Compounds Containing The Carbonyl Group

Question 1

What does a carbonyl group consist of?

Answer 1

•A carbon-oxygen double bond.

Question 2

Where is a carbonyl group present?

Answer 2

•In aldehydes and ketones.

Question 3

What is the general formula of an aldehyde? (2)

Answer 3

• RCHO.
• C (bonded to alkyl/aryl group and hydrogen)=O.
• Usually at the end.

Question 4

What is the general formula of a ketone? (3)

Answer 4

• RCOR.
• C (bonded to two alkyl/aryl groups)=O.
• Usually in the middle.

Question 5

How do you name aldehydes? (3)

Answer 5

• Name the alkane.
• Lose the ‘e’ and add ‘al’.
• The aldehyde group becomes carbon 1.

Question 6

How do you name ketones? (2)

Answer 6

• Name the alkane.

* Drop the ‘e’ and add ‘one’

Question 7

The carbonyl group is…

Answer 7

•Polar, it has a permanent dipole (uneven distribution of charge).

Question 8

What are the physical properties of carbonyl compounds? (2)

Answer 8

• The strong polar molecules with permanent dipole-dipole forces between the molecules (C delta positive = O delta negative) mean the boiling points are higher than alkanes.
• As the length of the carbonyl chain increases, carbonyl compounds become less soluble in water. (Hydrogen bonds form between the oxygen).

Question 9

What are the typical reactions of carbonyl groups? (2)

Answer 9

• Nucleophilic addition- due to the big difference in electronegativity between the carbon and oxygen, nucleophilic reagents can attack the carbon.
• Addition reactions are possible as carbonyl compounds are unsaturated.

Question 10

What is a nucleophile?

Answer 10

•Lone pair donor.

Question 11

What is addition?

Answer 11

• Adding to the double bond (no other product).

* Compound becomes saturated.

Question 12

Describe and draw the mechanism for nucleophilic addition. (5)

Answer 12

• 1). :Nu^-, arrow going from the lone pair to the carbon with delta positive.
• The arrow goes from the double bond to the oxygen with delta negative.
• 2). Intermediate compound with O:^-, arrow from the lone pair goes to the H^+ ion (occurs in an acidified solution (aqueous)).
• 3). Product with a hydroxy group.
• Overall equation: RCHO + HNU —> (displayed formula of product).

Question 13

What is reduction? (4)

Answer 13

• Gain of e^-.
• Gain of hydrogen.
• Loss of O2.
• Decrease in oxidation state.

Question 14

What does a reduction reaction in aldehydes and ketones involve? (3)

Answer 14

• Nucleophilic addition- hydrogen is added.
• :H^- is the nucleophile.
• NaBH4 in an acidified solution.

Question 15

What are the conditions for the nucleophilic addition mechanism of cyanide ions (CN^-)? (5)

Answer 15

• Reagent: acidified potassium or sodium cyanide solution (KCN (aq) followed by dilute acid) - easier to work with a solution as HCN is a gas.
• Heat.
• Aqueous.
• Nucleophile: :CN^-
• Product is a hydroxynitrile.

Question 16

RCN =

Answer 16

•Nitrile.

Question 17

How do you name a nitrile? (3)

Answer 17

• Name the alkane.
• Carbon included in the nitrile is the highest priority group, so it is carbon 1.
• Nitrile goes at the end with other groups in front before the alkane with prefixes.

Question 18

CN^- is…

Answer 18

•Toxic.

Question 19

Aldehydes are oxidised to…

Answer 19

•Carboxylic acids.

Question 20

What is the oxidising agent usually used [O]?

Answer 20

•Acidified (with dilute H2SO4) potassium dichromate (VI), K2Cr2O7/H^+.

Question 21

Why can’t ketones be oxidised easily to carboxylic acids? (2)

Answer 21

• The C-C bond must be broken.

* Stronger oxidising agents break the hydrocarbon chain of ketone molecules, resulting in a shorter chain molecule.

Question 22

What is aspirin? (2)

Answer 22

• An ester.

* Known as 2-ethanoyloxybenzenecarboxylic acid.

Question 23

How is aspirin manufactured?

Answer 23

•By reacting salicyclic acid (2-hydroxybenzenecarboxylic acid) with ethanoic anhydride or ethanoyl chloride.

Question 24

Why is ethanoic anhydride usually used in industry to make aspirin? (2)

Answer 24

• It’s cheaper than ethanoyl chloride.
• It’s safer to use than ethanoyl chloride - it’s less corrosive, reacts more slowly with water and doesn’t produce dangerous hydrogen chloride fumes.

Question 25

What are aldehydes reduced to?

Answer 25

•Primary alcohols.

Question 26

What are ketones reduced to?

Answer 26

•Secondary alcohols.

Question 27

Write the overall equation for the reduction of an aldehyde to a primary alcohol.

Answer 27

RCHO + 2[H] —> RCH2OH

Question 28

Write the overall equation for the reduction of a ketone to a secondary alcohol.

Answer 28

RCOR’ + 2[H] —> RR’CHOH

Question 29

What does ‘[H]’ represent?

Answer 29

•The reducing agent e.g. LiAlH4 or NaBH4.

Question 30

What does LiAlH4 (in dry ether) usually reduce?

Answer 30

•Carboxylic acids (to alcohols), esters (to alcohols) and amides (to amines), nitriles (to amines).

Question 31

What does NaBH4 usually reduce?

Answer 31

•Aldehydes and ketones (to alcohols).

Question 32

What are the conditions for reduction reactions of H^- ions? (Nucleophilic addition mechanism) (3)

Answer 32

• Reagent: NaBH4 (sodium borohydride) in acidic solution.
• Aqueous.
• OR Dissolved in alcohol, then boiling water = dry ether.

Question 33

What does the planar nature of the carbonyl group of aldehydes and ketones allow in nucleophilic addition reactions?

Answer 33

•The :CN^- nucleophile to have an equal chance of attacking the delta positive carbon above or below, so a racemic mixture is formed (hydroxynitrile).

Question 34

What are the tests to distinguish between aldehydes and ketones? (11)

Answer 34

•1). Silver mirror: Tollen’s reagent + heat:
•Ag^+ gets reduced to Ag = metallic silver:
[Ag(NH3)2]^+ + e^- —> Ag + 2NH3
•Aldehydes oxidised to carboxylic acids.
•2). Fehling’s/Benedict’s solution + heat in waterbath:
•Blue Cu^2+ ions are reduced to red Cu^+ ions in CuO2.
•Aldehydes oxidised to carboxylic acids
•3). Acidified potassium dichromate:
•Orange Cr2O7^- ions are reduced to green Cr^3+ ions.
•Aldehydes are oxidised to carboxylic acids.
•Ketones have no reaction, NVC, cannot break c-c bonds.

Question 35

Butanone is reduced in a two-step reaction using NaBH4 followed by dilute hydrochloric acid. Write an overall equation for the reduction of butanone using [H] to represent the reductant.

Answer 35

CH3CH2COCH3 + 2[H] CH3CH2CH(OH)CH3

Question 36

In practice, KCN rather than HCN is added to the carbonyl compound.
Given that Ka for HCN = 4.0 × 10^10 mol dm^-3, suggest why the reaction with HCN is very slow.

Answer 36

•It is a weak acid; it only slightly dissociates, since concentration of [CN^-] is very low.

Question 37

Suggest why Tollens’ reagent is used as the oxidising agent in the specific test for aldehydes rather than the less expensive acidified potassium dichromate (VI).

Answer 37

•It oxidises other organic materials e.g. it will give a positive test with primary alcohols.

Question 38

Ethanol can be oxidised by acidified potassium dichromate(VI) to ethanoic acid in a two-step
process.
ethanol —> ethanal —> ethanoic acid
In order to ensure that the oxidation to ethanoic acid is complete, the reaction is carried out under reflux.
Describe what happens when a reaction mixture is refluxed and why it is necessary, in this case, for complete oxidation to ethanoic acid. (3)

Answer 38

• A mixture of liquids is heated to boiling point for a prolonged time.
• Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture.
• Any ethanal and ethanol that initially evaporates can then be oxidised.

Question 39

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

Answer 39

CH3CH2OH + H2O CH3COOH + 4H^+

+ 4e^-

Question 40

Describe how you would obtain a sample of ethanal from a
mixture of these three compounds (ethanol, ethanal, ethanoic acid). Ethanal has the lowest boiling point. 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. (5)

Answer 40

•Mixture heated in a suitable flask/container.
•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 41

Give the industrial advantages of using ethanoic anhydride rather than ethanoyl chloride in the production of aspirin. (7)

Answer 41

• Cheaper.
• Less corrosive.
• Less vulnerable to hydrolysis.
• Less dangerous to use.
• Less violent/exothermic/vigorous reaction = more controllable reaction.
• Does not produce toxic/harmful fumes of HCl(g).
• Less volatile.

Question 42

What is observed when an alcohol reacts with ethanoyl chloride?

Answer 42

•Misty/steamy fumes of HCl(g).