Organic Tutorial
3rd Year
Hilary 2012
Spectroscopy
Reading:
The best way to prepare for this topic is by practicing lots of questions. However, you may wish
to consult “Spectroscopic Methods in Organic Chemistry” by Williams and Fleming or “Modern
NMR Spectroscopy: a Guide for Chemists” by JK Sanders.
Problems – General Paper Questions – attached:
1996 II Q7
1997 II Q5
1999 II Q6
2005 General Q7
2006 General Q6
GOII '96
GOII '97
GOII '99 6.
For THREE of the following reaction sequences assign as fully as possible all
the spectral data provided.
[ 3 x 6 2/3 ]
(a)
CO2Me
NO2
Ni, H2
A
+
PhCH2NMe3OH
B
high pressure
C6H11NO
C7H13NO4
Data for A
IR: 1735 cm–1
Data for B
IR: 1699 cm–1
1
H NMR
1.29 (6H, s)
1.92 (2H, t, J 8 Hz)
2.42 (2H, t, J 8 Hz)
6.92 (1H, s)*
* on addition of D2O the resonance
at 6.92 is removed.
(b)
+
O3, CH2Cl2
OSiMe2t-Bu
C
then Pd/C, H2
(PhCH2)2NH2 O2CCF3
D
CH2Cl2, heat
C13H26O3Si
C13H24O2Si
Data for C
IR: 1730 cm–1
Data for D
IR: 1690 cm–1
1
H NMR
0.05 (6H, s)
0.86 (9H,s)
1.10 (3H, d, J 7 Hz)
2.35 (1H, m)
2.77 (2H, m)
4.02 (1H, m)
6.64 (1H, d, J 4 Hz)
9.67 (1H, s)
Question continues
(c)
8
NO2
NO2
S S
OH
Cl2
O2N
NO2
E
, Et3N
F
CH2Cl2
C6H3ClN2O 4S
C9H6N2O5S
Data for F
IR: 1950 cm–1
5.31 (1H, dd, J 13 and 6 Hz)
5.54 (1H, dd, J 13 and 6 Hz)
6.35 (1H, t, J 6 Hz)
8.52 (1H, d, J 9 Hz)
8.82 (1H, dd, J 9 and 3 Hz)
9.16 (1H, d, J 3 Hz)
1
H NMR
(d)
O
O
heat
G
NaBH4
H
O
C8H14O
C8H16O
Data for G
IR: 1710 cm–1
Data for H
IR: 3400 cm–1
1
H NMR
1.09 (3H, d, J 6 Hz)
1.40 (2H, m)
1.55 (3H, s)
1.64 (3H, s)
2.00((2H, m)
2.50 (1H, broad s)*
3.65 (1H, sextet, J 6 Hz)
5.10 (1H, t, J 7 Hz)
* on addition of D2O the resonance
at 2.50 is removed.
Turn over
9
1B '05 7.
Answer both Parts A and B. (N.B. This question continues over three pages)
Part A Answer both of the following:
(a) Attribute the three proton-decoupled 13C NMR spectra 1, 2, and 3 shown below and on the
facing page to the three isomers A, B and C of formula C6H12O2 below, and explain your
reasoning for each case. [Note: you do not need to comment here on the resonances of the
solvent (CDCl3) around 77 ppm or the reference TMS at 0 ppm].
[9]
H3C
O
CH3
H3C
OH
O
H3C
O
OH
C
B
A
OH
(b) Explain why the solvent resonance of CDCl3 appears as a 1:1:1 three-line multiplet.
[2]
1
200
150
100
50
0
p.p.m.)
Question continues
DCHC 2781
-8-
2
200
150
100
50
0
p.p.m.)
3
200
150
100
50
0
p.p.m.)
Question continues
DCHC 2781
TURN OVER
-9-
Part B
Identify significant fragment ion peaks and hence assign the electron impact mass spectra 4, 5 and 6
below to the following isomeric ketones D, E and F. Explain your reasoning.
O
O
D
E
O
F
4
5
6
[3 3]
DCHC 2781
-10-
1B '06
4. Answer Parts A, B and C.
Part A. Answer both of the following:
(a) 1D 13C NMR spectra are routinely recorded as proton-decoupled spectra:
(i) What are the advantages of collecting 13C spectra in this manner?
[4]
(ii) What information is provided by KDEPT-editingL of 1D 13C spectra?
13
(b) Assign the three DEPT-edited C spectra 1, 2 and 3, shown below to the three isomers
A, B and C below, stating briefly your reasoning. (The TMS reference signal is at 0 ppm.) [6]
A
OH
H 3C
OH
B
H 3C
H3C
OH
H3C
CH3
C
CH3
1
70
60
70
60
50
40
30
20
10
0
ppm
2
50
40
30
20
10
0
ppm
QUESTION CONTINUES, Turn over
DCHC 2781
5
3
70
60
50
40
30
20
10
0
ppm
Part B. Outline briefly what is meant by the term Knuclear Overhauser effectL (nOe) in NMR
spectroscopy, and exemplify how the nOe could be employed to differentiate between the
alternative substitution patterns for isomers D and E shown below. [4]
HO
D
E
N
CH3
HO
N
CH3
QUESTION CONTINUES
DCHC 2781
6
Part C. Assign the benzoic acid derivatives 113 to mass spectra A to C. Give your reasoning, and
explain why the molecular ions of these isomers fragment differently.
CH 3
CO2H
CO2CH 3
1
2
[6]
CO2H
CH 3
3
Turn over
DCHC 2781
7