`
`
`
`1
`
`CON1054
`
`

`

`Editor: Paul F. Corey
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`Production Manager: Pamela Kennedy Oborski
`Text and Cover Designer: Sheree L. Goodman
`Cover photograph: © Richard Megna/Fundamental Photographs
`Photo Researcher: Chris Migdol
`
`I l
`i
`-! !
`
`This book was set in Times Roman and Goudy Old Style by York
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`The cover was printed by Von Hoffmann Press.
`
`Copyright © 1992 by Macmillan Publishing Company, a division of Macmillan, Inc.
`
`Printed in the United States of America
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`by any means, electronic or mechanical, including photocopying, recording, or any in(cid:173)
`formation storage and retrieval system, without permission in writing from the Publisher.
`
`Earlier editions, copyright © 1985, 1976, and 1981 by Macmillan Publishing Company.
`Selected illustrations have been reprinted from Orbital and Elect:ran Density Diagrams: An
`Application of Computer Graphics, by Andrew Streitwieser, Jr., and Peter H. Owens,
`copyright© 1973 by Macmillan Publishing Company.
`
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`Library of Congress Cataloging-in-Publication Data
`
`Streitwieser, Andrew (date)
`Introduction to organic chemistry.-4th ed. I Andrew
`Streitwieser, Clayton H. Heathcock, Edward M. Kosower.
`em.
`p.
`Includes index.
`ISBN 0-02-418170-6
`1. Chemistry, Organic.
`Edward M.
`III. Title.
`QD251.2.S76 1992
`547-dc20
`
`I. Heathcock, Clayton H.
`
`II. Kosower,
`
`91-33039
`CIP
`
`Printing: 1 2 3 4 5 6 7 8
`
`Year: 2 3 4 5 6 7 8 9 0 1
`
`2
`
`

`

`CHAPTER 34
`
`MASS SPECTROMETRY
`
`34.1 Introduction
`
`When a beam of electrons of energy greater than the ionization energy [about 8-13
`
`electron volts (eV) (185- 300 kcal mole- 1) for most compounds] is passed through a
`sample of an organic compound in the vapor state, ionization of some molecules occurs.
`In one form of ionization, one of the valence electrons of the molecule is lost, leaving
`behind a radical cation.
`
`+
`e- + CH4 ~ 2 e- + [CH4] ·
`In CH4 eight valence electrons bond the four hydrogens to carbon. The symbol [CH4ft
`represents a structure in which seven valence electrons bond the four hydrogens to carbon.
`The + sign shows that the species has a net positive charge. The · signifies that the
`species has an odd number of electrons. If a mixture of compounds is bombarded with
`electrons, a mixture of radical cations differing in mass will obviously be produced.
`
`CH4, C2H6 ~ [C~(, [C2H6]~
`16
`30
`m/z:
`
`where mlz = mass-to-charge ratio (m is the mass of the molecular radical cation and z is
`the number of charges).
`In practice, even when a pure substance is bombarded with electrons, a mixture of
`cations is produced. As will be discussed later, some of the ions initially formed break up
`or fragment into smaller ions. For example, methane can give cations with masses of 16,
`15, 14, 13, and 12.
`
`mlz:
`
`16
`
`CH3+,
`15
`
`+
`[CH2] · , CH+,
`14
`13
`
`[C(
`12
`
`1179
`
`3
`
`

`

`1180
`
`CHAPTER 34
`Mass Spectrometry
`
`Lewis structures for these cations are
`
`H
`H:c+
`H
`
`H:c+
`
`_Note that the cations having an even number of hydrogens are radical cations, whereas the
`cations having an odd number of hydrogens are normal carbocations.
`
`A mass spectrometer is an instrument that is designed to ionize molecules, often i
`the gas phase, separate the ions produced on the basis of their mass-to-charge ratio, an~
`record the relative amounts of different ions produced. A mass spectrum is a plot of the
`data obtained from the mass spectrometer. It is customary for the mass-to-charge ratio
`(mlz) to be plotted as the abscissa and the number of ions or relative intensity (height of
`each peak) to appear as the ordinate. Mass spectrometry differs from spectroscopy in that
`no absorption of light is involved. Nevertheless, it has been called a "spectroscopy"
`. because the mass "spectrum" resembles other kinds of spectra.
`
`f
`
`magnetic field (B)
`
`ion source , .
`---~,\------- ------~~-------
`
`/
`sample inlet (a)
`
`source slit (d)
`
`g
`
`1
`
`(a) schematic diagram
`
`\JUU
`~
`
`b
`
`c d
`
`~mplov•~,J :J ~ lo"' ,
`1-!T
`J--= ~
`ionization chamber--r- L I
`j-v..J
`
`accelerating voltage
`(b) detail of ion source
`
`FIGURE 34.1 90° sector magnetic deflection mass spectrometer.
`
`4
`
`

`

`1181
`
`SEC. 34.2
`Instrumentation
`
`34.2 Instrumentation
`One of !}le most common types of mass spectrometer currently in use is the magnetic
`sector mass spectrometer. A sketch of a 90° magnetic sector instrument is shown in
`Figure 34.1. The sample vapor is introduced at the sample inlet a, usually at low pressure
`(10-5 to 10-6 torr). A low pressure is used to minimize the number of collisions between
`ions and nonionized molecules. Such collisions lead to reactions that produce new ions
`containing parts of both collision partners. Such ions are often interesting in their own
`right but lead to difficulties in interpretation of the data. After the sample vapor enters .the
`ion source (see enlarged insert, Figure 34.1 b), it passes through the electron beam (b)
`where ionization occurs. The resulting ions pass out of the ionization chamber and be(cid:173)
`tween two charged plates (c), which serve to focus the ion beam. There is a difference in
`potential of several thousand volts between the ionization chamber and the source slit (d).
`In thi_s region, the ions are accelerated and pass through slit (d); after traveling a short
`distance they pass into the magnetic field.
`The radius of the path followed by an ion of mass m in a magnetic field depends on its
`charge ze (e is the electronic charge) and the accelerating potential (V). The energy
`acquired by an ion accelerated across a potential drop is equal to its kinetic 'energy.
`zeV = !mv2
`
`(34-1)
`
`In a uniform perpendicular magnetic field of strength B the ion experiences a centripetal
`force Bzev, where vis the velocity of the ion. Because the ion path is circular, the force on
`the ion is equal to mv2/r, where r is the radius of the path followed by the ion.
`
`mv2
`Bzev =-(cid:173)
`r
`mv
`r=--
`zeB
`
`(34-2)
`
`(34-3)
`
`Most of the ions are singly charged (z = 1). As a collection of ions of different masses
`enters the magnetic field region (f), each ion follows a circular path with a radius given by
`the foregoing equation. Ions of larger mlz follow a path of greater radius, and ions of
`lesser m/z follow a path of smaller radius. In the example diagrammed in Figure 34.2 the
`ions of mlz = y are passing through the collector slit (g) and impinging upon the ion
`collector (i).
`
`"'
`--; '\~------
`
`/
`
`source slit
`collector slit (g)
`FIGURE 34.2 Ions with mlz = y are focused on the collector (i) through slit (g) .
`
`5
`
`

`

`1182
`
`CHAPTER 34
`Mass Spectrometry
`
`Elimination of the velocity term from equations (34-1) and (34-3) gives
`m B2re
`z
`2V
`
`(34-4)
`
`This relationship shows that for an ion of give!J. mass-to-charge ratio (m!z), the radius f
`deflection r can be increased by decreasing B, the magnetic field strength. (Less defle~­
`tion with lower magnetic field = larger radius of deflection.) For example, in the case
`diagrammed in Figure 34.2 a slight decrease in B will cause the radius of deflection of aU
`of the ions to increase somewhat. In Figure 34.3 ions of m!z = y no longer pass through
`the slit and into the collector, but ions of m!z = x do.
`N.ote that the same. eff~ct might have been obtained ~y in~reasing V ~lightly or by
`movmg the collector sht slightly to the left. In actual practice this last techmque is incon(cid:173)
`venient, and scanning V has other disadvantages. Scanning of the spectrum is usually
`achieved by magnetic scanning; that is, the accelerating voltage Vis kept constant while
`the magnetic field strength B is increased. As B is increased, ions of progressively higher
`mlz attain the necessary radius of deflection to pass through the collector slit (g) and into
`the ion collector (i).
`As the ions enter the collector, they impinge upon an electron multiplier detector where
`a minute current is produced and amplified. The magnitude of this current is proportional
`to the intensity of the ion beam. The current produced is fed to a computer, which
`processes the data. The computer may have a stored library of thousands of spectra with
`which the sample spectrum can be compared. The current produced for various values of
`
`"' 7 "~----------;/ ~
`
`source slit
`collector slit (g)
`FIGURE 34.3 At lower B, ions with m!z = x are now focused on the collector (i).
`
`o~~~~~--~w.---T~-r----
`20
`30
`40
`50
`60
`70
`80
`m!z
`
`FIGURE 34.4 Mass spectrum of 2-butanone.
`
`6
`
`

`

`mlz is printed in a tabular manner and usually plotted as a bar graph. The most intense
`peak (the "base peak") is assigned the arbitrary intensity value of 100, and all other peaks
`are given their proportionate value. A mass spectrum recorded in this manner is shown in
`Figure 34.4.
`
`1183
`
`SEC. 34.3
`The Molecular Ion:
`Molecular Formula
`
`34.3 The Molecular Ion: Molecular Formula
`The molecular weight of a compound is one datum that can usually be obtained by visual
`inspection of a mass spectrum. Although the radical cations produced by the initial elec(cid:173)
`tron ionization usually undergo extensive fragmentation to give cations of smaller mlz
`(next section), the particle of highest mlz generally (but not always) corresponds to the
`ionized molecule, and mlz = M for this particle (called the molecular ion and abbreviated
`M+) gives the molecular weight of the compound.
`If the spectrum is measured with a "high-resolution" spectrometer, it is possible to
`determine a unique molecular formula for any peak in a mass spectrum, including the
`molecular ion. This is possible because atomic masses are not integers. For example,
`consider the molecules CO, N2 , and C2H4 , all of which have a nominal mass of 28. The
`actual masses of the four atomic particles are H = 1.007825, C = 12.000000 (by defini(cid:173)
`tion), N = 14.003074, 0 = 15.994915. Therefore, the actual masses of CO, N2 and
`CzH4 are as follows.
`12C 12.oooo
`160 15.9949
`27.9949
`
`14N2 28.0061
`
`12Cz 24. oooo
`1H4
`4.0314
`
`28.0314
`
`Since a high-resolution spectrometer can readily measure mass with an accuracy of better
`than 1 part in 100,000, and can separate masses that differ by 1 part in 10,000, the above
`three masses are readily distinguishable, as shown in Figure 34.5.
`
`FIGURE 34.5 High-resolution mass spectrum of a mixture of ethylene, nitrogen, and carbon
`monoxide.
`
`7
`
`

`

`1184
`
`CHAPTER 34
`Mass Spectrometry
`
`Because the mass spectrometer measures the exact ml z for each ion and because most
`of the elements commonly found in organic compounds have more than one naturally
`occurring isotope, a given peak will usually be accompanied by several isotope peaks.
`Table 34.1 shows the common isotopes of some of the elements.
`
`TABLE 34.1 Natural Abundance of Common Isotopes
`
`Element
`
`hydrogen
`
`carbon
`
`nitrogen
`
`oxygen
`
`sulfur
`
`fluorine
`
`chlorine
`
`bromine
`
`iodine
`
`99.985 1H
`98.893 12C
`99.634 14N
`99.759 160
`32S
`95.0
`
`19p
`
`100
`75.77 35Cl
`50.69 79Br
`1211
`100
`
`Abundance, %
`
`O.Ql5 2H
`1.107 13C
`0.366 15N
`0.037 170
`0.76 33S
`
`24.23 37Cl
`49.31 81Br
`
`0.204 180
`4.22 34S
`
`0.014 3ss
`
`The 13C abundance, 1.107%, is the content of oceanic carbonate. Organic compounds in the biosphere run around
`1.08% because of isotope effects.
`·
`
`The 13C abundance, 1.107%, is the content of oceanic carbonate. Organic compounds
`in the biosphere have 13C contents of around 1.08% because of isotope effects.
`Consider the molecular ion derived from methane. Most of the methane molecules are
`12C1H4 and have the nominal mass 16. However, a few molecules are either 13C1H4 or
`1H3 and have the nominal mass 17. An even smaller number of molecules have
`12C2H1
`both a 13C and an 2H or have two 2H isotopes and therefore have the nominal mass 18. An
`exact expression for the ratio of isotopic massed (M + 1)/M can be derived from probabil(cid:173)
`ity mathematics but is rather complex. The theoretical intensities of the various isotope
`peaks can be looked up in special tables compiled for this purpose. However, the contri(cid:173)
`butions of 2H and 170 to (M + 1)/M are relatively small and the ratio is given to a
`satisfactory approximation for most compounds having few N and S atoms by equation
`(34-5).
`
`M:; 1 = ~:~!!~; c + 0.00015h + 0.00367n + 0.00037o + 0.0080s
`
`(34-5)
`
`where M =intensity of the molecular ion (ions containing no heavy isotopes), M + 1 =
`intensity of the molecular ion+ 1 peak (ions containing one 13C, 2H, 15N, 170, or 33S)
`and c, h, n, o, s = the number of carbons, hydrogens , nitrogens, oxygens, sulfurs.
`Using this relationship, we can readily estimate the intensity of theM+ 1 peak in the
`mass spectrum of methane.
`
`M:; 1
`
`= 0.01119(1) + 0.00015(4) = 0.01179
`
`Thus the peak at m!z 17 in the mass spectrum of methane should be approximately 1.18%
`as intense as the peak at mlz 16.
`
`8
`
`

`

`.1185
`
`SEC. 34.3
`The Molecular Ion:
`Molecular Formula
`
`Note that the principal contributor to theM + 1 peak is 13C. This is partly because of the
`relatively large relative abundance of 13C (see Table 34.1) and partly because most or(cid:173)
`ganic compounds contain many more carbon atoms than they do oxygens or nitrogens. In
`fact, a useful rule of thumb is that the M + 1 peak will be 1.1% for each carbon in the
`molecule.
`A similar relationship may be derived for calculation of the intensity of the M + 2
`peak. However, in order to obtain an exact figure, a lengthy computation is required. For
`most compounds theM+ 2 peak is small. However, for compounds containing chlorine
`or bromine, the M + 2 isotopic peak is substantial. The characteristic doublets observed
`in the mass spectra of compounds containing chlorine and bromine are an excellent way of
`diagnosing for the presence of these elements, as shown in Figures 34.6 and 34.7.
`One use to which isotope peaks may be put is in approximating the molecular formula
`of the parent ion in the mass spectrum of an unknown compound. However, one must
`exercise caution when applying the foregoing computations. First, the M + 1 peak is
`generally much less intense than the parent ion. Unless the parent ion is a fairly strong
`one, its isotope peak may be too weak to measure accurately. Second, intermolecular
`proton transfer reaction can give M + 1 peaks that are not due to isotopes. Third, the
`presence of a small amount of impurity with a strong peak at M + 1 of the sample will
`interfere with accurate measurement.
`
`100
`
`80
`
`....
`> 60
`·u;
`c:
`....
`c: 40
`
`Q)
`
`20
`
`rr= 78 (C,H,"CII
`
`80 (C3H737CI)
`
`20
`
`30
`
`40
`
`50
`
`60
`
`70
`
`80
`
`90
`
`m/z
`
`FIGURE 34.6 Mass spectrum of 2-chloropropane.
`
`10
`
`20
`
`30
`
`40
`
`50
`
`60
`
`70
`m/z
`
`80
`
`90 100 110 120 130
`
`FIGURE 34.7 Mass spectrum of 1-bromopropane.
`
`9
`
`

`

`1186
`
`CHAPTER 34
`Mass Spectrometry
`
`34.4 Fragmentation
`A. Simple Bond Cleavage
`When an electron interacts with a molecule in the ionization chamber of the mass spec(cid:173)
`trometer, ionization will occur if the impinging electron transfers to the molecule an
`amount of energy equal to or greater than its ionization potential. The ionization potentials
`for several organic molecules are given in Table 34.2. When the colliding electron trans(cid:173)
`fers m~re energy than is re.quired ~or ionization: a part of .t~e excess energy will nonnally
`be earned away by the rad1cal cat10n produced m the colhs10n. If the molecular ion gains
`enough surplus energy, bond cleavage (fragmentation) may occur, with the resultant
`formation of a new cation and a free radical. Typically, the electron beams employed in
`the ionization process have an energy of 50-70 eV (1150-1610 kcal mole- 1). Since this
`is far in excess of the typical bond energies encountered in organic compounds (50-
`
`130 kcal mole- 1), fragmentation is normally extensive.
`
`TABLE 34.2 Ionization Energies
`
`Compound
`
`benzene
`aniline
`acetylene
`ethylene
`methane
`methanol
`methyl chloride
`
`Ionization Energy,
`electron volts ( e V)
`
`9.25
`7.70
`11.40
`10.52
`12.98
`10.85
`11.35
`
`Consider the case of the simplest hydrocarbon, methane. The mass spectrum of meth(cid:173)
`ane is shown in Figure 34.8 in bar graph form as well as tabular form. Note that the base
`peak (most intense peak) corresponds to the molecular ion (mlz 16). Note also the mono(cid:173)
`isotopic peak at m/z 17 (M + 1), which has an intensity 1.11% that of the molecular ion,
`within 0.07% of the intensity predicted by theory. Examination of the mass spectrum
`
`m/z
`
`Intensity
`
`1
`2
`12
`13
`14
`15
`16
`17
`
`3.4
`0.2
`2.8
`8.0
`16.0
`86.0
`100.0
`1 .11
`
`100
`
`80
`
`~ 60
`·u;
`c
`....
`Ql
`c 40
`
`20
`
`5
`
`15
`10
`m!z
`
`20
`
`FIGURE 34.8 Mass spectrum of methane.
`
`10
`
`

`

`reveals that cations are also produced and measured that have mlz values of 15, 14, 13,
`12, 2, and 1. The following rl'lOdes of fragmentation can be postulated to explain these
`various cationic fragments. Initial ionization yields the molecular ion, with mlz 16.
`CH4 + e- ~ [CH4t + 2e(cid:173)
`
`1187
`
`SEC. 34.4
`Fragmentation
`
`m/z 16
`
`Some of these ions move into the accelerating region and are passed into the magnetic
`field. However, since they possess a large amount of excess energy, many undergo frag(cid:173)
`mentation before leaving the ionization chamber, giving a methyl cation (m/z 15) and a
`hydrogen atom.
`
`+
`[CH4 ]· ~ CH3+ + H ·
`mjz 15
`
`Occasionally this cleavage occurs in such a way as to produce a methyl radical and a bare
`proton (mlz 1).
`
`+
`[CH4] · ~ CH3 · + H+
`m/z I
`
`The fragment CH3 +can be accelerated, deflected, and collected as a cation of m!z 15, or
`it too may undergo fragmentation, giving a hydrogen atom and a new radical cation of mlz
`14.
`
`+
`CH3+ ~ [CH2]· + H ·
`mjz 14
`
`Similar events give rise to fragment ions of mlz 13 and 12.
`+
`[CH2]· ~ CH+ + H·
`m/z 13
`+
`CH+ ~ [C]· + H·
`m/z 12
`
`Occasionally an ion ejects an ionized hydrogen molecule, giving rise to the weak peak at
`m!z 2.
`
`+
`+
`[CH4]· ~ CH2 + [H2]·
`m/z2
`
`More complicated alkanes give very complicated spectra, containing a large number of
`peaks. ~However, most of these fragment peaks are of low intensity. The more intense
`fragment peaks have mlz values of M - 15, M - 29, M - 43, M - 57, and so on, corre(cid:173)
`sponding to scission of the hydrocarbon chain at various places along its length. The
`spectrum of n-dodecane, plotted in Figure 34.9, is illustrative. There is a reason~.bly
`intense molecular ion (4% of the base peak) at mlz 170. The peak at mlz 155, correspond(cid:173)
`ing to loss of CH3 (M - 15) is so weak as not to be noticeable. However, the peaks at mlz
`141 (M- 29), 127 (M- 43), and so on, are apparent. Note that intensity decreases
`regularly as mass increases beyond mlz 43 (corresponding to C3H7 +). The modes of
`fragmentation responsible for the spectrum of n-dodecane are indicated in Figure 34.10.
`When there is a branch point in the chain, an unusually large arnount of fragmentation
`occurs there because a more stable carbocation results. Thus, in 2-methylpen!ane, loss of
`C3H7 or CH3 is much greater than loss of C2H5 , since the former modes give secondary
`carbocations, whereas the latter gives a primary carbocation.
`
`11
`
`

`

`,,
`
`The spectrum of 2-methylpentane, plotted in Figure 34.11, illustrates this behavior.
`On the other hand, the isomeric hydrocarbon 3-methylpentane can cleave in three ways
`
`m/z 57
`
`100
`
`80
`
`...
`> 60
`·;;;
`Cl> ... 40
`c
`c
`
`20
`
`c3
`
`c4
`
`Cs
`
`o~~~~~-.rw~--~L--.~-.--~--~U--.~~--~--.---.---~--.--
`20
`30
`40
`50
`60
`70
`80
`90
`100 110 120 130 140 150 160 170 180
`
`m/z
`
`FIGURE 34.9 Mass spectrum of n-dodecane.
`
`43
`
`57
`
`71
`
`29
`
`85
`
`99
`
`1188
`
`CH 3-CH 2-CH2 -CH 2-CH 2-CH 2-CH 2 -CH 2 -CH 2 -CH 2-CH2 -CH 3
`FIGURE 34.10 Fragmentation of n-dodecane.
`
`l13 1'27 1'41
`
`1170
`
`12
`
`

`

`43
`
`71
`
`1189
`
`SEC . 34.4
`Fragmentation
`
`o.
`
`10
`
`20
`
`30
`
`40
`
`60
`
`70
`
`80
`
`90
`
`100
`
`50
`m!z
`
`FIGURE 34.11 Mass spectrum of 2-methylpentane.
`
`so as to give a secondary carbocation. Two of these cleavages amount to loss of C2H5 .
`Correspondingly, the M - 29 peak in its spectrum, shown in Figure 34.12, is the most
`intense peak.
`
`[
`
`CH3CH):H~H2CH3
`
`CH
`
`m/z 86
`
`+
`·
`
`yH 3
`CH 3CH 2 • + +cHCH 2CH3
`m/z 57
`
`]
`
`Note that 3-methylpentane cannot undergo a simple cleavage to give an ion with mlz 43.
`The peak in its spectrum with this value must arise by a process involving some sort of
`skeletal rearrangement.
`The mode of fragmentation in the preceding discussion is common in mass spectrome(cid:173)
`try. A radical cation usually undergoes bond cleavage in such a manner as to give the most
`stable cationic fragment. What we know about the relative stabilities of various cations
`from other areas of organic chemistry may often be used to predict how fragmentation will
`occur in a mass spectrometer. The case of the methylpentanes is a good example of this
`principle. In Chapter 9 we discussed the SNl reactions of alkyl halides to give car(cid:173)
`bocationic intermediates and found a reactivity order tertiary > secondary > primary.
`From this order, and other data, we concluded that tertiary carbocations are more stable
`than secondary ones, which are , in tum, more stable than primary carbocations. Although
`these results are for solution processes and mass spectrometry measures the results of
`
`100 r-
`
`80 f-
`
`> 60 -
`+-'
`'iii
`c
`!!l 40 -
`..:
`20 -
`
`0
`0
`
`I
`
`· .1
`
`I
`10
`
`I
`20
`
`30
`
`57
`
`43
`
`71, M 15
`
`J
`
`I
`70
`
`I
`80
`
`86, M+
`...
`
`I
`I
`90 100
`
`I.
`I
`40
`
`.. II
`I
`I
`50
`60
`
`m/z
`
`FIGURE 34.12 Mass spectrum of 3-methylpentane.
`
`13
`
`

`

`1190
`
`CHAPTER 34
`Mass Spectrometry
`
`vapor phase processes, we can use our qualitative knowledge of carbocation stabilities to
`"interpret" the fragmentation pattern of hydrocarbons.
`
`Some of the enthalpy data for ionization of alkyl chlorides given in Table 9. 7 on page 195
`were actually obtained by mass spectrometric methods.
`
`In alkanes with a quaternary carbon, fragmentation to give tertiary carbocations is so
`facile that such hydrocarbons frequently give no detectable molecular ion peak. On the
`other hand, alkenes and aromatic hydrocarbons generally give rather intense molecular
`ion peaks.
`
`Simple one-bond cleavage is also a prominent fragmentation mode in amines. Cleav(cid:173)
`age of a bond adjacent to a carbon-nitrogen bond gives an alkyl radical and an immonium
`ion. Primary amines that are not branched at the carbon attached to nitrogen show an
`intense fragment with mlz 30.
`[CH3CHzCH2NH2]-:- -----7 CH3CH2 • + CH2= NH2 +
`m/z = 30
`When the amine is branched at the nitrogen-bearing carbon, an analogous cleavage oc(cid:173)
`curs, leading to a homologous immonium ion; loss of the larger group is preferred.
`
`[cH3CH2CH2~=~H,];--> CH3CH2CH,- + CH3CH~NH,+
`
`mjz = 44
`These cleavage patterns are illustrated by the spectra of isobutylamine and t-butylamine
`shown in Figures 34.13 and 34.14.
`
`0~--~~r-~~~~--~~~--rU--r-~r--,
`0
`10
`20
`30
`40
`50
`60
`70
`80
`90 1 00
`
`m/z
`
`FIGURE 34.13 Mass spectrum of isobutylamine, (CH3)zCHCH2NH2 .
`
`j:
`
`14
`
`

`

`1191
`
`SEC. 34.4
`Fragmentation
`
`0 ~--~~~~--~~-+~wr--~--~--~~
`0
`1 0
`20
`30
`40
`50
`60
`70
`80 90 100
`
`m/z
`
`FIGURE 34.14 Mass spectrum of t-butylamine, (CH3hCNH2 .
`
`100 r-
`
`80 1-
`
`> 60
`+"
`·u;
`c:::
`Cl.l
`+" 40
`c:::
`
`1-
`
`1-
`
`20 1-
`
`0
`20
`
`M-29
`
`(M-18-15)
`
`M-15
`
`M-18
`
`+
`
`It
`
`60
`
`70
`
`m/z
`
`M+
`
`t
`
`I
`90
`
`I
`80
`
`I
`100
`
`Ill
`
`I
`
`I
`
`30
`
`40
`
`.. I 111
`I
`50
`
`FIGURE 34.15 Mass spectrum of 2-methyl-2-butanol.
`
`B. Two .. Bond Cleavage: Elimination of a Neutral Molecule
`Some compounds give extremely weak molecular ion peaks. This tends to happen when
`some form of fragmentation is particularly easy. Such behavior is typical of alcohols,
`which often give no detectable molecular ion whatsoever. The spectrum of 2-methyl-2-
`butanol in Figure 34.15 illustrates this phenomenon.
`The molecular ion, which would appear at mlz 88, is not observed. Instead, sizeable
`peaks are observed at m!z values of 73 (M- 15) and 59 (M- 29), corresponding to
`cleavage of the radical ion so as to give stable oxonium ions.
`
`l ?H
`CH,CH,-L CH, ~ I ?H
`CH3CHz· + l CH39CH3 ~ CH3CCH3
`
`?iw]
`r l
`
`mlz 59
`
`15
`
`

`

`1192
`
`CHAPTER 34
`Mass Spectrometry
`
`In addition, there is a substantial peak at m/z 70, corresponding to loss of water from the
`molecular ion. This type of fragmentation, in which a radical cation expels a neutral
`molecule to give a new radical cation, is common with alcohols and ethers .
`
`or
`
`m/z 70
`
`mlz 70
`
`Of course, these new radical cations ions can undergo fragmentation of the type first
`discussed. The peak at m/z 55 probably arises from such a stepwise path.
`
`The mlz 55 fragment is a substituted allyl cation, and the special stability of this ion
`(Section 20.l.A) is the reason that this fragment is so intense.
`
`When the mass spectrum of an unknown compound does not contain a peak corresponding to
`the molecular ion, it is easy to be led astray in deducing the structure of the material.
`However, a careful examination of the mass spectrum of such a compound usually allows one
`to deduce that elimination of a neutral molecule has occurred and that the even peak of
`highest mlz is not the molecular ion. As an example, consider the following mass spectrum.
`
`100 r-
`
`80 f-
`
`...
`> 60 ._
`·c:;;
`Q) ... 40 '--
`s::
`s::
`
`20 f-
`
`0
`20
`
`55
`
`57
`
`70
`
`Ill,
`40
`
`h I
`I
`I
`50
`60
`
`30
`
`I
`80
`
`70
`
`m/z
`
`M+
`
`t
`
`I
`I
`90 100
`
`The mlz fragment could be considered to be the molecular ion of a compound with the
`molecular weight C5H10 , and the mlz 55 fragment would then be due to loss of methyl.
`However, the rather intense m!z 57 fragment would have to correspond to loss of CH, a
`mechanistically unreaspnable process. Thus , the internal evidence in this mass spectrum
`suggests that the mlz 70 fragment is not, in fact, a molecular ion.
`In addition, the chemist usually has other evidence that might not be consistent with the
`obvious interpretation of a mass spectrum that does not contain a molecular ion. In the
`present case, for example, the infrared spectrum contains a strong absorption at 3400 cm- 1
`,
`strongly suggesting the presence of a hydroxy group. If the m/z 70 fragment were the molecu(cid:173)
`lar ion of an alcohol, then we would expect aM - 18 fragment with mlz 52. The absence of
`such a peak is further evidence that the spectrum does not contain a molecular ion peak.
`
`16
`
`

`

`> ... =;;
`Q) ... c
`
`c
`
`100 ;-
`
`80 1-
`
`60 1-
`
`40 1-
`
`20 1-
`
`0
`0
`
`M-28
`
`1193
`
`SEC. 34.4
`Fragmentation
`
`.. .
`
`I
`20
`
`I
`10
`
`I
`30
`
`I
`
`40
`
`·•·
`
`I
`50
`
`I
`60
`
`I
`I
`70
`
`I
`80
`
`m/z
`
`FIGURE 34.16 Mass spectrum of butyraldehyde.
`
`There is one other type of fragmentation, also involving expulsion of a neutral mole(cid:173)
`cule that we will develop here. The spectrum of butyraldehyde is plotted in Figure 34.16.
`The most striking thing about the spectrum is the fact that the base peak (mlz 44) is an
`even number. Thus it must correspond to expulsion of a molecule, rather than a radical,
`from the molecular ion. Extensive studies suggest that this fragment arises in the follow-
`ing way.
`
`~
`
`+
`
`H
`/
`CH2
`CH2
`0
`I
`II ~ II
`CH2
`C~/CH
`CH2
`m/ z72
`
`+
`
`H'"'-
`
`0
`I
`#CH
`CH f/
`2
`
`m/z44
`
`There is some evidence that suggests that this fragmentation may involve two distinct
`steps, transfer of a hydrogen atom to the carbonyl oxygen from they-carbon followed by
`scission of the a,,B-bond.
`
`+
`
`CH "'o+
`CH
`~ I 2
`II ~ II
`2 +
`H
`C-!:_12 /CH
`CH2
`"cH
`
`2
`
`[
`
`HO
`
`tH
`.
`J+
`clJ
`
`2
`
`This rearrangement reaction is called a McLafferty rearrangement. It can provide useful
`information concerning the structure of isomeric aldehydes and ketones. For example,
`2-methylbutanal and 3-methylbutanal both undergo the rearrangement. In the former case
`one observes an intense peak at mlz 58, but in the latter the rearrangement peak occurs at
`mlz 44.
`
`!~
`
`'-1
`<
`
`H
`
`+
`
`\
`
`T~
`CH2
`~~
`~ II
`CH2
`CH2 YCH
`~ CH
`I
`CH3
`mjz86
`
`+
`
`~
`
`H
`~0
`I
`/CH
`CH
`I
`CH3
`m/z58
`
`:··. ,, ... ~
`J'·i\.f"ii
`
`17
`
`

`

`1194
`
`CHAPTER 34
`Mass Spectrometry
`
`H
`
`+
`
`elf) \__O
`CH2
`II
`I
`II ~ CH
`cH(
`CH
`CH
`cH( ?R(
`
`ml z 86
`
`· 1'
`
`+
`
`+
`
`H
`""-o
`I
`§CH
`CH[
`m/z 44
`
`An additional fragmentation common to ketones is cleavage of a bond to the carbonyl
`group to give a cation of the oxonium ion type.
`
`l-:
`R-~-Rj +
`
`0
`
`[
`
`34.5 Advanced Techniques
`Mass spectrometers vary in price from those attached to a gas chromatograph (system
`$50,000) to sophisticated double mass spectrometers (MS/MS) (about $1,000,000). Mass
`spectroscopy is one .of the ultimate tools of organic chemical analysis, and is effective for
`as little as a few femtograms (10- 15 g) of material. Although a detailed discussion of these
`advanced techniques goes beyond the scope of this text, the student should be aware of the
`possibilities.
`The general scheme for mass spectroscopic analysis can be summarized in a flow
`diagram.
`
`--- ~ sputtering - _ ::l>
`
`I Sample I ~ gas phase ~ ionization ~ mass separation ~ detection
`!l
`(4kV) \~ ~~~~:~ment (FAB)
`
`electrospray
`ionization
`
`~ argon, xenon, or cesium
`
`atoms (2-10 ke V)
`
`laser desorption
`californium-252 CZ52Cf) desorption (100 MeV)
`
`90° sector
`quadrupole
`time-of-flight
`
`Electrospray ionization is induced by a high voltage being applied to fine droplets
`emanating from a stainless steel needle, then removing the solvent as, for example, from a
`sample of the protein carbonic anhydrase in methanol-water by a stream of nitrogen. The
`protein acquires a number of protons, and gives mass species with mlz between 700 and
`1400 Da (Da = Daltons, a unit of molecular weight). A computer analysis of the multiple
`ions leads to a molecular weight of 29,012. It seems to be possible to measure molecular
`weights for proteins up to 100,000 Da. The mass separation is effected with a quadrupole
`mass spectrometer and the procedure can be carried out directly on the fractions obtained
`from high performance liquid chromatographic (HPLC) fractions. Organic molecules of
`considerable cmp.plexity can be investigated by mass spectrometry.
`
`18
`
`

`

`1195
`
`SEC. 34.5
`Advanced Techniques
`
`ASIDE
`
`Bubbles of gas entrapped in meteorites older than life on Earth, or in amber 100
`million years old, or in sealed jars taken from ancient tombs untouched for thousands
`of years are all minute samples that might be analyzed by mass spectrometry. A mass
`spectrometer will be the primary analytical instrument on board the Cassini-Huygens
`mission _to Titan, the sixth moon of the sixth plant, Saturn. The purpose is to evaluate
`the nature of prebiotic molecules. If we are the operators for this mission, we must
`practice on simple but realistic samples. The atmospheric pressure on Titan must be
`low, but the temperature is also low. Suppose over the eons, deuterated compounds
`had accumulated because of their slightly lower vapor pressure. Sketch the mass
`spectra and mark the species expected for CH4, CH3D, CH2D2, CHD3 , and CD4.
`Write out the expected fragmentation reactions.
`
`PROBLEMS
`
`1. a. Estimate the relative intensity of the peaks at mlz 112, 114, and 116 in the mass
`spectrum of 1 ,2-dichloropropane.
`b. A compound shows a molecular ion at mlz 138 with a ratio of (M + 1)/M of
`0.111. Show how this piece of information can be used to distinguish among the
`three formulas CwH1s, CsHwOz, and CsH14N2.
`2. Estimate the intensity of the M + 1 peak, relative to the M+ peak, for each of the
`following compounds.
`a. dimethyl adipate
`c. 1-phenylheptane
`e. methyl iodide
`
`b. 1 ,2-diaminonaphthalene
`d. n-hexacosane (C60H122)
`f. hexafluoroethane
`
`1
`
`3. The mass spectrum of N-propylaniline has a substantial fragment with m!z 106 (M-
`29). Account for the fact that N-propyl-p-nitroaniline shows almost noM- 29 peak.
`
`4. An unknown compound contains only carbon and hydrogen. Its mass spectrum is
`shown below. Propose a structure for the compound.
`
`43
`
`100
`
`~ ·o;
`c
`2l
`c
`
`80
`
`60
`
`40
`
`20
`
`Q r--UU,--~~L--,~UL.---~~--.-~-,----;----,~---,--~,--
`50
`70
`80
`60
`40
`90
`m!z
`
`20
`
`30
`
`100
`
`110
`
`120
`
`130
`
`19
`
`

`

`1196
`
`CHAPTER 34
`Mass Spectrometry
`
`5. The following mass spectra are of 2,2-dimethylpentane, 2,3-dimethylpentane, and
`2,4-dimethylpentane. Assign str