomer.
Study Check 14.10
Draw the Fischer projections for the D and L stereoisomers of 2-hydroxypropanal and label the D and L isomers.
Answer
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Concept Map
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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