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Lecture 3
Mass Spectrometry
and
Infrared Spectroscopy
The Fragmentation Pattern of Alkyl Halides
Simple heterolytic cleavage
CH3
-e
CH
CH
CH
H3C
CH3
CH3
-
Cl
H3C
H3C
Cl
Cl
Electron sink
Cl is more electronegative than C
Cl has a positive formal charge
CH3
+
C
H3C
H
m/z 43
Cl
The Fragmentation Pattern of Alkyl Halides
a-cleavage
CH3
●
●
C
H3C
H
H
●+
Cl
C
H3C
Cl
CH3
- e-
CH
CH
CH
H3C
CH3
CH3
Cl
H3C
Cl
H3C
Cl
- CH3
H
C
H3C
63 65 m/z
Cl
The Fragmentation Pattern of Ethers
Simple heterolytic cleavage
CH3
H3C
H3C
CH
O
CH3
H3C
CH3
CH3
O
C
H
CH3
H3C
CH3
O
CH3
CH3
CH
CH3
CH3
CH3
CH
O
H
CH3
a-cleavage
The Fragmentation of Alcohols
Elimination of H2O
g
b
a
The elimination undergoes a six-member ring
transition state
Common Fragmentation Behavior in
Alkyl Halides,
Ethers, and Alcohols
1. A bond between carbon and a more electronegative
atom—heterolytic cleavage
2. A bond between carbon and an atom of similar
electronegativity break homolytically, e.g. a-cleavage
3. The bonds most likely to break are those which lead
to formation of the most stable cation
Stability of Carbocations
• Tertiary > Secondary > Primary
H
H
H
H
H
C
H
H
C
H
C
C
H
H
H
H
H
C
>
H
H
>
H
H
C
H
H
H
H
H
hyperconjugation
• Carbocations conjugated with p-bonds
or lone paired electrons are stable
lone paried
electrons
p-bond
N
vacant
p-orbital
• Aromaticity
vacant
p-orbital
Fragmentation Pattern of Ketones
An intense molecular ion peak
McLafferty rearrangement may occur
A six-member ring transition state
H
●+
O
●
Molecular Spectroscopy
Study of interactions between electromagetic
radiation and matter
Electreomagnetic Radiation
E = hn = hc/l
h is called Planck’s constant
Wavenumber n~= 1/l (cm-1)
Infrared radiation excites covalent bond’s
vibration
E
The covalent bonds in molecules are constantly vibrating
• Stretching Vibration
• Bending Vibration
• Each stretching and bending vibration
of a bond occurs with a characteristic
frequency
• Molecule absorbs infrared radiation at
its characteristic vibrational frequency
• How an IR Spectrometer Works
Sample
IR
radiation
Detector
Computer
% transmittance
IR Spectrum
wavenumber
An Infrared Spectrum
4000 – 625 cm-1
2.5 – 16 mm
The spikes are absorption bands
1400
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
H
H
H
C
C
C
H
H
H
H
Dm=0
H
C=C stretch is IR inactive
1.51 Debye
1.34 Debye
H
F
C
H
C
H
C
F
C
H
r(C=C) = 1.330 Å
D m = 0.17 Debye
H
C
H
r(C=C) = 1.332 Å
C=C stretch is IR active