Carboxylic acidsEdexcel A-Level Chemistry: Subtopic test

1. 1
   Ethanoic acid (relative molecular mass 60.0, boiling temperature 118 °C) has a much higher boiling temperature than propanal (relative molecular mass 58.0, boiling temperature 49 °C). Ethanoic acid mixes with water in all proportions, but hexanoic acid, CH₃(CH₂)₄COOH, is only slightly soluble.

   (a)

   Which statement about liquid ethanoic acid is correct?

   [1 mark]

   • AIts molecules do not hydrogen bond because carbon is not very electronegative
   • BPairs of molecules are held together by hydrogen bonds between the O–H and C=O groups
   • CIts molecules are held together only by London forces
   • DIts molecules contain ionic bonds between CH₃COO⁻ and H⁺

   (b)

   Why is hexanoic acid much less soluble in water than ethanoic acid?

   [1 mark]

   • AHexanoic acid cannot form hydrogen bonds with water
   • BHexanoic acid has no carboxyl group
   • CHexanoic acid is a much stronger acid
   • DThe long non-polar hydrocarbon chain outweighs the hydrogen bonding of the carboxyl group

   (c)

   Explain why ethanoic acid has a much higher boiling temperature than propanal.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student plans to make propanoic acid in two different ways. One route starts from propan-1-ol. The other route starts from propanenitrile.

   (a)

   Which conditions convert propan-1-ol into propanoic acid in the highest yield?

   [1 mark]

   • ADistil the product immediately from acidified potassium dichromate(VI)
   • BWarm with lithium tetrahydridoaluminate in dry ether
   • CHeat under reflux with an excess of acidified potassium dichromate(VI)
   • DHeat under reflux with dilute sodium hydroxide

   (b)

   Which reagent and conditions convert propanenitrile into propanoic acid?

   [1 mark]

   • AHeat under reflux with dilute hydrochloric acid
   • BWarm with lithium tetrahydridoaluminate in dry ether
   • CWarm with phosphorus(V) chloride
   • DHeat under reflux with potassium cyanide in ethanol

   (c)

   State the colour change seen when propan-1-ol is oxidised to propanoic acid by acidified potassium dichromate(VI), and explain why the mixture must be heated under reflux.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ethanoic acid is used as a starting material in several reactions. In one test, aqueous sodium carbonate is added to ethanoic acid. In other reactions, ethanoic acid is converted into an acyl chloride by phosphorus(V) chloride and into an ester by reaction with ethanol.

   (a)

   Describe what is seen when aqueous sodium carbonate is added to ethanoic acid, and write the equation for the reaction.

   [3 marks]

   (b)

   Give the organic product and one other product when ethanoic acid reacts with phosphorus(V) chloride, and state the product and conditions for its reaction with ethanol.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fine-chemicals company starts from propan-1-ol and 1-bromopropane. It wishes to make propyl propanoate, an ester with a fruity smell, and also butanoic acid.

   (a)

   Outline a two-step synthesis of propyl propanoate from propan-1-ol, giving reagents, conditions, a colour change and equations.

   [6 marks]

   (b)

   Outline how butanoic acid can be made from 1-bromopropane, and explain why oxidising propan-1-ol cannot be used to make it.

   [6 marks]

   Total for question 4: 12 marks

End of questions