3.3.7 Optical Isomerism (A2) Flashcards
what is the definition of a positional isomer?
same molecular formula
different functional group position
what is the definition of a functional group isomer?
same molecular formula
different functional group
what are chain isomers?
same molecular formula
different chain length
what is a stereoisomer?
same molecular formula
different arrangement of atoms in space
what are the two types of stereoisomer?
E/Z
optical
when does optical isomerism occur?
when a carbon atom has 4 different substituent groups attached via bonds (chiral)
what are the rules when drawing a pair of enantiomers?
draw shapes as a tetrahedral around a chiral carbon
show a mirror as a dotted line and label as ‘imaginary mirror’
draw the image as an exact mirror image with the same groups having dotted and wedge bonds
how do optical isomers of the same molecule differ?
how they rotate plane-polarised light
what is plane-polarised light?
where light wave moves in only one plane
what is unpolarised light?
waves vibrate in all directions
how can separate samples of enantiomers be distinguished?
pass through plane polarised light
measure angle and direction light is rotated by
one enantiomers rotates clockwise
other rotates anticlockwise
what is a racemic mixture?
optically active products are produced in organic synthesis pathways we get a 50:50 mixture of the +isomer and the -isomer
no optical activity because the effect of the two enantiomers cancels out
what is a nucleophile?
electron donor
lone pairs of e-
“love positive charge”
what is a cyanide ion?
C triple bond N
what type of (x)phile is the cyanide ion?
nucleophile
does 2-hydroxylpentanenitrile have a chiral carbon? why?
yes
4 different groups attached to carbon
why does the oxygen not re form the double bond in nucleophilic attack (addition)
the carbon is not delta positive enough
what between C and ? makes the carbon delta positive?
double bond oxygen
what does the carbon have to be for the oxygen to reform the double bond?
delta positive (from at least 1 other group attached) eg. Cl, N etc.
what are the similarities and differences between two optical isomers?
similarities - same atoms and bonds, but they are non-superimposable mirror images of one another.
differences - not identical in chemical properties necessarily
differ in the way they rotate plane-polarised light - rotate plane of polarisation by the same angle but in different directions.
what does the hydrolysis of 2-hydroxypentanenitrile produce?
racemic mixture of 2-hydroxypropanoic acid enantiomers
why is nucleophillic addition of a nitrile important in chemical synthesis?
increases the number of carbons in the chain
this is difficult to do
why is only one enantiomers of lactic acid made in the body naturally?
the enzyme that catalyses the reaction has a very specific active site which can differentiate between enantiomers
what is the problem with producing a racemic mixture of a drug?
most drugs work by fitting to a complementary receptor within the body - shape is very important
therefore only one optically active isomer will fit and the other is inactive.
what are the solutions for producing a racemic mixture of a drug for the body where only one optically active enantiomer fits to the receptor?
separate the two enantiomers
sell as a mixture
design a synthesis pathway that only produces one isomer rather than a racemic mixture (usually incorporates enzymes)
what is the downside to separating enantiomers?
difficult and too expensive
what is the downside to selling drugs as a racemic mixture?
wasteful
what happens with ibuprofen?
exists as two enantiomers
one is an anti-inflammatory
one is inactive (harmless)
enzyme used to convert inactive to active enantiomer but is only X% effective
sold as a mixture
what happens with naproxen?
one enantiomer relieves pain from arthritis
one enantiomer causes liver failure
synthesis pathway designed to get one enantiomer
what happened with thalidomide?
one enantiomer cured morning sickness
one enantiomer caused limb shortening in unborn babies
withdrawn from use
what are the pair of isomers called?
enantiomers
what is the chiral centre?
the carbon that has 4 different substituents attached to it
how is the chiral centre denoted?
C*
how is light polarised?
by passing it through a polarised filter, so oscillations are only in one plane
what effect does the racemic mixture have on plane polarised light?
none, as the rotation by each enantiomer cancels out to nothing.
what effect does the + enantiomer have on plane polarised light?
rotates plane of polarisation by x degrees clockwise
what effect does the - enantiomer have on plane polarised light?
rotates plane polarised light by x degrees anti-clockwise
what is the first stage of the synthesis of lactic acid from ethanal?
CH3CHO + HCN —> CH3CH(OH)CN
what is the second stage of the synthesis of lactic acid from ethanal? how does this affect the chirality?
hydrolysis: CH3CH(OH)CN + HCl + 2H2O —> CH3CH(OH)COOH + NH4Cl
does not affect it - still racemic as chirality not affected by this stage.
are racemic mixtures formed in nature? why?
not often, enzyme mechanisms are 3D so only form one enantiomer
why is optical isomerism a problem for the drug industry?
sometimes, only one enantiomer is effective due to enzyme’s active site / cell receptors being 3D.
