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Mass Spectrometry, Infrared
Spectroscopy,
and
Ultraviolet/Visible Spectroscopy
Classes of Organic Compounds
[Insert Table 14.1]
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Mass Spectrometry
 A technique used to elucidate structural information of a
molecule via the measurement of its mass
 MS
does not involve electromagnetic radiation, therefore
spectrometry and not spectroscopy.
An electron is ejected from the compound, thereby forming a molecular ion.
A Mass Spectrometer
Only positively charged species reach the recorder.
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The Mass Spectrum of Pentane
m/z = mass-to-charge ratio of the fragment because z = 1
The Molecular Ion
Pentane forms a molecular ion with m/z = 72.
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Fragmentation of the Molecular Ion
The more stable the fragments, the more abundant they will be.
C-2—C-3 fragmentation forms more stable fragments (base peak at m/z 43) .
Loss of H2 From a Fragment
We often see peaks with one and two units less than
the m/z values for the carbocations
Further fragmentation causes the loss of one or two
hydrogen atoms
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More Stable Fragments are More Abundant
2-methylbutane also has the same molecular formula as
pentane, so it also has M+ = 72.
The peak at m/z = 57 is more abundant for isopentane than for pentane
because a secondary carbocation is more stable than a primary carbocation.
Secondary Carbocations are More Stable
Than Primary Carbocations
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Isotopes and their natural Abundance
Isotopes: Species having same atomic number but different atomic mass
High Resolution Mass Spectrometry Can Distinguish
Between Compound with the Same Molecular Mass
Exact Masses of Isotopes
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The Carbon—Bromine Bond
Breaks Heterolytically
The ratio of 79Br and 81Br is almost 1, therefore two M+ peaks, 2 mass
units apart with almost equal intensities indicate a bromo compound
35Cl is three times more abundant than 37Cl. We expect the height
of the M + 2 peak to be 1/3 the height of the M+ peak. If this
occurs, we can identify a chloro compound.
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The Carbon—Chlorine Bond Breaks Heterolytically
The Carbon—Carbon Bond Breaks Homolytically
α-Cleavage in an Alkyl Chloride
The homolytic cleavage of the carbon—carbon bond is called α-cleavage.
The bonds that break are
• the weakest bonds, and
• the bonds that form the most stable fragments.
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The Mass Spectrum of 2-Chloropropane
α-Cleavage Occurs in Alkyl Chlorides
but is Less Likely to Occur in Alkyl Bromides
The carbon—carbon bond and the carbon—chlorine bond have similar strengths.
The carbon—carbon bond is much stronger than the carbon—bromine bond.
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The Carbon—Oxygen Bond
Breaks Heterolytically
α-Cleavage in an Ether
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α-Cleavage in an Alcohol
Loss of a Hydrogen from a γ-Carbon
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α-Cleavage in a Ketone
The McLafferty rearrangement
 Loss of a Hydrogen from a γ-Carbon
 The bond between the α-C and the β-C breaks homolytically and a H
atom from the γ-C migrates to the oxygen atom.
 This type of fragmentation produces a resonance stabilized cation.
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Common fragmentation behaviour in alkyl Halides,
Ethers, Alcohols, and Ketones
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.
Infrared and Ultraviolet/Visible Spectroscopy
(IR and UV/Vis)
The Electromagnetic Spectrum
high energy
low energy
high frequency
low frequency
short wavelengths
long wavelengths
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Infrared Spectroscopy
Infrared refers (IR) seen between the visible and
microwave regions of the electromagnetic spectrum.
IR radiations are low energy radiations, do not cause
emission of electrons, only vibrate the bonds.
The Greater the Energy - Higher the Frequency
Shorter the Wavelength
Stretching and Bending Vibrations
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The More Polar the Bond,
the More Intense the Absorption
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The Greater the Bond Order the Larger the Wavenumber
Electron Delocalization (Resonance)
Affects the Frequency of the Absorption
The more double bond character - the
greater the frequency (wavenumber)
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This C═O Bond Is Essentially a Pure Double Bond
1720 cm-1
This C═O Bond has significant Single Bond Character
1680 cm-1
The less double bond character, the lower the frequency.
Resonance Electron Donation Decreases the Frequency Inductive
Electron Withdrawal Increases the Frequency
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The IR Spectrum of an Ester and amide
1740 cm-1
1660 cm-1
Carbon—Oxygen Bonds
 Alcohols and Ethers - The carbon—oxygen bond is a pure single bond.
 Carboxylic acids and Esters - The carbon—oxygen bond is a has partial
double bond character.
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The IR Spectrum of an Alcohol
The IR Spectrum of a Carboxylic Acid
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The Strength of a Carbon—Hydrogen Bond Depends on the
Hybridization of the Carbon
Stretching vibrations
require more energy than
bending vibrations.
• An sp3-carbon — Weakest hydrogen bond shortest wavenumber (< 3000 cm–1).
An sp2-carbon— stronger hydrogen stretch occurs at > 3000 cm–1.
Where Carbon—Hydrogen Bonds Bend
An sp3-carbon—hydrogen bend of a methyl occurs at < 1400 cm–1
An sp2- carbon—hydrogen bend of a methyl/methylene occurs at > 1400 cm–1
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The IR Spectrum of an Aldehyde
The carbon—hydrogen stretch of an aldehyde hydrogen occurs at 2820 cm–1
and at 2720 cm–1.
The IR Spectrum of an Amine
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Some Vibrations are Infrared Inactive
A bond absorbs IR radiation only if its dipole moment changes when it vibrates.
The IR Spectrum of 2-Methyl-1-pentene
sp2 CH
sp3 CH
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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The IR Spectrum of Benzaldehyde
sp2 CH
C-H
C=C
C=C
C=O
Wavenumber (cm–1)
Assignment
3050
sp2 CH an aldehyde
2810 and 2730
sp2 CH benzene ring
1600 and 1460
a partial single-bond
1700
C=O carbonyl
The IR Spectrum of 2-Propyn-1-ol
C≡C
C-H
O-H
Wavenumber (cm–1)
Assignment
3300
2950
2100
OH group
sp3 CH
alkyne
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The IR spectrum of N‐Methylethanamide
sp3 CH
amide
carbonyl
N—H
streching
N—H
bend
Wavenumber (cm–1)
Assignment
3300
2950
1660
1560
N—H
sp3 CH
amide carbonyl
N—H bend
IR Spectrum of Ethyl Benzyl Ketone
sp2 CH
sp3 CH
C=C
C=C
C=O
Wavenumber (cm–1)
>3000
<3000
1605 and 1500
1720
1380
Assignment
sp2 CH
sp3 CH
a benzene ring
a ketone carbonyl
a methyl group
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Ultraviolet and Visible Spectroscopy
 Spectroscopy is the study of the interaction between matter
and electromagnetic radiation
 UV/Vis spectroscopy provides information about compounds
with conjugated double bonds
Electronic transition
Only  electrons can produce UV/Vis spectra.
A UV spectrum is obtained if UV light is absorbed.
A visible spectrum is obtained if visible light is absorbed.
A UV Spectrum
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UV/Vis Absorption Bands are Broad
UV/Vis absorption bands are broad
because an electronic state has vibrational sublevels.
Chromophore
A chromophore is that part of a molecule
that is responsible for a UV/Vis spectrum.
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The More Conjugated Double Bonds,
the Longer the Wavelength
Conjugation Makes
the Electronic Transition Easier
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Colored Compounds Absorb
Visible Light (> 400 nm)
Auxochrome
An auxochrome is a substituent that alters
the position and intensity of the absorption.
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The Color Observed Depends on
the Color Absorbed
Common Dyes
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Anthocyanins
The Beer–Lambert Law
A = c l
A = absorbance of the sample
c = concentration of substance in solution
l = length of the cell in cm
 = molar absorptivity of the sample
(a measure of the probability of the transition)
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UV/Vis Spectroscopy Can Be Used
to Measure the Rate of a Reaction
UV/Vis Spectroscopy Can Be Used
to Measure the Rate of a Reaction
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NMR Spectroscopy
 Provided the carbon–hydrogen framework of an organic compound
 Certain nuclei, such as 1H, 13C, 15N, 19F, and 31P
 Spin quantum number (nonzero value) allows them to be studied by NMR.
 The ∆ Energy Between Two Spin States depends on the Strength of the
Applied Magnetic Field (Bo)
How Many Signals?
• All the hydrogens in a compound do not experience the same magnetic
field due to local current generated by electrons.
• Each set of chemically equivalent protons give a signal in the
1H NMR spectrum.
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(CH3)4Si
Most organic compounds the 1H
NMR ranges between 0.8‐12 ppm
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Important regions in 1H NMR spectrum
Methine proton is most deshielded relative to methylene and methyl protons as it is
attached to more number of carbon atoms (C is more electronegative than H atom)
Integration
 Integration identifies the relative number of protons
1 proton integrates to
1.6/2 = 0.8
• The area under each signal is proportional to the number of protons giving rise to
the signal.
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Splitting of signal (N+1 rule)
N is the number of non-equivalent protons on adjacent carbons that split the signal of the proton
H
Examples:
H
Cl
C
C
H
Cl
H
a is a triplet
b is a quartet
c is a singlet
a is a triplet
b is a multiplet
c is a triplet
d is a singlet
Equivalent Protons Do Not Split Each Other’s Signals
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(b)
(c)
(a)
(d)
(e)
Summary
 The number of signals tells us the number of sets of equivalent
protons in the compound.
 The value of the chemical shifts tells us the nature of the chemical
environment: alkyl, alkene, benzene, etc.
 The integration values tells us the relative number of protons.
 The splitting tells us the number of neighboring protons.
 The coupling constants identifies coupled protons.
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13C NMR Spectroscopy

13C
is also NMR active atom but lower in abundance (1.11%), thus
requires large number of scans.
 No coupling with other atoms in13C, so single signal for equivalent
carbons.
 The number of signals reflects the number of different kinds of carbons in
a compound.
Drawbacks:
•
Requires large number of scans due to low abundance.
•
The area under signal do not correspond to actual number of carbons
contributing to that signal.
Where 13C NMR Signals Appear
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The 13C NMR Spectrum of 2-Butanol
b
d
c
a
b
d
a
c
Carbons that are not attached to hydrogens give very small signals
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A DEPT (distortionless enhancement by polarization transfer) 13C NMR Spectrum
(Distinguishes CH3, CH2, and CH groups)
DEPT-135
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