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It is important to recognize, however, that
if all of the components of the starting state (reactants, catalysts,
solvents, etc.) are achiral, any chiral product will be formed as a racemic
mixture. |
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This generalization can be more simply stated as "Optically
inactive starting materials can't give optically active
products." (Remember: In order for a substance to be optically
active, it must be chiral and one enantiomer must be present in greater
amounts than the other. |
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However, if the reactant is racemic, the
product will be racemic also. |
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Remember: "Optically
inactive starting materials can't give optically active products." |
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Reactions in living systems are catalyzed by
enzymes, which are enantiomerically homogeneous. |
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The enzyme (catalyst) is part of the reacting system, so such reactions
don't violate the generalization that "Optically inactive starting
materials can't give optically active products." |
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How many stereoisomers when a particular
molecule contains two stereogenic centers? |
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What are all the possible R and S
combinations of the two stereogenic centers in this molecule? |
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What are all the possible R and S
combinations of the two stereogenic centers in this molecule? |
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4 Combinations = 4 Stereoisomers |
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4 Combinations = 4 Stereoisomers |
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What is the relationship between these
stereoisomers? |
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stereoisomers that are not enantiomers are
diastereomers |
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recall for Fischer projection: horizontal bonds point toward you; vertical bonds point away |
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staggered conformation does not
have correct orientation of bonds for Fischer projection |
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transform molecule to eclipsed conformation
in order to construct Fischer projection |
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stereochemical prefixes used to specify relative
configuration in molecules with two stereogenic centers |
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easiest to apply using Fischer
projections |
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orientation: vertical carbon chain |
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when carbon chain is vertical, same (or
analogous) substituents on same
side of
Fischer projection |
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when carbon chain is vertical, same (or
analogous) substituents on opposite
sides of
Fischer projection |
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nonsuperposable mirror images; enantiomers |
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nonsuperposable mirror images; enantiomers |
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stereoisomers that are not
enantiomers; diastereomers |
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It is possible for a molecule to have
stereogenic centers yet be achiral. |
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Consider a molecule with two equivalently
substituted stereogenic centers such as 2,3-butanediol. |
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therefore, this structure and its mirror
image
are the same |
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it is called a meso form |
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a meso form is an achiral molecule that has
stereogenic centers |
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therefore, this structure and its mirror
image
are the same |
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it is called a meso form |
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a meso form is an achiral molecule that has
stereogenic centers |
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meso forms have a plane of symmetry and/or a
center of symmetry |
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plane of symmetry is most common case |
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top half of molecule is mirror image
of bottom half |
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maximum number of stereoisomers = 2n |
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where n = number of structural units capable
of stereochemical variation |
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structural units include stereogenic centers
and cis and/or trans double bonds |
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number is reduced to less than 2n
if meso forms are possible |
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4 stereogenic centers |
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16 stereoisomers |
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11 stereogenic centers |
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211 = 2048 stereoisomers |
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one is "natural" cholic acid |
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a second is the enantiomer of natural cholic
acid |
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2046 are diastereomers of cholic acid |
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maximum number of stereoisomers = 2n |
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where n = number of structural units capable
of stereochemical variation |
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structural units include stereogenic centers
and cis and/or trans double bonds |
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number is reduced to less than 2n
if meso forms are possible |
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Notes
