usyd chem 2401 exam paper 2011

2244A Semester 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 ________________________________________________________________________________ SEMESTER 1 2011 Time allowed: 3 hours

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 Max Score Mark Page Max Score Mark 2 5 15 8 3 10 16 6 4 9 17 8 5-6 14 19 8 7 5 20 12 8 5 21 8 9 5 22 12 10 10 23 4 11 6 24 8 13 10 25 8 14 11 26 8 Total Mark 180 /180


Question 1 5 Marks Show your understanding of the use of curly arrow notation by drawing the structures for all products in the following reactions.


Question 2 10 marks Provide curly arrows to show electron movement in the following reactions. Note that non-bonding electron pairs are not shown, and you should add them to the starting materials where appropriate.


Question 3 5 marks The diketone dimedone shown below undergoes acid catalysed tautomerisation to give the corresponding enol. Complete the scheme using curly arrow notation. Note that non-bonding electron pairs are not shown, and you should add them where appropriate.

Question 4 4 marks

a) In the boxes provided, rank the following bases from best (1) to worst (4) base for performing an E2 reaction.

b) In the boxes provided, rank the following species from best (1) to worst (4) nucleophile for performing an SN2 reaction.


Question 5 14 marks Under certain reaction conditions, organic compounds can predictably undergo substitution reactions (via SN1 or SN2 mechanisms) and/or elimination reactions (via E1, E2 or E1CB mechanisms). 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, 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)


b)


Question 6 5 marks The mechanism of the peroxide mediated epoxidation of cyclohexenone is shown below.

Given that:

a. The overall reaction is endothermic. b. The Hammond postulate predicts that the structure of the first transition state will

most closely resemble the intermediate. c. The second step of the reaction is the rate determining step.

i) Sketch a reaction profile that accurately represents the reaction, and ii) Label each transition state, intermediate, and indicate the activation energy for the rate

determining step.


Question 7 5 marks The reaction below shows equilibration between two conformational isomers of cyclohexane (R = H).

(a) In the boxes provided, identify each conformer as either chair or boat.

(b) Draw the hydrogen atoms Ha and Hb onto the right hand structure. 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.

(d) What effect does changing R from H to methyl (Me) to tert-butyl (tBu) have on the position of the equilibrium? Why?


Question 8 5 marks The amino acid L-phenylalanine is shown below

a) Assuming replacement by deuterium (D), 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. Draw a Newman projection showing the arrangement of atoms around the C2C3 bond in phenylalanine, and use it to explain why these protons do not give rise to a simple two-hydrogen doublet. (3 marks)


Question 9 6 marks For each of the following pericyclic reactions, add curly arrows to complete a mechanism, and classify the reaction as either a sigmatropic rearrangement, an electrocyclisation, or a cycloaddition/ cycloreversion.

(a)

Reaction type:

(b)

Reaction type:

(c)

Reaction type:

Question 10 4 marks Draw structures for the products of the following Diels-Alder reactions. Indicate the relative stereochemistry where appropriate.

(a)

(b)


Question 11 6 marks

a) Propose a mechanism to explain the formation of 1-methylcyclohexane (the major product) in this reaction. Make sure to indicate the initiation and propagation stages of your mechanism. (5 marks)

b) In the above reaction, 1-methylcyclohexane is formed in preference to cycloheptane (9:1 product ratio). Why? (1 mark)


This page is left blank for additional space (if required).

If you need more space for an answer, use this page. Ensure that any answers are clearly labeled with the page number from which they have been continued. _______________________________________________________________________________


Question 12 10 marks

Complete the following table to show the major organic products formed.

REACTANT REAGENT(S)/CONDITIONS MAJOR ORGANIC PRODUCT(S)

(i)

Na+ OCH3, MeOH

(ii)

KMnO4 (excess) / H2O 95 C

(iii) O

NO2

H2, PdC

(iv) Br

SO3 / conc. H2SO4

(v) OCH3

O

Cli) ,

AlCl3 (1 equiv.)

ii) H3O


Question 13 11 marks Suggest appropriate reagents and conditions for the following conversions (more than one step is required in each case). Assume that any mixtures of ortho- and para- substituted compounds can be separated. Draw the structures of intermediate compounds.

(7 marks)

(4 marks)


Question 14 8 marks Complete the following equations by drawing stick diagrams of the organic products, and name the functional group formed in each case.

+ H2O

functional group?

PhSO3H (catalyst)

functional group?

functional group?

PhSO3H (catalyst)

NH

functional group?2. H3O

1. NaBH4

PhSO3H (catalyst)

NCH3

CH2OH

+ H2O

O

ONH2

O


Question 15 14 marks

Using curly arrows, provide a detailed mechanism for each of the following reactions. (6 marks)

a)


(8 marks)

b)


Question 16 20 marks The energy levels of the orbitals of a polyene such as hexatriene, pictured below, 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.

a) Draw the first three wavefunctions for hexatriene, showing the position of any nodes relative to the position of the carbon atoms in the molecule.

(3 marks)

b) Use de Broglies equation, p = mv =h/, 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:

2

2

812

mLnhE

(5 marks)


c) If each C-C bond is 0.139 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. (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)


Question 17 24 marks The infrared spectrum of CH4(g) contains a single strong band in the C-H stretching region, at 3020 cm-1. The Raman spectrum shows a band at 2914 cm-1. (a) Sketch the form of the vibrational modes which correspond to these bands and explain their

infrared and Raman activity. (5 marks)

(b) Explain why there is only one band in the C-H region of the infrared spectrum, even though

there are four C-H bonds in methane. (3 marks)


(c) Predict the wavenumber for the band in the infrared spectrum of deuterated methane, CD4

(4 marks)

(d) The infrared spectrum of methane also contains weak bands at 6006 and 9047 cm-1. Explain

the origin of these bands and hence calculate (in cm-1) the harmonic frequency, e, the anharmonicity constant, exe, and an estimate of the bond dissociation, D0.

(8 marks)


(e) Would you expect the experimental value for D0 to be larger, 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. (2 marks)


Question 18 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. (a) Draw a Jablonski diagram, indicating the processes which account for the spectra labelled A

and B in the figure below.

(8 marks)

40


(b) Where is the absorption maximum of the So S1 transition? Explain.

(4 marks)

(c) Add state(s) to your Jablonski diagram in (a) to explain the appearance of the spectrum

labelled C. (4 marks)


(d) The molecular orbitals of HBC can be deduced by interacting seven aromatic rings linked by

"single" bonds. The HOMOs of HBC may be described by (in-phase) benzene HOMOs on each of these rings. Indicate on the two HBC structures, by using shaded and unshaded lobes, the HOMOs of HBC. (8 marks)

usyd chem 2401 exam paper 2011

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