Chapter 25/26 - Optical Isomerism, Aldehydes and Ketones/ Compounds containing the carbonyl group

Chapter 25

Optical Isomerism

What is an optical isomer?

an isomer that has 4 different substituents and a chiral carbon

what is a racemic mixture?

equal amounts of each enantiomer

how can you identify different optical isomers

• plane polarised light

• rotates each enantiomer in opposite directions

Chapter 26

Aldehydes and Ketones

Aldehydes functional group

RCHO

ketones functional group

RCHOR^’

state the test to distinguish between aldehydes and ketones

Tollens Reagent

• silver mirror for aldehydes

• no reaction for ketones

what are the 2 reagents that can be used to reduce aldehydes:

1. NaBH₄

2. KCN / HCN

state the mechanism for reducing an aldehyde or a ketone into an alcohol

1. nucleophilic addition

2. Reagent: NaBH₄

3. Nucleophile: :H^-

state the mechanism for reducing an aldehyde/ketone into a hydroxynitrile

1. nucleophilic addition

2. KCN + H₂SO₄ or HCN

3. nucleophile: :CN^-

why is KCN preferred over HCN

1. HCN is toxic

2. KCN dissociates to CN

why might these reactions produce a racemic mixture

1. planar carbonyl bond

2. equal chance of nucleophilic attack from either side

3. equal amounts of each enantiomer produced

4. each optical isomer rotates plane polarised light in opposite directions

5. rotation cancels out

how is an aldehyde oxidised into a carboxylic acid

1. oxidation

2. reagent: K₂Cr₂O₇

3. Conditions: Conc. H₂SO₄ + heat under reflux and excess potassium dichromate

Chapter 26

Compounds containing carbonyl group

what are the 2 types of hydrolysis for esters:

1. acid hydrolysis

2. base hydrolysis

Acid hydrolysis

1. dilute sulphuric or hydrochloric acid

2. heat under reflux

3. produces carboxylic acid and alcohol

Base hydrolysis

1. sodium hydroxide

2. fractional distillation

3. carboxylation ion and alcohol

uses of esters:

soap

Forming an ester from carboxylic acid mechanism

1. nucleophilic addition elimination

2. alcohol + H₂SO₄

3. esterification

4. heat

forming an ester from acid anhydride mechanism

1. nucleophilic addition elimination

2. alcohol

3. room temperature

4. by product of carboxylic acid

forming an ester by acyl chloride (acylation) mechanism

1. nucleophilic addition elimination

2. alcohol

3. room temp

4. by product of HCl