1. Mass Spectrometry
O
O
29
57
O
59
O
87
102
O
A
154
B
139
strongest
D
C
111
125
weakest
1. Mass Spectrometry (cont’d)
F
OH
O
C6H13FO
C 6 x 12.0000
H 13 x 1.0078
F 1 x 18.9984
O 1 x 15.9949
120.0950
O
C5H12O3
C 5 x 12.0000
H 12 x 1.0078
O 3 x 15.9949
120.0780
O
O
C9H12
C 9 x 12.0000
H 12 x 1.0078
120.0940
C8H8O
C 8 x 12.0000
H 8 x 1.0078
O 1 x 15.9949
120.0570
1. Mass Spectrometry (cont’d) 2
70
45
73
88
0
10
88 = Molecular ion
73 = M+ -15 (loss of methyl)
70 = M+ -18 (loss of water)
45 = M+ -43 (loss of propyl)
20
30
40
50
60
70
80
90
100
Loss of water indicates the compound
is an alcohol. Alcohols also undergo αcleavage. Alcohol that can lose methyl
and propyl via α -cleavage:
OH
OH
or
2. Infrared Spectroscopy
a. Which signal would be more intense in an IR spectrum, a C-O stretch or a C-N stretch? Why?
Intensity of absorption is correlated to change in dipole moment with the vibration (stretch). The C-O bond is
more polar and will thus experience a greater change in dipole moment during a stretching vibration, giving a
more intense absorption.
b. Which would give a stronger C=C stretch in an IR spectrum, 1-butene or 2-butene? Why?
Intensity of absorption is correlated to change in dipole moment with the vibration (stretch). The 2-butene is
symmetrical and will not experience a change in dipole moment during a stretching vibration. The 1-butene
is not symmetrical about the C=C bond, and will this give a small change in dipole during stretching vibration
and this the more intense absorption.
c. Which would have a higher frequency absorption, an N-H bond or a P-H bond? Why?
Absorption frequency is correlated to bond strength and atomic masses. Phosphorus is a larger atom than
nitrogen, giving it a larger mass and weaker bonding via 3rd shell valence orbitals compared to nitrogen’s 2nd
shell valence orbitals. Both of these factors cause the N-H bond to have a higher frequency absorption than
the P-H bond.
2. Infrared Spectroscopy (continued)
Cyclohexene
- Presence of C=C at 1600 cm-1
- Absence of C=O around 1700 cm-1
- Absence of O-H around 3300 cm-1
Cyclohexenol
OH
- Presence of O-H at 3300 cm-1
- Absence of C=O around 1700 cm-1
- Absence of C=C around 1600 cm-1
Cyclohexanone
- Presence of C=O at 1700 cm-1
- Absence of C=C around 1600 cm-1
- Absence of O-H around 3300 cm-1
O
3. Ultraviolet Spectroscopy
O
O
O
O
OH
O
O
3. Ultraviolet Spectroscopy (cont.)
4
3
2
1
This would have the largest molar absorptivity (ε) and thus the most
intense absorption
4. NMR Spectroscopy
a. If an average carbon requires irradiation at 75 MHz to resonate in a 7.046 T
magnetic field, what frequency will be required to resonate an average carbon in a
14.092 T magnetic field?
ν = γB0
2π
so doubling the magnetic field doubles the frequency needed to flip the same nucleus,
therefore 150 MHz is needed.
b. Use the following molecule to answer the following.
a
c
b
d
Cl
Which carbon is furthest downfield?
_____d_____
Which carbon is furthest upfield?
_____a_____
Which carbon resonates at the highest frequency?
_____d_____
Which carbon resonates at the highest chemical shift?
_____d_____
Which carbon resonates at the lowest chemical shift?
_____a_____
Which carbon is the most shielded?
_____a_____
Which carbon is the most deshielded?
_____d_____
4. NMR Spectroscopy (cont’d)
O
Br
OCH3
O
3
5
5
6
5
Cl
Cl
7
140
6
120
100
3
80
PPM
60
4
40
20
CH3
7
0
4. NMR Spectroscopy
O
Br
OCH3
O
3
5
5
5
5
Cl
Cl
7
8
s
q
3
CH3
3
5
m (dq)
m (7)
t
O
F
O
O
s
d
t
d
t
Cl
s
m (dq)
HO
t
t
s
q
CH3
d
t
d
4. NMR Spectroscopy
3H
3H
2H
2H
3
2
PPM
1
O
OCH3
0