Organic 2 Section 3 - Unit 26: Compounds containing the carbonyl groups

Question 1

Explain why ethanoyl chloride reacts readily with nucleophiles. Write an equation for one nucleophilic addition–elimination reaction of ethanoyl chloride (4 marks)

Answer 1

• Large charge on carbonyl carbon atom due to bonding to O and Cl
• Nucleophiles have electron pairs which can be donated
• CH3COCl (l) + CH3CH2OH → CH3CO2CH2CH3 + HCl (g)

Question 2

Oxidation of aldehydes (3):

• Product
• Reagent
• Conditions

Answer 2

• Carboxylic Acid
• Potassium dichromate and dilute sulfuric acid
• Heat under reflux

Question 3

Reduction of carbonyls (3):

• Product
• Reagent
• Conditions

Answer 3

• Alcohol
• NaBH4 in aqueous ethanol
• Room temperature and pressure

Question 4

Addition of hydrogen cyanide to carbonyls (4):

• Mechanism
• Product
• Reagent
• Condition

Answer 4

• Nucleophilic addition
• Hydroxynitrile
• Sodium cyanide and dilute sulfuric acid
• Room temperature and pressure

Question 5

Write an overall equation for the reduction of butanone using [H] to represent the reductant (1 mark)

Answer 5

• CH3CH2COCH3 + 2[H] → CH3CH2CH(OH)CH3

Question 6

Give one advantage of the use of propanoyl chloride instead of propanoic acid in the laboratory preparation of methyl propanoate from methanol (1 mark)

Answer 6

• Faster
• Irreversible
• Bigger yield
• Purer product
• No acid catalyst required

Question 7

Give one advantage of the use of propanoic anhydride instead of propanoyl chloride in the laboratory preparation of methyl propanoate from methanol (1 mark)

Answer 7

• Anhydride less easily hydrolysed

- Reaction less exothermic

Question 8

Acyl chloride → carboxylic acid (2)

• Reagent
• Conditions

Answer 8

• Water

- Room Temperature

Question 9

Acid Anhydride → carboxylic acid (2)

• Reagent
• Conditions

Answer 9

• Water

- Room Temperature

Question 10

Acyl chloride → ester (2)

• Reagent
• Conditions

Answer 10

• Alcohol

- Room Temperature

Question 11

Acid Anhydride → ester (2)

• Reagent
• Conditions

Answer 11

• Alcohol

- Room Temperature

Question 12

Acyl chloride → primary amide (2)

• Reagent
• Conditions

Answer 12

• Ammonia

- Room Temperature

Question 13

Acid Anhydride → Primary amide (2)

• Reagent
• Conditions

Answer 13

• Ammonia

- Room Temperature

Question 14

Acyl chloride → secondary amide (2)

• Reagent
• Conditions

Answer 14

• Primary amine

- Room Temperature

Question 15

Acid Anhydride → secondary amide (2)

• Reagent
• Conditions

Answer 15

• Primary amine

- Room Temperature

Question 16

State two observations, during the determination of melting point that would indicate that a sample of aspirin is not pure (2 marks)

Answer 16

• Melting point range would be wide

- And below the true melting point

Question 17

Give one advantage and one disadvantage of preparing ethyl ethanoate by the reaction of carboxylic acid + alcohol rather than acyl chloride and alcohol (2 marks)

Answer 17

Advantages:
- Reaction goes to completion, not reversible
or faster

Disadvantages:
- Reaction vigorous/dangerous or HCl is toxic

Question 18

Explain why butylamine is a stronger base than ammonia (2 marks)

Answer 18

• Lone pair (on N) (in correct context)

- R group increases electron density / donates electrons

Question 19

What sort of a reaction is Tollens’ reagent (1 mark)

Answer 19

• Redox reaction

Question 20

Explain why ethanoyl chloride reacts readily with nucleophiles (2 marks)

Answer 20

• Large charge on carbonyl carbon atom due to bonding to O and Cl
• Nucleophiles have electron pairs which can be donated

Question 21

Reagents for oxidation (1 mark)

Answer 21

Potassium dichromate

Question 22

Reagents and conditions for nucleophilic addition (2 marks)

Answer 22

• NaBH4

- In methanol or water

Question 23

Reagents and conditions for esterification (3 marks)

Answer 23

• Concentrated H2SO4
• Heat under reflux
• Warm

Question 24

State one use of methyl esters (1 mark)

Answer 24

Biodiesel

Question 25

Explain how you would purify a solid product by recrystallisation (4 marks)

Answer 25

• Dissolve the product in the minimum volume of water
• Hot water
• Allow the solution to cool and allow crystals to form
• Filter off the pure product under reduced pressure / using a Buchner funnel and side arm flask

Question 26

Butanone is reduced in a two-step reaction using NaBH4 followed by dilute hydrochloric acid. Explain why the product has no effect on plane polarised light (6 marks)

Answer 26

Formation of product

• Nucleophilic attack
• Planar carbonyl group
• H – attacks from either side (stated or drawn)

Nature of product

• Product of step 1 shown
• This exists in two chiral forms (stated or drawn)
• Equal amounts of each enantiomer / racemic mixture formed

Optical activity

• Optical isomers / enantiomers rotate the plane of polarised light equally in
• With a racemic / equal mixture the effects cancel

Question 27

Describe how you would obtain a sample of ethanal from a mixture of ethanal, ethanol and ethanoic acid (6 marks)

Answer 27

• 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 28

Give one reason why ethanoyl chloride is not often used in nucleophilic addition-elimination (1 mark)

Answer 28

• Corrosive

- Forms strong acid / HCl fumes