CHM 181 EXAMINATION / EKSAMEN

Transcription

1 Copyright exists / Kopiereg voorbehou. DEPARTEMENT OF CHEMISTRY DEPARTEMENT CHEMIE DATE / DATUM: TIME / TYD: 170 min + 10min reading/lees MARKS / PUNTE: 83 Fakulteit Natuur- en Landbouwetenskappe CHM 181 EXAMINATION / EKSAMEN Internal Examiners / Interne Eksaminatore: External Examiner / Eksterne Eksaminator: Dr MM Nkwelo Dr N October Dr EMM Venter Prof WJ Schoeman Surname and Initials Van en Voorletters Student number Studentenommer Signature Handtekening Section A/Afdeling A Section B/Afdeling B Question 1 / Vraag 1 20 Question 2 / Vraag 2 12 Question 3 / Vraag 3 10 Question 4 / Vraag 4 8 Question 5 / Vraag 5 9 Question 6 / Vraag 6 9 Question 7 / Vraag 7 15 Total / Totaal 50 Total / Totaal 33 Sub (20) ( / X) Sub (13) ( / X) 1 Tot A/B 1) Use skeletal (line or zig-zag) structures for the organic compounds. Organic Reactions Data Sheet included. 2) Show ALL calculations in the Analytical-Physical Section. Data page included. 3) Summer School: 23 Jan 31 Jan 2012, for students with a fail between 40-49% after exam. 4) Viewing of exam papers: See Click-UP, info on exam page. 1) Gebruik skelet (lyn- of sig-sag)strukture vir die organiese verbindings. Organiese Reaksies Inligtingsblad ingesluit. 2) Toon ALLE berekeninge in die Analities-Fisiese Afdeling. Inligtingsblad ingesluit. 3) Somerskool: 23 Jan -31 Jan 2012, vir studente met n druip van tussen 40-49% na die eksamen, sien info eksamenblad, Click-UP. 4) Insae in vraestelle: Sien Click-UP, info op die eksamenblad. 83

2 Section A: Organic Chemistry / Afdeling A: Organiese Chemie. [48] Question 1 / Vraag 1 Consider the two compounds A and B: i) What is the relationship between the two compounds, conformers, enantiomers, diastereomers, constitutional isomers or identical? Name compound B and give the absolute configuration of each stereogenic center. Show clearly the group s priorities on the drawing. [20] Beskou die twee verbindings A en B: i) Wat is die verband tussen A en B, konformere, enantiomere, diastereomere, konstitusionele isomere of identies? Benaam die verbinding B en gee die absolute konfigurasie van elke stereogeniese sentrum. Toon duidelik die groepe se voorkeur op die tekening. (8) ii) Draw the line structure for (3S,5Z)-6-amino-5-hepten-3-ol. ii) Teken die lynstruktuur van (3S,5Z)-6-amino-5-hepten-3-ol. (4) 2

3 iii) Draw the line structure for sec-butanol. iii) Teken die lynstruktuur van sek-butanol. (1) iv) Assign only E or Z configuration (double bond geometry) to the following compound. Show clearly the group s priorities on the drawing. iv) Ken slegs E of Z konfigurasie toe (dubbelbindinggeometrie) aan die volgende verbinding. Toon duidelik die groepe se prioriteit op die tekening. (3) v) Draw the most stable conformer of (1R, 3S)-1,3 cyclohexanediol. Start with C-1 on the given point. Is this compound optically active? v) Teken die stabielste konformeer van (1R, 3S)-1,3- sikloheksaandiol. Begin met C-1 op die gegewe punt. Is hierdie verbinding opties aktief? (4) C-1 3

4 Question 2 / Vraag 2 [12] i) Consider the compounds C and D. A compound A with molecular formula, C 6 H 10 O, is treated with POCl 3 / Pyridine to give the compound B, with molecular formula C 6 H 8. Reductive ozonolysis of compound B give the products C and D, structures given. Treatment of A with (1) BH 3 and (2) H 2 O 2 /base, give the constitutional isomers, E and F, with molecular formula, C 6 H 12 O 2. Identify compounds A, B, E and F and give all the reactions involved. i) Beskou die verbindings C en D. n Verbinding A met molekulêre formule, C 6 H 10 O, word behandel met POCl 3 / Pyridien om die produk B met die molekulêre formule, C 6 H 8 te gee. Reduktiewe osonolise van B gee die produkte C en D, strukture gegee. Behandeling van A met (1) BH 3 en (2) H 2 O 2 / basis, gee die konstitusionele isomere, E en F, met molekulêre formule, C 6 H 12 O 2. Identifiseer die verbindings A, B, E en F en gee al die reaksies betrokke. (6) O O O H H H O C D 4

5 ii) Consider the three isomeric alkenes, A, B and C, C 5 H 10. These alkenes are hydrogenated to yield 2- methylbutane. A and B give the same 3 alcohol after acid-catalyzed hydration. B and C give different 1 alcohols on hydroboration-oxidation. Give the line structures A, B and C. [Hint: Write all the reactions involved in schematic format.] ii) Beskou die drie alkeenisomere, A, B en C, C 5 H 10, Hierdie alkene word gehidrogeneer om 2- metielbutaan te lewer. A en B gee dieselfde 3 alkohol na suurgekataliseerde hidrasie. B en C gee verskillende 1 alkohole na hidroborering-oksidasie. Gee die lynstrukture van A, B en C. [Wenk: Skryf al die reaksies wat betrokke is in skematiese formaat neer.] (6) A B C H 2 Pd/C or/ of PtO 2 5

6 Question 3 / Vraag 3 i) Draw the product(s) formed when the alcohol A is treated with each of the following reagents. Indicate the stereochemistry of the stereoisomers formed: [10] i) Teken die produk(te) wat vorm wanneer die alkohol A reageer met elk van die volgende reagense. Dui die stereochemie van die stereomere wat vorm aan: (4) KMnO 4, H 2 O, HO - OH [1] CH 3 CO 3 H A [2] H 2 O, HO ii) Use line structures and suggest syntheses to accomplish the following transformations. More than one step is involved. Assume that the solvents and any other inorganic reagents are available. Show all steps complete with reagents and organic products. No mechanisms are required. ii) Gebruik lynstrukture en stel n sintese voor om die volgende omskakelings te bewerkstellig. Meer as een stap is betrokke. Neem aan dat oplosmiddels en enige ander anorganiese reagense beskikbaar is. Toon alle stappe volledig met reagense en organiese produkte. Geen meganismes word vereis nie. (6) Br? O OH + O minor product neweproduk major product hoofproduk 6

7 7

8 Question 4 / Vraag 4 [8] 8

9 In a reaction, optical active (4R)-4-methyl-octan-4-ol, A is heated for 2 hours in an acid/water medium. Thin layer chromatography indicates the formation of two new optically inactive spots, B and C, and a third spot that corresponds to unreacted material of compound A, as depicted in Figure 1. Figure 1 show that compounds B and C have the same R f -values, and compound A has a different R f -value. These compounds (spots) are not isolated and allowed to continue to react overnight under the same reaction conditions to allow that all the starting material, compound A reacts. However, analysis of the thin layer chromatography plate indicates the total conversion of the two newly formed spots B and C into an optically inactive compound D (see, Figure 1). (Small amounts < 2% of other compounds were also present but can be ignored in analysis.) Explain the formation of each spot B, C and D by way of a complete stepwise mechanism using line structures and curved arrows. Clearly indicate the following: i) flow of electrons; ii) position of partial and formal charges; iii) the intermediates formed and the stability thereof; and iv) hybridization changes of all C-atoms taking part in the transformation. In n reaksie word opties aktiewe (4R)-4-metiel-oktan- 4-ol, A verhit vir 2 ure in n suur/water medium. Dunlaagchromatografie dui die vorming van twee nuwe opties onaktiewe kolletjies, B en C aan, en n derde kolletjie wat ooreenstem met onreageerde materiaal van verbinding A, soos uitgebeeld in Figuur 1. Figuur 1 toon dat verbindings B en C dieselfde R f - waardes het en dat verbinding A n verskillende R f - waarde het. Hierdie verbindings (kolletjies) word nie geïsoleer nie en toegelaat om oornag onder dieselfde reaksiekondisies te reageer om toe te laat dat al die uitgangstof, verbinding A reageer. Analise egter van die dunlaagchromatografie plaat dui die totale omskakeling van die twee nuwe gevormde kolletjies na n opties onaktiewe verbinding D aan. (sien Figuur 1). (Klein hoeveelhede < 2% van ander produkte was ook teenwoordig, maar kan in die analise geїgnoreer word.) Verduidelik die vorming van elke kolletjie B, C en D deur middel van n volledige stapsgewyse meganisme deur gebruik te maak van lyn(skelet)strukture en geboë pyle. Toon die volgende duidelik aan: i) vloei van elektrone; ii) iii) iv) posisie van gedeeltelike en formele ladings; die tussenprodukte wat vorm en die stabiliteit daarvan; en die verandering in hibridisasie van al die C-atome betrokke by die omskakeling. 9

10 10

11 Section B: Physical and Analytical Chemistry. Show all calculations where applicable. Afdeling B: Fisiese en Analitiese Chemie. Toon alle toepaslike berekeninge. [33] Question 5 / Vraag 5 [9] i) What is the density in kg/l of sulfuric acid that is 36% (mass by mass) with molarity i) Wat is die digtheid in kg/l van swaelsuur wat 36% (massa tot massa) is met molariteit (4) ii) Sulfur dioxide reacts with oxygen at 900K to form sulfur trioxide. Stoichiometric amounts of sulfur dioxide and oxygen are introduced into a 2.00 L flask, initial pressure 1.95 atm at this temperature. The pressure eventually falls to 1.65 atm. ii.a) Explain why the pressure falls. ii.b) Calculate how many moles sulfur trioxide formed as well as the partial pressure of oxygen at the start of the process. Assume a 100% yield. ii) Swaeldioksied reageer met suurstof by 900K om swaeltrioksied te vorm. Stoїgiometriese hoeveelhede swaeldioksied en suurstof word in n 2.00 L fles gevoeg, aanvanklike druk 1.95 atm by hierdie temperatuur. Die druk val eventueel na 1.65 atm. ii.a) Verduidelik hoekom die druk val. ii.b) Bereken hoeveel mol swaeltrioksied gevorm het, asook die parsiȅle druk van suurstof aan die begin van die proses. Aanvaar n 100% opbrengs. (5) 11

12 Question 6 / Vraag 6 [9] i) Arrange the following compounds in order of increasing vapour pressure at room temperature. Explain in detail, using molecular structures. i) Rangskik die volgende verbondings in orde van toenemende dampdruk by kamertemperatuur. Verduidelik in detail, gebruik molekulêre strukture. (4) pentane / pentaan LiCl ammonia / ammoniak 1-butanol cyclopentane / siklopentaan 12

13 ii) A biochemical engineer dissolves mg of a gene fragment in enough water to make a clear solution of exactly ml. The osmotic pressure of the solution was measured as torr at 25 C. ii.a) What is the molar mass of the gene fragment? ii.b) If the solution density is g/ml, how large is the freezing point depression? ii.c) Which functional groups might be present in this molecule, based on its properties in water? ii) n Biochemiese ingenieur los mg van n geenfragment op in genoeg water om presies ml van n helder oplossing te maak. Die osmotiese druk van die oplossing word gemeet as torr by 25 C. ii.a) Wat is die molêre massa van die geenfragment? ii.b) Indien die oplossing se digtheid g/ml is, hoe groot is die vriespuntverlaging? ii.c) Watter funksionele groepe kan teenwoordig wees in hierdie molekule, gebaseer op die eienskappe in water? (5) 13

14 Question 7 / Vraag 7 [15] K a1 = C 8 H 6 O 4 KC 8 H 5 O 4 K a2 = Consider the weak diprotic acid, phtalic acid and its potassium salts. i) Calculate the ph of the solution if g KC 8 H 5 O 4 is added to cm 3 distilled water. Use balanced reaction equations to illustrate the amphiprotic properties of this salt and indicate clearly which reaction equation must be applied for the ph calculation. Use specific data to explain your choice, not words. Beskou die swak diprotiese suur, ftaalsuur en dié se kaliumsoute. i) Bereken die ph van die oplossing indien g KC 8 H 5 O 4 by cm 3 gedistilleerde water gevoeg word. Gebruik gebalanseerde reaksievergelykings om die amfiprotiese eienskappe van hierdie sout te illustreer en toon duidelik watter reaksievergelyking gebruik moet word om die ph te bereken. Gebruik spesifieke data om jou keuse te illustreer, nie woorde nie. (6) 14

15 ii) You would like to make an effective buffer solution with ph 9.5 using the above solution. ii.a) Which compound must be added to the above solution to make this buffer? ii.b) Calculate the concentration of this compound in the above solution to give such a buffer. ii) Jy wil graag n effektiewe bufferoplossing met ph 9.5 maak deur KC 8 H 5 O 4 te gebruik. ii.a) Watter verbinding moet by bogenoemde oplossing gevoeg word om hierdie buffer te maak? ii.b) Bereken die konsentrasie van hierdie verbinding in bogenoemde oplossing om hierdie buffer te gee. (5) 15

16 iii) A solution consists of 0.20 M magnesium chloride and 0.10 M copper(ii) chloride. Calculate the ph at which these two metal ions can be separated as the hydroxides. iii) n Oplossing bestaan uit 0.20 M magnesium chloried en 0.10 M koper(ii) chloried. Bereken die ph waarby hierdie twee metaalione as die hidroksiede geskei kan word.. (4) K sp Magnesium hydroxide / Magnesiumhidroksied Copper(II) hydroxide / Koper(II)hidroksied

17 CHM 181 Organic Reagents Data Sheet / Organiese Reagense Datablad (Ref / Verw: Organic Chemistry, Smith 2 nd edition / uitgawe) Halogenating agents X 2, CCl 4 (X = Cl, Br) X 2, H 2 O (X = Cl, Br) or NBS, DMSO (aq) HX (X = Cl, Br, I) PBr 3 SOCl 2, pyridine Halogeneermiddels X 2, CCl 4 (X = Cl, Br) X 2, H 2 O (X = Cl, Br) of NBS, DMSO (aq) HX (X = Cl, Br, I) PBr 3 SOCl 2, piridien Hydrating agents H 2 O, H + or H 3 O + 1. BH 3, THF 2. H 2 O 2, HO Dehydrating agents Conc. acid, e.g. H 3 PO 4, H 2 SO 4 POCl 3, pyridine Hidreermiddels H 2 O, H + of H 3 O + 1. BH 3, THF 2. H 2 O 2, HO Dehidreermiddels Gekonsentreerde suur vb. H 3 PO 4, H 2 SO 4 POCl 3, piridien Oxidising agents 1. OsO 4 2. NaHSO 3, H 2 O KMnO 4, H 2 O, HO 1. CH 3 CO 3 H 2. H 3 O + or HO 1. O 3 2. Zn, H 2 O or Me 2 S CrO 3, H 2 SO 4 (aq) (Jones reagent) Na 2 Cr 2 O 7 or K 2 Cr 2 O 7, H 2 SO 4 (aq) = (H 2 CrO 4 ) PCC (C 5 H 6 N + ClCrO 3 ), CH 2 Cl 2 Tollens reagent: 1. Ag 2 O NH 4 OH 2. H 3 O + Oksideermiddels 1. OsO 4 2. NaHSO 3, H 2 O KMnO 4, H 2 O, HO 1. CH 3 CO 3 H 2. H 3 O + of HO 1. O 3 2. Zn, H 2 O of Me 2 S CrO 3, H 2 SO 4 (aq) (Jones se reagens) Na 2 Cr 2 O 7 or K 2 Cr 2 O 7, H 2 SO 4 (aq) = (H 2 CrO 4 ) PCC (C 5 H 6 N + ClCrO 3 ), CH 2 Cl 2 Tollens se reagens: 1. Ag 2 O NH 4 OH 2. H 3 O + Reducing agents H 2, Pd/C or PtO 2 Reduseermiddels H 2, Pd/C of PtO 2 Other reagents Na (s) or K (s) or NaH Note: Specify symbols such as R, X, etc.! Ander reagense Na (s) of K (s) of NaH Nota: Spesifiseer simbole soos R, X ens.! Formulae used in Analitical and Physical Chemistry / Formules gebruik in Analitiese en Fisiese Chemie K a K b = K w ; K w = [H 3 O + ][OH ] = 1.01 x at 25 C ph = pk a + log {[conjugate base] / [acid]} poh = pk b + log {[conjugate acid] / [base]} Avogadro s number / Avogadro se getal : Gas constant: / Gaskonstante: 17

18 R: L.atm.mol 1.K 1 ; J.mol 1.K 1 1 atm = 760 mmhg = 760 torr. Gas Equations:/ Gasvergelykings: P tot =P 1 + P 2 + P ; P tot =n tot (RT/V); P a = χ a (P tot ); P a V = n a RT; (P + a[n/v] 2 )(V-bn)=nRT Solvent and Solute Laws: / Oplosmiddel en Opgeloste Stof Wette: bp = bpt = boiling point / kookpunt. fp = fpt = freezing point / vriespunt. Sg=k H P g ; P solv = χ solv P solv P solv = -χ solute P solv ; T bp = K bp m solute ; T fp = K fp m solute ; T fp (measured) = K fp m solute i Osmotic pressure / Osmotiese druk: or for ionic compounds / of vir ioniese verbindings π= crt π = icrt Water: (25 ºC) K fp = C / m K bp = C / m Density: / Digtheid: = 1.01 g/cm 3 18

19 19

20 Periodic table of the elements / Periodieke tabel van die elemente 1A 8A 1 H 1,0079 2A 3A 4A 5A 6A 7A 2 He 4, Li 6,941 4 Be 9, B 10,811 6 C 12,011 7 N 14, O 15, F 18, Ne 20, Na 22, Mg 24,3050 3B 4B 5B 6B 7B 8B 1B 2B 13 Al 26, Si 28, P 30, S 32, Cl 35, Ar 39, K 39, Ca 40, Sc 44, Ti 47,88 23 V 50, Cr 51, Mn 54, Fe 55, Co 58, Ni 58, Cu 63, Zn 65,39 31 Ga 69, Ge 72,61 33 As 74, Se 78,96 35 Br 79, Kr 83,80 37 Rb 85, Sr 87,62 39 Y 88, Zr 91, Nb 92, Mo 95,94 43 Tc (98) 44 Ru 101,07 45 Rh 102, Pd 106,42 47 Ag 107, Cd 112, In 114,82 50 Sn 118, Sb 121, Te 127,60 53 I 126, Xe 131,29 55 Cs 132, Ba 137, La* 138, Hf 178,49 73 Ta 180, W 183,85 75 Re 186, Os 190,2 77 Ir 192,22 78 Pt 195,08 79 Au 196, Hg 200,59 81 Tl 204, Pb 207,2 83 Bi 208, Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra 226, Ac** Rf (261) 105 Ha (262) 106 Sg (263) 107 Ns (262) 108 Hs (265) 109 Mt (266) 110 Discovered Nov Discovered Dec 94 LANTHANIDES* 58 Ce 140, Pr 140, Nd 144,24 61 Pm (145) 62 Sm 150,36 63 Eu 151, Gd 157,25 65 Tb 158, Dy 162,50 67 Ho 164, Er 167,26 69 Tm 168, Yb 173,04 71 Lu 174,967 ACTINIDES** 90 Th 232, Pa 231, U 238, Np 237, Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 100 Fm (257) 101 Md (258) 102 No (259) 103 Lr (260) R = J KΓ 1 molγ 1 F = 9.65 x 10 4 C 20