School of Chemistry & Physics
Pietermaritzburg CHEM320
MAIN EXAMINATION November 2013 100 MARKS; 3 HOURS Organic Chemistry External Examiner: Internal Examiner: Prof C W McCleland Prof R. S. Robinson (Nelson Mandela Metropolitan University) Dr V. Jeena Instructions: 
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This paper consists of 13 pages, including this cover page. Please see that you have them all. Please hand in pages 12 & 13 (printed on GREEN paper) with your answer book. Please place your barcode sticker on the top right side of your answer book. To ensure accurate marking, students are required to write neatly and clearly. Non‐programmable calculators are permitted for calculations. The use of molecular models are permitted Answer ALL questions Scho
ool of Chemistry and Phyysics (Pieterm
maritzburg) Orgaanic Chemisttry, CHEM32
20 MAIN EXAM
MINATION: NO
OVEMBER 20
013 QU
UESTION
N ONE 1. You aree provided w
with a numb
ber of specttra for a com
mpound witth a molecuular formulaa of C8H8O2
Answerr the questio
ons that follow. NOTE: yyou should annotate yyour answeer on the NM
MR spectra provided oon the green pages. Th
hese pages m
must be han
nded in for marking. a. Whaat informattion can you
u derive fro m the IR spectrum abo
out the com
mpound? (2) b. Whaat informattion can you
u deduce froom the masss spectrum
m? (2) c. Discu
uss the inforrmation you
u can deducce from the NMR specttra providedd. (5) d. Propo
ose a plausible structu
ure based onn the inform
mation gained above. (2) Page 2 o
of 13 Scho
ool of Chemistry and Phyysics (Pieterm
maritzburg) Orgaanic Chemisttry, CHEM32
20 MAIN EXAM
MINATION: NO
OVEMBER 20
013 2.
2
Given th
he followingg molecule,, show the fformation o
of the McLafferty rearrrangement ffragment (mechanism), as w
well as one o
of the otherr possible m
major fragme
ents that is likely to forrm in a masss spectro
ometer. O
CH
C 3
CH3 C CH2 C CH3
H
(3) 3. With th
he aid of a suitable illusstration (a N
Newman typ
pe projectio
on) explain what the Kaarplus equaation is, and h
how it can b
be used in N
NMR spectrroscopy. (3) Page 3 o
of 13 School of Chemistry and Physics (Pietermaritzburg) Organic Chemistry, CHEM320 MAIN EXAMINATION: NOVEMBER 2013 4. Given the following compound and spectroscopic data, H
H
5
H
3
4
2
H
O
1
O
δH 6.67 (4‐H), 7.34 (3‐H), 7.81 (5‐H), 9.67 (CHO) J3,4=3.6Hz; J4,5=1.65Hz, J3,5=0.8Hz; J5,CHO=0.81Hz a. With the aid of a suitable splitting tree diagram show the splitting for protons H‐5 and H‐3. NOTE use the scale 1Hz = 1 cm (2) b. Discuss, using the splitting diagram for H‐5 in the above question, what spin decoupling is useful for in NMR spectroscopy. (2) [21] QUESTION TWO 1.
The following reaction is an example of the Haloform type reaction, which typically undergoes α‐
halogenation. Provide a detailed mechanism to account for the product formed in this reaction. I2/ OH-
O
R
C
CH3
(5) 2.
Provide a detailed synthesis for the following compound using the malonic ester type approach. CH3CH2CH2 CH COOH
CH3
(5) Page 4 of 13 School of Chemistry and Physics (Pietermaritzburg) Organic Chemistry, CHEM320 MAIN EXAMINATION: NOVEMBER 2013 3. Provide a detailed mechanism for the following aldol reaction. O
1) NaOEt/ EtOH
CH3CH2 C H
2) H2O
(3) 4. Given the following reactants, in a typical “mixed aldol” type reaction. Discuss what is meant by the term “mixed aldol”, and for this particular example explain why one is able to obtain a major product. O
O
+
O
EtO
(2) 5. Provide a mechanism to account for the reaction between the following two species. Name this reaction. O
O
O
1) NaOMe
+
OCH3
2) H3O+
(4) [19] QUESTION THREE 1.
Given the following molecule:‐ a. Show a complete retrosynthesis, showing all synthons and synthetic equivalents. b. Provide a synthesis for the following reaction. (Forward reaction). N
O
(5) Page 5 of 13 School of Chemistry and Physics (Pietermaritzburg) Organic Chemistry, CHEM320 MAIN EXAMINATION: NOVEMBER 2013 2.
Provide a retrosynthesis for the following compound at the disconnection point shown as well as a detailed method to synthesise this compound (include mechanistic detail). Disconnect Here
HO
Ph
(3) 3.
Given the following molecule:‐ a. Show a complete retrosynthesis, showing all synthons and synthetic equivalents. b. Provide a synthesis for the following reaction. (Forward reaction). Disconnect here
O
O
S
H
(4) 4.
Given the following molecule:‐ a. Show a complete retrosynthesis, showing all synthons and synthetic equivalents. b. Provide a synthesis for the following reaction. (Forward reaction). Ph
CO2Et
CO2Et
(3) [15] Page 6 of 13 School of Chemistry and Physics (Pietermaritzburg) Organic Chemistry, CHEM320 MAIN EXAMINATION: NOVEMBER 2013 QUESTION FOUR 1.
Define term ‘dihedral angle’ (1) 2.
The structural formulae of 1,2‐dichloroethane is given below: Cl
H
H
C
C
H
H
Cl
a. Draw Newman projections for all staggered conformations (with dihedral angle) formed by rotation from 0o to 360o about the carbon‐carbon single bond. (4.5) b. Which staggered conformation(s) have the highest energy and which has the lowest energy? (2) 3.
Following is a chair conformation of cyclohexane showing one methyl group and one hydrogen. CH3
H
4.
a. Indicate by a label which group is axial and which is equatorial (1) b. Draw the alternate chair form and label the axial and equatorial groups (2) The following substituted cyclohexane is subjected to an E2 reaction. Provide appropriate schemes to describe the formation of the products A and B, briefly explaining your reasoning. CH3
E2 conditions
Cl
H
HO
-
Product A
major
+
Product B
minor
H
(4.5) [15] Page 7 of 13 School of Chemistry and Physics (Pietermaritzburg) Organic Chemistry, CHEM320 MAIN EXAMINATION: NOVEMBER 2013 QUESTION FIVE 1. Define the term ‘heterocyclic compound’ (1) 2. Using appropriate resonance contributors to aid your discussion, provide an in‐ depth explanation why pyrrole undergoes electrophilic substitution at C2 rather than C3. (7) 3. Provide the missing product(s) in the following reactions. Br
-
+
N
NH2
OMe
CH3
CH3O -
+
N
Cl
NBS
N
CH2CH3
+
Br2
N
H
O O
+
CH3COCCH3
O
+
S
Br2
(7) [15] Page 8 of 13 School of Chemistry and Physics (Pietermaritzburg) Organic Chemistry, CHEM320 MAIN EXAMINATION: NOVEMBER 2013 QUESTION SIX 1.
2.
3.
Explain what is meant by a ‘pericyclic reaction’ (1) Explain, using molecular orbitals, what is meant by ‘disrotatory’ and ‘conrotatory’ (3)
The simplest example of a Diels‐Alder reaction ̶ the reaction between butadiene and ethylene requires fairly rigorous conditions and gives unsatisfactory yields of cyclohexene because of two competing reactions. Draw out the two competing reactions (clearly identifying the ‘diene’ and the ‘dienophile’), explaining how this type of situation(s) may arise. (6) 4. Define the phrase ‘sigmatropic rearrangement’ (1) 5. The following compound undergoes a sigmatropic rearrangement. Give the product of this reaction, showing the movement of electrons. Using the structure of the reactant and product, explain why this is classified as a sigmatropic rearrangement reaction. H3C
(4) [15] TOTAL MARKS [100] END Page 9 of 13 School of Chemistry and Physics (Pietermaritzburg) Organic Chemistry, CHEM320 MAIN EXAMINATION: NOVEMBER 2013 Page 10 of 13 School of Chemistry and Physics (Pietermaritzburg) Organic Chemistry, CHEM320 MAIN EXAMINATION: NOVEMBER 2013 Page 11 of 13 Scho
ool of Chemistry and Phyysics (Pieterm
maritzburg) Orgaanic Chemisttry, CHEM32
20 MAIN EXAM
MINATION: NO
OVEMBER 20
013 STUDENT NUMB
BER: QUEESTION O
ONE Page 12 o
of 13 Scho
ool of Chemistry and Phyysics (Pieterm
maritzburg) Orgaanic Chemisttry, CHEM32
20 MAIN EXAM
MINATION: NO
OVEMBER 20
013 Page 13 o
of 13