Organic Chemistry: Carbonyls, Carboxylic Acids and ChiralityEdexcel International A Level Chemistry: Topic test

1. 1
   2-Methylbutanoic acid, CH₃CH₂CH(CH₃)COOH, is used in the flavouring industry. A solution of one pure optical isomer of the acid rotates the plane of plane-polarised monochromatic light by +12.0° under a given set of conditions.

   (a)

   Which carbon atom in 2-methylbutanoic acid is the chiral centre?

   [1 mark]

   • AThe carbon atom of the COOH group
   • BThe carbon atom bonded to H, CH₃, CH₂CH₃ and COOH
   • CThe carbon atom of the CH₂ group
   • DThe carbon atom of the CH₃ group in the ethyl group

   (b)

   A solution of the other optical isomer, at the same concentration and under the same conditions, is examined. What rotation of the plane of polarisation is expected?

   [1 mark]

   • A+12.0°
   • B0°
   • C−12.0°
   • D+6.0°

   (c)

   A synthetic sample of 2-methylbutanoic acid made in the laboratory does not rotate the plane of plane-polarised light. Explain why.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist has three unlabelled liquids: pentanal, pentan-2-one and pentan-3-one.

   (a)

   Which liquid or liquids give a yellow precipitate when warmed with iodine and aqueous sodium hydroxide?

   [1 mark]

   • APentanal only
   • BPentan-3-one only
   • CPentanal and pentan-3-one
   • DPentan-2-one only

   (b)

   Which test distinguishes pentanal from both of the ketones?

   [1 mark]

   • AWarm with Fehling's solution: a red precipitate forms with pentanal only
   • BAdd 2,4-dinitrophenylhydrazine: an orange precipitate forms with pentanal only
   • CAdd 2,4-dinitrophenylhydrazine: an orange precipitate forms with the ketones only
   • DWarm with iodine and sodium hydroxide: a yellow precipitate forms with pentanal only

   (c)

   Describe what is observed when pentanal is warmed with Tollens' reagent, and write an equation for the reaction using [O] to represent the oxidising agent.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Butanone, CH₃COCH₂CH₃, is used as an industrial solvent. A chemist records its ¹³C and ¹H NMR spectra.

   (a)

   State the number of peaks in the ¹³C NMR spectrum of butanone, explain your answer, and identify the peak at the highest chemical shift.

   [3 marks]

   (b)

   Predict the number of peaks in the low-resolution ¹H NMR spectrum of butanone. State the relative peak areas and the splitting pattern of each peak in the high-resolution spectrum.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Propanal reacts with hydrogen cyanide in the presence of a little potassium cyanide to give 2-hydroxybutanenitrile, CH₃CH₂CH(OH)CN. This is hydrolysed by heating with dilute hydrochloric acid to 2-hydroxybutanoic acid, CH₃CH₂CH(OH)COOH. Relative atomic masses to four decimal places: C = 12.0000, H = 1.0078, O = 15.9949.

   (a)

   Describe, in words, the mechanism for the reaction of propanal with hydrogen cyanide in the presence of potassium cyanide, referring to curly arrows, lone pairs and dipoles. Explain why the product shows no optical activity even though it contains a chiral centre.

   [6 marks]

   (b)

   2-Hydroxybutanoic acid is heated with ethanol and a few drops of concentrated sulfuric acid. Name the type of reaction, the organic product and the role of the sulfuric acid. Calculate the accurate relative molecular mass of 2-hydroxybutanoic acid to four decimal places and explain how high-resolution mass spectrometry could distinguish it from C₅H₁₂O₂, which has an accurate relative molecular mass of 104.0834.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Pentanoic acid, CH₃CH₂CH₂CH₂COOH, is made in the laboratory from pentan-1-ol and, separately, from a nitrile.

   (a)

   Which conditions oxidise pentan-1-ol to pentanoic acid?

   [1 mark]

   • AHeat under reflux with an excess of acidified potassium dichromate(VI)
   • BDistil immediately from a limited amount of acidified potassium dichromate(VI)
   • CWarm with lithium tetrahydridoaluminate(III) in dry ether
   • DHeat under reflux with aqueous sodium hydroxide

   (b)

   Which nitrile gives pentanoic acid when it is heated with dilute hydrochloric acid?

   [1 mark]

   • AButanenitrile, CH₃CH₂CH₂CN
   • B2-Methylbutanenitrile, CH₃CH₂CH(CH₃)CN
   • CHexanenitrile, CH₃CH₂CH₂CH₂CH₂CN
   • DPentanenitrile, CH₃CH₂CH₂CH₂CN

   (c)

   Pentanoic acid is reduced with lithium tetrahydridoaluminate(III) in dry ether. Write an equation for the reaction, using [H] for the reducing agent, and explain why dry ether is used.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Hexanedioyl dichloride, ClCO(CH₂)₄COCl, is a reactive liquid. It reacts with water and, in a separate reaction, with ethane-1,2-diol, HOCH₂CH₂OH, to form a polyester.

   (a)

   Which organic product forms when hexanedioyl dichloride reacts with excess water?

   [1 mark]

   • AHexane-1,6-diol, HOCH₂(CH₂)₄CH₂OH
   • BHexanoic acid, CH₃(CH₂)₄COOH
   • CHexanedioic acid, HOOC(CH₂)₄COOH
   • DHexanedial, OHC(CH₂)₄CHO

   (b)

   Which small molecule is eliminated when the polyester forms from hexanedioyl dichloride and ethane-1,2-diol?

   [1 mark]

   • AWater
   • BHydrogen chloride
   • CChlorine
   • DHydrogen

   (c)

   State the type of polymerisation that forms the polyester and give the repeat unit of the polymer.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A tablet contains three compounds. A sample of its extract is separated by thin-layer chromatography on a silica gel plate, using a less polar solvent mixture as the mobile phase. The solvent front travels 6.0 cm from the baseline and the three spots are 1.5 cm, 3.6 cm and 4.8 cm from the baseline. Relative atomic masses to four decimal places: C = 12.0000, H = 1.0078, N = 14.0031, O = 15.9949.

   (a)

   Calculate the $R_f$ value of each spot. State, with a reason, which compound is most strongly adsorbed on the stationary phase.

   [3 marks]

   (b)

   The compound with $R_f$ 0.80 is extracted and analysed by high-resolution mass spectrometry. The molecular ion has m/z = 194.0804. It is either C₈H₁₀N₄O₂ or C₁₀H₁₄N₂O₂. Calculate the accurate relative molecular mass of each, deduce the molecular formula and explain why low-resolution mass spectrometry could not decide.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Compound W has the molecular formula C₅H₁₀O₂ and its high-resolution mass spectrum shows a molecular ion at m/z = 102.0677. Its ¹³C NMR spectrum has five peaks. Its high-resolution ¹H NMR spectrum has four peaks, with no peak above δ 5: δ 0.9 (triplet, 3H), δ 1.7 (sextet, 2H), δ 2.0 (singlet, 3H) and δ 4.0 (triplet, 2H). Relative atomic masses to four decimal places: C = 12.0000, H = 1.0078, O = 15.9949.

   (a)

   Use all of the data to deduce the structure of compound W. Explain your reasoning.

   [6 marks]

   (b)

   Compound W is propyl ethanoate, CH₃COOCH₂CH₂CH₃. Describe how it is hydrolysed with aqueous sodium hydroxide name the products and state how the carboxylic acid is then obtained. Compare making propyl ethanoate from ethanoyl chloride and propan-1-ol with making it from ethanoic acid and propan-1-ol.

   [6 marks]

   Total for question 8: 12 marks

End of questions