4.4 - ALDEHYDES AND KETONES

Question 1

Name the type of reaction alcohols undergo to form aldehydes and ketones

Answer 1

Oxidation

Question 2

How can you tell whether the oxidation of an alcohol will produce an aldehyde or ketone?

Answer 2

A primary alcohol will form an aldehyde when oxidised.
A secondary alcohol will form a ketone when oxidised.

Question 3

Why is immediate distillation required for the oxidation of an alcohol to an aldehyde?

Answer 3

The alcohol undergoes oxidation to form the aldehyde and the aldehyde must be distilled off immediately because otherwise it will undergo further oxidation to form a carboxylic acid.

Question 4

What is a common oxidising agent used in the oxidation of alcohols?

Answer 4

Acidified potassium dichromate(VI)

Question 5

What is observed when an alcohol undergoes oxidation by acidified potassium dichromate(VI)?

Answer 5

The orange dichromate(VI) ion, Cr O 2-, is 27
reduced to the green chromium(III) ion, Cr3+.
This means the solution changes colour from orange to green.

Question 6

What would be observed when acidified potassium dichromate is added to a tertiary alcohol?

Answer 6

The solution will remain orange because potassium dichromate(VI) does not react with tertiary alcohols as they cannot be oxidised.

Question 7

Give the chemical equation for the oxidation of propan-2-ol to propanone

Answer 7

CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O

Question 8

Give the chemical equation for the oxidation of ethanol to ethanal

Answer 8

CH3CH2OH + [O] → CH3CHO + H2O

Question 9

Name the two reagents which can be used to distinguish between aldehydes and ketones

Answer 9

Tollens’ reagent
Fehling’s reagent

Question 10

How can Tollens’ reagent be used to determine whether you have an aldehyde or a ketone?

Answer 10

Tollen’s reagent forms a silver mirror on the test tube only when warmed with an aldehyde and not a ketone.

Question 11

How can Fehling’s reagent be used to determine whether you have an aldehyde or a ketone?

Answer 11

Fehling’s reagent is blue.
If Fehling’s reagent is added to an aldehyde then the blue solution will form a brick red precipitate.
If Fehling’s reagent is added to a ketone, nothing happens.

Question 12

Name the type of reaction aldehydes and ketones undergo to form alcohols

Answer 12

Reduction

Question 13

What will be the classification of the alcohols formed by the reduction of aldehydes and ketones?

Answer 13

Aldehydes will be reduced to a primary alcohol.
Ketones will be reduced to a secondary alcohol.

Question 14

What is a common reducing agent used in the reduction of aldehydes and ketones to alcohols?

Answer 14

NaBH4 dissolved in water with methanol

Question 15

Give the chemical equation for the reduction of butanone to butan-2-ol

Answer 15

CH3COCH2CH3 + 2[H] → CH3C(OH)HCH2CH3

Question 16

Name the mechanism for the reduction of propanal to propan-1-ol

Answer 16

Nucleophilic addition

Question 17

Give the reactants and conditions required for the formation of a hydroxynitrile from an aldehyde/ketone

Answer 17

Potassium or sodium cyanide is added to sulfuric acid to produce hydrogen cyanide.
Temperature of 20oC.

Question 18

Why is hydrogen cyanide not added directly to an aldehyde/ketone for a nucleophilic addition reaction?

Answer 18

Hydrogen cyanide is a very poisonous gas and is also hard to store.

Question 19

Draw and name the mechanism for the formation of 2-hydroxypropanenitrile from ethanal and hydrogen cyanide

Answer 19

Question 20

Why are hydroxynitriles important?

Answer 20

Hydroxynitriles can be hydrolysed to form hydroxy acids which are very useful and are commonly used in the cosmetics industry.

Question 21

How does the addition of HCN to ethanal affect the C-C chain length?

Answer 21

The C-C chain length will increase by one carbon atom due to the carbon atom in the cyanide ion, CN-.

Question 22

Give the structural formula for 2,4-dinitrophenylhydrazine (2,4-DNP)

Answer 22

Question 23

Name the type of reaction taking place when aldehydes and ketones react with 2,4-dinitrophenylhydrazine

Answer 23

Nucleophilic addition-elimination reaction (condensation reaction)

Question 24

How can 2,4-dinitrophenylhydrazine be used to identify specific aldehydes and ketones?

Answer 24

Add 2,4-DNP so that a precipitate forms. Purify the solid by recrystallisation. Compare the melting point of the pure crystals formed with the melting points of 2,4-dinitrophenylhydrazones of all the common aldehydes and ketones.

Question 25

What reagents can be used for the triiodomethane (iodoform) test?

Answer 25

Iodine and sodium hydroxide solution
or
Potassium iodide and sodium chlorate(I) solutions

Question 26

Describe the method and results of the triiodomethane test

Answer 26

Iodine is added to a small amount of aldehyde/ketone, followed by some sodium hydroxide to remove the iodine colour. A very pale yellow precipitate will be formed if an aldehyde or ketone is present.

Question 27

What is the triiodomethane (iodoform) test used to identify?

Answer 27

The triiodomethane test is used to identify CH3CO- groups. This means it can be used to identify aldehydes and ketones.