3.3.7 Optical isomerism Flashcards
How does optical isomerism occur?
- Occurs in carbon compounds with 4 different groups of atoms attached to a carbon } asymmetric or chiral carbon atom
What does an asymmetric or chiral carbon atom give rise to?
- Optical isomers = enantiomers
- Enaniomers exist as non-superimposable mirror images and differ in their effect of plane polarised light.
What is a racemic mixture?
- A mixture of equal or 50/50 amounts of enantiomers
- +ve isomer and -ve isomer.
How are racemic mixtures (racemates) formed?
- formed in a reaction mechanism when a reactant or intermediate has a trigonal planar group in the molecule which is approached from both sides by an attacking species.
Why are racemic mixtures (racemates) optically inactive?
- As the effect of the +ve isomer rotating the plane of polarised light clockwise is equal to the effect of the -ve isomer rotating the plane anticlockwise.
- The effects of each isomer cancels out.
How is the chiral centre denoted?
C *
Give an example of a chiral molecule.
- lactic acid / 2-hydroxypropanoic acid.
- H3C-H-C * -OH - COOH
How is light polarised?
- Passing it through a polarised filter, so oscilliations are only in one plane.
What effect does the +ve isomer have on plane polarised light?
- Rotates plane of polarisation X * clockwise.
What effect does the -ve isomer have on the plane polarised light?
- Rotates plane of polarisation X * anticlockwise. (same angle, opposite direction)
What is the structure of a polarimeter?
- Light source (unpolarised light)
- Polarising the filter (polarised light)
- Polarised light passes through the compartment containing the sample.
- Detector determines the angle of rotation of the plane polarised light.
What are polarimeters used for?
- To identify which enantimoer is present, the purifty / concentration of the sample etc.
Why do most synthetic processes form racemic mixtures?
- they are formed from nucleophillic addition reactions with alcohols.
- The nucleophile attacks the C=C bond.
- C=C bond is planar.
- Equal chance of attack.
- So equal amounts of isomers are formed.
Why do living organisms only produce one of a pair of enantiomers?
- Molecules made in living systems like lactic acid in muscles or sour milk require enzymes catalyst which have a specific shaped active site so only one of the possible isomers are formed.
What is the first equation of the synthesis of lactic acid (2-hydroxypropanoic acid) from ethanal? (Equation)
REAGENTS
* KCN or HCl
* acceptable to write HCN
EQUATION
* CH3CHO + HCN -> CH3CH(OH)CN
Why is the CH3CH(OH)CN molecule formed chiral?
- H, CH3, OH and CN attached to the chiral carbon (centre carbon)
- 4 substituents.
What is the second stage of sythesis of lactic acid from ethanal?
HYDROLYSIS
CH3CH(OH)CN + HCl + 2H2O -> CH3CH(OH)COOH + NH4Cl
How does this second stage affect chirality?
- Does not affect it, still racemic mixture.
Why is optical isomerism an issue for the drug industry?
- One isomer works, for some, as the protein it binds with in the body has a specific shaped active site.
- enzymes active site is 3D.
What are the options to resolve the issue of only one enantiomer being effective?
- Seperate enantiomers: difficult and expensive as they have similar properties.
- ** Sell racemate:** wasteful as half is inactive.
- Develop a new way of producing the drug where only one isomer / enantiomer is made
Name some optically active drugs.
- Ibruprofen.
- Thalidomide.
Why is ibruprofen able to be sold as a racemate, even though the + isomer is needed to treat inflammation?
- Sold as 50% racemate
- But body covers 60% of R-isomer to S-isomer -> end up with 80% S-isomer.
eq State the meaning of the term racemic mixture and explain why a mixture formed from a reaction of a ketone and HCN could produce such a mixture. (3)
- a racemic mixture is a mixture (50:50) of equal amounts of a pair of optical isomers meaning is is optically inactive as the effect they each have on plane polarised light cancels out.
- the HCN will attack the C=O, which is a planar carbonyl group…
- so the HCN could attack either side meaning it is equally as likely for each of the isomers to be formed.
