usyd chem 2401 exam paper 2010

2244A Semester 1, 2010 Page 1 of 32

CONFIDENTIAL THE UNIVERSITY OF SYDNEY

Faculties of Science, Engineering and Arts

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CHEM2401/CHEM2911/CHEM2915 MOLECULAR REACTIVITY & SPECTROSCOPY ________________________________________________________________________________ SEMESTER 1 2010 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 9, 18 and 32.

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 32 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 .........................................................

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Page Max Score Mark Page Max Score Mark 2 5 17 8 3 5 19 7 4 10 20 4 5 4 21 3 6 14 22 8 8 5 23 4 10 10 24 6 11 8 25 6 12 8 26 3 13 11 27 2 14 10 28 715 8 29 616 6 30 8 31 4 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.

O

S

O

H

H

H

OMe

OH

H

H

Br Br

O

CH3O

O

OCH3

O

OS

O

O

H


Question 2 5 marks The cyclohexenol shown below undergoes acid catalysed tautomerisation to give the corresponding cyclohexanone. Complete the scheme using curly arrow notation. Note that non-bonding electron pairs are not shown, and you should add them where appropriate.


Question 3 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.

O

EtOCH3

O

H

N

H3C Br

MeO Br

NO O

NO2

NO O

OMeNO2

Br

NH

O

Br Br FeBr3

H Br

NH

O

FeBr3

O

EtOCH3

O

H

NBr

BrO

O

HO

Br


Question 4 4 marks a) In the boxes provided, rank the following bases from best (1) to worst (4) base for performing the reaction shown below.

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


Question 5 14 marks Alkyl halides and related compounds can undergo substitution reactions (via SN1 or SN2 mechanisms) and/or elimination reactions (via E1 or E2 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 the conditions shown; and iii) providing the structure(s) of the major product(s) of the reaction.

a)


b)


Question 6 5 marks Consider the reaction profiles A-D shown below.

i) in profile A, which product N or O would be favoured under conditions of high temperatures and long reaction times? ii) does reaction profile A more accurately describe an SN1 or SN2 reaction? iii) in profile B, what term is given to R? iv) Using your knowledge of the Hammond postulate, which species in profile C does Y most closely resemble? v) Which reaction profile most accurately depicts an E1CB reaction?


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 7 10 marks Propose mechanisms to explain each of the following transformations. Include a brief explanation of any selectivity that is observed.

(a) 4 marks

(b) 6 marks

BrBu3SnH

AIBN


Question 8 8 marks (a) Suggest suitable diene and dienophile partners that could combine to form each of the

following cycloadducts. (6 marks)

(a) 2 marks

CO2Et

diene dienophile

+heat

(b) 2 marks

O

O

O

H

H

diene dienophile

+heat

(c) 2 marks

O

HH

OO

O

diene dienophile

+heat

(b) Briefly explain why the product in (c) above has the relative stereochemistry shown. (2 marks)


Question 9 6 marks Consider the two isomers of 1-bromo-4-tert-butylcyclohexane shown below. (a) Draw the lowest energy conformation of each isomer. (3 marks)

Br

cis

Br

trans

(b) Briefly explain why reaction of the cis isomer with potassium tert-butoxide (tBuO-K+)

proceeds 500 times faster than reaction of the trans isomer. (3 marks)

Question 10 2 marks The biological oxidation of ethanol (CH3CH2OH) to acetaldehyde (CH3CHO) involves removal of the pro-R hydrogen. Draw 3-dimensional structures (ie using wedged and dashed bonds) to illustrate the stereochemistry of this reaction.


Question 11 11 marks Complete the following table to show the major organic products formed.

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

KI

CO2CH3

Br2, FeBr3

Cl

NO2

NO2

NaOH, 130 oC

KMnO4 (excess)

H2O, 95 oC

SnCl2, H+

Conc HNO3, conc H2SO4


Question 12 10 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 products can be separated. Draw the structures of intermediate compounds.

6 marks CO2CH3CO2CH3

CN

4 marks

NH2


Question 13 8 marks Complete the following equations by drawing stick diagrams of the organic products, and name the functional group formed in each case.


Question 14 14 marks

Using curly arrows, provide a detailed mechanism for each of the following reactions.

(a) 6 marks

catalytic NaOHdry ethanol

O

heat

O


(b) 8 marks


Question 15 14 marks (a) Use de Broglies equation, p = mv = , to show that the energy levels of a particle inside a

one-dimensional box of length L and infinitely high sides are given by:

En = (4 marks)

(b) Draw the corresponding wavefunctions and probability densities for n = 1 3. Mark the

positions of the nodes for each function. (3 marks)


(c) Explain why (i) the energy levels of a particle in a box are quantized but the energy levels of

a free particle are not and (ii) the lowest possible energy for a particle in a box is not zero. (4 marks)


Question 15 (continued) (d) By considering the effect of large n and L for a particle in a box, discuss how the classical

and quantum mechanical pictures can be reconciled. (3 marks)


Question 16 27 marks Porphyrins are naturally occurring chromophores featuring a highly conjugated -system. The simplest porphyrin structure is porphin (pictured).

(a) Shade the pathway of unhybridized p-orbitals in porphin (all nitrogens are sp2). (2 marks) (b) How many electrons are there in porphin? (2 marks)

(c) Despite the Hckel 4n+2 rule strictly applying to monocyclic systems, in each resonance

form there is a path around the molecule which follows alternating single and double bonds. How many bonds comprise this path and how many electrons are along this path? Does this path obey the Hckel rule? (4 marks)


Question 16 (continued) (d) Approximating porphin as the cyclic system from (c), the allowed energy levels form

degenerate pairs. Fill the following electronic energy diagram with electrons. (2 marks)

(e) What are the j quantum numbers for the HOMO and LUMO? (2 marks)


Question 16 (continued) (f) From molecular orbital theory, the energy levels are given by:

N

jj

2cos2 .

If benzene absorbs at 260 nm, with a transition of -2 magnitude, calculate where porphin is expected to absorb given the above approximations. (4 marks)

(g) Experimental porphin spectra are shown above. Comment on how your prediction compares to the lowest absorptions of porphin? (2 marks)

Fig. The absorption spectrum of porphin (solid line), and the fluorescence spectrum obtained by 396nm excitation (dashed line). Both peaks a and b belong to the Q band (S1-S0).


Question 16 (continued) (h) Experimentally, the HOMO-LUMO transitions give rise to two distinct band systems. The

higher energy one is known as the Soret, or B band, and the lower energy one is known as the Q band. These are evident in the absorption spectrum. With the aid of a Jablonski diagram, explain how absorption in the B band yields fluorescence consistent with population with the upper state of the Q band system. Name all processes involved. (4 marks)

(i) The fluorescence yield of porphin is only 0.043. Explain possible processes which may

explain non-unity fluorescence yield. (2 marks)


Question 16 (continued) (j) Peaks b and c, as indicated, correspond to transitions between the ground vibrational levels

in S1 and S0, with the small Stokes shift being due to solvent reorganization. With the aid of diagrams, and invoking the Franck-Condon principle, explain why bands a and d appear more intense than b and c. (3 marks)


Question 17 15 marks The vibrational absorption spectrum of liquid water extends from the infrared into the visible region. The spectrum of the liquid is largely explained by that of the water monomer, albeit in a hydrogen-bonded environment. The water molecule (in water) has three vibrational modes:

mode frequency description v1 3277 symmetric stretch v2 1645 bend v3 3490 asymmetric stretch

Additionally, there are librations vibrations of the entire framework of H2O about its equilibrium position in the liquid environment. Important librations occur at 396 cm-1 and 686 cm-1. (a) The highly heterogeneous environment causes the symmetric and antisymmetric O-H

stretches to blend into one feature, while the bend remains distinct. Indicate these features as 1/3 and 2 on the following spectrum. (2 marks)


Question 17 (continued) The smaller features are much more easily shown using a logarithmic absorption axis. The following spectrum extends into the visible region.

(b) Indicate on this spectrum the onset of visible light (700 nm). (2 marks)

(c) The following table assigns the observed vibrational absorption features of liquid water,

noting that multiple quanta of 1 and 3 are not resolved, and simply denoted S. There are five features left unassigned. Write assignments for these features in the table, using the assigned features as a guide. (5 marks)

Assignment of the IR vibrational absorption spectrum of liquid water

Wavelength cm-1 Assignment Wavelength cm-1 Assignment

0.2 mm 50 intermolecular bend 1470 nm 6800 2S 55 m 183.4 intermolecular stretch 1200 nm 8330 2S + v2 25 m 395.5 L1, librations 970 nm 10310 15 m 686.3 L2, librations 836 nm 11960 6.08 m 1645 v2, bend 739 nm 13530 4.65 m 2150 v2 + L2 660 nm 15150 3.05 m 3277 v1, symmetric stretch 606 nm 16500

2.87 m 3490 v3, asymmetric stretch

514 nm 19460 6S 1900 nm 5260 S + v2


Question 17 (continued) (d) It turns out that, if one looks at white light through a long pathlength of pure water, one

perceives a blue coloration. What is responsible for this blue colour? (2 marks)

(e) Using that data in the above table, calculate effective spectroscopic constants, e and exe,

for the OH stretching motion for water (in water). Use the average of the symmetric and antisymmetric stretches, (3277+3490)/2, as the fundamental anharmonic frequency. (4 marks)


Question 18 12 marks The vibrational spectrum of a triatomic molecule XY2 contains three strong bands. Two bands, at 667.3 and 2349.3 cm-1, are only observed in its IR spectrum. The third band, at 1340 cm-1, is only observed in its Raman spectrum.

(a) Is XY2 a linear or bent molecule? Briefly explain your answer including an explanation for

the observation of only three bands. (2 marks)

(b) Assign, with explanation, the observed bands to the symmetric and antisymmetric stretches

and the bending vibration of the molecule. (2 marks)

(c) For each of these vibrations, explain why they are IR or Raman active or inactive. (4 marks)


Question 18 (continued)

(d) The molecule N2O is linear with the form NNO. Predict the number of bands in its IR and Raman spectrum. Briefly explain your answer. (4 marks)

usyd chem 2401 exam paper 2010

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