CHEM10022 EXAM Paper 2021
Chemistry for Bioscientists 2
Question 1. Answer part (a) AND EITHER (b) OR (c).
(a) Answer ALL parts (i) – (iii).
(i) For each reaction A-C, give the curly arrow mechanism and classify them by reaction type (addition / elimination / rearrangement / substitution).
(8 marks)
(ii) Draw the conjugate bases for the following compounds, 1 – 3.
(3 marks)
(iii) Rank 1 – 3 in terms of acidity, assigning one of the following pKa values to each compound: 4.8, 10, 12.5. Explain your answer.
(4 marks)
(b) Answer ALL parts (i) - (v).
Tripeptide ester 4, ThrSerAlaOCH3, reacts with one equivalent of methyl iodide and a mild base (Na2CO3) to form. 5.
(i) Identify the nucleophilic sites in tripeptide ester 4, ThrSerAlaOCH3. (2 marks)
(ii) Give the site of methylation, justifying your answer. (3 marks)
(iii) Give the rate equation for the reaction to form. 5. How would doubling the volume of solvent affect the rate of reaction? (2 marks)
(iv) Sketch a labelled energy profile diagram for the reaction to form. 5. Include the structure of the transition state. (6 marks)
(v) Using the alternative electrophile, 6, give the product of the reaction with tripeptide ester 4. Pay attention to the stereochemistry of any relevant chiral centres.
(2 marks)
(c) Answer ALL parts (i) - (ii).
The reaction of the bromo compound 7 with weak base (Na2CO3) to give 8 is given below.
(i) Classify the reaction that leads to product 8, showing the curly arrow mechanism of the formation of one potential product. (6 marks)
(ii) When 7 is instead treated with methanol, an alternative product 9, is formed. Draw the product (9) of this reaction. Classify the reaction that leads to product 9, providing the curly arrow mechanism. Comment on the stereochemistry of the product obtained and provide an explanation for its occurrence. (9 marks)
Question 2. Answer part (a) AND EITHER (b) OR (c).
(a) Answer ALL parts.
The structure of a tetrapeptide A is given below.
(i) Identify all of the stereogenic centres in A. (2 marks)
(ii) Discuss the properties of the amino acid side chains in A and explain how these side chains could contribute to protein interactions and enzyme catalysis. (9 marks)
(iii) Chymotrypsin is a protease enzyme that cleaves at the C-terminal side of aromatic amino acid residues in peptides and proteins. Draw the structures of products that would be formed when A is treated with chymotrypsin. (4 marks)
(iv) With the aid of diagrams, explain why the backbone amide bonds in proteins and peptides adopt a planar geometry and are typically in the trans configuration. (5 marks)
(b) Answer ALL parts.
The structure of a triacylglycerol B is given below.
(i) Draw the structures of the products resulting from the hydrolysis of B, using aqueous NaOH, and provide a mechanism for this reaction. (7 marks)
(ii) Draw the structure of the products formed from the reaction of B with ethanol and HCl under anhydrous conditions. (3 marks)
(c) Answer ALL parts.
(i) The open chain form. of D-N-acetylmannosamine (D-ManNac) is shown below. Draw the chair structures of both pyranose forms of D-mannose.
(4 marks)
(ii) When 6-Deoxy-β-D-glucose reacts with methanol under acid catalysis two products are formed. Provide a mechanism for this reaction and the structures of both products.
(6 marks)
Question 3. Answer part (a) AND EITHER (b) OR (c).
(a) Answer ALL parts.
(i) Calculate the frequency in hertz and the wavenumber (cm‒1) corresponding to light of wavelength 250 nm. Give the photon energy in eV and indicate what region of the electromagnetic spectrum it corresponds to? (8 marks)
(ii) Carbon monoxide has an infrared absorption band at 2143 cm-1. Calculate the bond force constant, k, in N m-1
. (6 marks)
(iii) Calculate the relative populations at 298 K between two energy levels, each with a degeneracy of one, separated by 200 kJ mol‒1
. (6 marks)
(b) Answer BOTH parts.
(i) A 1H nuclear magnetic resonance (NMR) line is shifted from the reference TMS in a 400 MHz NMR spectrometer by + 455 Hz. Calculate the chemical shift on the δ scale. (4 marks)
(ii) Obtain the number of infrared modes are for each of the following molecules? Give reasons for your answers.
A. CCl4 B. Cyclohexane C. Pyridine
(6 marks)
(c) Answer BOTH parts.
(i) A copper(II) sulfate solution of unknown concentration is placed in a colorimeter and an absorbance reading of 0.46 is recorded. Using the same solution cell, a 0.055 Μ solution of copper(II) sulfate gives an absorbance reading of 0.34. Calculate the concentration of the first solution? (3 marks)
(ii) Discuss the origin and give examples of spin-spin splitting in Nuclear Magnetic Resonance spectroscopy? (7 marks)