Aldehydes and Ketones

Question 1

Aldehydes and ketones are carbonyls. What is a carbonyl group?

Answer 1

C=O A carbon with a double bond to an oxygen

Question 2

If the C=O is on the end of the chain with an H attached, what homologous series is present?

Answer 2

aldehyde

Question 3

If a carbonyl is present in the middle of a carbon chain and has alkyl groups to both sides, which homologous series is present?

Answer 3

Ketone

Question 4

What are the strongest kind of intermolecular forces present in aldehydes and ketones?

Answer 4

permanent dipole interactions

Question 5

Why is the carbonyl group polarised?

Answer 5

The C=O bond is polarised because oxygen is more electronegative than carbon

Question 6

Why does the carbonyl group in aldehydes and ketone react?

Answer 6

The positive carbon atom attracts nucleophiles.

Question 7

What is a nucleophile?

Answer 7

Electron pair donor

Question 8

Which oxidising agent is used to oxidise aldehydes and ketones?

Answer 8

acidified potassium dichromate solution OR (H⁺(aq) / K₂Cr₂O_7(aq))

Question 9

What can primary alcohols be oxidised to?

Answer 9

Primary alcohol → aldehydes → carboxylic acid

Question 10

What are the conditions to oxidise a primary alcohol to an aldehyde?

Answer 10

excess alcohol; acidified potassium dichromate; distillation of product

Question 11

What do you see when acidified potassium dichromate succesfully oxidises a carbonyl compound?

Answer 11

Colour change of orange to green

Question 12

What are the conditions to oxidise an alcohol or aldehyde to a carboxylic acid?

Answer 12

heat under reflux; acidified potassium dichromate

Question 13

Which acid can be used to acidify potassium dichromate?

Answer 13

dilute suphuric acid

Question 14

What product is formed when you oxidise a secondary alcohol with acidified potassium dichromate?

Answer 14

ketone

Question 15

What product is formed when you oxidise a tertiary alcohol with acidified potassium dichromate?

Answer 15

no reaction

Question 16

What is Tollens Reagent?

Answer 16

Tollen’s reagent is formed by mixing aqueous ammonia and silver nitrate.

Question 17

What is the active part of Tollens’ Reagent?

Answer 17

The active substance is the complex ion of [Ag(NH₃)₂]⁺ .

Question 18

What do you see if Tollens Reagent acts as an oxidising agent?

Answer 18

a silver precipitate is formed

Question 19

Give an equation for Tollens reagent acting as an oxidising agent

Answer 19

Ag⁺_(aq) + e^- → Ag _(s)

Question 20

Do aldehydes and/or ketones react with Tollens?

Answer 20

Just aldehydes (they are oxidised by the Tollens to form carboxylic acids - ketones can’t be oxidised)

Question 21

What ions are present in Fehlings Reagent?

Answer 21

blue Cu ²⁺ ions.

Question 22

Which substances are oxidised by Fehlings?

Answer 22

Just aldehydes (they are oxidised to form carboxylic acids - ketones can’t be oxidised)

Question 23

What do you observe when Fehlings oxidises an aldehyde?

Answer 23

blue solution forms a red/brown precipitate of Cu⁺ oxide (Cu₂O)

Question 24

Name a reducing agent that can reduce an aldehyde to a primary alcohol?

Answer 24

NaBH₄ In aqueous ethanol. Room temp and pressure.

Question 25

Name a reducing agent that can reduce a carboxylic acid to a primary alcohol?

Answer 25

NaBH₄ In aqueous ethanol. Room temp and pressure.

Question 26

Name a reducing agent that can reduce a ketone to a secondary alcohol?

Answer 26

NaBH₄ In aqueous ethanol. Room temp and pressure.

Question 27

Give a balanced equation for the reduction of propanal. What are the conditions?

Answer 27

Conditions: NaBH₄ in aqueous ethanol

Question 28

Give a balanced equation for the reduction of propanone. What are the conditions?

Answer 28

Conditions: NaBH₄ in aqueous ethanol.

Question 29

Give the mechanism for the reduction of propanone.

Answer 29

Question 30

Nae the mechanism

Answer 30

Nucleophilic substitution.

Question 31

What are the conditions for catalytic hydrogenation?

Answer 31

Reagent: hydrogen (H_2(g)) and nickel catalyst (Ni) Conditions: high pressure

Question 32

Give the mechanism for the addition of cyanide ions to carbonyls

Answer 32

Question 33

Name the mechanism for the addition of cyanide ions to carbonyls

Answer 33

Nucleophilic substitution

Question 34

What are the conditions for nucleophilic additions of CN^- to carbonyls?

Answer 34

Reagent: sodium or potassium cyanide (NaCN or KCN) and dilute sulfuric acid. Conditions: Room temperature and pressure

Question 35

Why do chemists use H⁺/KCN and not HCN to make hydroxynitriles?

Answer 35

HCN is a toxic gas that is difficult to contain. The KCN is still toxic but it is a solution.

Question 36

Why does nucleophilic addition of HCN to unsymmetrical carbonyls results in the formation of a racemate?

Answer 36

The carbonyl bond is planar. it can be attacked from above or below by CN- with an equal chance. This means that both enantiomers are formed in equal amounts.