3.3.7 Optical isomerism

Question 1

How does optical isomerism occur?

Answer 1

• Occurs in carbon compounds with 4 different groups of atoms attached to a carbon } asymmetric or chiral carbon atom

Question 2

What does an asymmetric or chiral carbon atom give rise to?

Answer 2

• Optical isomers = enantiomers
• Enaniomers exist as non-superimposable mirror images and differ in their effect of plane polarised light.

Question 3

What is a racemic mixture?

Answer 3

• A mixture of equal or 50/50 amounts of enantiomers
• +ve isomer and -ve isomer.

Question 4

How are racemic mixtures (racemates) formed?

Answer 4

• 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.

Question 5

Why are racemic mixtures (racemates) optically inactive?

Answer 5

• 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.

Question 6

How is the chiral centre denoted?

Answer 6

C *

Question 7

Give an example of a chiral molecule.

Answer 7

• lactic acid / 2-hydroxypropanoic acid.
• H3C-H-C * -OH - COOH

Question 8

How is light polarised?

Answer 8

• Passing it through a polarised filter, so oscilliations are only in one plane.

Question 9

What effect does the +ve isomer have on plane polarised light?

Answer 9

• Rotates plane of polarisation X * clockwise.

Question 10

What effect does the -ve isomer have on the plane polarised light?

Answer 10

• Rotates plane of polarisation X * anticlockwise. (same angle, opposite direction)

Question 11

What is the structure of a polarimeter?

Answer 11

• 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.

Question 12

What are polarimeters used for?

Answer 12

• To identify which enantimoer is present, the purifty / concentration of the sample etc.

Question 13

Why do most synthetic processes form racemic mixtures?

Answer 13

• 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.

Question 14

Why do living organisms only produce one of a pair of enantiomers?

Answer 14

• 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.

Question 15

What is the first equation of the synthesis of lactic acid (2-hydroxypropanoic acid) from ethanal? (Equation)

Answer 15

REAGENTS
* KCN or HCl
* acceptable to write HCN

EQUATION
* CH3CHO + HCN -> CH3CH(OH)CN

Question 16

Why is the CH3CH(OH)CN molecule formed chiral?

Answer 16

• H, CH3, OH and CN attached to the chiral carbon (centre carbon)
• 4 substituents.

Question 17

What is the second stage of sythesis of lactic acid from ethanal?

Answer 17

HYDROLYSIS
CH3CH(OH)CN + HCl + 2H2O -> CH3CH(OH)COOH + NH4Cl

Question 18

How does this second stage affect chirality?

Answer 18

• Does not affect it, still racemic mixture.

Question 19

Why is optical isomerism an issue for the drug industry?

Answer 19

• 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.

Question 20

What are the options to resolve the issue of only one enantiomer being effective?

Answer 20

• 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

Question 21

Name some optically active drugs.

Answer 21

• Ibruprofen.
• Thalidomide.

Question 22

Why is ibruprofen able to be sold as a racemate, even though the + isomer is needed to treat inflammation?

Answer 22

• Sold as 50% racemate
• But body covers 60% of R-isomer to S-isomer -> end up with 80% S-isomer.

Question 23

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)

Answer 23

1. 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.
2. the HCN will attack the C=O, which is a planar carbonyl group…
3. so the HCN could attack either side meaning it is equally as likely for each of the isomers to be formed.

Question 24

**eq** State how separate samples of optical isomers can be distinguished. (2)

Answer 24

1. by shining plane polarised light through each of the samples 2. each isomer will rotate the light in opposite directions

Question 25

**eq** State the meaning of the term stereoisomerism.

Answer 25

when molecules have the same molecular and structural formula but atoms are arranged differently in space, either over a c=c or around a chiral carbon centre

Question 26

**eq** Why is a racemic mixture not optically active?

Answer 26

the effects of the +ve isomer cancel out those of the -ve isomer on plane polarised light.

Question 27

Passing polarised light through a solution of sucrose (4)

Answer 27

Polarised light passed through 2 sol. of sucrose same conc each contains a different optical isomer 1 sol. will rotate plane of polarisation by an angle, clockwise: (+)-isomer Another solution will rotate plane of polarisation by the same angle, anticlockwise: (-)-isomer