Name………………….
Student Number……………………
CH437 ORGANIC STRUCTURAL ANALYSIS
EXAMINATION 1, Tuesday 20 October 2009, 13.00 – 14.30
Attempt all the questions. Write your answers in the boxes
1. Write down the full terms for each of the following mass spectrometry abbreviations (a) TIC
(b) SIM (c) ESI (d) CI (e) BE (f) TOF (3 points)
(a) Total ion chromatogram
(b) Selected ion monitoring
(c) Electrospray ionization
(d) Chemical ionization
(e) Magnetic sector (= magnetic-electric field)
(f) Time of flight
2. Write down the method of ionization (full name) that is most likely to result in the production of
the following molecular ions:
(a) M+. (b) MH+ (c) (M + nH)n+ (3 points)
(a) Electron ionization
(b) Chemical ionization (or ESI)
(c) ESI (or MALDI)
3. (i) Give a name for the tandem mass spectrometric technique below. (1 point)
A
B
C
CID
(ii) State what box A, B and C mean, and give the full name for CID. (2 points)
(i) Precursor ion scan
(ii) A Ions scanned
B Fragmentation or decomposition
C Ion selected
CID Collision-induced decomposition.
4. Rationalize the following observations using fragmentation schemes. The EI mass spectrum of
o-nitrotoluene (C7H7NO2) shows the base peak (100%) at m/z = 120, whereas that of pnitrotoluene has the base peak at m/z = 91 and no peak at m/z = 120.
(5 points)
1
.
Mr = 137, so m/z = 120 indicates loss of 17 amu (= OH ). This is rationalized for o-nitrotoluene by
.
CH2 H
+O
N
O
-OH
.
CH2
+
N O
(17)
.
This is not possible for p-nitrotoluene, whose molecular
ion fragments by loss of NO2
H3C
m/z = 137
.
.
-NO 2
NO2+
(46)
m/z = 120
m/z = 137
(many resonance forms)
Rearranges
+ to benzyl or
tropyllium
ion
H3C
m/z = 91
5. A straight chain saturated ester has the EI mass spectrum below. No other functional groups
are present. Identify the ester and rationalize formation of the ions of m/z 143, 127, 88 and 73. (8
points)
+
O C OC2H 5
100
% Abundance
.
88
-C7H 15
(99)
m/z = 73
McLafferty

-C4H 9CH=CH2
+
H +
C4 H9
(84) = C6H 12
O
CH2 O
C
H 2C
C
H 2C
OC2H 5
C
OC2H 5
H2
-C2H 5
m/z = 88
m/z = 172
(29)
.
.
50
.
101
57
73
41
115
20
40
-OC2 H5
(45)
127
29
60
80
100
.
120
143
140
172
160
180
+
C7 H15 C O
m/z = 127
O
C7 H15 C +
O
m/z = 143
m/z
6. The picture below represents the bulk magnetization vector of a collection of nuclear spins in a
strong magnetic field before the arrival of a radiofrequency pulse. Draw corresponding diagrams
for the spin system immediately after the application of (a) a /2 pulse and (b) a  pulse. (2
points)
7. Decide whether the hydrogen atoms marked a and b are magnetically equivalent. (5 points)
2
HO2C
Hb
(c)
(b)
(a)
(a)
(b)
(c)
(d)
(e)
Ha
tBu
C
Ha Hb
Structure
H3C
Ha
Hb
CO2H
CH3
CH3
O
C
a H3C C
HO
C
(d)
CH3 b
H2N
H
C
H C
CO 2H
a 3 C
H
CH3 b
(e)
Magnetic equivalence
of a and b (YES or NO)
YES
NO
NO
NO
NO
8. Identify the methyl groups a, b and c of –pinene from their 1H chemical shifts of 1.63, 1.27
and 0.85 p.p.m. (3 points)
b
CH3
a
CH3
CH3 c
0.83 p.p.m. b
1.27 p.p.m. a
1.63 p.p.m. c
9. Write the structure of 3-methyl-2-butanol and account for the peaks in its proton-decoupled 13C
NMR spectrum at 17.9, 18.2, 20.0, 35.0 and 72.0 p.p.m. by writing, as far as you can, the
chemical shift values against the corresponding carbon atoms in the structure. (5 points)
CH3
17.9, 18.2
35.0
CH
H3 C
OH
CH *
72.0
CH3
20.0
* Chiral center
10. Below is the proton decoupled 13C NMR spectrum of ethyl isopropyl ketone (2-methyl-3pentanone).
(i) Assign the peaks according to carbon numbering in the inset diagram: write a number over
each peak in the spectrum. (3 points)
3
CDCl3
4,5
3 2
6
1
(ii) Write down the expected multiplicity of each peak if the spectrum of the same compound is
run with off-resonance decoupling. Multiplicity symbols; d = doublet, q = quartet, s = singlet and t
= triplet. (3 points)
1
q
2
t
3
d
4
q
5
q
6
s
11. Two stretching vibrations are characteristic of the nitro group; one at ~1350 cm -1 and the
other at ~1550 cm-1. Using the figures in the box, write arrows to illustrate the two vibrations, write
down their names and assign a frequency (cm -1) to each. (3 points)
O
O
+
+
N
N
_
_
O
O
O
O
+
+
N
N
_
_
O
O
1350 cm-1
Symmetric stretch
1550 cm-1
Asymmetric stretch
12. Vibrational frequencies of covalent bonds depend on the bond force constant, the reduced
mass and the %s character of the atoms in the bond. Assign the following pairs of vibrational
stretching frequencies to the corresponding molecules. Write the appropriate frequency below the
structure. R is a saturated hydrocarbon group. (4 points)
(a)
R3C
Cl
R3C
750 cm
-1
1200 cm
O
(b)
C
R
R
1715 cm
(c)
RN
C
~1200 cm-1 ~750 cm-1
-1
O
C
R
C-Hal
F
-1
NR
2145 cm -1
CH
1690 cm
CH2
-1
R2C
NR
1670
cm -1
C=O
~1715 cm-1
~1690 cm-1
C=N
~2145 cm-1
~1670 cm-1
C=O
~1715 cm-1
~1780 cm-1
O
O
(d)
1780 cm -1
1715 cm -1
Hint: consider the external
C-C-O bond angle
4