Optical isomers, aldehydes + Ketones (P2+P3)

Question 1

What is optical isomerism?

Answer 1

• shine plane polarised light
• one isomer will rotate clockwise and one isomer will rotate anti clockwise

Question 1

When does optical isomerism occur?

Answer 1

when a carbon atom is bonded to 4 different groups

Question 2

When does optical isomerism occur?

Answer 2

when a carbon atom is bonded to 4 different groups

Question 3

What are enantiomers?

Answer 3

each optical isomer

Question 4

What does optically active mean?

Answer 4

plane polarised light will rotate in one direction only.

Question 5

What does optically inactive mean?

Answer 5

plane polarised light will rotate in opposite directions.

Question 6

What is a racemic mixture?

Answer 6

Where equal amount of enantiomers are formed

Question 7

Name the mechanism, reagents and conditions of reducing propanal to Propan-1-ol
and state the equation
(same mechanism for ketone to secondary alcohol)

Answer 7

Nucleophilic addition
reagent: NaBH4
nucleophile: H:-
conditions: acidic solvent
CH3CH2CHO + 2[H] –> CH3CH2CH2OH

Question 8

Name the mechanism, reagents and conditions of reacting propanal or propanone with KCN
state the equation for propanal with KCN and name the product

Answer 8

Nucleophilic addition
reagent: KCN OR HCl
nucleophile: -:CN
conditions: acidic solvent
CH3CH2CHO + -:CN + H+ –> CH3CH2CH(CN)(OH)
Name of product: 2-hydroxybutanenitrile

Question 9

Why can a nucleophilic addition to aldehydes and ketones result in a racemic mixture of optical isomers?

Answer 9

bonding in C=O is planar
50/50 chance of nucleophile attacking on both sides of the molecule
equal proportions of enantiomers are formed resulting in a racemic mixture.

Question 10

Why is HCl not used in the reaction with aldehydes and ketones?

Answer 10

HCl is a weak acid so slightly dissociates into H+ ions
whereas KCN fully dissociates in water so more cyanide forms