13_Lecture.ppt

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Organic Chemistry 6th Edition Paula Yurkanis
Bruice
Chapter 13 Mass Spectrometry, Infrared
Spectroscopy, and Ultraviolet/Visible Spectrosco
py
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Spectrally Identifiable Functional Groups
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The Mass Spectrometer
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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Information obtained from a mass spectrum

• 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.).

Typically M and the isotope peaks are the highest
masses in the spectrum
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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• Fragment masses and intensities together provide
  structural information.

• The base peak has the greatest intensity in the
  spectrum.
• Intense peaks correspond to relatively stable
  cationic and/or relatively stable radical species
  lost.
• The fragments lost also provide structural
  information.

M-15
For example

• Fragment m/z 57 resulted from the loss of methyl
  (m/z 15) from the molecular ion.
• Given its intensity, m/z 57 must be the sec-butyl
  carbocation (not the primary butyl carbocation).

M
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The Mass Spectrum of Pentane
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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
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2-Methylbutane is more likely than pentane to
lose a methyl radical because a secondary
carbocation can be formed
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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
Note All fragments originate from the molecular
ion. Exception Tandem Mass spectrometry where
fragments of fragments are observed.
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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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The Mass Spectrum of 2-Chloropropane
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a-Cleavage results from the homolytic cleavage of
a CC bond at the a carbon
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a-Cleavage occurs because the CCl and CC bonds
have similar strengths, and the species that is
formed is a relatively stable cation
a-Cleavage is less likely to occur in alkyl
bromide because CC bond is stronger than CBr
bond
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Fragmentation Patterns of Ethers
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A CO bond is cleaved heterolytically, with the
electrons going to the more electronegative atom
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A CC bond is cleaved homolytically at an
a-position because it leads to a relatively
stable cation
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Fragmentation Patterns of Alcohols
Because they fragment, molecular ions obtained
from alcohols usually are not observed
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Like alkyl halides and ethers, alcohols undergo
a-cleavage
In alcohols, loss of water results in a
fragmentation peak at m/z M-18
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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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Fragmentation Patterns of Ketones
An intense molecular ion peak
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McLafferty rearrangement may occur
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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
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Vibrational Transitions Observed in IR
Spectroscopy
Functional groups stretch at different
frequencies, and IR spectroscopy is used to
identify functional groups
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Infrared transitions require a bond dipole to
occur
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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The Vibrating Bond as a Quantized Harmonic
Oscillator
Quantum levels for a stretching vibration
Ball-and-spring model
Fundamental transition ?o ? ?1 Overtone ?o ? ?2
Overtones are twice the frequency of the
fundamental transition and are always weak
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The approximate wavenumber of an absorption can
be calculated from Hookes law
?? wavenumber c speed of light K force
constant M1 and M2 masses of atoms
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Hookes law predicts that lighter atoms will
vibrate at a higher frequency than heavy atoms
CH 3000 cm1 CD 2200 cm1 CO 1100
cm1 CCl 700 cm1
Increasing the s character of a bond (higher K
value) increases the stretching frequency
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Note the influence of mass and s character on
stretching frequency
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An Infrared Spectrum
The functional group region (40001400 cm1)
The fingerprint region (1400600 cm1)
High energy
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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Functional group regions Both compounds are
alcohols
Fingerprint regions Compounds are different
alcohols
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The exact position of the absorption band depends
on electron delocalization, the electronic
effect of neighboring substituents, and hydrogen
bonding
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Esters have a carbonyl and a CO stretch
Ketones have only a carbonyl stretch
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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
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The position of a CO absorption varies because
of resonance release in acids and esters
1050 cm1
1050 cm1
1250 cm1
1250 cm1 and 1050 cm1
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Acids are readily distinguished from alcohols
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The position and the breadth of the OH
absorption band depend on the concentration of
the solution
It is easier to stretch an OH bond if it is
hydrogen bonded
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The strength of a CH bond depends on the
hybridization of the carbon
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Examine the absorption bands in the vicinity of
3000 cm1
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Benzene ring

• Sharp absorption bands at 1600 cm1 and
  15001430 cm1.
• Overtones at 17001900 cm1 for the in-plane and
  out-of-plane benzene CH bends.
• The benzene overtones in the diagnostic region
  are readily recognized.

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Stretch of CH Bond in an Aldehyde
The stretch of the CH bond of an aldehyde shows
one absorption band at 2820 cm1 and another
one at 2720 cm1
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Identifying a functional group by the bending
vibrations

• Primary amine two NH stretches at 3350 cm1.
• Amine NH bend.
• Isopropyl split at 1380 cm1 indicates the
  presence of an isopropyl group.

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Analyzing Infrared Spectra
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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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

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UV and Vis light cause only two kinds of
electronic transition
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A chromophore is the part of a molecule that
absorbs UV or visible light
Only compounds with p electrons can produce
UV/Vis spectra
Allowed
Forbidden
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The BeerLambert Law
? 10,000 M1cm1, Allowed
A e c l
? lt100 M1cm1, Forbidden
A log(I0/I) c concentration of substance in
solution l length of the cell in cm e 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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Effect of Conjugation on lmax
The lmax and ? 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 (lmax gt400 nm), and
the compound will be colored
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An auxochrome is a substituent in a chromophore
that alters the lmax and the intensity of the
absorption
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The Visible Spectrum and Color
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Uses of UV/Vis Spectroscopy