Mass Spectra of Aromatic Esters E. M. EMERY Corporate Research and Development, Colgafe-Palmolive
b The mass spectra of seven homologous series of aromatic esters have been correlated with molecular structures. Both normal fragments and specific rearrangement fragments largely characterize these esters. The mass spectra can b e used for the identification of unknown esters and the analysis of these esters in mixtures.
T
of mass spectrometry in the petroleum and allied fields is well established. One application of the technique in this laboratory has been the examination of the numerous synthetic and natural essential oil chemicals. K o r k toward this goal has included the correlation of the mass spectra of 25 aliphatic e s t m of aromatic acids, and 17 aromatic esters of aliphatic acids. The fragmentation patterns were found to be characteristic of molwular structure. Correlations of mass spectra by other 1%-orkershave shown that characteristic molecular breakdowns occur, dependent on chemical structure. Xormal and rearrangement fragment ions have been reported extensively. Oxygen-bearing hydrocarbons that have been studied include alcohols ( 6 ) ) aldehydes (7), ketones (16). ethers ( 1 1 ) , and acids (6, 1 3 ) . Work on esters included both aliphatic (1, 8, 9, 14, 1 7 ) and aromatic types (3. 10, I S ) . HE WIDE USE
EXPERIMENTAL
Spectra were obtained on a Consolidated Model 21-103C mass spectrometer equipped with a heated inlet and gallium-covered sintered disk system for sample introduction. The entire inlet system mas operated at 150" C., while the ion source was constant a t 250' C. The ionizing voltage was 70 volts. The magnet current was adjusted to a n appropriate value to include the molecular weight (parent) peaks, and a single
The editor has noted that over one fourth of the spectra included here repeat work reported earlier [McASAL. Lafferty, F IT., Gohlke, R. S., CHEM.31, 2074 (1959)l. However, as the experimental work of the two groups was done independently and these spectra are an integral part of the over-all interpretation in this article, it seemed of value to include them.
Co., Jersey City, N. I.
Table I. Mass Spectra
Source4 Mol. wt. m/e 27 28 29 30 31 37 38 39 40 41 42 43 44 45 45.5 49 50 51 52 53 55 56 57 57.5 58 58.5 59 60 61 62 63 64
65 66 69 70
71 72 73 74 75 76 77 78 79
Formate
Acetate
T 136 10.40 6.00 37.80b 1.60 5.19 3.26 8.21 29.30 2.21 4.12 ... 0.99 1.35 0.73
T 150 5.37 3.57 6.37 0.63 0.76 1.89 5.05 19.20 1.44 4.02 3.86 74.50b 2.43 0.92 2.22 1.02 11.40 24.50 6.12 2.20 ...
...
1.65 17.10 34.60 8.99 4.50 .
I
.
... ... ... ...
... ...
... 2.31 6.20 15.10 4.36 25.80 1.63 ...
...
...
... 0.70
3.40 2.39 2.27 36.10 7.98 39.40 80 4.43 85 0.54 86 0.99 87 0.81 88 1.09 1.09 88.3m 89 19.10 90 79.80 91 100 O O d + 92 8.01 93 101 ... 102 ... 103 ... 104 ... 105 3.65 106 1.22m 107 30.00 108 35.20
...
... ... ...
...
... ...
1.26 3.78 10.20 3.24 18.40 1.15 ... ...
... ... ... 2.26 2.08 2.16 25.10 7.06 27.20 2.88 ... 0.50 ... 2.16 2.16 13.40 48,90 78. OOd 6.93
... ... ...
...
... 4.93 2.18 19.20 100.00'
of Aromatic Esters of Aliphatic Acids Propionate Butanoate i-Butanoate &Valerate BENZYL T V v v 192 164 178 178 13.50 23.60 16.30 25 90 9.20 9.48 4.91 2.77 14.30 45.60c 4.91 2.05 1.23 ... ... ... 0.50 0.77 0.92 ... 0.57 0.84 0.63 0.67 2.00 2.15 2.76 2.45 16.40 14.20 15.00 18.10 2.04 1.62 1.92 1.19 17.30 12.50 14.20 3.35 1.14 1.93 3.54 0.83 9.63 0.90 36. 70e 36. loc 0.53 1.40 0.52 1.52 1.08 ... ... 0.68 ... ... ... ... ... ... ... ... 2.99 3.09 5.76 4.06 7.94 7.97 15.20 10.60 2.29 2.23 4.02 3.06 1.24 0.85 1.46 1.01 1.97 1.01 0.68 1.69 1.09 ... 0.97 ... 31, 5OC 42, 70* 0.95 ... ... ... ... ... 1.54 ... ... ... ...
...
...
... 1.93 6.90 2.25 18.80 1.08
...
... ... ...
...
0.94 0.91 1.03 14.80 3.66 14.70 1.32 ... ... ... I . .
...
...
...
...
... ...
...
...
1.38 5.06 1.75 14.60 0.90 ... 28.30b 1.28
...
0.62 0.81 0.76 10.40 2.87 8.98 0.75
...
... ...
... ...
7.79 25.50 100.OOd'O 7.81
5.52 18.00 loo,OOd" 9.39
...
...
... ... 3.38 0.87 9.88 80.20
...
... ...
. . ~ ...
... 2.80 0.76 7.98 72.70
...
...
2.63
1.05
0.95 3.70 1.32 11.80 0.68
3.76 1.28
11.oo
0.66 ... 0.51 13.lob 0.63 ... 0.57 0.50 0.55 7.84 1.84 5.77
... ...
1.22
...
0.78 ... ...
0.51 ...
0.55 8.16 2.30 6.80 0.58 ... 18.80b ... 1.14 ... ... ... ... ... ... ... 3.97 3.61 10.30 7.80 100.OOd~~ 100,OOdse 11.40 8.62 0.56 ... , . .
... I . .
... 1.71
...
... ...
...
2.31 0.58 ... 6.06 4.25 47, 30 22.30 (Continued on nezt page)
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1502
ANALYTICAL CHEMISTRY
.
.
.
.
.
.
.
.
.
.
.
.
.
.
os
N a-
.
.
scan u-as used. Consequently, the spectra of the phthalates began a t m/e 41, while all others began a t mle 27. Compounds were used as obtained without further purification. Infrared spectra of these materials showed or less of their respective alcohols. Appropriate corrections were made in the mass spectra data. The source for each compound is given in Table I. Patterns, calculated from the corrected spectral data in the usual manner, are presented in Tables I to 111. Values below 0.50% of the largest peak have been omitted.
.
CORRELATIONS OF MASS SPECTRA MOLECULAR STRUCTURES
. . . . . . . .f . . . . . .
a
.
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.. .. .. .. . . . . . . . . . . .
C
EL M?
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LI
WITH
The significant peaks of ester spectra of monofunctional acids are giyen in Tables I and 11. Those of esters of difunctional aromatic acids are found in Table 111. I n this work, alkyl groups and phenyl (or phenylene) rings are designated R a n d d r , respectively. The ions from normal fragmentation and from rearrangement are discussed below for each homologous series studied. All percentages cited are relative to the base peak.
Benzyl
Esters. 0
Ar-CH2-0-C-R
II
The major mass peaks, predictable from the structural formula above, correspond to the structural fragments .. .. . . . . . . . .
0 I1
R, R-C, d r C H 2 , and ArCH20, along with relatively intense niolecular ions (>147,). It appears probable that tiyo predominant fragmentations occur initially:
.. ..
0 (a)
h
01
. .
01
ATCH+~O(!!R -,hrCH2+ 0
11
( b ) BrCH +gOCR -c ArCH20
+
. .
. .
nz/e 01
m/e 107
The latter process also shows marked hydrogen rearrangement to ;IrCH20H, m / e 108. Subsequent loss of water from the normal or rearranged ion of (bj above would yield the fairly intense 89 and 90 mass peaks. 'It is significant that the intensities at these masses, with those of 107 and 108, generally fall off markedly with the increasing size of the R group. Closely paralleling this condition is the increasing prevalance of the benzyl ion over all other ions. The ArCH2+ ion is logically the tropylium ion studied extensively by Rylander, Meyerson, and Grubb (16). It was depicted here, however, as the benzyl ion, being an integral part of the basic structural formula for these esters, and consistent with their nomenclature. VOL. 32, NO. 1 1 , OCTOBER 1960
1503
p-Phenylethyl
corresponding structural fragments of R,
Esters.
0
0 .4rCH2-CH~-O-C-R
I/
/I
The phenyl ring is isolated from the electron-rich carboxyl portion of the molecule by the insulating methylene groupings. Fragmentation more nearly like the totally aliphatic esters reported by Sharkey, Shultz, and Friedel (17 ) would be predicted. The absence of a molecular ion species, and the major
0
II
fragments of R, RC, ArCH2, ArCHzCHt, and ArCH=CH2 are consistent with
0
/I
their work. However, the RCO(2H) ion was not observed. Correlations with recently published work, presented below, readily explain the absence of this ion. Two general trends relative to the styryl ion (the base peak) are apparent as the acid radical increases from acetate to i-valerate: Increasing ArCH*CH*+ion (13 to 24%) (1)
0
Decreasing R-b+
RC, ArCH=CH-CH2, ArCH= CHCH,O, and strong molecular ions (10% or more) are observed. The corresponding alcohol, formed by hydrogen rearrangement is also found, though its intensity is markedly less. It is also apparent that, in contrast to the
0
I1
benzyl esters, either the R or R C group is the most intense ion fragment. Moderate to intense peaks (between 4 and 60%) found a t m/e’s 91, 92, and 105 are not predictable from the structural considerations. It has not been determined whether these fragments represent simple hydrogen rearrangements or multiple bond cleavage and rearrangement. Benzoate Esters.
(2)
These trends suggest that the formation of the styryl ion via the six-membered transition state of McLafferty and Gohlke (19) is influenced by the nature of the R group.
0 Ar-CH=CH-CH,-O-C-R
/I
Fragmentation basically similar to that of benzyl esters would be expected, since the vinyl group maintains the ring character b y conjugation. Indeed, the
1504
ANALYTICAL CHEMISTRY
I n spite of the added OH functional group in the ortho position, the salicylate homologs show fragmentation markedly similar to that of benzoates. Fragments are R or R-(lH),
Ar-C-0-R
It
+
R-(lH), Ar, A r c , and ArCO (2H). These same ions are found in the work of McLafferty and Gohlke (13). The lower members of this series show intense peaks corresponding to R, possibly
0
0
0
/I
dominate. With the higher homologs, the R group is typified by R-(IH), while the other major fragments are 0 0 0
I/
OArC,
II
/I
HOArC,
HOArCOH,
0
Cinnamate Esters. 0
I/
Cinnamate esters show analogous behavior to benzoates described above, Ar, The expected R or R-(lH),
ArCH=CHC,
I1
OArC, m/e 120, had no benzoate analog. The spectra are influenced by the size of the R groups involved. For the lower homologs, R, 0-Ar,
OAr,
corresponding to the ArCO(2H) rearrangement ion are also observed. Formation of each rearrangement fragment illustrates the double rearrangement processes described by McLafferty (12). Accompanying this energetically favored rearrangement is the nearly complete loss of the molecular ion species. The normal and rearrangement ions of the benzoate esters, and those for cinnamates below, appear to follow the fragmentation characteristics of the aliphatic esters (17).
II
0
OArC, HOArC, and molecular ions pre-
RO, Ar, and A r c , and molecular ion species. Beyond propyl, however, the R group is characterized by the rearrangement fragment R- (1H). At the same time, intense peaks (over 25%)
0
I1
HOArCOH, and a strong molecular ion species (>lo%). The base peak,
II
II
Ar-CH=CH-C-0-R
II
HOArC,
0
I n contrast to the benzyl esters characterized earlier, the major fragments of benzoate homologs are R or 0 0
II
Cinnamyl Esters.
II
HO-Ar-C-0-R
0
II
0 The same authors state t h a t ethyl esters seldom undergo rearrangement of a second hydrogen. The above structure is essentially that of an ethyl ester. RTithoutrearrangement of a second hydrogen, the RCOO(2H) ion could not result. Another phenomenon observed was the comparatively small m/e 91 (1%) and the unusually large m/e 65 (22%) found in the spectrum of the propionate ester. This variation from the other phenylethyl esters included in this study has not been explored.
Salicylate Esters. 0
0
II
ion (82 to 11%)
are readily evident. The decreasing intensity of the molecular ion .ivith increasing size of the R group is not as pronounced. Fragments corresponding to the styryl ion, XrCH=CH, and phenylacetylene, drC=S3H, are observed as expected. Correlations of the esters of the difunctional aromatic acid homologs are vastly more difficult, especially for lower mass peaks, due to the increased modes of fragmentation, rearrangement, and interactions that are possible. Sonetheless, certain structural features are correlated with the mass spectra.
0
!I
and ArCH=CHCO(2H)
I1
HOArCO(2H), and a decreased molecular ion. The question as to which oxygens were present in the major fragment ions was answered by consideration of the various possible structures. Formation
0
II
of the base pcak ion, OArC, a t m/e 120, was of primary concern. For the lower homologs (methyl, ethyl, and allyl), metastable peaks corresponding to the direct loss of ROH from the molecular ion species were observed. This loss of ROH logically indicated which oxygens remained in the base peak ion. However, for the higher homologs, the intermediate fragment ion a t mle 138 had great significance. Certainly the mle 138 contains all three oxygen atoms, corresponding to aalicylic acid. Dehydration of this acid fragment yields the base peak, m/e 120. The first step of this two-step proc-
ess
was
supported 0
by
fragments
il
R- (1H) and HOArCOH. both typical of the higher homologs. Metastable peaks in these spectra, however, verified both steps. Metastable peaks were found corresponding to the loss of R- (1H) from the parent ion, along with the mutually common metastable peak a t m/e 104.3 for the step: (138)++ (120)+
+ 18
Further, these higher homologs showed no metastable peaks supporting a singlestep transition through loss of ROH. The formation of the base peak a t m j e 120 through the dehydration of the intermediate acid ion n u s t have followed either of two possible processes. These alternate steps, along with the probable ions of further fragmentation, were postulated as being:
ion species with increasing size of the R group. Discernable fragments of moderate to low intensities include R 0 0 0
II II
0
0 0
Il
Phthalate 0
Esters. 0
93
Table V. ?ti
e
1.40
105 121 122
Work on these five speculative structures as well as the normal and rearranged fragments cited initially has becn performed. Of primary interest was the base peak of m/e 149, found for all esters except the dimethyl. There must exist a common path to this mle 149 fragment regardless of bulk or configuration of the R group. The following sequence was proposed :
The outlined fragmentation was tentatively confirmed by the small, but observable, metastable peaks resulting from
150 151 152
153
Sormal
00
100 00
50 32
10 00
Metastable Peaks of Selected Phthalates
160 5 141.0 125.7 175,O 116.6 106.5 175.0 116.5
18
Di-i-propyl
m/e’s 195 to 177 222 to 177 177 to 149 209 t o 191 191 to 149 209 to 149 209 t o 191 191 to 149
+ ROH ++ H20 H20
The observed metastable peak for this ester is typically broad and diffuse, spanning from m / e 97 to m/e 101, with a n apparent peak maximum a t m/e 98.9. Even with the metastable peaks as partial evidence of the steps I-IV, the possibility of radical rearrangements t o produce the observed ions still esisted. Further characterization of the m/e 149 fragment was performed, using a specially prepared diethyl phthalate having the two carbonyl oxygens enriched with OI8 (4). A strictly statistical approach to the isotopic distribution of the m/e 149 fragment was discarded because of the difficulty of interpreting bond fission probability. It ivas possible to examine critically the mass regions loeginning a t mle 149. 177, and 222, for the influence of the tagged oxygen. Partial spectra of normal and 0 1 8 enriched diethyl phthalate are presented in Table V. Both m/e 149 and m/e 177 show some hydrogen rearrangement to the corresponding higher masses. The molecular weight region of m/e 222 shows exactly normal isotopic abundance a t mle 223 in the untagged material, as would be expected. Comparison of the “A-pattern” columns of Table V shows a remarkably
Table IV. Ifetastable
m/e
Diethyl
m/e 100.0 (222) + (149) m/e 98.8 (149) + (121) m/e 99.2 (150) -P (122)
Loss
45
28 18
42 60
18 42
Probable
Process
Loss
Water CzH& GHa Water C3He CIHTOH Water C3H6
Step I1 t o I11 I t o I11 I11 t o IV I1 to I11 I11 to IV I1 to IV I1 to I11 I11 to IV
Partial Mass Spectral Patterns of Normal and O18-Enriched Diethyl Phthalates
Pattern Tagged
100 12 1 0 0
Ar-0 -Ar-C=O Ar-C=O Ar-COO Ar-COOH
104
R-0--C-Alr-C-O-R
The spectrd of the phthalate esters show 3 iapid decay of the molecular
Il /I
C0(2H), ROCArC, and ROCArCO(2H). The base peak for all the phthalates studied except dimethyl is the m/e 149. This mass has been found to be a monohydrogenated phthalic anhydride species, independently verifying the structure suggested by RIcLafferty and Gohlke (13). Confirmatory data supporting this conclusion are described later. Peaks a t m/e values of 76, 93, 104, 105, 121, and 122 require extensive rearrangement or multiple bond cleavages. Possible structural forms for all but the m/e 76 have been assumed.
Di-n-propyl
I
0 0
I1 I1
m/e m/e m/e m/e m/e
The occurrence of ions a t masses 64 and 92, and the absence of both masses 76 and 104 established that path 2 was correct. This path also holds for the lower esters, from m/e 120 down. Such processes, involving the loss of one and two neutral CO groups from molecules, have been described by Beynon (2). Subsequent comparison of the salicylate data to the degradation processes proposed b y McLafferty and Gohlke (13) showed that fragmentation corresponds to their Types I1 and IV mechanism. The loLver homologs are typified by such one-step processes. The higher esters, however, with the intermediate formation of acid ions, apparently follow their cyclic transition mechanism for acids having a-hydrogen in the orthoposition.
It
and/or R- (IH), CArCO(lH), HOCAr-
such steps. Csing metastables from the diethyl and two propyl esters data are given in Table IV. It was noted also, that there were no metastable peaks representing a onestep process I + IV, in the spectra of propyl and higher esters. The ethyl ester spectrum is complicated, since a t least three metastable peaks can be calculated for the same approximate mass region:
13 26 4 61 0 46 0 06
Pattern A
0 0 3 0 0
64 29 36 06
m/e
1TT 178 1T9 180 181
Sormal 100 00 22 09 2 33 0 20 0 02
Pattern
Tagged
A
100 00
0
0 15
0 3 0 0
22 47 5 38 0 91
38
07 71
13
m/e
222 223 224 225 226
Xormal 100 12 1 0 0
00
74 42 14 00
Tagged
100 00 13 40 4 65 0 49 0 05
VOL. 32, NO. 1 1 , OCTOBER 1960
~A
0 0 3 0 0
66 23
35 05
1505
close correspondence, Of particular note are m/e 151, 179, and 224, which show values of 3.29, 3.07, and 3.23, respectively, reflecting the 0 1 8 effect a t these masses. The reasonable constancy of these delta values was taken as proof that the carbonyl osygens are present a t these masses, This, in turn, indicates that steps I-IV are definitely involved in the production of m/e 149, the base peak. The sole assumption made n-as that hydrogen rearrangement occurs with approsimately equal facility on the normal and tagged molecules. Having established the structure of the m/e 149, several of the lower mass fragments mere tentatively identified. The metastable peak a t mle 98.9 indicated the step:
Table VII. Mass Spectra Analysis of Aromatic Ester Synthetic Blends
Blend Methyl cinnamate Ethyl cinnamate Phenylethyl acetate Diethyl phthalate Ethyl benzoate Benzyl acetate Methyl benzoate Benzyl acetate Ethyl cinnamate Cinnamyl acetate Methyl salicylate Butyl benzoate Phen lothyl acetate EthyY benzoate Diethyl phthalate
U/E
149
M/E
Mole Theoretical 36.7 63.3
% M.S. 36.0 64.0
79.9 20.1 50.2 49.8 62.6 37.4 60.1 39.9
80.3 19.7 50,4 49.6 62.0 38.0 59.2 40.8
85.1 14.9 40.0 50.0 10.0
84.6 15.4 39.2 50.5 10.3
Ill
The two structures for m/e 121 correspond to monohydrogenated variations of the two alternate structures of the salicylate esters of m/e 120. The postulation of both structures in phthalate fragmentation ryas indicated by the masses 93 and 105, derivable from analogous independent paths from the m/e 121 ion.
acteristics have been useful also in examining mass spectra of esters of unknown constituents,
Table VIII. Partial Mass Spectrum of Natural Oil No. 526
IDENTIFICATION OF ESTERS
The identification of the aromatic ester type can frequently be made by reference to Table VI. Analyses of several synthetic blends are presented in Table VII. These aromatic ester char-
Table VI.
Mass Spectra
ml e 45 92 93 120 135 136 152
Pattern 100.00 5.04 2.67 5.99 4.33 2.60 2.50
of Aromatic
Adjusted Pattern 1669,OO 84.19 44.60
100~00
72.30' 43,35 41.74
Esters
Origins of Base Peaks for Mixed Alkyl Aryl Esters Ester Type Benzyl
Base Peak, mle 91
8-Phenylethyl Cinnamyl
Probable Fragment Ion
ocHl EX.
acetate (m/e 108)
104 Varies
Cinnamatee
131
Benzoates
105
Salicylates
120
Phthalates
149
R-4'
:3
R*
ck
ck
0=
ANALYTICAL CHEMISTRY
~ 2formate . ( m / e 115) - 20.
&*
p,=cp* v
=
3
0
1506
I n essential oil systems, the preclominantly nonaromatic materials often do not interfere with the determination of aromatic ester types present. One natural oil, for esample, showed a partial pattern (Table VIII). Reference to Table VI indicated a salicylate ester v;as possible. An adjusted pattern rvas calculated, using m/e 120 as its base peak. The value now found for mass 152 was exactly that corresponding to methyl salicylate Subsequent verification of its presence was made by other means. The masses of 45, 135, and 136 had no relationship to this ester, while only part of the observed intensities a t masses 92 and 93 were due to it. Another example is the qualitative identification of this same ester (methyl salicylate) as the major component in oil of \Tintergreen, through a process analogous to that just described.
Ex. methyl (m/e 163); allyl (m/e 41)
ACKNOWLEDGMENT
The 01*enriched diethyl phthalate used in characterizing phthalate fragmentation was kindly supplied by D. B. Denney. The author acknowledges the help of 11.' T. Casazza in recording, reading, and tabulating spectra, and J. V. Schurman for his helpful comments and suggestions. LITERATURE CITED
(1) Asselineau, J., Ryhage, R., Stenhagen,
E., Acta Chem. Scand. 11, 196 (1957). (2) Beynon, J. H., J. Phys. Chem. 63, 1861 (1959). (3) Beynon, J. H., Mikrochim. Acta 1956, 437. (4) Denney, D. B., Rutgers, The State University, New Brunswick, S. J., private communication. ( 5 ) Friedel, R . A., Shultz, J. L., Sharkey, A. G., Jr., A s . 4 ~ .C H w . 28, 926 (1956). (6) Genge, C. A,, Ibid., 31, 1715 (1959). (7) Gilpin, J. A., Mclafferty, F. W., Ibid., 29,990 (1957). (8) Hallgren, B., Ryhage, R., Stenhagen, E., Acta Chem.Scand. 13,845 (1959). (9) Kourey, R. E., Tuffly, B. L., Yarborough, V. A., ANAL.C H m . 31, 1760, 1959. (10) AIcLafferty, F. R.,Ibid., 28, 306 (1956). (11) Ibid., 29, 1782 (1957). (12) Ibid., 31, 82 (1959). (13) LIcLafferty, F. W., Gohlke, R. S., Ibid., 31,2076 (1959). (14) Ryhage, R., Stenhagen, E., Arkiv Kenti 13,523 (1959). (15) Rvlander, P. S . , lleyerson, S., Gruth, H. M.,J . Am. Chem. SOC. 79, 842 (1957). (16) Sharkev, A. G., Jr., Shultz, J. L., Friedel, R. A., ~ A L CHEx . 28, 93.4 (1956). (17) Ibid., 31, 87 (1959). RECEIVEDfor review February 8, 1960. Accepted July 27, 1960.
