Chapter 5 Stereochemistry: Chiral Molecules Flashcards
What are the subdivision of isomers?
isomers to constitutional isomers and stereoisomers to enantiomers and diastereomers
isomers with the same molecular
formula and same connectivity of atoms but different arrangement of atoms in space
Stereoisomers
Different compounds with same molecular formula
Stereoisomers
isomers whose atoms have a different connectivity
Constitutional isomers
isomers that have the same connectivity but that differ in the same arrangement of their atoms in space
stereoisomers
stereoisomers that are non superposable mirror images of each other
enantiomers
The ability for an isomers to be placed over another isomers, usually in such a way that both will be visible
superposable
Not superposable on its mirror image and can exist as a pair of enantiomers
chiral molecules
A chiral molecule and its mirror image
Pair of enantiomers
Superposable on its mirror image
Achiral molecule
A molecule with a _________ bonded to four different groups will always be chiral
single tetrahedral carbon
A molecule with ______________ bonded to four different groups is not always chiral
more than one tetrahedral carbon
Switching two groups at the tetrahedral center leads to the ______ in a molecule with one tetrahedral carbon
enantiomeric molecule
An atom bearing groups of such nature that an interchange of any two groups will produce a stereoisomer
Stereogenic center
Carbons at a tetrahedral stereogenic center are designated with an _________
asterisk(*)
The binding specificity of a chiral receptor site for a chiral molecule is usually only favorable in one way
The Biological Importance of Chirality
consider this when testing for chirality
planes of symmetry
An imaginary plane that bisects a molecule in such a way that the two halves of the molecule are mirror images of each other
Plane of symmetry
A molecule with a plane of symmetry cannot be _____
chiral
Nomenclature of Enantiomers
R,S System
The R,S System is also called
Cahn-Ingold-Prelog system
The four groups attached to the stereogenic carbon are assigned priorities from ___________
highest to lowest
Priorities are assigned as follows:
1. Atoms directly attached to the _____ are compared
2. Atoms with _______ are given higher priority
- stereogenic center
- higher atomic number
If priority cannot be assigned based on directly attached atoms, the _________ of atoms is examined
next layer
The molecule is rotated to put the lowest priority group at the_____
back
If the groups descend in priority (a,b then c) in clockwise direction
the enantiomer is ____
R
If the groups descend in priority in counterclockwise direction the
enantiomer is ____
S
Groups with double or triple bonds are assigned priorities as if their atoms were ________ or _______
duplicated or triplicated
_________ have almost all identical physical properties (melting point, boiling point, density)
enantiomers
An empty sample tube or one containing an achiral molecule will
not rotate the plane-polarized light
Specific Rotation
The amount the analyzer needs to be turned to permit light through is called ______
observed rotation (a)
If the analyzer is rotated clockwise the rotation is (+) and the molecule is _________
dextrorotatory
If the analyzer is rotated counterclockwise the rotation is (-) and the molecule is ________
levorotatory
what is the formula for specific rotation
[a] = a/c*l
[a]
specific rotation
a
the observed rotation
c
concentration of the solution in g/ml
l
length of the tube in decimeters
= A 1:1 mixture of enantiomers
= No net optical rotation
= Often designated as (+)
Racemic mixture
Often a mixture of enantiomers will be enriched in one enantiomer
Enantiomeric Excess
formula for %enantiomeric excess
- % enantiomeric excess = moles of one enantiomer - moles of other enantiomer/ total moles of both enantiomers * 100
- % enantiomeric excess* = observed specific rotation/ specific rotation of the pure enantiomer * 100
The optical rotation of a sample of 2-butanol is +6.76o. What is the enantiomeric excess? its [a] = +13.52o
50%
The maximum number of stereoisomers available will
not exceed ____, where n is equal to the number of tetrahedral stereogenic centers
2^n
achiral despite the presence of
stereogenic centers
Meso compound
= Not optically active
= Superposable on its mirror image
= Has a plane of symmetry
meso compound
A 2-dimensional representation of chiral molecules
Fischer Projection Formulas
______ represent bonds that project behind the plane of the paper
vertical lines
_________ represent bonds that project out of the plane of
the paper
horizontal lines
A reaction which takes place in a way that no bonds to the
stereogenic carbon are broken is said to proceed with _______
retention of configuration
the relationship between
comparable stereogenic centers in two different molecules
Relative configuration
the actual 3-dimensional orientation of the atoms in a chiral molecule
Absolute configuration
absolute configuratoon can be determined by ______
x-ray crystallography
Chiral Molecules that Do Not Possess a Tetrahedral Atom with Four Different Groups
- atropoisomer 2. allenes
conformational isomers that are stable
Atropoisomer
contain two consecutive double bonds
Allenes
