18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
Mastering Chemistry Hints
1. Read the chapter in your textbook before coming to lecture.
2. Take a brief look at all the assigned on-line questions for the current chapter.
3. Test yourself with Question Sets in the Study Area (very helpful: has hints and NO penalties!)
4. Come to lecture and Office Hours and ask questions.
5. Do the on-line questions for credit.
Chapter 18.5-18.6
3.! Why do amino acids have “handedness”? Be able to explain what is responsible
for handedness and recognize simple molecules that display this property.!
Chapter 22.2-22.3
2.! Why are monosaccharides handed, and how does this influence the numbers
and types of their isomers? Be able to identify the chiral carbon atoms in
monosaccharides, predict the number of isomers for different monosaccharides, and
identify pairs of enantiomers.!
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
18.5 Handedness or Organic Molecules
Chiral:
One hand does not match the other when superimposed.
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
(18.42)
Chiral Carbon:
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18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
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Achiral:
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It is easy to visualize the chair on top of its mirror image.
Achiral Carbon:
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18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
Fisher Projections:
Can you identify the chiral carbon? (18.44)
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
Enantiomers or optical isomers:
(22.43)
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
Stereoisomers:
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
Handedness of Carbohydrates:
Carboyhdrates:
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
Entantiomers:
D-Form:
L-Form:
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
(22.42)
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
• Solutions of chiral chemical compounds change the plane in which the light is polarized.
Each enantiomer of a pair rotates the plane of the light by the same amount, but the
directions of rotation are opposite. If one enantiomer rotates the plane of the light to the
left, the other rotates it to the right.
What is the maximum number of stereoisomers and enantiomers?
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
Diastereomers:
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
The D and L Family of Sugars: Drawing Sugar Moleculess
Fischer Projection:
Bonds that point up and out of the page are shown as horizontal lines, and bonds that point
behind the page are shown as vertical lines.
In a Fischer projection, the aldehyde or ketone carbonyl group of a monosaccharide is
always placed at the top.
The result is that -H and -OH groups projecting above the page are on the left and right of
the chiral carbons, and groups projecting behind the page are above and below the chiral
carbons.
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
Tetrahedral C atoms
can be drawn with
wedges and dashes
or as Fischer
projections.
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness
D Sugars and L Sugars
D and L stand for dextro and levo , respectively.
The D and L relate only to the position of the -OH group on the chiral carbon furthest from
the carbonyl group in a Fischer projection.
The D and L isomers do rotate plane-polarized light in opposite directions, but the direction
of rotation cannot be predicted.
There are D isomers that rotate polarized light to the left and L isomers that rotate it to the
right.
D-Sugars:
L-Sugars:
18.5-18.6 Stereoisomers; 22.2-22.3 Handedness