2244ASemester 1, 2011 Page 1 of 27 CONFIDENTIAL THE UNIVERSITY OF SYDNEY Faculties of Science, Engineering and Arts TABLE NO....................................................................................... CANDIDATE'S SURNAME ........................................................... OTHER NAMES ............................................................................. SID NUMBER.................................................................................. CHEM2401/CHEM2911/CHEM2915 MOLECULAR REACTIVITY & SPECTROSCOPY ________________________________________________________________________________ Time allowed: 3 hours SEMESTER 1 2011 Instructions to Candidates • • • • All questions are to be attempted. All information the candidate wishes to present for examination is to be written in the spaces provided. The space provided is an indication of the length of response required. However, additional space has been provided on pages 12, 18 and 27. Only University-approved non-programmable calculators can be used in the examinations. Students are warned, however, that credit may not be given, even for a correct answer, where there is insufficient evidence of working required to obtain the solution. Required data as well as a Periodic Table are printed on a separate data sheet. This examination paper consists of 27 pages. Please check that this booklet is complete and affirm this by signing below. I have checked this booklet and affirm that it is complete. SIGNATURE .......................................................……….. If you have an incomplete booklet, obtain a replacement from the Examination Supervisor immediately. THIS EXAMINATION BOOKLET IS NOT TO BE TAKEN FROM THE EXAMINATION ROOM. ________________________________________________________________________________ FOR OFFICIAL USE ONLY Page 2 3 4 5-6 7 8 9 10 11 13 14 Max Score 5 10 9 14 5 5 5 10 6 10 11 Mark Page 15 16 17 19 20 21 22 23 24 25 26 Total Mark Max Score 8 6 8 8 12 8 12 4 8 8 8 180 Mark /180 2244A Question 1 Semester 1, 2011 Page 2 of 27 5 Marks Show your understanding of the use of “curly arrow notation” by drawing the structures for all products in the following reactions. 2244A Question 2 Semester 1. Note that non-bonding electron pairs are not shown. 2011 Page 3 of 27 10 marks Provide curly arrows to show electron movement in the following reactions. . and you should add them to the starting materials where appropriate. rank the following bases from best (1) to worst (4) base for performing an E2 reaction. 2011 Page 4 of 27 5 marks The diketone dimedone shown below undergoes acid catalysed tautomerisation to give the corresponding enol. b) In the boxes provided.2244A Question 3 Semester 1. and you should add them where appropriate. rank the following species from best (1) to worst (4) nucleophile for performing an SN2 reaction. Note that non-bonding electron pairs are not shown. . Question 4 4 marks a) In the boxes provided. Complete the scheme using curly arrow notation. Illustrate your understanding of these pathways in the following examples by: i) indicating the reaction mechanism(s) favoured by the particular type of: leaving group. E2 or E1CB mechanisms). organic compounds can predictably undergo substitution reactions (via SN1 or SN2 mechanisms) and/or elimination reactions (via E1.2244A Question 5 Semester 1. substrate. nucleophile/base and solvent employed ii) predicting the major pathway(s) that will occur under these conditions and iii) providing the structure(s) of the major product(s) of the reaction a) . 2011 Page 5 of 27 14 marks Under certain reaction conditions. 2011 Page 6 of 27 .2244A b) Semester 1. intermediate. The overall reaction is endothermic. and indicate the activation energy for the rate determining step. b. Given that: a. c.2244A Semester 1. and ii) Label each transition state. The second step of the reaction is the rate determining step. . i) Sketch a reaction profile that accurately represents the reaction. 2011 Question 6 Page 7 of 27 5 marks The mechanism of the peroxide mediated epoxidation of cyclohexenone is shown below. The Hammond postulate predicts that the structure of the first transition state will most closely resemble the intermediate. (b) Draw the hydrogen atoms Ha and Hb onto the right hand structure. (d) What effect does changing R from H to methyl (Me) to tert-butyl (tBu) have on the position of the equilibrium? Why? . Take care with your bond angles. (c) How many signals would you expect in the 1H NMR spectrum of cyclohexane (R = H)? Explain your answer. 2011 Question 7 Page 8 of 27 5 marks The reaction below shows equilibration between two conformational isomers of cyclohexane (R = H).2244A Semester 1. (a) In the boxes provided. identify each conformer as either chair or boat. which of Hc and Hd is the pro-S hydrogen? Which is the pro-R? Show your working. (2 marks) b) The 1H NMR signals due to Hc and Hd are shown below. and use it to explain why these protons do not give rise to a simple two-hydrogen doublet. 2011 Page 9 of 27 5 marks The amino acid L-phenylalanine is shown below a) Assuming replacement by deuterium (D). Draw a Newman projection showing the arrangement of atoms around the C2–C3 bond in phenylalanine.2244A Question 8 Semester 1. (3 marks) . (a) Reaction type: (b) Reaction type: (c) Reaction type: Question 10 4 marks Draw structures for the products of the following Diels-Alder reactions. 2011 Question 9 Page 10 of 27 6 marks For each of the following pericyclic reactions. and classify the reaction as either a sigmatropic rearrangement. add curly arrows to complete a mechanism. or a cycloaddition/ cycloreversion.2244A Semester 1. Indicate the relative stereochemistry where appropriate. an electrocyclisation. (a) (b) . 1-methylcyclohexane is formed in preference to cycloheptane (9:1 product ratio). Make sure to indicate the initiation and propagation stages of your mechanism. 2011 Page 11 of 27 Question 11 6 marks a) Propose a mechanism to explain the formation of 1-methylcyclohexane (the major product) in this reaction.2244A Semester 1. (5 marks) b) In the above reaction. Why? (1 mark) . 2244A Semester 1. Ensure that any answers are clearly labeled with the page number from which they have been continued. 2011 Page 12 of 27 This page is left blank for additional space (if required). use this page. If you need more space for an answer. _______________________________________________________________________________ . ) ii) H3O MAJOR ORGANIC PRODUCT(S) .2244A Semester 1. Pd–C NO2 (iv) Br SO3 / conc. AlCl3 (1 equiv. REACTANT REAGENT(S)/CONDITIONS Na+ –OCH3. H2SO4 (v) O OCH3 i) Cl . 2011 Question 12 Page 13 of 27 10 marks Complete the following table to show the major organic products formed. MeOH (i) (ii) KMnO4 (excess) / H2O 95 °C (iii) O H2. 2011 Page 14 of 27 11 marks Suggest appropriate reagents and conditions for the following conversions (more than one step is required in each case).substituted compounds can be separated. (7 marks) (4 marks) .2244A Question 13 Semester 1. Draw the structures of intermediate compounds.and para. Assume that any mixtures of ortho. 2244A Semester 1. 2011 Question 14 Page 15 of 27 8 marks Complete the following equations by drawing stick diagrams of the organic products. NaBH4 2. H3O f unctional group? . and name the functional group formed in each case. O N H + H2O PhSO3H (catalyst) f unctional group? NH2 O + H2O PhSO3H (catalyst) f unctional group? CH2OH O PhSO3H (catalyst) f unctional group? NCH3 1. 2244A Semester 1. provide a detailed mechanism for each of the following reactions. 2011 Page 16 of 27 Question 15 14 marks Using curly arrows. a) (6 marks) . 2011 Page 17 of 27 (8 marks) b) .2244A Semester 1. _______________________________________________________________________________ . Ensure that any answers are clearly labeled with the page number from which they have been continued.2244A Semester 1. 2011 Page 18 of 27 This page is left blank for additional space (if required). use this page. If you need more space for an answer. a) Draw the first three wavefunctions for hexatriene.2244A Question 16 Semester 1. can be approximated to those of a particle in a one-dimensional box whose length is equal to the sum of the bonds in the conjugated system. showing the position of any nodes relative to the position of the carbon atoms in the molecule. (3 marks) b) Use de Broglie’s equation. p = mv =h/. 2011 Page 19 of 27 20 marks The energy levels of the π orbitals of a polyene such as hexatriene. to show that the separation between adjacent energy levels for a particle inside a one-dimensional box of length L and infinitely high sides is given by: E h 2 2 n 1 8 mL 2 (5 marks) . pictured below. 2011 Page 20 of 27 c) If each C-C bond is 0.2244A Semester 1. (4 marks) e) Predict what will happen to the energy of the equivalent transition as the length of the polyene increases. (3 marks) f) Explain why the energy levels for the electrons in a molecule such as hexatriene are observed to be quantized whereas the energy levels for macroscopic objects are not (3 marks) .139 nm in length. what is the length of the box for hexatriene? (2 marks) d) Calculate the energy (in J) required to excite an electron from the highest occupied to the lowest unoccupied orbital in hexatriene using the particle in a box model. at 3020 cm-1. The Raman spectrum shows a band at 2914 cm-1. (3 marks) .2244A Semester 1. even though there are four C-H bonds in methane. (5 marks) (b) Explain why there is only one band in the C-H region of the infrared spectrum. (a) Sketch the form of the vibrational modes which correspond to these bands and explain their infrared and Raman activity. 2011 Question 17 Page 21 of 27 24 marks The infrared spectrum of CH4(g) contains a single strong band in the C-H stretching region. 2011 Page 22 of 27 Predict the wavenumber for the band in the infrared spectrum of deuterated methane. (8 marks) . D0. the anharmonicity constant. and an estimate of the bond dissociation. Explain the origin of these bands and hence calculate (in cm-1) the harmonic frequency. e. CD4 (4 marks) (d) The infrared spectrum of methane also contains weak bands at 6006 and 9047 cm-1. exe.2244A (c) Semester 1. smaller or the same as the value you calculated above? (2 marks) (f) The upper atmosphere of Neptune contains 1% methane which is responsible for the blue colour of the planet. Explain the molecular origin of this colour. 2011 Page 23 of 27 (e) Would you expect the experimental value for D0 to be larger.2244A Semester 1. (2 marks) . indicating the processes which account for the spectra labelled A and B in the figure below. 2011 Page 24 of 27 24 marks Hexa-peri-benzocoronene (HBC) is a large polycyclic aromatic hydrocarbon molecule with formula C42H18. It is used in molecular solar cells and is a proposed component of interstellar material. ×40 (8 marks) . (a) Draw a Jablonski diagram.2244A Question 18 Semester 1. 2011 Page 25 of 27 Where is the absorption maximum of the So →S1 transition? Explain.2244A (b) Semester 1. (4 marks) (c) Add state(s) to your Jablonski diagram in (a) to explain the appearance of the spectrum labelled C. (4 marks) . the HOMOs of HBC. Indicate on the two HBC structures. The HOMOs of HBC may be described by (in-phase) benzene HOMOs on each of these rings. by using shaded and unshaded lobes.2244A (d) Semester 1. 2011 Page 26 of 27 The molecular orbitals of HBC can be deduced by interacting seven aromatic rings linked by "single" bonds. (8 marks) . If you need more space for an answer.2244A Semester 1. Ensure that any answers are clearly labeled with the page number from which they have been continued. use this page. 2011 Page 27 of 27 This page is left blank for additional space (if required). _______________________________________________________________________________ .