Orgo Ch 5 Pt.3
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What is the equation for observed rotation?
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
What does racemic not do?
does not rotate the plane because plus and minus cancel each other out
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
What can't you do?
- look at a structure and say its plus.
- R and S allow looking at the structure
What do enantiomers do in term of light?
bend it in equal degrees but different signs
A racemic mixture contains __.
Equal quantities of __
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
Which physical property differs in enantiomers?
optical rotation in terms of direction, not magnitude
If optically inactive reagents combine to form a chiral molecule, what happens?
a racemic mixture is formed
- sometimes called enantiomeric excess
- one enantiomer is present in greater amounts
- op= (observed rotation)/ (rotation of pure enantiomer) x100
If equilibrium exists between two chiral conformers, what happens?
the molecule is not chiral
How do we judge chirality?
by looking at the most symmetrical conformer
Cyclohexane can be considered __.
planar, on average
Because cyclohexanes flip, what?
they don't rotate the plane of polarized light. On average, they are flat and conformationally mobile.
Although cyclohexanes contain mirrors, by flipping it, what happens?
it becomes its mirror image, rendering it optically inactive
When is the only time a cyclohexane is not superimposable?
if it is frozen in the same conformation
Because the interconversion for cyclohexanes is fast, what are the molecules?
What about a sample of it?
- any sample would be racemic and, as such, optically inactive
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
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
__ can be chiral, although they lack a __.
What is the chiral center considered in an allene?
True or False:
It is possible to have chirality without a saturated center.
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
In Fischer projections, the horizontal are __; the vertical are __.
90 degree rotations for Fischer projections do what?
If oyu rotate the fischer projection 180 degrees, what do you get?
the same molecule
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)
A rotation of 180 is allowed because what?
it will not charge the configuration
A 90 degree rotation will do what?
change the orientation of the horizontal and vertical groups
Fischer projections are all __ one another.
If you have an internal plane of symmetry, what's the situation?
If you have chiral centers but no chirality, what is it?
What is meso?
has at least two asymmetric carbons but is achiral
What are two ways to have optically inactive molecules with chiral centers?
- -having an internal plane of symmetry
- -racemic mixtures
How do racemic mixtures behave?
as pure substances independent from others; different characteristics than its enantiomers
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
Molecules with two or more chiral carbons; stereoisomers that are not mirror images
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 __.
- meso compounds
- opposite configurations
- some matching, some opposite config
- internal mirror planes
- 2^n, where n= # of chiral carbons
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
- chiral centers
The 2^n rule will not apply to compounds that may have a __.
plane of symmetry
Diastereomers have __, so they can be __.
Enantiomers differ only in __.
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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