Question 9 Question 10 (5 marks) (a) Disconnection of paracetamol (2) leads to synthons A and B (as shown below). Draw the structures of synthons A and B with an appropriate sign to indicate the electrophilic or nucleophilic site. HO H N W CH3 Synthon A + Synthon B 2 (b) If synthon B corresponds to acetic anhydride, (CH3CO)2O, draw the structure of the reagent corresponding synthon A. (5 marks) (a) Balance the following chemical equations. (i) Al(s) + Cr2O3(s) = Al2O3(s) + Cr(s) (ii) Fe2O3(s) + CO(g) = Fe(s) + CO2(g) (b) (15 marks) Scheme 2 (below) shows both the forward (synthetic) and backward (retro- synthetic) steps in a proposed synthesis of an anaesthetic, 18. The conventional symbols are used in Scheme 2. Steps in the retrosynthetic analysis (starting from the target molecule 18) are indicated by the symbol , and are designated by the letters a-e. Steps in the actual, or forward, synthesis of 18 (starting from compound 20) are indicated by single arrows, →, and are designated by the numbers 1-5. C 18 a 5(C) NH b NH 19 CN 3 (B) 2 1 (A) Br OH NH NH e d NH 20 20 Scheme 2 Cl The following questions relate to the retrosynthetic analysis shown in Scheme 2. (i) (3 marks) Identify all C-X bond and C-C bond disconnections. (ii) (2 marks) Identify all functional group interconversions (FGIS). (iii) (2 marks) Draw the structures of synthons R and S produced by the disconnection shown below and add + and - signs, as appropriate, to indicate the electrophilic and nucleophilic sites. NH synthon R + synthon S (iv) (2 marks) If synthon R corresponds to compound 19. Write the structure of the compound to which synthon S corresponds? Question 2 S215 June 2022 the stability of the +2 state varies on descending Group 14. (10 marks) (a) (3 marks) Draw chemical structures of E-1-bromo-2-chloro-but-2-ene and its geometric isomer. (b) (7 marks) The structure of compound 1 is shown below. HO. HO 1 H NH, (i) Assign priority to the groups around the chiral centre in compound 1. Using this information assign the absolute configuration (R/S) of the chiral centre. (ii) If the NH2 group in compound 1 is replaced by an SH group to produce compound 2, draw chemical structures of compound 2 and its enantiomer (2a). (iii) If a solution consists of 2 and 2a, in equal amounts, predict whether or not the solution is optically active and give a brief explanation (one sentence). TURN OVER 3 Question 7 (a) (15 marks) A multi-step synthetic route to a dye (7.7) from toluene (7.1) involves the reactions shown in Scheme 7.1. Examine the synthetic route carefully and answer the following questions. Scheme 7.1 CH3 CH3 Reagents (7A) HNO3/H2SO4 7.3 Step 1 Step 2 CH3 7.1 7.2 CH3 CH3 Step 3 Reagents (7B) == NH2 NH2 Reagents (7C) Step 4 CH3 CH3 7.4 7.5 7.1 NaNO2/HCI 7.6 7.7 Step 5 Step 6 (i) (3 marks) Suggest suitable reagents (7A, 7B and 7C) for the reactions in step 1, step 3 and step 4. (ii) (5 marks) Draw the structure of the product (7.3) and using curly arrows, draw a mechanism for the nitration in step 2 to give the product (7.3). (Your answer should also include the generation of the nitronium ion.) (iii) (3 marks) In the nitration reaction of 7.2, a small amount of another product (7.3a) is also detected and found to be an isomer of 7.3. Draw the structure of 7.3a. Give a brief explanation why 7.3 is the major product while 7.3a is the minor one. (iv) (2 marks) Draw the structure of the intermediate (7.6). Briefly comment on the reaction temperature under which this transformation (from 7.5 to 7.6) would take place. (v) (2 marks) Step 6 involves the reaction of the intermediate (7.6) with the starting material (7.1) to give a dye (7.7). Draw the chemical structure of the product (7.7). Question 9 (a) (15 marks) (i) (9 marks) Construct a partial orbital energy level diagram for the molecule SiS. Show only the molecular orbitals made from the sulfur 3p orbitals (ionisation energy 1.66 × 10-18 J) and the silicon 3p orbitals (ionisation energy 1.31 × 10-18 J). Label the molecular orbitals as σ or and indicate whether they are bonding or antibonding. (ii) (4 marks) Calculate the bond order for SiS and for the ions SiS+ and SiS. Which molecule would you expect to have the highest bond dissociation energy? Explain how you arrived at your answer. (iii) (2 marks) Which of the species in (ii) would be paramagnetic? (b) (15 marks) Scheme 9.1 shows the forward and retrosynthetic steps in a proposed synthesis of compound 9.1 from an oxirane (9.7). The conventional symbols are used in Scheme 9.1. Steps in the retrosynthetic analysis (starting from the target molecule 9.1) are indicated by the symbol, and are designated by the letters a-f. Steps in the actual, or forward, synthesis of 9.1 (starting from compound 9.7) are indicated by single arrows →. Examine the scheme carefully, and then answer the questions that follow. Scheme 9.1 CH3 CH3 HO CH3 CH3 H H3C CH3 9.7 CH3 9.1 HO CH3 CH3 9.2 CH3 CH3 b CH3 9.3 CH3 CH3 CH3 NC. Br N CH3 CH3 9.4 9.5 9.6 (i) (5 marks) Identify three C-C bond disconnections, one FGI and one C-X bond disconnection from steps af in Scheme 9.1.