Optical Isomerism

Question 1

What are optical isomers

Answer 1

Mirror Images of each other 🪞

Question 2

Optical isomerism is a type of what?

Answer 2

Stereoisomerism

Question 3

What are stereoisomers

Answer 3

Stereoisomers have the same structural formula but have their atoms arranged differently in space

Question 4

What’s a chiral molecule?

Answer 4

Asymmetric carbon atom that has four different groups attached to it

Question 5

What are enantiomers/ optical isomers

Answer 5

• Can be arranged in two different ways around the carbon atom so that two different molecules are made
• Chiral molecules
• Mirrored images of each other
• Can’t be superimposed

Question 6

What does superimposed mean?

Answer 6

• They are the same and you can’t tell them apart
• If molecules can be superimposed, they’re achiral and there’s no optical isomerism

Question 7

What does achiral mean?

Answer 7

Symmetrical

Question 8

Asymmetrical

Answer 8

Chiral molecule

Question 9

How can optical isomers be drawn?

Answer 9

• By locating the chiral centre: carbon atom with four different groups attached
• Draw one enantiomer in a tetrahedral shape, then draw a mirror image beside it

Question 10

What do optical isomers rotate?

Answer 10

Optical isomers rotate plane-polarised light

Question 11

Why do optical isomers rotate plane-polarised light?

Answer 11

Because they are optically active

Question 12

What is the difference in the direction of vibration of normal light and plane-polarised light?

Answer 12

• Normal light- vibrates in all directions
• Plane-polarised light- only vibrates in one direction

Question 13

How do enantiomers differentiate in their rotation

Answer 13

• One enantiomer rotates in a clockwise direction
• The other enantiomer roasters in an anti-clockwise direction

Question 14

What is a racemate

Answer 14

• A mixture of enantiomers
• Contains equal quantities of each enantiomer of an optically active compound

Question 15

Why don’t racemates show any optical activity and how are they formed?

Answer 15

• Racemates don’t show any optical activity- the two enantiomers cancel each other’s light rotating effect
• Racemates are made when chemists react two achiral things together to get a race mix mixture of a chiral product
• There’s an equal chance of forming each of the enantiomers

Question 16

How is an enantiomer formed with butane and chlorine, give the formula

Answer 16

Check physical flashcards for answers

Question 17

What other reactions produce Racemates?

Answer 17

Reactions involving planar bonds

Question 18

What is a planar bond and give examples and where they can be found

Answer 18

• Planar bond is a double bond, flat
• C=O, C=C
• Found in carbonyl groups: aldehydes and unsymmetrical ketones

Question 19

Describe the reaction between propanal and acidified potassium cyanide

Answer 19

• Involves CN- ion attacking s+ carbon of propanal’s planar C=O group

• CN- ion can attack from two directions, above the plane of the molecule or from below it

• One of the two enantiomers is formed depending on the direction of the nucleophilic attack

• Because C=O bond is planar, there is an equal chance the nucleophile will attack from either directions, an equal amount of enantiomer will be formed

• Leads to a race mixture product

Question 20

What happens when you react a symmetrical ketone with cyanide

Answer 20

• Makes a product that doesn’t have a chiral centre so it won’t display optical isomerism

Question 21

What are the issues with creating a single enantiomer

Answer 21

It is is difficult and expensive

Question 22

Draw the reaction of propanal with acidified potassium cyanide

Answer 22

Check physical flash cards for answers

Question 23

Explain the meaning of the term stereoisomerism

Answer 23

The property of having stereoisomers which are molecules with the same molecular formula and with their atoms arranged in the same way but with a different orientation of the bonds in space

Question 24

State how you can distinguish between two enantiomers

Answer 24

Shine (monochromatic) plane-polarised light through a solution of the molecule. The enantiomers will rotate the light in opposite directions

Question 25

Explain why the enantiomers of the reaction of PENTANAL and acidified potassium cyanide is optically inactive

Answer 25

• Pentanal has a planar C=O bond
• Depending on which direction the CN- ion attacks the s+ carbons from, two different enantiomers can be formed
• The CN- ion is equally likely to attack from either direction
• So equal amounts of each enantiomer are formed, giving an optically inactive racemic mixture

Question 26

Why are optical isomers called ‘optical’

Answer 26

Because of how they interact with polarised light