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 (3 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

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

Answer 2

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

Question 3

Give three advantages of the use of propanoyl chloride instead of propanoic acid in the laboratory preparation of methyl propanoate from methanol (3 mark)

Answer 3

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

Question 4

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 4

• Anhydride less easily hydrolysed

- Reaction less exothermic

Question 5

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

Answer 5

• Melting point range would be wide

- And below the true melting point

Question 6

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

Answer 6

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

Disadvantages:
- Reaction vigorous/dangerous or HCl is toxic

Question 7

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

Answer 7

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

- R group increases electron density / donates electrons

Question 8

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

Answer 8

• Redox reaction

Question 9

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

Answer 9

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

Question 10

Reagents for oxidation (1 mark)

Answer 10

Potassium dichromate

Question 11

Reagents and conditions for nucleophilic addition (2 marks)

Answer 11

• NaBH4

- In methanol or water

Question 12

Reagents and conditions for esterification (3 marks)

Answer 12

• Concentrated H2SO4
• Heat under reflux
• Warm

Question 13

State one use of methyl esters (1 mark)

Answer 13

Biodiesel

Question 14

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

Answer 14

• 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 15

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 15

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 16

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

Answer 16

• 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 17

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

Answer 17

• Corrosive

- Forms strong acid / HCl fumes