Mass spectrometry detail to up

Molecular weight can be obtained from a very small sample.
It does not involve the absorption or emission of light.
A beam of high-energy electrons breaks the molecule apart.
The masses of the fragments and their relative abundance reveal information about the structure of the molecule
DETERMINE THE MOLECULAR MASS OF
ORGANIC COMPOUNDS
DETERMINE THE MOLECULAR FORMULA OF
ORGANIC COMPOUNDS

1. Mass spectroscopy
SHOUVIK KR NANDY

2. Mass Spectrometry
•Molecular weight can be obtained from a
very small sample.
•It does not involve the absorption or emission
of light.
•A beam of high-energy electrons breaks the
molecule apart.
•The masses of the fragments and their
relative abundance reveal information about
the structure of the molecule.

3. Mass Spectrometry
THE MAIN USE OF MS IN ORG CHEM IS:
•DETERMINE THE MOLECULAR MASS OF
ORGANIC COMPOUNDS
•DETERMINE THE MOLECULAR FORMULA OF
ORGANIC COMPOUNDS

4. HOW DO WE ACHIEVE THIS?
•PERSUADE THE MOLECULE TO ENTER
THE VAPOR PHASE (CAN BE DIFFICULT)
•PRODUCE IONS FROM THE MOLECULES THAT ENTER
THE GAS PHASE
•SEPARATE THE IONS ACCORDING TO THEIR
MASS-TO-CHARGE RATIOS (m/z))
•MEASURE AND RECORD THESE IONS

5. IONIZING METHODS
•ELECTRON IMPACT -HIGH ENERGY
ELECTRONS ABOUT 70 EV!!
•CHEMICAL IONIZATION LOW ENERGY

6. ELECTRON IMPACT
RADICAL CATION
ONLY CATIONS ARE CARRIED TO DETECTOR
BOND-BREAKING

7. Electron Impact Ionization
A high-energy electron can dislodge an electron
from a bond, creating a radical cation (a
positive ion with an unpaired e
-
).e- + HC
H
H
C
H
H
H HC
H
H
C
H
H
H HC
H
H
C
H
H
+H HC
H
H
C
H
H
H
+
=>

8. Separation of Ions
•Only the cations are deflected by the
magnetic field.
•Amount of deflection depends on m/z.
•The detector signal is proportional to the
number of ions hitting it.
•By varying the magnetic field, ions of all
masses are collected and counted. =>

9. The GC-MS
=>
A mixture of compounds is separated
by gas chromatography, then identified
by mass spectrometry.

10. High Resolution MS
•Masses measured to 1 part in 20,000.
•A molecule with mass of 44 could be C
3H
8,
C
2H
4O, CO
2, or CN
2H
4.
•If a more exact mass is 44.029, pick the
correct structure from the table:
C
3H
8 C
2H
4O CO
2 CN
2H
4
44.06260 44.02620 43.98983 44.03740
=>

11. Molecules with
Heteroatoms
•Isotopes: present in their usual abundance.
•Hydrocarbons contain 1.1% C-13, so there will
be a small M+1 peak.
•If Br is present, M+2 is equal to M
+
.
•If Cl is present, M+2 is one-third of M
+
.
•If iodine is present, peak at 127, large gap.
•If N is present, M
+
will be an odd number.
•If S is present, M+2 will be 4% of M
+
. =>

12. Isotopic Abundance
=>
81
Br

13. Mass Spectrometry
•Introduction
–General overview
•Mass Spectrometry is the generation, separation and
characterization of gas phase ions according to their
relative mass as a function of charge
•Previously, the requirement was that the sample be
able to be vaporized (similar limitation to GC), but
modern ionization techniques allow the study of
such non-volatile molecules as proteins and
nucleotides

14. Mass Spectrometry
II.The Mass Spectrometer
–General Schematic
•A mass spectrometer needs to perform three
functions:
•Creation of ions–the sample molecules are subjected
to a high energy beam of electrons, converting some
of them to ions
•Separation of ions–as they are accelerated in an
electric field, the ions are separated according to
mass-to-charge ratio (m/z)
•Detection of ions–as each separated population of
ions is generated, the spectrometer needs to qualify
and quantify them

15. Mass Spectrometry
II.The Mass Spectrometer
B.Single Focusing Mass Spectrometer
•A small quantity of sample is injected and
vaporized under high vacuum
•The sample is then bombarded with electrons
having 25-80 eV of energy
•A valence electron is “punched”off of the
molecule, and an ion is formed

16. Mass Spectrometry
II.The Mass Spectrometer
B.The Single Focusing Mass Spectrometer
4.Ions (+) are accelerated using a (-) anode towards the focusing
magnet
5.At a given potential (1 –10 kV) each ion will have a kinetic energy:
½ mv
2
= eV
As the ions enter a magnetic field, their path is curved; the radius of
the curvature is given by:
r = mv
eH
If the two equations are combined to factor out velocity:
m/e = H
2
r
2
2V
m = mass of ion
v = velocity
V = potential difference
e = charge on ion
H= strength of magnetic
field
r = radius of ion path

17. Mass Spectrometry
II.The Mass Spectrometer
B.Single Focusing Mass Spectrometer
6.At a given potential, only one mass would have
the correct radius path to pass through the
magnet towards the detector
7.“Incorrect”mass particles would strike the
magnet

18. Mass Spectrometry
II.The Mass Spectrometer
B.Single Focusing Mass Spectrometer
8.By varying the applied potential difference that
accelerates each ion, different masses can be
discerned by the focusing magnet
9.The detector is basically a counter, that produces
a current proportional to the number of ions that
strike it
10.This data is sent to a computer interface for
graphical analysis of the mass spectrum

19. Mass Spectrometry
II.The Mass Spectrometer
C.Double Focusing Mass Spectrometer
Resolution of mass is an important consideration for MS
Resolution is defined as R = M/DM, where M is the mass of the particle observed and
DM is the difference in mass between M and the next higher particle that can be observed
Suppose you are observing the mass spectrum of a typical terpene (MW 136) and you
would like to observe integer values of the fragments:
For a large fragment: R = 136 / (135 –136) = 136
For a smaller fragment: R = 31 / (32 –31) = 31
Even a low resolution instrument can produce R values of ~2000!
4.If higher resolution is required, the crude separation of ions by a single focusing
MS can be further separated by a double-focusing instrument

20. Mass Spectrometry
II.The Mass Spectrometer
C.Double Focusing Mass Spectrometer
4.Here, the beam of sorted ions from the focusing
magnet are focused again by an electrostatic
analyzer where the ions of identical mass are
separated on the basis of differences in energy
5.The “cost”of increased resolution is that more
ions are “lost”in the second focusing, so there is
a decrease in sensitivity

21. Mass Spectrometry
II.The Mass Spectrometer
D.Quadrupole Mass Spectrometer
•Four magnets, hyperbolic in cross section are
arranged ; one pair has an applied direct current,
the other an alternating current
•Only a particular mass ion can “resonate”
properly and reach the detector

22. Mass Spectrometry
II.The Mass Spectrometer
D.Quadrupole Mass Spectrometer
3.The compact size and speed of the quadrupole
instruments lends them to be efficient and
powerful detectors for gas chromatography (GC)
4.Since the compounds are already vaporized,
only the carrier gas needs to be eliminated for
the process to take place
5.The interface between the GC and MS is shown;
a “roughing”pump is used to evacuate the

23. Mass Spectrometry
III.The Mass Spectrum
–Presentation of data
•The mass spectrum is presented in terms of ion
abundance vs. m/e ratio (mass)
•The most abundant ion formed in ionization
gives rise to the tallest peak on the mass
spectrum –this is the base peak

24. Mass Spectrometry
III.The Mass Spectrum
–Presentation of data
3.All other peak intensities are relative to the base
peak as a percentage
4.If a molecule loses only one electron in the
ionization process, a molecular ionis observed
that gives its molecular weight –this is
designated as M
+
on the spectrumM
+
,m/e 114

25. Mass Spectrometry
III.The Mass Spectrum
–Presentation of data
5.In most cases, when a molecule loses a valence
electron, bonds are broken, or the ion formed
quickly fragment to lower energy ions
6.The masses of charged ions are recorded as
fragment ionsby the spectrometer –neutral
fragments are not recorded !
fragment ions

26. Mass Spectrometry
III.The Mass Spectrum
B.Determination of Molecular Mass
•When a M+ peak is observed it gives the molecular mass –assuming
that every atom is in its most abundant isotopic form
•Remember that carbon is a mixture of 98.9%
12
C (mass 12), 1.1%
13
C
(mass 13) and <0.1%
14
C (mass 14)
•We look at a periodic table and see the atomic weight of carbon as
12.011 –an average molecular weight
•The mass spectrometer, by its very nature would see a peak at mass 12
for atomic carbon and a M + 1 peak at 13 that would be 1.1% as high

27. Mass Spectrometry
III.The Mass Spectrum
B.Determination of Molecular Mass
5.Some molecules are highly fragile and M+ peaks are not observed –one
method used to confirm the presence of a proper M+ peak is to lower the
ionizing voltage –lower energy ions do not fragment as readily
6.Three facts must apply for a molecular ion peak:
1)The peak must correspond to the highest mass ion on the spectrum
excluding the isotopic peaks
2)The ion must have an odd number of electrons –usually a radical
cation
3)The ion must be able to form the other fragments on the spectrum
by loss of logical neutral fragments

28. Mass Spectrometry
III.The Mass Spectrum
B.Determination of Molecular Mass
5.The Nitrogen Ruleis another means of confirming the observance of a
molecular ion peak
6.If a molecule contains an even number of nitrogen atoms (only
“common”organic atom with an odd valence) or no nitrogen atoms the
molecular ion will have an even mass value
7.If a molecule contains an odd number of nitrogen atoms, the molecular
ion will have an odd mass value
8.If the molecule contains chlorine or bromine, each with two common
isotopes, the determination of M+ can be made much easier, or much
more complex as we will see

29. Molecular Formulas –What can be learned from them
Remember and Review!
The Rule of Thirteen–Molecular Formulas from Molecular Mass –
When a molecular mass, M
+
, is known, a base formula can be generated from the
following equation:
M = n + r
13 13
the base formula being: C
nH
n+ r
For this formula, the HDI can be calculated from the following formula:
HDI = ( n –r + 2 )
2

30. Molecular Formulas –What can be learned from them
Remember and Review!
The Rule of Thirteen
The following table gives the carbon-hydrogen equivalents and change in HDI for
elements also commonly found in organic compounds:
Element
added
Subtrac
t:
DHDI
(DU in
text)
Element
added
Subtract: DHDI
(DU in text)
C H
12 7
35
Cl C
2H
11 3
H
12 C -7
79
Br C
6H
7 -3
O CH
4 1 F CH
7 2
N CH
2 1/2 Si C
2H
4 1
S C
2H
8 2 P C
2H
7 2
I C
9H
19 0

31. Mass Spectrometry
III.The Mass Spectrum
C.High Resolution Mass Spectrometry
•If sufficient resolution (R > 5000) exists, mass numbers can be recorded to
precise values (6 to 8 significant figures)
•From tables of combinations of formula masses with the natural isotopic
weights of each element, it is often possible to find anexactmolecular
formula from HRMS
Example: HRMS gives you a molecular ion of 98.0372; from mass 98 data:
C
3H
6N
4 98.0594
C
4H
4NO
2 98.0242
C
4H
6N
2O 98.0480
C
4H
8N
3 98.0719
C
5H
6O
2 98.0368 gives us the exact formula
C
5H
8NO 98.0606
C
5H
10N
2 98.0845
C
7H
14 98.1096

32. Mass Spectrometry
IV.The Mass Spectrum and Structural
Analysis
–Inferences from Isotopic Ratios
• If a M
+
can be observed at sufficient intensity, information leading to a molecular
formula can be attained
• Consider ethane, C
2H
6 –on this mass spectrum a M
+
ion would be observed at 30:
(2 x
12
C) + (6 x
1
H) = 30
– However, 1.08% of carbon is
13
C –there is a 1.08% chance that either
carbon in a bulk sample of ethane is
13
C (2 x 1.08% or 2.16%)
– In the mass spectrum we would expect to see a peak at 31 (one of the
carbons being
13
C) that was 2.16% of the intensity of the M
+
signal -this is
called the M+1peak

33. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
–Inferences from Isotopic Ratios
5.To calculate the expected M+1 peak for a known molecular formula:
%(M+1) = 100 (M+1) = 1.1 x # of carbon atoms
M + 0.016 x # of hydrogen atoms
+ 0.38 x # of nitrogen atoms…etc.
6.Due to the typical low intensity of the M
+
peak, one does not typically “back
calculate”the intensity M+1 peak to attain a formula
7.However if it is observed, it can give a rough estimate of the number of carbon
atoms in the sample:
Example: M
+
peak at 78 has a M+1 at 79 that is 7% as intense:
#C x 1.1 = 7%
#C = 7%/1.1 = ~6

34. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
–Inferences from Isotopic Ratios
5.For very large molecules the M+1, M+2, M+3… bands
become very important
The M+2, 3, … peaks become even more prominent and
molecules that contain nothing but the most common
isotopes become rare!

35. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
–Inferences from Isotopic Ratios
5.For very large molecules the M+1, M+2, M+3… bands
become very important
Remarkably, here is the molecular ion(s) of insulin (257
carbon atoms):
Molecules that
are completely
12
C are now
rare

36. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
–Inferences from Isotopic Ratios
6.For molecules that contain Cl or Br, the isotopic peaks are
diagnostic
–In both cases the M+2 isotope is prevalent:

35
Cl is 75.77% and
37
Cl is 24.23% of naturally occurring
chlorine atoms

79
Br is 50.52% and
81
Br is 49.48% of naturally occurring
bromine atoms
a)If a molecule contains a single chlorine atom, the molecular ion
would appear:
m/e
M
+
M+2

37. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
–Inferences from Isotopic Ratios
6.For molecules that contain Cl or Br, the isotopic peaks are
diagnostic
c)If a molecule contains a single bromine atom, the molecular ion
would appear:
c)The effects of multiple Cl and Br atoms is additive –your text has
a complete table of the combinations possible with 1-3 of either
atom
7.Sulfur will give a M+2 peak of 4% relative intensity and silicon 3%
m/e
M
+
M+2
The M+2 peak
would be about
the size of the M
+
if one Br is present

38. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
B.Inferences from M
+
-(A summary before moving on…)
•If M
+
is visible be sure to test for its validity:
1)The peak must correspond to the highest mass ion on the spectrum
excluding the isotopic peaks
2)The ion must have an odd number of electrons –test with an HDI
calculation
»If the HDI is a whole number the ion is an odd-electron ion
and therefore could be M
+
»If the HDI is not a whole number, it suggests that the ion is an
even-electron ion and cannotbe a molecular ion.
3)The ion must be able to form the other fragments on the spectrum
by loss of logical neutral fragments

39. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
B.Inferences from M
+
-(A summary before moving on…)
2.Using the the M
+
peak, make any inferences about the
approximate formula
–Nitrogen Rule
–Rule of Thirteen
–HDI
3.Using the M+1 peak (if visible) make some inference as to
the number of carbon atoms (for small molecules this
works as H, N and O give very low contributions to M+1)
4.If M+2 becomes apparent, analyze for the presence of one
or more Cl or Br atoms (sulfur and silicon can also give
prominent M+2s)

40. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
C.Fragmentation -General
•The collision of a high energy electron with a molecule not
only causes the loss of a valence electron, it imparts some
of the kinetic energy of collision into the remaining ion
•This energy typically resides in an increased vibrational
energy state for the molecule –this energy may be lost by
the molecule breaking into fragments
•The time between ionization and detection in most mass
spectrometer is 10
-5
sec.
–If a particular ionized molecule can “hold together”for greater
than 10
-5
sec. a M
+
ion is observed
–If a particular ionized molecule fragments in less than this time, the

41. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
C.Fragmentation -General
4.Due to the low concentration of molecules in the ionization
chamber, all fragmentation processes are unimolecular
5.Fragmentation of a molecule that is missing one electron in
most cases results in a covalent bond breaking
homolytically –one fragment is then missing a full pair of
electrons and has a + charge and the other fragment is a
neutral radical
6.Only the + charged ions will be observed; but the loss of
a neutral fragment is inferred by the difference of the
M+ and the m/e of the fragment

42. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
D.Fragmentation –Chemistry of Ions
•One bond s-cleavages:
a.cleavage of C-C
b.cleavage of C-heteroatomCC C
C+ CZ C
Z+

43. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
D.Fragmentation –Chemistry of Ions
•One bond s-cleavages:
c.a-cleavage of C-heteroatomCCZ C CZ+ CCZ CC Z+ CCZ C Z+ C

44. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
D.Fragmentation –Chemistry of Ions
2.Two bond s-cleavages/rearrangements:
a.Elimination of a vicinal H and heteroatom:
b.Retro-Diels-AlderCCZ
Z+H
H
CC +
+
Full mechanism
Abbreviated:

45. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
D.Fragmentation –Chemistry of Ions
2.Two bond s-cleavages/rearrangements:
c.McLafferty Rearrangement
3.Other types of fragmentation are less common, but in specific cases are dominant
processes
These include: fragmentations from rearrangement, migrations, and fragmentation of
fragments
Full mechanism
Abbreviated:H
+
H
H
+
H

46. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
D.Fragmentation –Chemistry of Ions
4.When deducing any fragmentation scheme:
–The even-odd electron ruleapplies: “thermodynamics dictates that
even electron ions cannot cleave to a pair of odd electron
fragments”
–Mass losses of 14 are rare
–The order of carbocation/radical stability is
benzyl/3°> allyl/2°> 1°> methyl > H
* the loss of the longest carbon chain is preferred
–Fragment ion stability is more important than fragment radical
stability
–Fragmentation mechanisms should be in accord with the even-odd

47. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
Aside:Some nomenclature –rather than explicitly writing out
single bond cleavages each time:CH
2
+H
2C
CH
3 57
Fragment
obs. by MS
Neutral fragment
inferred by its loss
–not observed
Is written as:

48. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
•Alkanes
–Very predictable –apply the lessons of the stability of carbocations
(or radicals) to predict or explain the observation of the fragments
–Method of fragmentation is single bond cleavage in most cases
–This is governed by Stevenson’s Rule–the fragment with the
lowest ionization energy will take on the + charge –the other
fragment will still have an unpaired electron
Example: iso-butaneCH
3
+
CH
3
+

49. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
•Alkanes
Fragment Ions :n-alkanes
»For straight chain alkanes, a M
+
is often observed
»Ions observed: clusters of peaks C
nH
2n+1apart from the loss of
–CH
3, -C
2H
5, -C
3H
7, etc.
»Fragments lost: ·CH
3, ·C
2H
5, ·C
3H
7, etc.
»In longer chains –peaks at 43 and 57 are the most common

50. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
•Alkanes
Example MS:n-alkanes –n-heptane43
M
+57

51. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
•Alkanes
Fragment Ions :branched alkanes
»Where the possibility of forming 2°and 3°carbocations is
high, the molecule is susceptible to fragmentation
»Whereas in straight chain alkanes, a 1°carbocation is always
formed, its appearance is of lowered intensity with branched
structures
»M+ peaks become weak to non-existent as the size and
branching of the molecule increase
»Peaks at 43 and 57 are the most common as these are the iso-
propyl and tert-butyl cations

52. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
•Alkanes
Example MS:branched alkanes –2,2-dimethylhexane57
M
+
114

53. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
•Alkanes
Fragment Ions :cycloalkanes
»Molecular ions strong and commonly observed –cleavage of
the ring still gives same mass value
»A two-bond cleavage to form ethene (C
2H
4) is common –loss
of 28
»Side chains are easily fragmented H
2CCH
2
+
HCCR
HH
H
2CCH
2
C
H
2
n

54. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
Alkanes
Example MS:cycloalkanes –cyclohexane
M
+
84+
M -28 = 56

55. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
•Alkanes
Example MS:cycloalkanes –trans-p-menthane97
M
+
140

56. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
2.Alkenes
–The p-bond of an alkene can absorb substantial energy –molecular
ions are commonly observed
–After ionization, double bonds can migrate readily –determination
of isomers is often not possible
–Ions observed: clusters of peaks C
nH
2n-1apart from -C
3H
5, -C
4H
7, -
C
5H
9etc. at 41, 55, 69, etc.
–Terminal alkenes readily form the allyl carbocation, m/z 41R
H
2
C
+R
C
H
CH
2
H
2CC
H
CH
2

57. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
2.Alkenes
Example MS:alkenes –cis-2-pentene
M
+
7055

58. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
2.Alkenes
Example MS:alkenes –1-hexene
M
+
8441 56
Take home assignment:
What is M-42 and m/z 42?

59. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
2.Alkenes
Example MS:alkenes –1-pentene
M
+
70
Take home assignment 2:
What is m/z 42?

60. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
Comparison: Alkanes vs. alkenes
Octane (75 eV)
M
+
114
m/z 85, 71, 57, 43 (base), 29
Octene (75 eV)
M
+
112 (stronger @ 75eV than octane)
m/z 83, 69, 55, 41, 29

61. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
2.Alkenes
Fragment Ions :cycloalkenes
»Molecular ions strong and commonly observed –cleavage of
the ring still gives same mass value
»Retro-Diels-Alder is significant
observed loss of 28
»Side chains are easily fragmented +

62. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
2.Alkenes
Example MS:cycloalkenes –1-methyl-1-cyclohexene
M
+
9681
68

63. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
3.Alkynes–Fragment Ions
–The p-bond of an alkyne can also absorb substantial energy –
molecular ions are commonly observed
–For terminal alkynes, the loss of terminal hydrogen is observed (M-
1) –this may occur at such intensity to be the base peak or
eliminate the presence of M
+
–Terminal alkynes form the propargyl cation, m/z 39 (lower
intensity than the allyl cation)R
H
2
C
+R
CCH
H
2CCCH

64. Mass SpectrometryIV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
3.Alkynes
Example MS: alkynes –1-pentyne
M
+
68H
67 H
39

65. Mass SpectrometryIV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
3.Alkynes
Example MS: alkynes –2-pentyne
M
+
6853

66. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
4.Aromatic Hydrocarbons–Fragment Ions
–Very intense molecular ion peaks and little fragmentation of the
ring system are observed
–Where alkyl groups are attached to the ring, a favorable mode of
cleavage is to lose a H-radical to form the C
7H
7
+
ion (m/z 91)
–This ion is believed to be the tropylium ion; formed from
rearrangement of the benzyl cationCH
2CH
3 75 eV e
-

67. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
4.Aromatic Hydrocarbons–Fragment Ions
d)If a chain from the aromatic ring is sufficiently long, a McLafferty
rearrangement is possible
e)Substitution patterns for aromatic rings are able to be determined
by MS –with the exception of groups that have other ion chemistry

68. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
4.Aromatic Hydrocarbons
Example MS: aromatic hydrocarbons –p-xylene
M
+
106CH
3CH
3
H
3C
m/z 91

69. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
4.Aromatic Hydrocarbons
Example MS: aromatic hydrocarbons –n -butylbenzene
M
+
134H H
+
9291

70. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
5.Alcohols–Fragment Ions
–Additional modes of fragmentation will cause lower M
+
than for
the corresponding alkanes
1°and 2°alcohols have a low M
+
, 3°may be absent
b)The largest alkyl group is usually lost; the mode of cleavage
typically is similar for all alcohols:
primary
secondary
tertiaryOH
H
2C
OH
+
OH
+
OH OH
+
OH
m/z
31
59
45

71. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
5.Alcohols–Fragment Ions
c)Dehydration(M -18) is a common mode of fragmentation –
importance increases with alkyl chain length (>4 carbons)
»1,2-elimination –occurs from hot surface of ionization
chamber
»1,4-elimination –occurs from ionization
»both modes give M -18, with the appearance and possible
subsequent fragmentation of the remaining alkene
d)For longer chain alcohols, a McLafferty type rearrangement can
produce water and ethylene (M -18, M -28)O
H
R
H
O
H
R
H
+

72. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
5.Alcohols–Fragment Ions
e)Loss of H is notfavored for alkanols (M –1)
f)Cyclic alcohols fragment by similar pathways
»a-cleavage
»dehydrationOHH OHH
H
OHH
H
OHH
+ OHH
,
+ H
2O
m/z 57
M -18

73. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
5.Alcohols
Example MS: alcohols –n -pentanol
M
+
88
-H
2O
70OH
31 OH
H
OH
H
+
42

74. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
5.Alcohols
Example MS: alcohols –2-pentanol
M
+
88OH
45

75. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
5.Alcohols
Example MS: alcohols –2-methyl-2-pentanol
M
+
10287
OH OH
59

76. Mass SpectrometryIV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
5.Alcohols
Example MS: alcohols –cyclopentanol
M
+
86OHH
OHH
+
57

77. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
6.Phenols–Fragment Ions
–Do not fully combine observations for aromatic + alcohol; treat as a
unique group
b)For example, loss of H·is observed (M –1) –charge can be
delocalized by ring –most important for rings with EDGs
c)Loss of CO (extrusion) is commonly observed (M –28); Net loss
of the formyl radical (HCO·, M –29) is also observed from this
processO
H
O
H
H
O
C
O
-CO
-H

78. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
5.Example MS: phenols –phenol
M
+
94
-CO 66
-HCO 65

79. Mass SpectrometryIV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
An interesting combination of functionalities: benzyl alcohols
Upon ring expansion to tropylium ions, they become
phenols!
M
+
108
77
M –1,107
“tropyliol”HO H
H
+
“tropyliol”-CO
79+ H
2 OH

80. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
7.Ethers–Fragment Ions
–Slightly more intense M
+
than for the corresponding alcohols or
alkanes
b)The largest alkyl group is usually lost to a-cleavage; the mode of
cleavage typically is similar to alcohols:
c)Cleavage of the C-O bond to give carbocations is observed where
favorable R
H
2
COR R H
2COR+ R
H
COR RCH OR
RR
+

81. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
7.Ethers–Fragment Ions
d)Rearrangement can occur of the following type, if a-carbon is
branched:
e)Aromatic ethers, similar to phenols can generate the C
6H
5O
+
ion by
loss of the alkyl group rather than H; this can expel CO as in the
phenolic degradationRCOC RC
HH
R
CH
2
H
H
O
H
R
+ O
R
O
R+ CO + C
5H
5
+

82. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
7.Example MS: ethers –butyl methyl ether
M
+
88O
45

83. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
7.Example MS: ethers –anisole
M
+
108O
93
M-28 (-CH
3, -CO)
65O
77
Take home –what is m/z 78?

84. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
8.Aldehydes -Fragment Ions
–Weak M
+
for aliphatic, strong M
+
for aromatic aldehydes
b)a-cleavage is characteristic and often diagnostic for aldehydes –
can occur on either side of the carbonyl
c)b-cleavage is an additional mode of fragmentationRH
O
RCO + H
RH
O
HCO+R
M-1 peak
m/z 29H
O
+R
R
H
O
m/z R
+
M -41
can be R-subs.

85. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
8.Aldehydes -Fragment Ions
d)McLafferty rearrangement observed if g-Hs present
e)Aromatic aldehydes –a-cleavages are more favorable, both to lose
H·(M -1) and HCO· (M –29)
m/z 44
m/z R
+
Remember:
aromatic ring can
be subs.+
O
H
HR
O
HR CO + H
H
O
O
H
+
O
H

86. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
8.Example MS: aldehydes (aliphatic) –pentanal
M
+
86
M-1
85H
C
O
29 +
O
H
H
O
H
m/z44

87. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
8.Example MS: aldehydes (aromatic) –m-tolualdehyde
M
+
120
M-1
119O
H
91

88. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
9.Ketones -Fragment Ions
–Strong M
+
for aliphatic and aromatic ketones
b)a-cleavage can occur on either side of the carbonyl –the larger
alkyl group is lost more often
c)b-cleavage is not as important of a fragmentation mode for ketones
compared to aldehydes –but sometimes observedRR
1
O
RCO + R
1
R
1 is larger than R
M –15, 29, 43…
m/z 43, 58, 72, etc.R
1
O
+R
R
R
1
O

89. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
9.Ketones -Fragment Ions
d)McLafferty rearrangement observed if g-H’s present –if both alkyl
chains are sufficiently long –both can be observed
e)Aromatic ketones –a-cleavages are favorable primarily to lose R·
(M –15, 29…) to form the C
6H
5CO
+
ion, which can lose CO
Remember:
aromatic ring can
be subs.+
O
R
1
HR
O
HR
R
1 CO + R
O
R
+ CO
m/z 105
m/z 77

90. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
9.Ketones -Fragment Ions
f)cyclic ketones degrade in a similar fashion to cycloalkanes and
cycloalkanols:O
H
O
H
O
+
O
O O O
+
- CO
m/z 55
m/z 42
m/z 70

91. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
9.Example MS: ketones (aliphatic) –2-pentanone
M
+
86
M-15O
43 O
H
O
H
+
58

92. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
9.Example MS: ketones (aromatic) –propiophenone
M
+
134CO
O
m/z 105
m/z 77

93. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
10.Esters -Fragment Ions
–M
+
weak in most cases, aromatic esters give a stronger peak
b)Most important a-cleavage reactions involve loss of the alkoxy-
radical to leave the acylium ion
c)The other a-cleavage (most common with methyl esters, m/z 59)
involves the loss of the alkyl groupR
R
1
O
RCO + OR
1
O R
R
1
O
R C
O
+
O
OR
1

94. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
10.Esters -Fragment Ions
d)McLafferty occurs with sufficiently long esters
e)Ethyl and longer (alkoxy chain) esters can undergo the McLafferty
rearrangementR
1
O
+
O
H
R
1
O
O
H R
O
+
O R
O
O
HH

95. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
10.Esters -Fragment Ions
f)The most common fragmentation route is to lose the alkyl group by
a-cleavage, to form the C
6H
5CO
+
ion (m/z 105)R
O
RO
C
O
+
Can lose CO to
give m/z 77

96. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
10.Esters -Fragment Ions
g)One interesting fragmentation is shared by both benzyloxy esters
and aromatic esters that have an ortho-alkyl groupO
O
H
OH
fragmentation
+
CH
2
C
O
ketene O
R
O
C
H
2
H
C
HO
R
O
CH
2
+
benzyloxy ester
ortho-alkylbenzoate ester

97. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
10.Example MS: esters (aliphatic) –ethyl butyrate
M
+
116
both McLafferty
(take home exercise)
m/z 88O
O
71 O
O
43 O
O
29

98. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
10.Example MS: esters (aliphatic) –ethyl butyrate
M
+
116
both McLafferty
(take home exercise)
m/z 88O
O
71 O
O
43 O
O
29

99. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
10.Example MS: esters (benzoic) –methylortho-toluate
M
+
150C
O
CH
2 O
O
119 O
O
91 m/z 118

100. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
11.Carboxylic Acids -Fragment Ions
–As with esters, M
+
weak in most cases, aromatic acids give a
stronger peak
b)Most important a-cleavage reactions involve loss of the alkoxy-
radical to leave the acylium ion
c)The other a-cleavage (less common) involves the loss of the alkyl
radical. Although less common, the m/z 45 peak is somewhat
diagnostic for acids.R
H
O
RCO + OH
O R
H
O
R C
O
+
O
OH

101. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
11.Carboxylic Acids -Fragment Ions
d)McLafferty occurs with sufficiently long acids
e)aromatic acids degrade by a process similar to esters, loss of the
HO·gives the acylium ion which can lose CO:H
O
+
O
H
H
O
O
H
m/z 60H
O
HO
C
O
+
+ further loss of
CO to m/z 77

102. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
11.Carboxylic Acids -Fragment Ions
f)As with esters, those benzoic acids with an ortho-alkyl group will
lose water to give a ketene radical cationO
H
O
C
H
2
H
C
HO
H
O
CH
2
+
ortho-alkylbenzoic acid

103. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
11.Example MS: carboxylic acids (aliphatic) –pentanoic
acid
M
+
102O
OH
H
OH
OH
m/z 60

104. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
11.Example MS: carboxylic acids (aromatic) –p-toluic acid
M
+
136OH
O
119 OH
O
91

105. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
Summary–Carbonyl Compounds
For carbonyl compounds –there are 4 common modes of
fragmentation:
A
1& A
2 --two a-cleavages
B --b-cleavage
C –McLafferty RearrangementO
G
R
O
G
R+ O
G
RH
O
G
R H
+ R
O
G
OCG
2+ R
OCR + G
R
O
G

106. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
Summary–Carbonyl Compounds
In tabular format:
m/z of ion observed
FragmentationPath
Aldehydes
G = H
Ketones
G = R
Esters
G = OR’
Acids
G = OH
Amides
G = NH
2
A
1
a-cleavage
-R 29 43
b
59
b
45 44
d
A
2
a-cleavage
-G 43
b
43
b
43
b
43
b
43
b
B
b-cleavage
-G 43
a
57
b
73
b
59
a
58
a
C
McLafferty
44
a
58
b,c
74
b,c
60
a
59
a
b
= base, add other mass attached to this chain
a
= base,if a-carbon branched, add appropriate mass
c
= sufficiently long structures can undergo on either side of C=O
d
= if N-substituted, add appropriate mass

107. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
12.Amines -Fragment Ions
–Follow nitrogen rule –odd M
+
, odd # of nitrogens; nonetheless, M
+
weak in aliphatic amines
b)a-cleavage reactions are the most important fragmentations for
amines; for 1°n-aliphatic amines m/z 30 is diagnostic
c)McLafferty not often observed with amines, even with sufficiently
long alkyl chains
d)Loss of ammonia (M –17) is not typically observedR
C
N
R
C
N
+

108. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
12.Amines -Fragment Ions
e)Mass spectra of cyclic amines is complex and varies with ring size
f)Aromatic amines have intense M
+
g)Loss of a hydrogen atom, followed by the expulsion of HCN is
typical for anilines
h)Pyridines have similar stability (strong M
+
, simple MS) to
aromatics, expulsion of HCN is similar to anilinesNH
2
NH
+H
HH
+HCN
H
+H

109. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
12.Example MS: amines, 1°–pentylamine
M
+
87NH
2
30

110. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
12.Example MS: amines, 2°–dipropylamine
M
+
101N
H
72 N
H
H

111. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
12.Example MS: amines, 3°–tripropylamine
M
+
143N
114

112. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
13.Amides -Fragment Ions
–Follow nitrogen rule –odd M
+
, odd # of nitrogens; observable M
+
b)a-cleavage reactions afford a specific fragment of m/z 44 for
primary amides
c)McLafferty observed where g-hydrogens are presentR
C
NH
2
O
R +OCNH
2
m/z 44 O
NH
2
H
O
NH
2
H
+

113. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
13.Example MS: amides –butyramide
M
+
87C
NH
2
O
44 O
NH
2
H
59

114. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
13.Example MS: amides (aromatic) –benzamide
M
+
121C
NH
2O
77 C
NH
2O
105

115. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
14.Nitriles -Fragment Ions
–Follow nitrogen rule –odd M
+
, odd # of nitrogens; weak M
+
b)Principle degradation is the loss of an H-atom (M –1) from a-
carbon:
c)Loss of HCN observed (M –27)
d)McLafferty observed where g-hydrogens are present
Aromatic nitriles give a strong Mas the strongest peak, loss of H +R
H
2
CCN RC
H
CN C
N
H
H
2C
C
N
H
+
m/z 41

116. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
14.Example MS: nitriles –propionitrile
M
+
55
M-154
-HCN

117. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
14.Example MS: nitriles –valeronitrile (pentanenitrile)
M
+
83H
2C
C
N
H
m/z 41 C
N
43 C
N
54

118. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
15.Nitro -Fragment Ions
–Follow nitrogen rule –odd M
+
, odd # of nitrogens; M
+
almost
never observed, unless aromatic
b)Principle degradation is loss of NO
+
(m/z 30) and NO
2
+
(m/z 46)RN
O
O
R N
O
O
+
m/z 46
+
m/z 30
RN
O
O
RN
O
O
RONO RO NO

119. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
15.Nitro -Fragment Ions
c)Aromatic nitro groups show these peaks as well as the fragments of
the loss of all or parts of the nitro group NO
2 O
+NO +CO
NO
2
+NO
2 +HCCHC
4H
3
m/z 93 m/z 65
m/z 77 m/z 51

120. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
15.Example MS: nitro –1-nitropropane
M
+
89
NO
2
+
46
NO
+
30NO
2
43

121. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
15.Example MS: nitro (aromatic) –p-nitrotoluene
M
+
137O
m/z10791
NO
2 C
5H
5
+

122. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Halogens -Fragment Ions
–Halogenated compounds often give good M
+
–Fluoro-and iodo-compounds do not have appreciable contribution
from isotopes
–Chloro-and bromo-compounds are unique in that they will show
strong M+2 peaks for the contribution of higher isotopes
–For chlorinated compounds, the ratio of M
+
to M+2 is about 3:1
–For brominated compounds, the ratio of M
+
to M+2 is 1:1
–An appreciable M+4, 6, … peak is indicative of a combination of
these two halogens –use appropriate guide to discern number of

123. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Halogens -Fragment Ions
g)Principle fragmentation mode is to lose halogen atom, leaving a
carbocation –the intensity of the peak will increase with cation
stability
h)Leaving group ability contributes to the loss of halogen most
strongly for -I and -Br less so for -Cl, and least for –F
i)Loss of HX is the second most common mode of fragmentation –
here the conjugate basicity of the halogen contributes (HF > HCl >
HBr > HI)R +XRX R +
CXCR
H
HH
H
C
H
CH
2 HX

124. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Halogens -Fragment Ions
j)Less often, a-cleavage will occur:
k)For longer chain halides, the expulsion of a >dcarbon chain as the
radical is observed
l)Aromatic halides give stronger M
+
, and typically lose the halogen
atom to form C
6H
5
+R +CX
H
H
H
2CXR R+
R
X X

125. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Example MS: chlorine –1-chloropropane
M
+
78m/z 49, 5143
Cl H
2CCl
M+2

126. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Example MS: chlorine –p-chlorotoluene
M
+
126
M+291
Cl

127. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Example MS: bromine –1-bromobutane
M
+
136
M+257
Br H
2CBr

128. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Example MS: bromine –p-bromotoluene
M
+
170
M+291
Br

129. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Example MS: multiplebromines –3,4-dibromotoluene
M
+
248
M+4
M+2169,
171
Br
Br 90
Br
Br

130. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
E.Fragmentation Patterns of Groups
16.Example MS: iodine –iodobenzene
M
+
20477
I

131. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
F.Approach to analyzing a mass spectrum
•As with IR, get a general feel for the spectrum before you
analyze anything –is it simple, complex, groups of peaks,
etc.
•Squeeze everything you can out of the M
+
peak that you can
(once you have confirmed it is the M
+
)
–Strong or Weak?
–Isotopes? M+1? M+2, 4, …
–Apply the Nitrogen rule
–Apply the Rule of Thirteen to generate possible formulas (you can
quickly dispose of possibilities based on the absence of isotopic
peaks or the inference of the nitrogen rule)
–Use the HDI from the Rule of Thirteen to further reduce the
possibilities

132. Mass Spectrometry
IV.The Mass Spectrum and Structural Analysis
F.Approach to analyzing a mass spectrum
3.Squeeze everything you can out of the base peak
–What ions could give this peak? (m/z 43 doesn’t help much)
–What was lost from M
+
to give this peak?
–When considering the base peak initially, only think of the most
common cleavages for each group
4.Look for the loss of small neutral molecules from M
+
–H
2C=CH
2, HCCH, H
2O, HOR, HCN, HX
5.Now consider the possible diagnostic peaks on the spectrum
(e.g.: 29, 30, 31, 45, 59, 77, 91, 105 etc.)
6.Lastly, once you have a hypothetical molecule that explains
the data, see if you can verify it by use of other less intense

133. Mass Spectrometry

135. Ionization to Radical Cation
Molecular Ion (m
+
)

136. Glossary
•Molecular ion-The ion obtained by the loss of
one electron from the molecule(m
+
)
•Base peak-The most intense peak in the MS,
assigned 100% intensity
•Radical cation-positively charged species with
an odd number of electrons
•Fragment ions-Lighter cations (and radical
cations) formed by the decomposition of the
molecular ion.These often correspond to stable
carbcations.
•m/z-mass to charge ratio

137. The Mass Spectrum
Masses are graphed or tabulated according to their
relative abundance.
=>

138. Mass Spectrum
with Sulfur
=>

139. Mass Spectrum
with Chlorine
=>

140. Mass Spectrum
with Bromine
=>

141. Mass Spectra
of Alkanes
More stable carbocations will be more
abundant.
=>

142. Mass Spectra
of Alkenes
Resonance-stabilized cations favored.
=>

143. Methanol

144. Octane, m
+
= 114CH
3CH
2CH
2CH
2CH
2CH
2CH
2CH
3
m+ = 114
-15
-29
-43
-57
-71 (base)
m-29
m-43
m-57
m-71
Base peak
m
+

145. Isooctane, no molecular ionCH
3CCH
2CHCH
3
CH
3
CH
3
CH
3
m+ = 114
loss of
(isobutyl)
.
CH
3C
CH3
CH
3
+
m/z = 57

146. Decane

147. 2-Methylpentane

148. Effect of Branching in
Hydrocarbons

149. Isotopes
•Mass spectrometers are capable of separating and
detecting individual ions even those that only differ by
a single atomic mass unit.
•As aresult molecules containing different isotopes can
be distinguished.
•This is most apparent when atoms such as bromine or
chlorine are present (
79
Br :
81
Br, intensity 1:1 and
35
Cl :
37
Cl, intensity 3:1) where peaks at "M" and "M+2" are
obtained.
•The intensity ratios in the isotope patterns are due to
the natural abundance of the isotopes.
•"M+1" peaks are seen due the the presence of
13
C in
the sample.

150. Bromomethane

151. 1-Bromopropane

152. 2-Chloropropane

153. (3-Chloropropyl)benzeneCH2CH2CH2Cl
base peak m/e 91 (m-63)

154. ChloroacetoneCH3CCH2Cl
O
m+ = 92
m+2 = 94
- Cl
- CH
3
- CH2Cl
.
.
.
O
CH
3CCH
2
O
CCH
2Cl
O
CH3C
+
+
+
m/z = 57
m/z = 77,79
m/z = 43 (base)(3:1 ratio)

155. 3-PentanoneC
O
CH
2CH
3
CH
3CH
2
+
m/z = 86
.
CH
2CH
3
.
CH
3CH
2C=O
+
loss of
m/z = 57
m
+
m-29
base

156. 2-PentanoneC
O
CH2CH2CH3
CH
3
+
m/z = 86
.
CH
2CH
2CH
3
.
CH
3C=O
+
loss of
m/z = 43
m-43
base
m-15
m
+

157. (E) 2-Hexene

158. 2-methyl-2-pentene
and
2-hexene

159. 1-ButanolCH
3CH
2CH
2CH
2OH
+
.
m+ = 74
- H2O
CH
3CH
2CHCH
2
+
.
m/z = 56 CH
2OH CH
2=OH
++
m/z = 31
m-18

160. 1
o
Alcohols: Loss of H
2O

161. TolueneCH3
+
.
m/z = 92
loss of H
.
CH2
m/z = 91
+
+
tropylium ion
m
+
m-1

162. PropylbenzeneCH
2CH
2CH
3
+
.
loss of
CH
2CH
3
.
CH
2
+
+
m/z = 91m/z = 120
m
+
m-29

163. IsopropylbenzeneC
H3C CH
3
H
+
.
loss of
CH
3
.
CHCH3
+ CH
3
+
m/z = 105m/z = 120
m
+
m-15

164. n-Butylbenzenem/e 91 m/e 92 CH
2CH
2CH
2CH
3

165. 3-Methyl-1-penten-3-olOH
m/z = 71
m
+

166. 4-Methyl-1-penten-3-olOH
m
+
m/z = 57

167. McLafferty Rearrangement
link to SDBSH
O
H
m+ = 86
+
.
O
H
H
a
b
g
.
+
O
H
H
.
+
m/z = 44
+ H transfer from gcarbon
results in loss of a neutral alkene

168. McLafferty Rearrangements in
Alkyl Benzenesg
b
a
CH
2
CH
2
CHCH
3
H
m+ 134
loss of CH
3CH=CH
2
.
+
CH
2
H
H
+
.
m/e 92
CH2
+
m/e 91
- propyl
.

169. Benzamide

170. 2-OctanoneO
H
m+ = 128
O
+
.
+
loss of
C6H13
.
m/z = 43
loss of pentene
O
H
.
+
m/z = 58
via McLafferty
a
b
g

171. 2-OctanoneO
H
m+ = 128
O
+
.
+
loss of
C6H13
.
m/z = 43
loss of pentene
O
H
.
+
m/z = 58
via McLafferty
a
b
g

172. Decanoic Acid

173. p-Chloroacetophenone

174. Methyl Octanoate

175. 2,4-Dimethoxyacetophenone

176. Cyclohexanone

177. High Resolution Mass Spectrometry
Determination of Molecular FormulaCO
N2
C
2H
4
CH2N
all show m+ at 28 CO 27.9949
N2 28.0062
C
2H
4 28.0312
CH
2N 28.0187
exact mass

178. Isotope Ratios Can Help to
Determine Molecular Formula
Relative intensities (%)
MF MW M M+1 M+2
CO 28.0 1001.120.2
N
2 28.0 1000.76----
C
2H
428.0 1002.230.01

179. Comparisons of Molecular Weights
and Precise Masses
MF MW exact mass
C
3H
8O 60.1 60.05754
C
2H
8N
2 60.1 60.06884
C
2H
4O
2 60.1 60.02112
CH
4N
2O 60.1 60.03242

181. Determine the Formula
fragment finderMolecular mass m+1 m+2
110 111 112
rel. intensity (%) 100 6.96 0.60
exact mass = 110.0376

182. Determine the FormulaMolecular mass m+1 m+2
118 119 120
rel. intensity (%) 100 7.45 4.55

184. Subtract Sulfur’s contribution
fragment finderMolecular mass m+1 m+2
118 119 120
rel. intensity (%) 100 7.45 4.55
subtract sulfur (32)
86 87 88
100 6.67 0.15

185. Determine the Molecular
FormulaMolecular mass m+1 m+2
154 155 156
rel. intensity (%) 100 15.41 3.77

186. Compound gives four signals in
the C-13 NMR spectrumMolecular mass m+1 m+2 m+4
190 191 192 194
rel. intensity (%) 100 6.48 130.77 31.81