Stereochemistry Flashcards
(32 cards)
What is a constitutional isomer?
Same molecular formula but different patterns of covalent bonding. They have different IUPAC names, and have a cis or trans relationship.
What is an isomer?
Same molecular formula but different chemical structures
What is a stereoisomer?
Same patterns of covalent bonding but different spatial arrangements of their atoms/functional groups.
What is an enantiomer?
Optical isomers (non-superimposable) that are mirror images of each other.
What are diastereomers?
Optical isomers (non superimposable) that are not mirror images of each other.
Enantiomers rotate plane polarised light. If rotated anti-clockwise, is it L or D?
L and can also be notated with a “-“ sign
Enantiomers rotate plane polarised light. If rotated clockwise, is it L or D?
D, can also be noted with a “+” sign.
If you have equal amounts of both D and L enantiomers in a mixture, what is it called?
Racemic mixture.
What is a meso isomer?
A compound that has two or more chiral centres, however it has a plane of symmetry making it an achiral compound.
How do you assign priority groups?
Highest priority is the highest atomic number. Lowest priority is the lowest atomic number.
ONLY INCLUDE THE ATOM DIRECTLY ATTACHED TO THE CARBON, NOT THE WHOLE GROUP.
What if any of the atoms attached the carbon are the same?
Include the next atom along as well.
When assigning priority groups, what do you do with multiple bonds, such as C=C?
They are treated as having the equivalent number of single bonded species, so C=C would be C-C-C
If the priority groups go in a clockwise order, is it R or S?
R
If the priority groups go in an anti-clockwise order, is it R or S?
S
When assigning R or S, how do you look at the compound?
The lowest priority group, usually hydrogen, is projected backwards away from you.
What are conformational isomers?
Stereoisomers that have different spatial arrangements of their atoms, that are interconverted by rotation around single bonds.
Why is the rotation around a single bond restricted?
There is an energy barrier associated with bond strain to convert from one conformer to another.
What are energetically preferred conformers called?
Totamers.
In open chain molecules, what are the most energetically favourable and unfavourable conformers and why?
Favourable: Those with staggered bonds. Atoms are far apart as possible, repulsive forces are minimised and there is maximised stability.
Unfavourable: Those with eclipsed bonds. Atoms are closer, repulsive forces increased and there is reduced stability.
In non-aromatic cyclic molecules, what is the most energetically favourable and unfavourable forms, and why?
Favourable: Chair: Staggered bonds and minimal torsional strain.
Unfavourable: Boat: Eclipsed bonds, increased torsional strain and is less stable as steric interaction is being produced.
How do geometric isomers arise? What are the two types of geometric isomers?
When a group of atoms are arranged asymmetrically around a double bond.
E- isomer (trans)
Z- isomer (cis)
Why are E and Z isomers not readily interconverted?
This required the pi bond to be broken so that the groups at either side of the now sigma bond can rotate. Then, the pi bond is reformed. The whole process is energetically unfavourable.
Give examples of topological isomers.
Nucleic acids, proteins and polysaccharides
Alpha-helices
These biomolecules are polymers involving chiral molecules . Their macromolecular forms can also be chiral and form helices.
When naming molecules, what is the difference between alpha and beta?
Alpha: Going backwards. Bonds below the plane.
Beta: Towards you. Bonds above the plane.
