Chemistry Olympiad, Ontario Division March Examination 2009

Transcription

1 NAME: Chemistry lympiad, ntario Division March Examination 2009 Note: The Preparatory Problems published by the UK hosts of the 2009 International Chemistry lympiad contain topics concerning different aspects of inorganic, physical, analytical and organic chemistry. There is no particular emphasis on one sub-discipline of chemistry, hence this examination reflects that fact. It is understood that some of the organic chemistry topics are not covered by the curriculum of Canadian Secondary schools. You are urged to consult any current textbook on organic chemistry for help. Instructions: 1. Write your name at the top of this page and each of the following pages. 2. Using the spaces provided for answers, attempt all questions and show all necessary working. 3. Solutions should be mailed to the address below on or before April 17 th 2009 (they should be received on or before Friday April 24 th 2009 for credit). 4. Please D NT send anything other than this examination paper. Anything else submitted will not be marked. 5. Please D NT send your solutions by ! They must be sent by regular Canada Post delivery to the following address: Dr. David C. Stone Department of Chemistry University of Toronto 80 St. George Street TRNT, ntario M5S 3H6

2 1. (i). Thiamphenicol (formula C 12 H 15 Cl 2 N 5 S, structure below) is an antibacterial agent used in many countries as a veterinary antibiotic. H 3 C S 5 H 4 3 HN 2 H Cl 1 Cl thiamphenicol (a).calculate the percentage of nitrogen present in thiamphenicol. (b). Name the functional groups present in thiamphenicol. (c). What is the hybridization of the numbered carbon atoms and the nitrogen atom in the thiamphenicol structure shown above?

3 (d). Which (if any) of the numbered carbon atoms in thiamphenicol are stereocenters (chiral centers)? How many stereoisomers of thiamphenicol are there? (ii). As an essential amino acid, threonine ((2S, 3R)-2-amino-3-hydroxybutanoic acid) is not synthesized in humans and must be ingested in the form of threonine-containing proteins. Draw the Fischer projections of all stereoisomers of threonine, using the template given below. CH CH 3

4 2. Rohypnol is the trade name for the compound flunitrazepam whose structure is shown in the scheme below. It is a controversial sedative which has been misused to 'spike' people's drinks. (a). Draw the structures for the reagent phenylhydrazine and compounds A to L. A = B= C = D=

5 E= F= G= H= I= J= K= L= Phenylhydrazine =

6 3. A voltaic cell is constructed as follows: Ag (s) Ag + (0.015 M) Fe 3+ (0.055 M), Fe 2+ (0.045 M) Pt (s) (a). Calculate the initial value of E cell. (b). As the cell operates, will E cell increase, decrease, or remain constant? Explain (in less than 10 words). (c). Calculate the E cell when [Ag+] has increased to 0.02M. (d). What will be the [Ag+] when E cell = 0.01V? (e). What are the ion concentrations when E cell = 0?

7 4. The phosphorus isotope 32 P is used in biochemical studies to determine pathways of phosphorus atoms in living organisms. Its presence is detected through radioactive emission of β - particles. (a). What is the radioactive decay constant for 32 P, expressed in units of s -1? (b). What is the activity of a 1.00 mg sample of 32 P? (c). Approximately what mass of 32 P will remain in the original 1.00 mg sample after 57 days? (d). What will be the rate of radioactive decay after 57 days?

8 5. (i). Draw the likely structures of the following chelate complexes, showing any geometric isomers: (a) [Cr(ox) 3 ] 3- (b) [CrCl 2 (en) 2 ] + (en = 1,2-ethylenediamine; ox = oxalate) (ii). The color of [Cr(H 2 ) 6 ]Cl 3 is violet, whereas the color of [Cr(NH 3 ) 6 ]Cl 3 is yellow. Explain the difference in color, with the use of a diagram.

9 6. Peroxyacetyl nitrate (PAN) is an air pollutant produced in photochemical smog by the reaction of hydrocarbons, oxides of nitrogen, and sunlight. PAN is unstable and dissociates into peroxyacetyl radicals and N 2 (g). Its presence in polluted air is like a reservoir for N 2 storage. H 3 C C N 2 H 3 C C + N 2 The first order decomposition of PAN has a half life of 35 hours at 0 C and 30 minutes at 25 C. Calculate the temperature at which a sample of air containing 5.0 x PAN molecules per liter decomposes at the rate of 1.0 x PAN molecules per liter per minute. 7. Maleic acid is an organic compound used in dyeing and finishing fabrics and as a preservative of oils and fats. It contains only the elements C, H and. (a). In a combustion analysis, a gram sample of maleic acid yielded g of C 2 and g of H 2. In a freezing point depression experiment, a g sample of maleic acid dissolved in g of glacial acetic acid, (CH 3 CH, which has the freezing point depression constant K f = 3.90 C m -1 ), lowers the freezing point by 0.82 C. Determine the empirical and molecular formulae of maleic acid. (All calculations must be shown for full credit).

10 (b). In a titration experiment, a g sample of maleic acid is dissolved in water and requires ml of M KH for its complete neutralization. Use the results in part (a) and the titration data to rewrite the molecular formula to show the number of ionizable H atoms in the molecule. (c). The ph of a g sample of maleic acid dissolved in 50 ml of aqueous solution is found to be Determine the first ionization constant of maleic acid. (d). Calculate the expected ph of a 0.05 M aqueous solution of maleic acid. Assume all the [H 3 + ] arises from the first ionization.

11 8. (i). What fragments would you expect in the mass spectra of the following compounds? Show the structures of the fragments and the m/z values for each. H CH 3 N H (ii). Compound A (formula C 10 H 16 ) has IR absorbances at 1640 cm -1 and cm -1. Compound A undergoes ozonolysis under reducing conditions to form two fragments, B and C with formulae of C 6 H 8 3 and C 4 H 8, respectively. Compound B reacts with Tollens reagent (Ag 2, NH 3 ) to form a silver mirror in a reaction test tube, and compound C gives a positive iodoform test. Compound A does not react with Tollens reagent and does not give a positive iodoform test. Using the NMR data given below for compounds B and C and the data above, draw structures for compounds A, B and C. NMR data for compound B: δ = 1.95 ppm (quintet, 2H) δ = 2.4 ppm (multiplet, 4H) δ = 9.6 ppm (singlet, 1H) δ = 9.72 ppm (triplet, 1H) NMR data for compound C: δ = 1.06 ppm (triplet, 3H) δ = 2.09 ppm (singlet, 3H) δ = 2.49 ppm (quartet, 2H) (NMR peak format: singlet, triplet, quartet, multiplet refer to splitting of each signal and the number of H atoms refers to the integration of each peak in the spectra).

12 Compound A: Compound B: Compound C: 9. (i). Propose necessary organic and inorganic reagents for the following transformations: CH 2 CH 3 CH 2 CH 3 (ii). Lithocholic acid is a steroid found in human bile. Predict the product of reaction of lithocholic acid with each of the following reagents:

13 BH 3, H + Cr 3, H + CH Tollens' reagent CH 3 H CH 3 H (CH 3)3 SiCl, Et 3 N H H H H H Lithocholic acid CH 3 MgBr, H + LiAlH 4, H Provide a structural formula for each of the two compounds (X and Y) given below using the spectral data provided. (a). X: C 8 H 6 2 IR:

14 X: 1 H NMR: X: 13 C NMR: solvent

15 Structure X: (b). Y: C 9 H 16 5 IR: Y: 1 H NMR: solvent

16 Y: 13 C NMR: solvent Structure Y: - END F EXAM-