8: Aldehydes and Ketones

Question 1

What is the difference between an aldehyde and a ketone?

Answer 1

Aldehyde groups are found at the end of the carbon chain ( C=O H)
Ketones are found in the middle of the carbon chain (C=O)

Question 2

Primary alcohols can be oxidised into what type of compound?

Answer 2

Aldehyde

Question 3

After primary alcohols have been oxidised into aldehydes, what can the aldehydes be oxidised into?

Answer 3

Carboxylic Acids

Question 4

Secondary alcohols can be oxidised into what type of compound?

Answer 4

Ketone

Question 5

What can tertiary alcohols be oxidised into?

Answer 5

Nothing.

Question 6

Why is oxidation of aldehydes into carboxylic acids easy?

Answer 6

Aldehydes have a hydrogen attached to the carbonyl group. This means the hydrogen can be lost easily, allowing for oxidation.

Question 7

Why are ketones not oxidised as easily?

Answer 7

To oxidise a ketone, you need to break the C=O bond, which is more difficult.

Question 8

Name 2 reagents that can be used to distinguish aldehydes and ketones.

Answer 8

Tollens Reagent
Fehling’s Solution

Question 9

Describe how to test for aldehydes using Tollens reagent.

Answer 9

Heat a test tube containing your solution with Tollens.
If aldehydes are present, a silver mirror forms.

Question 10

What is contained in a solution of Tollens?
Name the complex ion.

Answer 10

Ammoniacal silver nitrate [Ag(NH3)2]

Question 11

Describe the reduction/oxidation that takes place in a positive test for aldehydes using Tollens.

Answer 11

The aldehyde is oxidised (loss of hydrogen)
The diamine silver ions in the Tollens are reduced, forming silver and ammonia.

Question 12

What happens when Tollens is added to a ketone?

Answer 12

Nothing, ketones don’t react with Tollens reagent.

Question 13

Describe how to test for aldehydes using Fehling’s solution.

Answer 13

Heat the Fehling’s with the unknown solution .
If aldehydes are present, copper (II) ions are reduced to form a brick-red precipitate of copper (I) oxide.

Question 14

Describe what you would see if you added ketones to Fehling’s solution.

Answer 14

Nothing, ketones don’t react with Fehling’s solution.

Question 15

Name the compound used to reverse the oxidation of primary/secondary alcohols (reduction).

Answer 15

NaBH4

Question 16

Give a general equation for the reduction of an aldehyde to a primary alcohol using a reducing agent (such as NaBH4)

Answer 16

RCOH + 2[H] -> RCH2OH

Question 17

Give a general equation for the reduction of a ketone to a secondary alcohol using a reducing agent (such as NaBH4)

Answer 17

RC=OR + 2[H] -> RCHROH

Question 18

Name the mechanism for aldehydes/ketones turning into alcohols.

Answer 18

Nucleophilic addition

Question 19

During reduction of aldehydes/ketones into alcohols, where does the nucleophile come from?
Name the nucleophile.

Answer 19

H- ions come from the reducing agent and act as a nucleophile.

Question 20

On an aldehyde, where does the H- ion attack and why?

Answer 20

H- ions attack the carbon. The C=O bond is polar, so the carbon is slightly positive and the oxygen is slightly negative.

Question 21

How does the H- ion form a bond with the carbon?

Answer 21

The H- donates is lone pair of electrons forming a bond with the carbon.

Question 22

Why does the addition of H- cause one of the carbon oxygen bond to break?
Where do electrons from the C=O bond go to?

Answer 22

Addition of the H- causes 5 bonds to form on the carbon atom. This can’t happen, so one of the bonds needs to break. This allows the carbon to have 4 bonds.
A lone pair of electrons is forced from the C=O bond onto the oxygen.

Question 23

The lone pair on the oxygen is then forced onto where?
Where does this molecule come from?

Answer 23

H+ ion, which comes from water or a weak acid.

Question 24

At the end of nucleophilic addition for aldehyde, what is produced?

Answer 24

Primary alcohol.

Question 25

At the end of nucleophilic addition for ketone, what is produced?

Answer 25

Secondary alcohol.

Question 26

What type of isomerism can occur during the formation of hydroxynitriles?

Answer 26

Optical isomers.

Question 27

Name the mechanism for how hydroxynitriles are formed.

Answer 27

Nucleophilic addition.

Question 28

Give the formula for a nitrile group.

Answer 28

C triple bond N

Question 29

Give the overall equation for making hydroxynitriles by reacting aldehydes or ketones with HCN

Answer 29

Aldehyde: RCHO + HCN -> RCH(OH)CN
Ketone: RCOR’ + HCN -> RC(OH)R’CN

Question 30

If C=O and C=C bonds are planar, how does this affect the attack of nucleophiles?
How does the attack affect which isomers are produced?

Answer 30

Nucleophiles attack from either above or below the plane of the bond.
Attack from either side of these bonds produces different enantiomers.

Question 31

Why is a racemic mixture likey to form during the production of hydroxynitriles?

Answer 31

Attack of the nucleophile on either side of the planar bond is equally likely.

Question 32

What happens during reflux?

Answer 32

The reaction mixture is vapourised. After it condenses, it drips back into the reaction vessel to prevent loss of reactants.

Question 33

Ethanol is oxidised to ethanoic acid in reflux. Explain how and why.

Answer 33

1. Reaction mixture heated for a prolonged time period.
2. Some vapours produced escape. But they recondense and therefore drip back into the reaction mixture so are not lost.
3. This ensures that any ethanol/ethanol lost initially in evaporation can be oxidised.

Question 34

Write a half equation for the oxidation of ethanol into ethanoic acid.

Answer 34

CH3CH2OH + H2O -> CH3COOH + 4e- + 4H+

Question 35

The boiling points of the organic compounds in a reaction mixture are shown in the following table.
Compound ethanol ethanal ethanoic acid
Boiling point / °C 78 21 118
Use these data to describe how you would obtain a sample of ethanal from a mixture of these three compounds. Include in your answer a description of the apparatus you would use and how you would minimise the loss of ethanal. Your description of the apparatus can be either a description in words or a labelled sketch.

Answer 35

Mixture heated in a suitable flask
With still head containing a thermometer

Water cooled condenser connected to the still head and suitable cooled collecting vessel

Collect sample at the boiling point of ethanal

Cooled collection vessel necessary to reduce evaporation of ethanal

Question 36

Why is it possible to separate ethanal, ethanol and ethanoic acid using distillation? Explain in terms of structure and bonding.

Answer 36

Ethanal does not have hydrogen bonds (the strongest IMF) because the oxygen is not bonded to any hydrogen. It only has VDW and permanent dipole dipole forces which are weaker. Carboxylic acids and ethanol do have hydrogen bonds, but carboxylic acids have more hydrogen bonds, so have the highest boiling point. As their boiling points are very different to each other, they can be collected at different points in distillation.

Question 37

State a hazard associate with the use of KCN but explain why its used instead of HCN when forming hydroxynitriles.

Answer 37

Hazard - Toxic
Why KCN used - HCN is too weak so the CN ion does not dissociate as well as in KCN.