Aldehydes and Ketones

Question 1

What are optical isomers?

Answer 1

• non super imposable mirror images
• occurs when C atoms bonded to 4 different groups (chiral carbon)
  -tetrahedral 109.5

Question 2

Chiral definition

Answer 2

molecule which exists as a pair of optical isomers

Question 3

How are optical isomers distinguished

Answer 3

shine plane of polarised light, one isomer rotates in clockwise the other rotates it anti clockwise

Question 4

Sample of enantiomers

Answer 4

1 enantiomer:
-pure
-rotates polarised light
-optically active

2 enantiomers:
-50/50
-racemic mixture
-not optically active
-rotation cancels

Question 5

Optically active definition

Answer 5

rotates plane of polarised light in one direction only

Question 6

Racemic Mixture definition

Answer 6

A mixture which contains equal quantities of enantiomers

Question 7

Identifying chiral carbons

Answer 7

Question 8

Properties of Optical isomers

Answer 8

• Same melting point and boiling point.
• Same solubility
• Same Mr
• Same peaks in their NMR spectrum, IR spectrum and Mass spectrum

Question 9

Optical isomers as drugs

Answer 9

Many optical isomers will have the same chemical reactivity, however if they are used as a drug both isomers may not work in same way when inside cells.

Thalidomide:
- The ‘R’ optical isomer had the desired effect.
- The ‘S’ optical isomer seemed to have no effect actually caused physical defects in the unborn baby.

Question 10

Options when producing drugs

Answer 10

(1) Supply the drug as a racemic mixture. runs the risk of unwanted side effects. 50% of the drug is ineffective which is wasteful and inefficient.

(2) Make the drug as a racemic mixture and then separate out the two isomers.

(3) Find a method of synthesis in which only one enantiomer is formed.

Question 11

Reduction of Aldehydes and Ketones

Answer 11

Reagent: NaBH4
Mechanism Nucleophilic Addition
Nucleophile H- (a hydride ion) LP donor
Condition: Acidic Solvent needed (H+)

Question 12

Reaction of Aldehydes and Ketones with -:CN

Answer 12

Reagent: KCN then HCl
Mechanism: Nucleophilic Addition
Nucleophile: -:CN
Condition: Acidic needed
Name of product: Hydroxy nitrile

Question 13

Name

Answer 13

1) 2-hydroxy-2-phenylpropanenitrile
2) 2-hydroxy-2-methylbutanenitrile

Question 14

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

Answer 14

• The bonding is about a >C=O group in an aldehyde and ketone is planar.
• This means there is a 50:50 chance of the nucleophile attacking from one side of the molecule or the other.
• This results in equal proportions of each optical isomer forming, and so a racemic mixture is formed.