Lecture 9 - Diastereoisomers Flashcards
If the sample rotates plane polarized light clockwise →
dextrorotatory (+)
If it rotates plane polarized light CCW →
laevorotatory ( - )
Optical rotation (a) is a:
quantitative measure of optical activity and a = [a]cl
[a] = specific rotation; c = concentration; l = path length
properties of enantiomers:
often have identical chemical properties too but not if they’re interacting with another chiral molecule, such as an enzyme - having very different biological effects
racemate:
a racemate is a mixture containing equal amounts of two enantiomers
racemization:
the process of forming a racemate from a pure enantiomer is called racemization
the separation of a pair of enantiomers is called:
resolution
difference between racemates and pure enantiomers:
racemates typically have different physical properties from that of the pure enantiomers
when are additional stereoisomers possible?
additional stereoisomers are possible when a molecule has 2 or more asymmetric carbons
stereoisomers that are not enantiomers are called:
diastereomers
diastereomers are not [x] and [x]:
diastereomers are not mirror images and differ in all their physical properties
what relationships are present in a set of four diastereomers?
all four compounds are diastereomers with one another however there are also two pairs of enantiomers within the four of them
meso compounds:
• meso compounds have two or more asymmetric carbons and are achiral
• they are not optically active
• they possess an internal mirror plane
a molecule with n stereocenters can exist as:
2n stereoisomers
a molecule with n stereocenters, can exist
as __ stereoisomers, this number is _______ if a ____ compound is present among the possibilities
(1) 2n
(2) reduced
(3) meso
can allenes be chiral?
allenes (cumulated dienes) may be chiral even though they lack an asymmetric carbon
amine inversion:
amines with an asymmetric nitrogen atom have the potential for chirality
diastereoisomers are stereoisomers that are not enantiomers
what governs the priority order in R&S stereochemistry?
the atomic number - higher the atomic number, higher the priority
R&S - which one is clockwise and which one is anticlockwise?
S - ANTICLOCKWISE - LEFT
R - CLOCKWISE - RIGHT
how do you view the lowest priority groups in R&S stereoisomerism?
you must always view the lowest priority group straight on so that it is projecting behind from your viewpoint
what is different between group placements in R&S stereoisomers?
in S isomers the highest-priority group will be found on the left and carry on around rightwards
in R isomers the highest-priority group will be found on the right and carry on around leftwards
what is E/Z isomerism used for?
determine the stereochemistry of double bonds
what is a newman projection?
the projection with a circle in the middle that looks like a mini-pie-chart
with R&S stereoisomers, when counting the priorities, what is different about double bonds?
with double bonds, you count their atomic mass twice O=O = 2(atomic mass)
