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
how is stereoisomerism different to structural isomerism?
- structural isomers have the same molecular formula but different structural formulas
- stereoisomers have the same molecular formula and the same structural formula but differ in the arrangement of the atoms in space
define optical isomerism:
- a type of stereoisomerism
- optical isomers have the same structure but are non-superimposable mirror images of each other
why is a chiral centre required for optical isomerism?
- a chiral centre has 4 different groups attached to it
- these groups can be arranged in different ways to give two different molecules known as optical isomers
explain the optical activity of optical isomers:
optic isomers are optically active which means they rotate plane polarised light in equal
how does the optical activity differ for a pair of enantiomers?
the two enantiomers will rotate plane polarised light in opposite directions
what is a racemic mixture?
- a mixture which contains equal quantities of each enantiomer of an optically active compound
why is optical isomerism a problem for the drug industry?
- often only one enantiomer is effective due to an enzyme’s active site / cell receptors being 3D
- a reaction can be modified to produce a single enantiomer but it is difficult and expensive
why do reactions involving planar bonds often produce racemates?
- planar bonds can be attacked by a nucleophile from either above or below
- if the carbon has different groups attached to it then to enantiomers will be produced depending on the direction of attack by the nucleophile on the planar bond
- a racemic mixture will be produced as there’s an equal chance of forming each of the enantiomers
