Organic chemistry IIIEdexcel A-Level Chemistry: Topic test

1. 1
   A student builds a model of a benzene molecule, C₆H₆, from six carbon atoms and six hydrogen atoms, and is asked to describe the bonding using the delocalised model.

   (a)

   How many electrons are in the delocalised π system of one benzene molecule?

   [1 mark]

   • A3
   • B4
   • C6
   • D12

   (b)

   Which statement about the delocalised model of benzene is correct?

   [1 mark]

   • AEach carbon atom contributes one p-orbital electron to the π system, and all six carbon–carbon bonds have the same length
   • BThe π electrons are held in three fixed double bonds that alternate with single bonds
   • CEach carbon atom uses all four outer electrons in σ bonds
   • DThe ring is puckered with C–C–C bond angles of 109.5°

   (c)

   Describe how the delocalised π system forms in a benzene molecule.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethylbenzene, C₆H₅CH₂CH₃, can be prepared in the laboratory by warming benzene with chloroethane in the presence of anhydrous aluminium chloride.

   (a)

   What is the role of the aluminium chloride in this reaction?

   [1 mark]

   • AIt is a nucleophile that attacks the benzene ring
   • BIt accepts a lone pair from the chlorine atom of chloroethane, helping to form the electrophile
   • CIt removes a hydrogen atom from benzene to form a free radical
   • DIt is the oxidising agent

   (b)

   Which species is the electrophile that attacks the benzene ring?

   [1 mark]

   • ACH₃CH₂Cl
   • BAlCl₃
   • CCH₃CH₂⁻
   • DCH₃CH₂⁺

   (c)

   Write equations to show (i) how the electrophile is generated and (ii) how the catalyst is regenerated in the final step of the mechanism.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   N-Propylethanamide, CH₃CONHCH₂CH₂CH₃, is made by adding ethanoyl chloride, CH₃COCl, to propylamine, CH₃CH₂CH₂NH₂.

   (a)

   Write the equation for the formation of N-propylethanamide from these two reagents. Explain why two moles of propylamine are used for each mole of ethanoyl chloride.

   [3 marks]

   (b)

   In an experiment, 5.90 g of propylamine ($M_\mathrm{r}$ = 59.0) was added to 4.71 g of ethanoyl chloride ($M_\mathrm{r}$ = 78.5). The mass of N-propylethanamide ($M_\mathrm{r}$ = 101.0) isolated was 3.54 g. Calculate the percentage yield, taking into account that two moles of the amine are needed for each mole of ethanoyl chloride.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Benzene reacts with chlorine in the presence of anhydrous aluminium chloride to form chlorobenzene and hydrogen chloride. Cyclohexene, C₆H₁₀, reacts with chlorine at room temperature without any catalyst, by addition across the C=C bond.

   (a)

   Describe the mechanism for the reaction of benzene with chlorine, including the generation of the electrophile and where each curly arrow starts and finishes.

   [6 marks]

   (b)

   Explain why benzene undergoes substitution rather than addition with chlorine, and why benzene needs a catalyst to react whereas cyclohexene does not.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Serine, HOCH₂CH(NH₂)COOH, is a 2-amino acid found in many proteins.

   (a)

   What is the structure of serine in a strongly acidic solution at pH 1?

   [1 mark]

   • AHOCH₂CH(NH₂)COOH
   • BHOCH₂CH(NH₃⁺)COO⁻
   • CHOCH₂CH(NH₂)COO⁻
   • DHOCH₂CH(NH₃⁺)COOH

   (b)

   A solution containing only the naturally occurring form of serine rotates the plane of plane-polarised light. What is the reason?

   [1 mark]

   • AIt is a zwitterion with both a positive and a negative charge
   • BThe second carbon atom is bonded to four different groups, so the molecule is chiral
   • CIt contains a hydroxyl group
   • DIt contains a peptide bond

   (c)

   Give the structural formula of serine in strongly alkaline solution and explain how it forms from the zwitterion.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Serine, HOCH₂CH(NH₂)COOH, and glycine, H₂NCH₂COOH, can join together to form dipeptides. A dipeptide is completely hydrolysed by boiling it with 6 mol dm⁻³ hydrochloric acid for 24 hours.

   (a)

   Which bond is broken when a dipeptide is hydrolysed?

   [1 mark]

   • AThe C–N bond of the peptide link
   • BThe C=O bond of the peptide link
   • CThe N–H bond of the peptide link
   • DThe C–C bond next to the carboxyl group

   (b)

   How many different dipeptides can be made that contain one glycine unit and one serine unit?

   [1 mark]

   • A1
   • B4
   • C2
   • D3

   (c)

   Give the structural formula of the dipeptide in which the glycine unit has the free –NH₂ group, and name the type of reaction by which it forms.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A compound Q contains 71.1% carbon, 6.7% hydrogen and 10.4% nitrogen by mass, the remainder being oxygen. Its mass spectrum has a molecular ion peak at m/z 135 and a strong fragment peak at m/z 43. Its infrared spectrum has strong absorptions at 3300 cm⁻¹ and 1670 cm⁻¹ and no broad absorption between 2500 and 3300 cm⁻¹.

   (a)

   Calculate the empirical formula of Q and deduce its molecular formula. (Use $A_\mathrm{r}$: C = 12.0, H = 1.0, N = 14.0, O = 16.0.)

   [3 marks]

   (b)

   Q contains a benzene ring and is made from phenylamine. Use the infrared and mass spectral data to identify the functional groups in Q and suggest its structure.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Benzoic acid, C₆H₅COOH, can be made from bromobenzene, C₆H₅Br, by way of a Grignard reagent. The crude product is then purified by recrystallisation from hot water.

   (a)

   Describe how benzoic acid is made from bromobenzene by this route, giving the reagents and conditions for each stage.

   [6 marks]

   (b)

   Describe how the crude benzoic acid is purified by recrystallisation and how its melting temperature is used to judge its purity.

   [6 marks]

   Total for question 8: 12 marks

End of questions