4.1 stereoisomerism Flashcards
what is stereoisomerism?
isomers that have the same structure and bond sequence but atoms are orientated differently in space
what are the 2 main types of stereoisomerism?
- geometric isomers (E/Z isomerism)
- optical isomers
in e/z isomers, what isomer is it if the higher priority groups are on the same side of the double bond?
z isomer
in e/z isomers, what isomer is it if the higher priority groups are on opposite sides of the double bond?
e isomers
why can molecules exhibit geometric isomerism?h
- as there is no rotation about the double bond
- AND there are two different groups either side of the carbon-carbon double bond
where does optical isomerism occur?
- in any compound that had any carbon with 4 single bonds, with each bonded to a different group
what do you call carbons that fit the description of optical isomerism?
chiral centres or asymmetric carbons
what does achiral mean?
not chiral
how do you denote a chiral centre?
with an asterisk next to it (*)
- you can spot if a compound is going to be chiral looking to see if it has a plane of symmetry
- if a compound has no plane of symmetry, it can be described as chiral
- molecules that are chiral are said to exhibit optical activity
what does Le Bel-Van’t Hoff rule state?
that the number of stereoisomers that an organic molecule contains is equal to 2^n
where n represents the number of chiral centres
any molecule with a chiral centre can occur as a different version of each other - what are these ‘versions’ called?
enantiomers
what is an enantiomer?
one of two stereoisomers that are non-superimposable mirror images of each other
how do you draw chiral molecules?
- 3D
- with the chiral centre in the middle
what does it mean if objects are super-imposable on each other?
- they can perfectly overlap with each other if they were in the same place
(anything that has a chiral centre therefore has enantiomers that are non superimposable)
do enantiomers have the same chemical properties?
no they often have completely different chemical properties
do enantiomers have the same physical properties?
yes - enantiomers can often have completely different chemical properties but often similar physical properties
(physical properties = melting point/boiling point/solubility)
how are enantiomers distinguished by name?
- by + / - or R / S
how do we distinguish between enantiomers?
- shine plane polarised light on each enantiomer sample
- and each rotates the plane of plane polarised light in equal but opposite directions
- using a polarimeter
why are we able to use a polarimeter to distinguish between enantiomers?
- enantiomers behave differently when they are exposed to plane polarised light
- they rotate in opposite directions
- (-) enantiomer = anticlockwise rotation
- (+) enantiomer = clockwise rotation
what is it called if we have equal amounts of each enantiomer?
a racemate or racemic mixture is when you have a 50:50 (equimolar) mixture of each enantiomer
what affects does a racemate or racemic mixture have on the rotation of plane polarised light?
no effect
- on the rotation of plane polarised light
- as each enantiomer rotates the plane of plane polarised light in equal but opposite angles - resulting in no net rotation
what is the importance of optical isomerism?
- many natural molecules are chiral and most natural reactions are affected by optical isomerism
- e.g most amino acids are chiral (and so proteins) along with many other molecules. This is the reason enzymes can build very specific active sites
- many drugs are optically active, with one enantiomer only having the beneficial effect
- in the case of some drugs the other enantiomer can even be harmful e.g thalidomide
- most drugs that are produced today are only 1 pure enantiomer
- however this process is expensive and is often difficult to get enantiomerically pure version of a substance
- this is an active area of research in chemistry - asymmetric synethesis
