4.4 Aldehydes And Ketones

Question 1

How to form aldehyde from primary alcohol

Answer 1

Oxidation
Distillation to prevent further oxidation to a carboxylic acid
Use of oxidising agent

Question 2

How to form ketones from secondary alcohols

Answer 2

Oxidation
Use oxidising agent
Warmed in a water bath to since alcohols are flammable

Question 3

Can tertiary alcohols be oxidised to form carbonyl groups

Answer 3

No

Question 4

Oxidising agents to use

Answer 4

• acidified potassium dichromate (VI)
• acidified potassium manganate (VII)

Question 5

Acidified potassium dichromate

Answer 5

(H+/K2Cr2O7)
Colour change from orange Cr6+ ion to green Cr3+ ion

Question 6

Acidified potassium manganate

Answer 6

(H+/KMnO4)
Colour change from purple Mn7+ion to colourless Mn2+ ion

Question 7

What to use Tollens’ and Fehling’s reagent for and why can you do this

Answer 7

Test to distinguish between aldehyde or ketone
Aldehydes will be oxidised to carboxylic acid and cause a reaction whilst a ketone will not

Question 8

Tollen’ reagent - silver mirror test

Answer 8

• add 5 drops of unknown solution into test tube followed by 2cm3 of Tollens’ reagent and warm in water bath
• Tollens’ reagent is a solution of silver nitrate in aqueous ammonia
• ketone - no reaction
• aldehyde - silver mirror is seen as the silver ions are reduced to silver

Question 9

Fehling’s reagent

Answer 9

• add 5 drops of unknown solution to a test tube followed by 2 cm3 of Fehling’s reagent
• Fehling’s reagent is a blue solution
• ketone - no reaction
• aldehyde - red-brown ppt of Cu2O(s) is seen as Cu2+ ions are reduced to Cu+ ions

Question 10

What happens when you reduce aldehydes and ketones using NaBH4

Answer 10

Form primary and secondary alcohols

Question 11

Reducing agents

Answer 11

• Sodium tetrahydridoborate (III) (NaBH4) dissolved in water with methanol
• lithium tetrahydrialuminate (III) (LiAlH4) in ethoxyethene

Question 12

What reducing agent to use if reducing aldehydes and ketones to primary and secondary alchols

Answer 12

NaBH4 dissolved in water with methanol

Question 13

Aldehydes reduce to

Answer 13

Primary alcohols

Question 14

Ketones reduce to

Answer 14

Secondary alcohols

Question 15

What makes aldehydes and ketones susceptible to be attacked by a nucleophile

Answer 15

The polar carbonyl group

Question 16

How is HCN created

Answer 16

In situ from a mixture of aqueous sodium or potassium cyanide and sulfuric acid

Question 17

What happens to change length when HCN added to aldehyde or ketone

Answer 17

Increases by 1C

Question 18

What does ethanal and hydrogen cyanide form

Answer 18

2-hydroxypropannitrile

Question 19

What does propanone and hydrogen cyanide form

Answer 19

2-hydroxy-2-methylpropannitrile

Question 20

What is significant about hydroxynitriles

Answer 20

The hydrolysis of them leads to important hydroxyacids which are often used in the cosmetic industry

Question 21

Test for a carbonyl group

Answer 21

2,4-dinitrophenylhydrazine

Question 22

Explain the 2,4-DNP test

Answer 22

• 2,4-DNP is dissolved in methanol and dilute sulfuric acid
• an aldehyde or ketone will react with the 2,4-DNP to form a orange-red ppt
• nucleophilic addition-elimination reaction
• condensation reaction

Question 23

What can be done with the orange-red ppt result from the 2,4-DNP test to identify the carbonyl compound

Answer 23

The ppt is filtered, recrystallised and dried
• the melting point of this can be compared to the data book and values can be used to identify the aldehyde or ketone

Question 24

Test for CH3CO- group

Answer 24

Triiodomethane (iodoform)

Question 25

What is iodoform

Answer 25

CHI3
Solid yellow
Used as antiseptic in dentistry and veterinary medicine

Question 26

Explain the iodoform test

Answer 26

• The unknown organic compound is warmed with a small amount of an oxidising agent (either I2/NaOH(aq) or KI/NaOCl(aq))
• the CH3CO- group will be oxidised and iodoform will be produced
• observations - yellow, antiseptic smell

Question 27

What to look out for whilst using iodoform test

Answer 27

Ethanol also creates a positive reaction as it is oxidised to ethanal and this will give a positive result