3.3.7 Optical Isomerism

Question 1

What is a chiral (or asymmetric) carbon?

Answer 1

Carbon atom that 4 different groups has attached to it

Question 2

What are optical isomers called?

Answer 2

enantiomers

Question 3

Enantiomers are _______ ____ ____

Answer 3

non-superimposed mirror images

Question 4

Draw 2-hydroxypropanoic acid and indentify its chiral centre

Answer 4

Question 5

Draw the 2 enantiomers of 2-hydroxypropanoic acid

Answer 5

Question 6

Plane-polarised light only vibrates in ___ direction

Answer 6

one

Question 7

Optical isomers are optically active. What is meant by this?

Answer 7

They rotate plane-polarised light

Question 8

Describe what happens when enantiomers rotate plane-polarised light

Answer 8

• One enantiomer rotates it in a clockwise direction
• & other rotates it in an anticlockwise direction

Question 9

What does a racemic mixture contain?

Answer 9

Racemic mixture contains equal quantities of each enantiomer of an optically active compound

Question 10

Racemates don’t show any ____ ______

Answer 10

optical activity

Question 11

Why don’t racemates show any optical activity?

Answer 11

2 enantiomers cancel each other’s light-rotating effect

Question 12

How do chemists get a racemic mixture of a chiral product? Explain why this works

Answer 12

• React 2 achiral things together
• ∵ when 2 molecules react there’s an equal chance of forming each of enantiomers

Question 13

Explain how reacting butane and chlorine (2 achiral compounds) yields a racemic mixture of a chiral product

Answer 13

• Cl atom replaces on the H atoms
• Either H atom can be replaced so reaction produces mixture of 2 possible enantiomers
• Each H has 50-50 chance of being replaced = 2 optical isomers formed in equal amounts

Question 14

What is the disadvantage of using chemical methods to modify a reaction to produce a single enantiomer?

Answer 14

It’s difficult and expensive

Question 15

Doubles bonds (C=O and C=C) are _____

Answer 15

planar (flat)

Question 16

When is a racemate formed in a reaction mechanism?

Answer 16

When reactant or intermediate has a trigonal planar group in molecule is approached from both sides by an attacking species

e.g. Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical)

Question 17

Draw the 1st stage of the mechanism for the reaction of propanal with acidified potassium cyanide (KCN)

Answer 17

Question 18

When propanal is reacted with acidified potassium cyanide (KCN), why is there an equal amount of each enantiomer formed?

Answer 18

C=O bond is planar = equal chance that nucleophile will attack from above the plane of the molecule or from below it

Question 19

Why does the product(s) of nucleophilic addition of HCN to symmetrical ketones not display optical isomerism?

Answer 19

Make a product that doesn’t have a chiral centre

Question 20

Name another mechanism where a racemate can be formed

Answer 20

electrophilic addition of HBr to an unsymmetrical alkene