Organic 2 Section 3 - Unit 26: Compounds containing the carbonyl groups
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Explain why ethanoyl chloride reacts readily with nucleophiles. Write an equation for one nucleophilic addition–elimination reaction of ethanoyl chloride (3 marks)
• 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)
Write an overall equation for the reduction of butanone using [H] to represent the reductant (1 mark)
• CH3CH2COCH3 + 2[H] → CH3CH2CH(OH)CH3
Give three advantages of the use of propanoyl chloride instead of propanoic acid in the laboratory preparation of methyl propanoate from methanol (3 mark)
• Faster • Irreversible • Bigger yield • Purer product • No acid catalyst required
Give one advantage of the use of propanoic anhydride instead of propanoyl chloride in the laboratory preparation of methyl propanoate from methanol (1 mark)
• Anhydride less easily hydrolysed - Reaction less exothermic
State two observations, during the determination of melting point that would indicate that a sample of aspirin is not pure (2 marks)
• Melting point range would be wide - And below the true melting point
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)
Advantages: - Reaction goes to completion, not reversible or faster Disadvantages: - Reaction vigorous/dangerous or HCl is toxic
Explain why butylamine is a stronger base than ammonia (2 marks)
• Lone pair (on N) (in correct context) - R group increases electron density / donates electrons
What sort of a reaction is Tollens’ reagent (1 mark)
• Redox reaction
Explain why ethanoyl chloride reacts readily with nucleophiles (2 marks)
• Large charge on carbonyl carbon atom due to bonding to O and Cl • Nucleophiles have electron pairs which can be donated
Reagents for oxidation (1 mark)
Potassium dichromate
Reagents and conditions for nucleophilic addition (2 marks)
• NaBH4 - In methanol or water
Reagents and conditions for esterification (3 marks)
• Concentrated H2SO4 • Heat under reflux • Warm
State one use of methyl esters (1 mark)
Biodiesel
Explain how you would purify a solid product by recrystallisation (4 marks)
• 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
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)
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
Describe how you would obtain a sample of ethanal from a mixture of ethanal, ethanol and ethanoic acid (6 marks)
• 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
Give one reason why ethanoyl chloride is not often used in nucleophilic addition-elimination (1 mark)
• Corrosive - Forms strong acid / HCl fumes