3.3.7 Optical Isomerism (A2)
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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.