Transcript Structure Determination: MS, IR, and UV

Chapter 11
Structure Determination:
Mass Spectrometry, Infrared
Spectroscopy, and Ultraviolet
Spectroscopy
© 2006 Thomson Higher Education
Introduction
Modern techniques for structure determination of
organic compounds include:
•
Mass spectrometry
•
•
What is the size and formula of the compound
Infrared spectroscopy
•
•
What functional groups are present in the compound
Ultraviolet spectroscopy
•
•
Is a conjugated p electron system present in the compound
Nuclear magnetic resonance spectroscopy
•
What is the carbon-hydrogen framework of the compound
11.1 Mass Spectrometry of Small Molecules:
Magnetic-Sector Instruments
Mass spectrometry (MS) measures the mass and
molecular weight (MW) of a molecule
• Provides structural information by finding the masses
of fragments produced when molecules break apart
• Three basic parts of mass spectrometers:
Mass Spectrometry of Small Molecules:
Magnetic-Sector Instruments
Electron-impact, magneticsector instrument
•
•
•
•
Sample is vaporized into
ionization source
Bombarded by electron beam
(70 eV) dislodging valence
electron of sample producing
cation-radical
Most cation-radicals fragment
and are separated in magnetic
field according to their mass-tocharge ratio (m/z)
Since z = 1 for most ions the
value of m/z is mass of ion
Mass Spectrometry of Small Molecules:
Magnetic-Sector Instruments
Mass spectrum of propane (C3H8; MW = 44)
•
Base peak
• Tallest peak
•
Base peak at m/z = 29 in propane mass spectrum
Parent peak
• Unfragmented cation radical – molecular ion (M+)
• Parent peak only 30% of base peak for propane
•
•
Assigned intensity of 100%
11.2 Interpreting Mass Spectra
Molecular weight determined from molecular ion
peak
• High resolution double-focusing mass
spectrometers are accurate to about 0.0005
amu
• M+1 peak results from presence of 13C and
2H
• Fragmentation occurs when high-energy
cation radical falls apart
•
•
One fragment retains positive charge and is a
carbocation
One fragment is a neutral radical fragment
Interpreting Mass Spectra
Mass spectrum of 2,2-dimethylpropane (MW = 72)
• No M+ peak observed when electron-impact ionization
is used
•
“Soft” ionization methods can prevent fragmentation of
molecular ion
Interpreting Mass Spectra
• Base peak in mass spectrum of 2,2-dimethylpropane
is at m/z = 57
•
•
m/z = 57 corresponds to t-butyl cation
Molecular ion fragments to give most stable
carbocation
Interpreting Mass Spectra
Mass spectrum of hexane (C6H14; MW = 86)
• All carbon-carbon bonds of hexane are electronically
similar and break to a similar extent
• Mass spectrum contains mixture of ions
Interpreting Mass Spectra
M+ = 86 for hexane
•
•
•
•
m/z = 71 arises from
loss of methyl radical
from hexane cation
radical
m/z = 57 arises from
loss of ethyl radical
from hexane cation
radical
m/z = 43 arises from
loss of propyl radical
from hexane cation
radical
m/z = 29 arises from
loss of butyl radical
from hexane cation
radical
Worked Example 11.1
Using Mass Spectra to Identify Compounds
Assume that you have two unlabeled samples, one of methylcyclohexane
and the other of ethylcyclopentane.
How could you use mass spectrometry to identify them?
Worked Example 11.1
Using Mass Spectra to Identify Compounds
Strategy
• Look at the two possible structures and
determine how they differ. Then think about
how any of these differences in structure
might give rise to differences in mass spectra.
Methylcyclohexane, for instance, has a –CH3
group, and ethylcyclopentane has a –CH2CH3
group, which should affect the fragmentation
process.
Worked Example 11.1
Using Mass Spectra to Identify Compounds
Solution
• The mass spectra of both samples show
molecular ions at M+ = 98, corresponding to
C7H14, but the two spectra differ in their
fragmentation patterns. Sample A has its base
peak at m/z = 69, corresponding to the loss of a
CH2CH3 group (29 mass units), but B has a
rather small peak at m/z = 69. Sample B shows
a base peak at m/z = 83, corresponding to the
loss of a CH3 group (15 mass units), but sample
A has only a small peak at m/z = 83. We can
therefore be reasonably certain that A is
ethylcyclopentane and B is methylcyclohexane.
11.3 Mass Spectrometry of Some
Common Functional Groups
Alcohols
• Fragment by two pathways
•
Alpha cleavage
•
Dehydration
Mass Spectrometry of Some
Common Functional Groups
Amines
• Aliphatic amines undergo characteristic a cleavage
Mass Spectrometry of Some
Common Functional Groups
Carbonyl compounds
• Ketones and aldehydes with C-H three atoms away
from carbonyl group undergo McLafferty rearrangement
Mass Spectrometry of Some
Common Functional Groups
• Ketones and aldehydes also undergo a cleavage of
bond between carbonyl group and neighboring carbon
Worked Example 11.2
Identifying Fragmentation Patterns in a Mass
Spectrum
The mass spectrum of 2-methylpentan-3-ol is shown in
Figure 11.8. What fragments can you identify?
Worked Example 11.2
Identifying Fragmentation Patterns in a Mass
Spectrum
Strategy
• Calculate the mass of the molecular ion, and
identify the functional groups in the molecule.
Then write the fragmentation processes you
might expect, and compare the masses of the
resultant fragments with the peaks present in
the spectrum.
Worked Example 11.2
Identifying Fragmentation Patterns in a Mass
Spectrum
Solution
•
2-Methylpentan-3-ol, an open-chain alcohol, has M+ = 102 and
might be expected to fragment by a cleavage and by dehydration.
These processes would lead to fragment ions of m/z = 84, 73, and
59. Of the three expected fragments, dehydration is not observed
(no m/z = 84 peak), but both a cleavages take place (m/z = 73, 59).
11.4 Mass Spectrometry in Biological Chemistry:
Time-of-Flight (TOF) Instruments
Most biological analyses use “soft” ionization methods:
• Electrospray ionization (ESI)
•
•
•
Sample dissolved in polar solvent and sprayed through steel
capillary tube
As sample exits tube it is subjected to high voltage producing
variably protonated sample ions (M + Hnn+)
Matrix-assisted laser desorption ionization (MALDI)
•
•
Sample is adsorbed onto a suitable matrix compound, such as
2,5-dihydroxybenzoic acid, which is ionized by laser light
Matrix compound then transfers energy to the sample and
protonates it, forming M + Hnn+ ions
Protonated sample ions are focused into small packet and hit
with energy from accelerator electrode
Ions begin moving with velocity(v) = 2E / m
Molecules separate based on different times of flight through
analyzer drift tube
Mass Spectrometry in Biological Chemistry:
Time-of-Flight (TOF) Instruments
MALDI-TOF mass spectrum of chicken egg-white
lysozyme
• Peak at 14,307.7578 daltons (amu) is due to the monoprotonated protein
Spectroscopy and the
Electromagnetic Spectrum
Absorption spectrum
• Spectrum of compound’s selective absorption of
electromagnetic radiation
• Infrared absorption spectrum of ethanol
11.6 Infrared Spectroscopy
Infrared (IR) region
• Ranges from 7.8 x 10-7 m to 10-4 m
•
2.5 x 10-6 m to 2.5 x 10-5 m used by organic chemists
• Wavelengths given in micrometers (1 mm = 10-6 m)
• Frequencies given in wavenumbers
• Wavenumber
• Reciprocal of wavelength in centimeters
• Expressed in units of cm-1
Infrared Spectroscopy
Molecules stretch or bend at specific frequencies
•
Energy is absorbed if the frequency of the radiation matches
the frequency of the vibration
IR spectrum
What molecular motions?
What functional groups?
11.7 Interpreting Infrared Spectra
Most functional groups have characteristic IR absorption bands
that don’t change from one compound to another
Interpreting Infrared Spectra
Interpreting Infrared Spectra
Interpreting Infrared Spectra
Hexane
Hex-1-ene
Hex-1-yne
Infrared Spectra of Some
Common Functional Groups
Infrared Spectra of Some
Common Functional Groups