chapter 17 stereoisomerism. ©2005 mark s. davis goals distinguish between constitutional and...
TRANSCRIPT
Chapter 17
Stereoisomerism
©2005 Mark S. Davis
Goals• Distinguish between Constitutional and
Stereoisomers
• Distinguish between Chiral and Achiral molecules
• Identify tetrahedral stereocenters
• Describe optical activity of chiral compounds
• Draw Enantiomers
©2005 Mark S. Davis
Biological Importance
• Biological systems can only use one stereoisomer
• Perception in light, Box 12.2
• Insect pheromones, Box 12.3
©2005 Mark S. Davis
Review of Isomerism
• Constitutional isomers (Structural Isomers)
– Same formula– Different connectivity
1.
2. Different
3. Different
©2005 Mark S. Davis
Stereoisomers• Same connectivity, different
• Enantiomers–
• Diastereomers– – cis- and trans- isomers
©2005 Mark S. Davis
Enantiomers• Must meet two requirements
1. The molecules are
2. They are not
• If the molecule meets the two conditions, it is said to be
– Hand,
• This is called
©2005 Mark S. Davis
Ultimately
• A pair of enantiomers is possible only when a compound contains a
©2005 Mark S. Davis
Ways to draw• Wedge-bond
• Fischer projection - rules– Horizontal =
– Vertical =
©2005 Mark S. Davis
Naming Enantiomers• Use prefix, either
– These are confirmed with x-ray crystallography
• RULES – must be followed1. The tetrahedral stereocenter must have
1.
2. A
3.
©2005 Mark S. Davis
Naming Continued2. Fischer projections with
1. R groups pointed
2. R1, the most substituted Carbon (least # of H) is pointed
3. R2 is pointed
3. The D form has the heteroatom on the
4. The L form has the heteroatom on the
©2005 Mark S. Davis
Properties of Enantiomers
• Chiral compounds are different than achiral compounds in – –
• A pair of enantiomers have the same physical properties, mp, bp, solubility
• They differ in
©2005 Mark S. Davis
Optical Activity• Ability to rotate
• Use a polarimeter and a solution of the desired enantiomer
• Light is either rotated
©2005 Mark S. Davis
Optical Activity
• Note that designations are based on Fischer drawings and NOT on the rotation of light.
• Some
• A mixture of equal concentrations of
©2005 Mark S. Davis
Chiral Recognition
• Enantiomers have the same reactivity with
• Enantiomers have large
– See figure 17.12, p. 509– This is called
©2005 Mark S. Davis
Chirality in Organisms
• Humans and most organisms use and synthesize only
• Reactions in living cells produce
• Reactions in laboratory produce a
©2005 Mark S. Davis
Chirality
• Different enantiomers can produce different effects in the body. See box 17.1 page 511
• Box 17.2 page 512 – Dopamine, L-Dopa and D-Dopa
©2005 Mark S. Davis
Review…• Enantiomers are
• The only difference in physical properties is
• No reactivity difference to achiral compounds
• Large differences in reactivity to chiral compounds or chiral enzymes
©2005 Mark S. Davis
Two or more Stereocenters• Increases the number of stereoisomers
• If there are
• D-Glucose has
• Starch (many glucose connected) has hundreds of stereocenters
©2005 Mark S. Davis
Consider…
• 2,3-pentanediol–
©2005 Mark S. Davis
Nutrasweet (Aspartame)
• Two stereocenters,
©2005 Mark S. Davis
Consider…• 2,3-butanediol
–
©2005 Mark S. Davis
Cyclic stereoisomers• Cyclic compounds can be enantiomers
– Carvone from box 17.1 p 511
• A ring carbon is a tetrahedral stereocenter if:–