organic 2 p2 Flashcards
what property must a carbon atom have for the molecule to display optical isomerism about that carbon atom?
4 different substituents attached to one carbon
what are the similarities and differences between two optical isomers?
same atoms and bonds, but they are non superimposable mirror images of one another NOT IDENTICAL in chemical properties necessarily
differ in the way they rotate plane polarised light - rotate by the same angle but in different directions
what word to used to describe optically active molcules?
chiral
example of a chiral molecule - lactic acid
what are a pair of isomers called?
enantiomers
what is the chiral centre?
the carbon that has four different substituents attached to it
denoted by a star on C - C*
how is light polarised?
by passing it through a polaroid filter, so oscillations are only in one plane
what effect does a racemic mixture have on plane polarised light?
none - the rotation by each enantiomer cancels out to nothing
what effect do the + and - isomers have on plane polarised light?
+ rotates plane clockwise
- rotates plane anti clockwise
what is the structure of a polarimeter?
light source (unpolarised light) –> polarising filter (polarised light) –> polarised light passes through compartment containing sample –> detector determines the angle of rotation of the plane polarised light
what are polarimeters used for?
to identify which enantiomer is present, the purify of the sample, the concentration of the sample etc.
are racemic mixtures formed in nature? why?
not often, as enzyme mechanisms are 3D so only form one enantiomer
why is optical isomerism a problem for the drug industry?
sometime, only one enantiomer is effective due to enzyme’s active site/cell receptors being 3D
what are the options to resolve the issue of only one enantiomer being effective?
1 separate enantiomers - difficult and expensive as have very similar properties
2 sell racemate - wasteful as half is inactive
3 design alternative synthesis to only produce one enantiomer
examples of optically active drugs?
ibuprofen, thalidomide
why is ibuprofen able to be sold as a racemate, even though the + isomer is needed to treat inflammation?
sold as 50% racemate
but body converts 60% of R- isomer to S+ isomer –> end up with 80% S+ isomer
what is the carbonyl group?
C=O
what is the functional group of an aldehyde?
RCHO (C double bonded to O, single bond to H and R) C=O is on end of chain
-al
what is the functional group for a ketone?
RCOR’ (C double bonded to O) C=O in middle of chain
-one
what kind of intermolecular forces do molecules with the carbonyl group have? why?
permanent dipole-dipole due to the polar C=O bond (O is delta -) not hydrogen bonds
how soluble are carbonyl groups in water? what influences solubility?
yes - form hydrogen bonds between water molecules and oxygen of C=O
as C chain length increases, solubility decreases
which bond in carbonyl compounds is usually involved in reactions? why?
C=O due to the polarity of the bond (large difference in electronegativity between C and O)
C=O is also the strongest bond in carbonyl compounds
why is adding HCN useful for carbonyl compounds?
increases the length of the carbon chain by one carbon atom - very useful!
will the product of HCN added to a carbonyl compound have optical isomers? why?
yes they will
in the aldehyde/ketone, the carbonyl carbon is planar, so the CN- can attack from either above or below forming two enantiomers
what is the name of the product when HCN is added to a carbonyl compound?
hydroxynitriles (have OH and CN groups)
what is Fehling’s solution? what colour is it?
copper complex ions
blue
what happens when an aldehyde is added to Fehling’s solution?
reduced to Cu+ ions –> colour change to brick red ppt
no change when a ketone is added
what is in Tollens’ reagent? what colour is it?
silver complex ions
colourless solution
what happens when an aldehyde is added to Tollens’ reagent?
silver mirror forms as Ag+ reduced to Ag(s)
ketone - no visible change
