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Organic Chemistry
6th Edition
Paula Yurkanis Bruice
Chapter 13
Mass Spectrometry,
Infrared Spectroscopy,
and Ultraviolet/Visible
Spectroscopy
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Spectrally Identifiable Functional Groups
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13.1 MASS SPECTROMETRY
A mass spectrum
records only positively
charged fragments,
either cations or radical
cations
m/z = mass-to-charge
ratio of the fragment
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13.2 THE MASS SPECTRUM • FRAGMENTATION
The mass spectrum of pentane
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• The molecular ion (M): measured to the nearest whole number or
up to four decimal places (high-resolution mass spectrometry).
• Isotope peaks (M + 1, M + 2 etc.).
M
…
M+1
Typically M and the isotope peaks
are the highest masses in the
spectrum (12C and 13C)
Exception: a compound whose
molecular ion completely fragments
• The high-resolution mass of the molecular ion provides the
molecular formula directly.
• The whole-number mass of the molecular ion and the relative
intensities of M + 1, M + 2, etc., can also provide the molecular
formula.
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The base peak of m/z 43 in the mass spectrum of pentane
indicates the preference for C-2 to C-3 fragmentation:
All fragments originate
from the molecular ion
The mass of the radical species lost in a fragmentation is
the difference between the m/z values of the fragment ion
and the molecular ion
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The Mass Spectrum of Isopentane
Note strong m/z = 57
peak, secondary butyl
carbocation
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Two-Fragment Loss from the Molecular Ion
What are the structures of m/z 42 and 41?
These ions arise from loss of the ethyl radical and either hydrogen
atom or H2 from the pentane molecular ion:
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13.3 ISOTOPES IN MASS SPECTROMETRY
• M + 1 peak: a contribution from 2H or 13C.
• M + 2 peak: a contribution from 18O or from two heavy
isotopes (2H or 13C) in the same molecule.
• A large M + 2 peak suggests a compound containing
either chlorine or bromine: a Cl if M + 2 is one-third the
intensity of M; a Br if M + 2 is the same intensity as M.
• To calculate the molecular masses of molecular ions
and fragments, the atom mass of a single isotope of an
atom must be used.
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Fragmentation Patterns of Alkyl Halides
79Br
81Br
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2-Chloropropane
35Cl
37Cl
35Cl
37Cl
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if a compound contains five carbon atoms, the relative
abundance of the M+1 ion is estimated to be 5 x 1.1%,
then, the number of carbon atoms in a compound
can be calculated if the relative intensities of both the M
and M+1 peaks are known.
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13.4 HIGH-RESOLUTION MASS
SPECTROMETRY
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13.5 THE FRAGMENTATION PATTERNS OF
FUNCTIONAL GROUPS
α-Cleavage results from the homolytic cleavage of a
C—C bond at the α carbon:
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α-Cleavage occurs because the C—Cl and C—C bonds
have similar strengths, and the species that is formed is a
relatively stable cation:
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α-Cleavage is less likely to occur in alkyl bromide
because C—C bond is stronger than C—Br bond
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Ethers
A C-O bond is cleaved heterolytically, with the electrons going to
the more electronegative oxygen atom
A C—C bond is cleaved homolytically at an α-position
because it leads to a relatively stable cation:
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Alcohols
Because they
fragment, molecular
ions obtained from
alcohols usually are
not observed
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Common Fragmentation Behavior in Alkyl Halides,
Ethers, and Alcohols
1. A bond between carbon and a more electronegative
atom breaks heterolytically
2. A bond between carbon and an atom of similar
electronegativity breaks homolytically
3. The bonds most likely to break are the weakest bonds
and those that lead to formation of the most stable
cation
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Ketones
An intense molecular ion peak:
McLafferty rearrangement may occur:
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13.7 SPECTROSCOPY AND THE
ELECTROMAGNETIC SPECTRUM
Spectroscopy is the study of the interaction of matter and
electromagnetic radiation
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Electromagnetic radiation has wave-like properties
High frequencies and short wavelengths are associated with
high energy
The relationship between wavenumber (in cm-1) and wavelength (in
nm) is given by
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13.8 INFRARED SPECTROSCOPY
Functional groups stretch
at different frequencies,
and IR spectroscopy is
used to identify functional
groups
When a molecule is
bombarded with radiation
of a frequency that exactly
matches the frequency of
the vibration of one of its
bonds, the molecule
absorbs energy.
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The functional group
region (4000–1400 cm–1)
High energy
The fingerprint
region (1400–600 cm–1)
Low energy
The functional group, or diagnostic region, is used to determine
the functional group present
The fingerprint region is used for structure elucidation by spectral
comparison
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13.9 CHARACTERISTIC INFRARED
ABSORPTION BANDS
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13.10 THE INTENSITY OF ABSORPTION
BANDS
Infrared transitions require a bond dipole to occur:
δ
δ
C H
hν
δ
δ
C
H
Higher Energy
Vibrational State
The more polar the bond, the more intense the absorptions:
The intensity of an absorption band also depends on
the number of bonds responsible for the absorption
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Influence of symmetry on IR activity of the alkene stretch:
1-butene — infrared active
2,3-dimethyl-2-butene — infrared inactive
2,3-dimethyl-2-heptene — infrared active, but very weak absorption band
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13.11 THE POSITION OF ABSORPTION BANDS
The approximate wavenumber of an absorption can be
calculated from Hooke’s law:
ν = wavenumber
v=
1
2πc
K
Reduced Mass =
M 1M 2
M1 + M 2
c = speed of light
K = force constant
M1 and M2 = masses of atoms
M (mass) ↑, ν ↓
C—H ( ~3000 cm–1) < C—D (~2200 cm–1) < C—O (~1100 cm–1) < C—Cl (~700 cm–1 )
K (bond strength)↑
ν↓
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13.12 THE POSITION OF AN ABSORPTION BAND
IS AFFECTED BY ELECTRON DELOCALIZATION,
ELECTRON DONATION AND
WITHDRAWAL, AND HYDROGEN BONDING
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Electron delocalization (less double-bond character) so it absorbs at a
lower frequency (1680 cm-1 )
2-cyclohexenone
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Carbonyl
overtone
The electron-withdrawing oxygen atom makes the carbonyl group of
an ester harder to stretch (1740 cm-1 ) than the carbonyl group of a
ketone (1720 cm-1).
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The Vibrating Bond as a Quantized Harmonic Oscillator
Quantum levels for a
stretching vibration:
Ball-and-spring model:
Fundamental transition: νo → ν1
Overtone: νo → ν2
Overtones are twice the
frequency of the fundamental
transition and are always weak
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The carbonyl group of an amide has less double bond
character than does the carbonyl group of a ketone, the carbonyl
group of an amide stretches more easily ('1660 cm-1).
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C-O bond :1250 - 1050 cm-1.
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Acids are readily distinguished from alcohols
Higher-frequency
C─O stretch
Broad
OH stretch
C═O stretch
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Hydrogen-bonded OH groups also have broader absorption bands
because hydrogen bonds vary in strength.
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Functional group regions: Both
compounds are alcohols
Fingerprint regions: Compounds
are different alcohols
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The strength of a
C—H bond depends
on the hybridization
of the carbon
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Examine the absorption bands in the vicinity of 3000 cm–1
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Benzene in-plane and
out-of-plane C—H bends
Benzene ring:
• Sharp absorption bands at ~1600 cm–1 and 1500–1430 cm–1.
• Overtones at 1700–1900 cm–1 for the in-plane and out-of-plane benzene
C—H bends.
• The benzene overtones in the diagnostic region are readily recognized.
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Stretch of C—H Bond in an Aldehyde
The stretch of the C—H bond of an aldehyde shows one
absorption band at ~2820 cm–1 and another one at ~2720
cm–1
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Identifying a functional group by the bending vibrations:
Primary amine: two N—H stretches at 3350 cm–1.
Amine: N—H bends tend absorption to be broader (due to hydrogen
bonding) and more intense (due tobeing more polar) than those caused by
C=C stretches (see cyclohexene)
“Isopropyl split” at 1380 cm–1 indicates the presence of an isopropyl
group.
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13.14 THE ABSENCE OF ABSORPTION
BANDS
diethyl ether
a strong absorption at 1100 cm-1 : C-O
no absorption above 3100 cm-1 : no O-H
no absorption ~ 1700 cm-1 : no carbonyl (C=O)
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13.16 HOW TO INTERPRET AN INFRARED
SPECTRUM
The position, intensity, and shape of an absorption band
are helpful in identifying functional groups
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
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13.16 HOW TO INTERPRET AN INFRARED
SPECTRUM
wavenumber (cm–1)
3075
2950
1650 and 890
absence of bands
1500–1430 and 720
assignment
sp2 CH
sp3 CH
a terminal alkene with two substituents
has less than four adjacent CH2 groups
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wavenumber (cm–1)
3050
2810 and 2730
1600 and 1460
1700
assignment
sp2 CH
an aldehyde
benzene ring
a partial single-bond
character carbonyl
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wavenumber (cm–1)
3300
2950
2100
assignment
OH group
sp3 CH
alkyne
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wavenumber (cm–1)
3300
2950
1660
1560
assignment
N—H
sp3 CH
amide carbonyl
N—H Bend
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wavenumber (cm–1)
assignment
>3000
<3000
1605 and 1500
1720
1380
sp2 CH
sp3 CH
a benzene ring
a ketone carbonyl
a methyl group
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13.17 ULTRAVIOLET AND VISIBLE
SPECTROSCOPY
- UV/Vis spectroscopy provides information about compounds with
conjugated double bonds
- Ultraviolet light : 180 to 400 nm
- visible light : 400 to 780 nm
The shorter the wavelength, the greater the energy. Ultraviolet light,
therefore, has greater energy
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UV and Vis light cause only two kinds of electronic
transition:
Symmetry:
allowed
transition
Forbidden transition:
lone pair orthogonal to π
system
• Only organic compounds with π electrons can produce UV/Vis
spectra.
• A visible spectrum is obtained if visible light is absorbed.
• A UV spectrum is obtained if UV light is absorbed.
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A chromophore is the part of a molecule that absorbs UV or
visible light
Only compounds with π electrons can produce UV/Vis spectra
The absorption bands are
broad because each
electronic state
has vibrational sublevels
Allowed
Forbidden
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13.18 THE BEER—LAMBERT LAW
A=εcl
ε = ~10,000 M–1cm–1,
ε = <100 M–1cm–1,
Allowed
Forbidden
A = log(I0/I)
c = concentration of substance in solution
l = length of the cell in cm
ε = molar absorptivity, a measure of the probability of the
transition
The molar absorptivity of a compound is a constant that is
characteristic of the compound at a particular wavelength
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13.19 THE EFFECT OF CONJUGATION ON
λmax
The λmax and ε values increase as the number of conjugated double
bonds increases
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If a compound has enough conjugated double bonds, it will absorb visible
light (λmax >400 nm), and the compound will be colored
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An auxochrome is a substituent in a chromophore that
alters the λmax and the intensity of the absorption:
anilinium ion does not have an auxochrome, its λmax is similar to that
of benzene.
A chromophore is the part of a molecule that absorbs UV
or visible light.
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Uses of UV/Vis Spectroscopy
• Measure the rates of a reaction
• Determine the pKa of a compound
• Estimate the nucleotide composition of DNA
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