Organic Chemistry
4th Edition
Paula Yurkanis Bruice
Chapter 13
Mass Spectrometry
and
Infrared Spectroscopy
Irene Lee
Case Western Reserve University
Cleveland, OH
©2004, Prentice Hall
Information Obtained from Mass
Spectrometry
The molecular mass
The molecular formula
Certain structural features
of the compound
A Mass Spectrometer
The mass spectrometer records a mass spectrum
A mass spectrum records only positively charged
fragments
m/z = mass to charge ratio of the fragment
• Nominal molecular mass: the molecular mass to the
nearest whole number
• Each m/z value is the nominal molecular mass of the
fragment
• The peak with the highest m/z value represents the
molecular ion (M)
• Peaks with smaller m/z values__called fragment ion
peaks__represent positively charged fragments of the
molecule
• The base peak is the peak with the greatest intensity,
due to its having the greatest abundance
• Weak bonds break in preference to strong bonds
• Bonds that break to form more stable fragments break
in preference to those that form less stable fragments
The base peak of 43 in the mass spectrum of pentane
indicates the preference for C-2 to C-3 fragmentation
To identify fragment ions in a spectrum, determine the
difference between the m/z value of a given fragment ion
and that of the molecular ion
Carbocations can undergo further fragmentation
2-methylbutane has the same m/z as pentane
but the peak at m/z = 57 (M – 15) is more intense
Isotopes in Mass Spectrometry
• peaks that are attributable to isotopes can help identify
the compound responsible for a mass spectrum
• M + 2 peak: a contribution from 18O or from two heavy
isotopes in the same molecule
• a large M + 2 peak suggests a compound containing
either chlorine or bromine: a Cl if M + 2 is 1/3 the height
of M; a Br if M + 2 is the same height as M
• In calculating the molecular masses of molecular ions
and fragments, the atom mass of a single isotope of an
atom must be used
The Mass Spectrum of Bromopropane
The weakest bond is the C–Br bond
The base peak is at m/z = 43 [M – 79, or (M + 2) – 81]
The propyl cation has the same fragmentation pattern
it exhibited when it was formed in the cleavage of
pentane
The Mass Spectrum of
2-Chloropropane
The compound contains a chlorine, because M + 2 peak
is 1/3 the height of the molecular ion peak
The base peak at m/z = 43 results from heterolytic
cleavage of the C–Cl bond
The peaks at m/z = 63 and m/z = 65 have a 3:1 ratio,
indicating the presence of a chlorine atom
a cleavage results from the homolytic cleavage of a
C–C bond at the a carbon
The Fragmentation Pattern of Ethers
The Fragmentation of Alcohols
Common Fragmentation Behavior in
Alkyl Halides,
Ethers, and Alcohols
1. A bond between carbon and a more electronegative
atom
2. A bond between carbon and an atom of similar
electronegativity break homolytically
3. The bonds most likely to break are those which lead
to formation of the most stable cation
Fragmentation Pattern of Ketones
An intense molecular ion peak
McLafferty rearrangement may occur
Infrared Spectroscopy
An infrared spectrum is obtained by passing infrared
radiation through the sample
E = hu =
hc
l
wavenumber (~
u) is another way to describe the frequency
of electromagnetic radiation
High frequencies, large wavenumbers, and short
wavelengths are associated with high energy
The covalent bonds in molecules are constantly vibrating
Each stretching and bending vibration of a bond occurs
with a characteristic frequency
An Infrared Spectrum
The spikes are absorption bands
The functional group
region
The fingerprint
region
It takes more energy to stretch a bond than to bend it
The greater the change in dipole moment, the more
intense the absorption
When the bond stretches, the increasing distance
between the atoms increases the dipole moment
The intensity of an absorption band depends on the
number of bonds responsible for the absorption
Bond order affects bond strength, so bond order affects
the position of absorption bands
The approximate wavenumber of an absorption can be
calculated from Hooke’s law
u~ = 1
f(m1 + m2) 1/2
2pc
u~
f
m1 and m2
m1m2
the wavenumber of the stretching vibration
the force constant
masses of the atoms
The exact position of the absorption band depends on
electron delocalization, the electronic effect of
neighboring substituents,and hydrogen bonding
O
O-
O
CH3CCH2CH2CH3
C O
at 1720 cm–1
C O
at 1680 cm–1
Putting an atom other than carbon next to the carbonyl
group causes the position of the carbonyl absorption
band to shift
The predominant effect of the nitrogen of an amide is
electron donation by resonance
The predominant effect of the oxygen of an ester is
inductive electron withdrawal
The position of a C–O absorption varies because
CH3CH2 OH
~1050 cm–1
CH3CH2 O CH2CH3
~1050 cm–1
H3C
H3C
O
C
O
C
O CH3
OH
OC
H3C OH
~1250 cm–1
OC
~1250 cm–1 and 1050 cm–1
H3C O CH3
The position and the breadth of the O–H absorption
band depend on the concentration of the solution
It is easier to stretch an O–H bond if it is hydrogen
bonded
The strength of a
C–H bond depends
on the hybridization
of the carbon
Examine the absorption bands in the vicinity of 3000 cm–1
If a compound has hydrogen bonded to sp2 carbon
The detection of sharp absorption bands at ~1600 cm–1
and 1500–1430 cm–1: benzene
The detection of absorption bands at ~1600 cm–1: alkene
The N–H bending vibrations also occur at ~1600 cm–1,
but they are broader and more intense
They will be accompanied by N–H stretching at
3500–3300 cm–1
Stretch of C–H Bond in an Aldehyde
If a compound has sp3 carbon, absorption at ~1400 cm–1
identifies the methyl group
Analyzing Infrared Spectra
The absence of absorption bands can be useful in
identifying a compound in IR spectroscopy
Bonds in molecules lacking dipole moments will not be
detected
wavenumber (cm–1)
3075
2950
1650 and 890
absence of 720
assignment
sp2 CH
sp3 CH
a terminal alkene with two substituents
has less than four adjacent CH2 groups
wavenumber (cm–1)
3050
2810 and 2730
1600 and 1460
1700
assignment
sp2 CH
an aldehyde
benzene ring
a partial single-bond
character carbonyl
wavenumber (cm–1)
3300
2950
2100
assignment
OH group
sp3 CH
alkyne
wavenumber (cm–1)
3300
2950
1660
1560
assignment
N–H
sp3 CH
amide carbonyl
N–H
wavenumber (cm–1)
assignment
>3000
<3000
1605 and 1500
1720
1380
sp2 CH
sp3 CH
a benzene ring
a ketone carbonyl
a methyl group