s&r topic 5&6 Flashcards
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define structural/constitutional isomer
same molecular formula but different connectivity
define stereoisomers
same structural formula but different spatial arrangement
name 2 types of stereoisomer
enantiomer
diastereomers
how do diastereomers arise
a molecule with two or more chiral centres
pair of enantiomers + pair of enantiomers = four diastereomers
can have different combination of R and S configurations
a pair of enantiomers has another pair of enantiomers with different S and R configurations
- the two enantiomers in each pair are enantiomers of each other
- the total four stereoisomers are diastereomers of each other
see topic5 vid1 for diol examples
how do you calculate the number of stereoisomers a molecule will have
in which cases does this not work
what are these cases called
for n number of chiral centres, there are 2ⁿ stereoisomers
doesn’t always work when molecules are chiral (superimposable)
- molecule has an internal plane of symmetry
- one pair of enantiomers are not actually a pair because they are superimposable
these molecules are called meso compounds = achiral molecules that have chiral centres
- meso compounds have opposite R/S labels for chiral centres either side of the internal plane of symmetry
see topic5 vid1 for cyclohexane-diol example
give 2 properties that differ between diastereomers
different melting point
different solubility in water
define trans isomer
similar priority groups on each C in a carbon-carbon double bond are on opposite sides of the double bond
define cis isomer
similar priority groups on each C in the carbon-carbon double bond are on the same side of the double bond
they are both above or both below
what are geometric isomers and how do their properties differ from one another
geometric isomers are cis-trans isomers they have different physical properties - diff density - diff mp - diff IR and NMR spectrum
what are E/Z isomers
aka imine isomers
E = high priority groups on opposite sides of double bond Z = high priority groups are on the same side of the double bond
lone pairs always take priority
in square-planar complexes, how does cis-trans isomerism arise
transition metals take on the square-planar complexes with four bonds to a central atom
in monodentate ligands if there are two different types of substituent, cis-trans isomerism arises
cis isomer = similar ligands at 90° to one another
trans isomer = similar ligands at 180° to one another
define monodentate ligand
substituent binds to metal with only one of its atoms
define bidentate ligand
substituent binds to metal with two of it’s atoms
see topic5 vid2 for examples
in octahedral complexes, how does cis-trans isomerism arise
in an MX₂Y₄ structure where M is the central metal and X and Y are substituents cis-trans isomerism an occur
isomerism is determined by the 2 X substituents
- cis = X substituents are 90° from one another
- trans = X substituents are 180° from one another
rest of the atoms fit around these two X atoms
in octahedral complexes, how does fac-mer isomerism occur
what is meant by fac isomers and mer isomers
in an MX₃Y₃ complex it can occur
fac (facial) isomers = ligands occupy one face of the 3D octahedral
mer (meridional) isomers = identical set of ligands span around meridian of the octahedral structure
what is a racemic mixture
why do S and R enantiomer form a racemic mixture
a 50:50 mixture of enantiomers
activation energy to form R and S enantiomers is the same so they both equally likely to form
what is an enantiopure substance
where are they found
substance composed of only one enantiomer
usually found in biology or drugs to make them more effective
name and describe two ways of separating chiral compounds/enantiomers
chiral resolution - reacting a racemic mixture with a single enantiomer (R or S) of another compound
- this forms two distereomers with different physical properties
- these physical properties can be used to separate them
- reverse the original reaction to get the desired enantiomer from the original racemic mixture
chiral stationary phase chromatography
- enantiomer with greater affinity for the stationary phase with elute from the column more slowly
- using HPLC
