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What is the equation for observed rotation?
[α]= 
where alpha (observed) is the rotation observed int eh polarimeter, c is the concentration in g/mL, and l is the length of sample cell in decimeters
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What does racemic not do?
does not rotate the plane because plus and minus cancel each other out
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What we find in chiral objects is that what?
- chiral compounds usually come in what?
- receptor sites are chiral in nature
- - chiral compounds usually come in one hand
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What can't you do?
- look at a structure and say its plus.
- R and S allow looking at the structure
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What do enantiomers do in term of light?
bend it in equal degrees but different signs
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Racemic Mixtures
A racemic mixture contains __.
Notation: __
Equal quantities of __
No __.
The mixture may have __ and __ from the __.
- equal amounts of the two enantiomers
- d- and l- enantiomers
- (d,l) or (±)
- optical activity
- different bp and mp from the enantiomers
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Which physical property differs in enantiomers?
optical rotation in terms of direction, not magnitude
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If optically inactive reagents combine to form a chiral molecule, what happens?
a racemic mixture is formed
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Optical purity
equation:
- sometimes called enantiomeric excess
- one enantiomer is present in greater amounts
- op= (observed rotation)/ (rotation of pure enantiomer) x100
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If equilibrium exists between two chiral conformers, what happens?
the molecule is not chiral
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How do we judge chirality?
by looking at the most symmetrical conformer
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Cyclohexane can be considered __.
planar, on average
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Because cyclohexanes flip, what?
they don't rotate the plane of polarized light. On average, they are flat and conformationally mobile.
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Although cyclohexanes contain mirrors, by flipping it, what happens?
it becomes its mirror image, rendering it optically inactive
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When is the only time a cyclohexane is not superimposable?
if it is frozen in the same conformation
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Because the interconversion for cyclohexanes is fast, what are the molecules?
What about a sample of it?
- equilibrium
- any sample would be racemic and, as such, optically inactive
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What would make a conformer nonmobile?
The staggered positions are __. Why?
- the planar conformation may be too sterically crowded, making the compound without rotation aroudn the central C-C bond and thus conformationally locked
- chiral: they are nonsuperimposable mirror images
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What does locked mean in terms of a molecule?
can't rotate because tehy bang into each other; doesn't exist in two conformations; its one or the other
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__ can be chiral, although they lack a __.
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What is the chiral center considered in an allene?
C=C=C
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True or False:
It is possible to have chirality without a saturated center.
True
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What is the Fischer projection?
- flat representation of a 3D molecule
- a chiral carbon is at the intersection of horizontal and vertical lines
- horizontal lines are forward, out of plane
- vertical lines are behind the plane
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In Fischer projections, the horizontal are __; the vertical are __.
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90 degree rotations for Fischer projections do what?
give enantiomers
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If oyu rotate the fischer projection 180 degrees, what do you get?
the same molecule
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What are the Fischer rules?
- carbon chain is on the vertical line
- highest oxidized carbon on to
- rotation of 180 doesn't change molecule
- rotation of 90 changes molecule adn gives an enantiomer (not allowed)
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A rotation of 180 is allowed because what?
it will not charge the configuration
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A 90 degree rotation will do what?
change the orientation of the horizontal and vertical groups
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Fischer projections are all __ one another.
eclipsing
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If you have an internal plane of symmetry, what's the situation?
achiral
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If you have chiral centers but no chirality, what is it?
meso
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What is meso?
has at least two asymmetric carbons but is achiral
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What are two ways to have optically inactive molecules with chiral centers?
- -having an internal plane of symmetry
- -racemic mixtures
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How do racemic mixtures behave?
as pure substances independent from others; different characteristics than its enantiomers
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What is 2 to the n power?
the maximum possible number of stereoisomers you CAN have, not MUST have; if you don't have all, its because you have a meso compound
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Molecules with two or more chiral carbons; stereoisomers that are not mirror images
diastereomers
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when compounds have two or more chiral centers they have __, __, or __.
Enantiomers have __ at each corresponding chiral center.
Diastereomers have __.
Meso compounds have __.
Maximum # of isomers is __.
- enantiomers
- diastereomers
- meso compounds
- opposite configurations
- some matching, some opposite config
- internal mirror planes
- 2^n, where n= # of chiral carbons
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Meso compounds have __. If one image was rotated 180 degrees, it could be __ on the other image.
Meso compounds are __ even though they have __.
- a plane of symmetry
- superimposed
- achiral
- chiral centers
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The 2^n rule will not apply to compounds that may have a __.
plane of symmetry
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Diastereomers have __, so they can be __.
Enantiomers differ only in __.
Enantiomers are__.
Convert enantiomers into diastereomers to be able ot __.
- different physical properties
- easily separated
- reaction with other chiral molecules and hte direction in which polarized light is rotated
- difficult to separate
- separate them
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