4. Stereochemistry and Isomers Flashcards
what are isomers?
two structures that have the same molecular formula but different structural arrangement in 3D
what are constitutional or structural isomers?
two molecules that have the same molecular formula but are bonded differently
what is the index of hydrogen deficiency?
-a calculation to find the degrees of unsaturation in a molecule
-2C+2+N-H-X/2
what are conformational isomers?
two molecules that have the same molecular formula but are bonded differently in terms of rotation about the axis
what is a condition where constitutional isomers can not be present?
when there is a double bond or pi bond
what does it mean that due to steric repulsions, different constitutional isomers are favoured for enthalpic reasons?
reducing steric strain can lower enthalpy and thus increase stability
what are staggered conformations?
an arrangement of Newman projections that minimizes steric repulsions because substituents are at the maximum distance from one another
what are eclipsed conformations?
an arrangement of Newman projections where steric repulsions are not minimized because substituents are not at the maximum distance from one another
what is anti-conformation?
when substituents are arranged as far as possible, typically 180 degrees apart to reduce steric strain
what is gauche conformation?
when substituents are not arranged as far apart as possible, typically 60 degrees apart it has more steric strain
what type of bond is needed for cis and trans conformations?
a double bond
what is a cis conformation?
when higher priority substituent groups are added to the same side of the double bond
what is trans conformation?
when higher priority groups are added to opposite sides of the double bond
what geometric conformation is E the similar to?
trans
what geometric conformation is Z similar to?
cis
what is the chiral center or a stereocenter?
sp3 hybridized atoms with four groups connected to it
what is the purpose of Cahn Ingold Prelog Rules?
allows you to assign priority to four different substituent groups
what does it mean when substituents contain a double bond according to the Cahn Ingold Prelog Rules?
if carbon is double bonded to an atom, there are now two of the atoms carbon is bonded to
how do you assign R and S nomenclature?
- find a chiral center
- rank the four substituents
- draw an arrow tracing from highest to lowest priority
- if the arrow is clockwise this is R
-if the arrow is counter-clockwise, this is S
what is the rule when assigning R and S nomenclature if hydrogen is pointing out of the page?
the answer is the opposite nomenclature
what are optical isomers also known as?
enantiomers
what are enantiomers?
-when comparing two molecules, if you flip one you will get the other molecule
-two molecules that are flipped mirror images of one another
to have enantiomers, what must a molecule have?
one or more chiral centers
what is the optical activity of enantiomers?
enantiomers rotate plane-polarized light in the opposite direction, one rotates light to the left and the other rotates light to the right
what are diastereomers?
two molecules that have the same formula but are bonded differently and are not enantiomers
to have diastereomers, what must a molecule have?
two or more chiral centers
what is the optical activity of diastereomers?
there is no optical activity
what is a meso compound?
a molecule where if you draw a line through the middle of it, one half mirrors the other half
what is the optical activity of meso compounds?
there is no optical activity
what does a molecule name with R or S stereochemistry look like?
the letter comes first and is in brackets
when naming a molecule, what should you do if there are two stereocenters?
give the position of the stereocenters with numbers
when naming a molecule, what should you do if there is one stereocenter?
you do not have to give the position of the stereocenters with numbers, you can just put the letter
what does a molecule name with E or Z stereochemistry look like?
the letter comes first and is in brackets
how can you find the number of possible stereoisomers?
2 to the power of n, where is the total number of chiral carbon atoms and double bonds
what is the exception to find the total number of stereoisomers of meso compounds?
use the number of possible stereoisomers formula but subtract two from the total
what is the formula for finding the number of geometric isomers when n is odd?
2^(n-1) +2^(n+1/2-1)
what is the formula to find the number of geometric isomers with n even?
2^(n-1)+2^(n/2-1)
