26.2- REACTIONS OF THE CARBONYL GROUP IN ALDEHYDES AND KETONES

Question 1

What type of reactions are many of the reactions of carbonyl compounds?

Answer 1

nucleophilic addition reactions

Question 2

What other reactions do carbonyl compounds undergo other than nucleophilic addition reactions?

Answer 2

redox reactions

Question 3

What is produced when a carbonyl compound reacts with sodium/potassium cyanide + HCl?

Answer 3

hydroxynitriles

Question 4

Why are hydroxynitriles useful in synthesis

Answer 4

as both -OH and -CN groups reactive + can be converted into other functional groups

Question 5

When is a racemic mixture of two optical isomers (enantiomers) produced? (addition of hydrogen cyanide)

Answer 5

when carried out with an aldehyde or unsymmetrical ketone

Question 6

Why is a racemic mixture produced of two optical isomers (enantiomers) produced when carried out with an aldehyde or an unsymmetrical ketone?

Answer 6

as :CN- ion may attack from above or below the flat C=O group

Question 7

What can aldehydes be oxidised into?

Answer 7

carboxylic acids

Question 8

What is used to represent the oxidising agent?

Answer 8

[O]

Question 9

What is a commonly used oxidising agent?

Answer 9

acidified (with sulphuric acid) potassium dichromate

Question 10

Can ketones be oxidised easily into carboxylic acids?

Answer 10

no

Question 11

Why can’t ketones be oxidised easily into carboxylic acids?

Answer 11

as, unlike aldehydes, a C-C bond must be broken

Question 12

What do stronger oxidising agents do to ketones? (oxidation)

Answer 12

stronger oxidising agents break hydrocarbon chain of ketone molecule resulting in shorter chain molecule, CO2 + water

Question 13

What is the basis of distinguishing between aldehydes and ketones?

Answer 13

weak oxidising agents can oxidise aldehydes but not ketones

Question 14

What is Fehling’s solution made up from?

Answer 14

mixture of two solution

Question 15

What two mixtures of solution is Fehling’s mixture made up of?

Answer 15

Fehling’s A which contains Cu2+ ion and so is coloured blue, and Fehling’s B which contains an alkali + complexing agent

Question 16

What happens when an aldehyde is warmed with Fehling’s solution?

Answer 16

brick red precipitate of copper (I) oxide produced as copper (II) oxidises aldehyde to carboxylic acid, and is itself reduced to copper (I)

Question 17

What happens when a ketone is warmed with Fehling’s solution?

Answer 17

no reaction

Question 18

What does Tollen’s reagent contain?

Answer 18

complex ion [Ag(NH3)2]+ which is formed when aqueous ammonia added to aqueous solution of silver nitrate

Question 19

What happens when an aldehyde is warmed with Tollen’s reagent?

Answer 19

metallic silver formed
aldehydes oxidised to carboxylic acids by Tollen’s reagent
Ag+ reduced to metallic silver
silver mirror will be formed on inside of test tube

Question 20

Equations for aldehyde being oxidised and silver being reduced with silver mirror test?

Answer 20

RCHO + [O] -> RCOOH

[Ag(NH3)2] + E- -> Ag + 2NH3

Question 21

What happens when a ketones is warmed with Tollen’s reagent?

Answer 21

no reaction to test

Question 22

Example of reducing agent that will reduce both aldehydes and ketones to alcohols?

Answer 22

sodium tetrahydridoborate (III) in aqueous solution

Question 23

When the reducing agent sodium tetrahydridoborate (III) acts on an aldehyde or ketone, what does this generate?

Answer 23

nucleophile :H-, hydride ion

Question 24

What does the nucleophile :H- do?

Answer 24

reduces partially positive C𝛿+ = O𝛿- but not C=C as it’s repelled by high electron density in C-C bond, but attracted to C𝛿+ of C=O bond

Question 25

What are aldehydes reduced to?

Answer 25

primary alcohols

Question 26

What are ketones reduced to?

Answer 26

secondary alcohols

Question 27

What sort of reaction is the reduction of an aldehyde or ketone?

Answer 27

nucleophilic addition reactions