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Sometimes, we deal with mixtures that are neither optically pure (all one enantiomer) nor racemic (equal amounts of two enantiomers). In these cases, we specify the __ of the mxiture, which is defined as what?
- optical purity (o.p)
- the ratio of its rotation to the rotation of a pure enantiomer
How do we compute o.p?
(observed roation)/ (rotation of pure enantiomer) x 100%
The __ is a similar method for expressing the relative amounts of enantiomers in a mixture.
enantiomeric excess (e.e)
How do we compute e.e.?
we calculate the excess of hte predominant enantiomer as a percentage of the entire mixture.
For a chemically pure compound, the calculation of enantiomeric excess generally gives the same result as the calculation of optical purity, and we often do what? What would this make the formula?
- use e.e. and o.p. interchangeably
- o.p.=e.e.= [abs(d-l)]/(d+l) x 100%
Since the two mirror-image conformations of cyclohexane interconvert and have identical energies, any sample of (for example) cis-1,2-dibromocyclohexane) must what?
contain equal amounts of hte two mirror images.
Similarly, most achiral compounds can exist in what?
transient chiral conformations that are in equilibrium with their mirror-image conformations
A molecule cannot be __ if its __ are in __ with their __. We consider such a molecule to be __.
- optically active
- chiral conformations
- mirror images
What is the general principle with conformationally mobile systems.
- to determine whether a conformationally mobile molecule can be optically active, consider its most symmetric conformation
- can also be said that a molecule cannot be optically active it if is in equilibrium with a structure (or conformaiton) that is achiral
Inherently dchiral compounds have __.
NO achievabel achiral conformaitons
Because conformers differ only by __ about __, they are generalyl in equilibrium at room temp.
We can consider cyclohexane rings as though what? And we should consider straight-chain compounds in what?
- they were flat
- (the most symmetric conformation)
- in their most symmetric conformations (often an eclipsed conformation)
Organic compounds commonly exist as rapidly interconverting __. Even ethane is __ in its __.
When we speak of __, we intend to focus on what rather than what?
- chiral conformations
- skew conformatiosn
- observable, persistent properties
- transient conformations
Most chiral organic compounds have at least __. Some compounds are chiral because what? Some compounds are chiral even though they have what? In these types of compounds, special characteristics of the molecules' shapes lend __ to the structure.
- one asymmetric carbon atom
- they have another asymmetric atom
- no asymmetric atoms at all
Some molecules are so bulky or so highly strained that whT?
These molecules are said to be what?
- they cannot easily convert form one chiral conformation to the mirror-image conformation; nor can they achieve the most symmetric conformation because it has too much steric strain or ring strain
- in a locked-in conformation
When a molecule cannot pass through a conformation, it is what?
It can exist in only one of the two __ conformations shown on the left and right. These conformations are __, and they do not __. They are __, and they can be __ and __. Each of them is __, and they have __ and __.
- conformationally locked
- nonsuperimposable mirror images
- optically active
- oppsoite specific rotations
__ are compounds that contain the C=C=C unit, with __ meeting at a single carbon atom. The parent compound, __, has the common name __.
- two C=C double bonds
In allene, the central carbon is __ and __, and the two outer carbons are __ and __.
- sp hybridized/ linear
- sp2 hybridized/ trigonal
Explain the p orbitals in an allene.
- the central sp carbon atom must use different p orbitals to form the pi bonds with the two outer carbon atoms
- the two unhybridized p orbitals on the sp hybrid carbon atom are perpendicular, so the two pi bonds must be perpendicular too
If we add substitutents to allene, what happens?
it may become chiral