SUSTAINABLE CHEMISTRY (CHEM10023)

1. Answer all parts. The industrial production of adipic acid uses nitric acid oxidation of a cyclohexanol/cyclohexanone 2:1 mixture, known as KA oil, as shown in Route A. Other proposed routes to adipic acid are Route B and Route C.

(a) Compare and contrast the three routes (A-C) against the 12 green chemistry principles. [8]

(b) An industrial company uses Route A, where the KA oil is made from cyclohexanone of purity 99.8 % and cyclohexanol of purity 98.0 %. The rest of the KA oil comprises, in equal stoichiometry, acetic acid, acetaldehyde, acetone and ethyl acetate. The KA oil is mixed at a rate of 100 kmol h−1 with 60 % HNO3 in a 3:1 molar ratio. Exiting the mixing tank, the reaction mixture is combined with the recycled stream (25 % mole fraction of H2O) and placed in a reactor. Once the reaction is complete, 90 % of the water is evaporated and the adipic acid is purified by crystallisation. The waste from the separation is then either recycled or purged. 90 % of KA oil is recycled, whereas the purge stream contains 0.7 mol % of adipic acid along with 0.45 mol % of impurities.

(i) Draw and label a flow diagram which illustrates this industrial process. [3]

(ii) Calculate the composition in the mixing tank in units of kg h−1 and the total mass of material which leaves the mixing tank. [2]

(iii) Calculate the composition of the purge stream in units of mol %. [2]

(iv) Calculate the composition of the waste stream in units of mol %. [1]

(v) Calculate the mass of water in the recycle stream. [3]

(vi) Calculate the rate of the flow entering the reactor in units of kmol h−1 . [1]

2. Answer all parts.

Compounds A to D are used as reagents to separate metals by solvent extraction.

For each of the following metal solutions, deduce the best single reagent for their separation. Not all of the reagents may be needed. In your answers, you should define the extraction equilibrium, draw the complexes that reside in the polar and non-polar phases, state how the organic-phase complexes may be characterised, and explain how separation is achieved.

(a) Dysprosium and neodymium in H2SO4. [7]

(b) Uranium and plutonium in HNO3. [7]

(c) Gold and platinum in HCl [6]

3. Answer all parts.

(a) Window frames are often made of poly(vinyl chloride) A, which can be produced by the polymerisation of vinyl chloride. Describe the benefits and disadvantages of using wooden window frames instead of window frames made from A. Your answer should include a Life-Cycle Assessment for a window frame made from A. [6]

(b) The metal-catalysed ring-opening polymerisation of ε-decalactone B can be used to prepare poly(ε-decalactone) C, in the presence of benzyl alcohol BnOH.

(i) Draw a mechanism for this polymerisation and identify the key steps. [5]

(ii) Predict the structure of the poly(ε-decalactone) produced if 1 equivalent of pentaerythritol D is used instead of BnOH. Explain your answer. [2]

(iii) The structure of poly(lactic acid) E is shown below. Explain why E has a higher glass transition temperature than polymer C. [1]

Question 3 continued.

(c) Polymer F was prepared from monomer G using a monomer:initiator ratio of 100:1, with 95% conversion of monomer to polymer. The sample was characterized by elemental analysis and found to have the following composition by weight: %C = 63.7%, %H = 9.8%, %O = 14.1%, %N = 12.4%.

The sample was analysed by gel permeation chromatography (GPC) and has Mn(GPC) = 10,750 g mol−1 . The 1H NMR spectrum of the monomer, G, shows several resonances between 1.42 and 3.20 ppm, as well as one resonance at 7.21 ppm. The resonance at 7.21 ppm integrates to [1H] and does not correlate to any 13C resonances in the HSQC spectrum. The 13C NMR spectrum includes a resonance at 179.6 ppm, which does not correlate to any 1H resonances in the HSQC spectrum. The FT-IR spectrum of monomer G is shown below and includes a resonance at 1661 cm−1 .

Use the data provided to suggest a chemical structure of monomer G, and explain your answer. [6]

4. Answer all parts.

(a) Syngas can be obtained from either coal by gasification, or from natural gas by steam reforming or autothermal reforming.

Describe the differences in the syngas streams resulting from each of the three processes. [4]

(b) Fischer-Tropsch (FT) synthesis is used to obtain liquid hydrocarbons from syngas.

When syngas arising from coal gasification is used in FT synthesis, Fe containing catalysts are preferred over Co containing catalysts. Rationalise the choice of catalyst. [3]

(c) Grey and blue hydrogen are both derived from natural gas, with syngas as an intermediate. Explain why blue hydrogen is considered greener than grey hydrogen. [2]

(d) Frustrated Lewis pairs (FLPs) can activate hydrogen for its use in various synthetic reactions and can act as catalysts in some cases. Shown below is the synthetic route to FLP C (Mes = mesityl, 2,4,6-trimethylphenyl).

(i) Draw B, and explain the role of the reagents in the synthesis. Comment on the reaction conditions and explain why inert conditions are required for this reaction. [4]

(ii) FLP C is unreactive towards hydrogen over 12 h whereas FLP D forms E in less than 5 minutes.

Draw E and provide an explanation for the observed reactivity of C and D. [3]

(iii) Describe a structural modification to D that would enable its efficient recovery when used as a catalyst. Explain your choice of modification strategy and how you would prepare your target molecule. [4]

5. Answer all parts.

(a) Draw the structures of A-C in each of the three biocatalysed reactions below. [6]

(b) The amino acid L-threonine (L-Thr) is used in the synthesis of the -hydroxyl amino acid D, using a pyridoxal 5’-phosphate (PLP)-dependent aldolase (ThrA) biocatalyst as shown below. The yield of this reaction was 60%.

(i) Draw a chemical mechanism for the ThrA-catalysed reaction that includes the structures of any intermediates. Suggest roles for the PLP cofactor and active site residue Lys100. [6]

(ii) Suggest a way of increasing the yield of D from 60%. [2]

(c) To confirm the stereochemistry of product D, the reaction was incubated with E, Lfluoro-2,4,-dinitrophenyl-5-L-alanine amide (L-FDAA). The resulting derivative F, (formula C18H19BrN5O8) was analysed by (I) hplc. The peak eluting at 4.363 minutes was also analysed by (II) electrospray ionisation mass spectrometry (ESIMS, + mode).

(i) Explain why preparing derivative F is a good choice to analyse the stereochemistry of product D. [3]

(ii) Use the data provided to suggest a chemical structure for F. Explain why two peaks are observed in the ESI mass spectrum. [3]