Chapter 9 - Stereoisomerism
9.1 Review of Isomerism
Isomers
9.2 Enantiomers
Enantiomer –
1)
2)
Chiral ex’s:
Achiral ex’s:
Build It! 2-butanol, 2-methyl-2-butanol
Wedge-bond Structual Drawing
Fischer Projection
Chiral Test:
Does it have a
tetrahedral
stereocenter?
mirror
mirror
• The solid, horizontal wedge bonds (
) extend in front of the plane of the paper.
• The dashed, vertical wedge bonds (
) extend behind the plane of the paper.
• Both solid and dashed wedge bonds are oriented with their wider ends nearer the viewer.
Build It! 1-bromo-1-chloroethane, 1,1-dichloroethane, 1,2-dichloroethane
Wedge-bond Structual Drawing
Fischer Projection
mirror
mirror
1
9.3 Interpreting Structural Formulas of Enantiomers
Build it: 1-bromo-1-chloroethane. The book is set up so that you can only rotate diagram 180°.
Determining whether two structural drawings represent a pair of enantiomers or the same
compound.
Indicate whether each of the following pairs of structural drawings represents the same
compound of a pair of enantiomers.
(a)
(b)
9.4 Nomenclature for Enantiomers
• configuration determined by x-ray crystallography
• Two systems R/S and D/L
D/L System (Works if substituents include –H, -heteratom substituent (-OH or –NH2), -R1, -R2.
Fischer Projections
-R1 and -R2 extend backward (__________)
-H and –heteroatom extend forward (__________)
Fischer projection is oriented so
-R1 (carbon w/ fewest H’s) is located upward and
- R2 (most H’s) is located downward.
9.5 Properties of Enantiomers – OMIT Optical Activity & Specific Rotation
Chemical Properties: Chiral Recognition
Chiral reactivity depends on other reactants or the enzyme (biological catalyst) or both.
• When reacting with achiral compounds, enantiomers usually have the same reactivity.
• When reacting with chiral compounds or in reactions catalyzed by chiral enzymes,
enantiomers usually have very large differences in reactivity.
2
Box 9.3 Synthetic Chiral Drugs
• 40-50% drugs consist of compounds with one or more stereocenters.
• Manufacturing produces a racemic mixture
Body Responds:
1) Both enantiomers react but in different processes, one of which may produce an
undesired effect on the organism.
2) One enantiomer reacts in a biological process and the other pases through the
organism with no effect.
Ex. Parkinson’s Disease
Consequence –
Drug – L-Dopa (L-3,4-dihydroxyphenylalanine)
Making non-racemic mixtures:
1)
2)
3