Mass Spectra of Derivatives of o-Aminobenzoic Acid RICHARD M. TEETER Chevron Research Co., Richmond, Calif.
b Aliphatic primary and secondary alcohols and amines usually have only very small molecular ion peaks in their mass spectra. These materials and thiols react readily with N-methylisatoic anhydride to produce omethylaminobenzoate esters, amides, and thioesters, all of which show prominent molecular ions, even a t low ionizing voltage. Qualitative and quantitative mass spectrometric analysis can thereby b e simplified in many cases. Complications in the fragmentation patterns were revealed and explained by the use of deuterium labeling and high resolution mass measurements.
A
GENERAL DISCUSSION of
the preparation of derivatives to aid in mass spectrometry has been given by Biemann (I). Volatile derivatives of alcohols previously reported include acetates and trimethylsilyl ethers ( 3 , 4 ) . Although both of these improve the volatility characteristics of the compounds to be studied, they do little to increase the size of molecular ion peaks. For this, introduction of a n aromatic functional group performs well. The greater increase in molecular weight thus introduced is not the problem it once was since all-glass hot inlet systems and direct insertion probes have become more available. The chemical reactions of isatoic anhydride have been reviewed in the literature by Staiger et al. (5, 6). The authors of Reference (6) suggest its use to prepare derivatives of alcohols, amines, thiols, and isocyanates for qualitative organic analysis by melting and boiling point comparison. I have found that these derivatives are useful for mass spectrometric analysis of alcohols, thiols, and amines which have only very small molecular ions in their spectra and which do not respond a t all well to low ionizing voltage. The principal reactions of interest are given in Equations 1, 2, and 3. Isatoic anhydride reacts with alcohols and thiols, in the presence of potassium hydroxide and either dioxan or dimethyl
DMF
1736
ANALYTICAL CHEMISTRY
formamide, to yield o-aminobenzoate esters and thioesters. Amines react, without added catalyst, to produce amides. Side reactions
are possible, as shown in Equations 4, 5 , and 6. The presence of water leads to the formation of anthranoyl anthranilate which dehydrates
water. The pentane layer was washed twice with water, dried with MgS04 and the solvent was evaporated. o - N - Methylaminobenzamides. Dry N-methylisatoic anhydride was slowly sifted into the amine with stirring and occasional gentle warming until COZ evolution stopped. The product was crystallized from methanol. N - M e t h y l - d l - Isatoic Anhydride. Isatoic anhydride (1.63 grams, 0.01 mole) and anhydrous potassium car-
(4) further in the mass spectrometer, a p parently t o a cyclic diamide, Tertiary alcohols are sterically unable to react to eliminate carbon dioxide but yield, instead, a carbamate. Both primary and secondary
bonate (1.40 grams, 0.01 mole) were stirred together in 15 ml. of dimethylformamide for 1 hour a t room temperature under an Nz blanket. A solution of C D J (1.46 grams, 0.01 mole) in 2 ml. of cold dimethyl-
(5)
amines form mixtures of amide and a substituted urea.
formamide was added over a period of 2 minutes and rinsed in with an
N-methyl isatoic anhydride forms neither the carbamate nor the urea. Therefore, ti^^ 5 is suppressed and Reaction leads Only to the o-methylaminobenzamide. Because of the few side reactions, 'methyl isatoic anhydride was used for this work.
additional 10-ml. portion of dimethylformamide. The mixture was warmed slowly to 45' C. and stirred for 1 hour. The suspended solid was removed by filtration and the resulting solution was wed directly in the ester or amide synthesis.
EXPERIMENTAL
All of the esters and amides which were examined show large molecular ion peaks which persist a t low ionizing voltage (9.5 volts). This is in contrast to simple benzoate esters and is probably due to the electron-releasing characteristics of the amino group on the benzene ring. Crable et al. in 1960 (2) suggested that low voltage molar sensitivities of monoalkylbenzenes are constant with increasing alkyl chain length beyond about a 3-carbon chain because the inductive effect of the remote carbons is insulated from the ring. The ester group in the o-aminobenzoates performs the same insulating function so that molar sensitivities of primary
The mass spectra were measured on either a Consolidated Electrodynamics Model 21-103C single-focusing instrument or on an Associated Electrical Industries h4S-9 high resolution mass spectrometer. o - N Methylaminobenzoates and Thiobenzoates. Approximately 0.2 gram of N-methylisatoic anhydride was mixed with an equimolar amount of alcohol or thiol in about 1.0 ml. of dry dioxane or dimethylforrnaniide and pellet of KOH (crushed) was added. The mixture was warmed on a hot plate until COz evolution ceased (4-8 minutes). After cooling, the mixture was diluted with n-pentane and
-
DISCUSSION
0
7
L
~
160
120
loo
1iO
220
2io
mle Figure 1.
Partial mass spectrum
n-Butyl o-methylaminobenzoate
o-aminobenzoates could also be independent of chain length a t low voltage. This was tested with a weighed mixture of n-amyl, n-heptyl, n-decyl, and ntetradecyl alcohols. The mixture was converted to N-methyl o-aminobenzoates and the low ionizing-voltage mass spectrum was observed. When the contributions from heavy isotope ions were included, molar sensitivities were constant within 4y0 average deviation as shown in Table I. Because the variations were apparently not systematic with molecular weight, the 4% probably reflects experimental error and impurities in the starting alcohols. This should permit the use of o-aminobeneoates for rough quantitative analysis of primary alcohol mixtures without prior calibration. o-Methylaminobenzoates which have been prepared and examined in the mass spectrometer are those of various primary and secondary alcohols from methanol through tetradecanol, 2butoxyethanol, phenol, benzyl alcohol, 4nitropheno1, a mixture of CU-CU alcohol-ethylene oxide adducts, and a mixture of Cl4-Cl$ oxo alcohols. All showed major peaks corresponding to the molecular weights. Moreover, four isomeric pentanols gave fragmentation patterns that were so similar that it was virtually impossible to tell them apart as will be shown later. This means, of course, that these esters are not useful
for determining the structures of unknown alcohols; but their employment in the quantitative analysis of alcohol mixtures of different molecular weights should prove very useful. Low voltage analysis of primary alcohol mixtures has already been mentioned, and this lack of sensitivity to structure implies that extension to secondary and branched chain alcohols should also lead to a useful quantitative analytical method. The low resolution mass spectrum of n-butyl o-methylaminobenzoate is given in Figure 1 as a n example of an ester spectrum. I n addition to the prominent molecular ion peak a t m/e 207, major peaks occur a t m/e 151, 134, 133, 132, and 105. The first of these is due to the loss of butene and corresponds in mass to o-methylaminobenzoic acid. The mass spectrum of one thioester was observed. The ester was prepared from N-methylisatoic anhydride and
Table 1.
Alcohol
n-Cs n-CT n-Clo n-Cl4 Av. sensitivity
m/e’s
221,222 249,250 291,292 347,348 = 15.3 f 4.17,.
1-butane-thiol; and its spectrum, given in Figure 2, is simpler than that of the oxygen ester. Two features are of interest. The molecular ion is the only major peak containing sulfur, and there is nothing corresponding to the acid peak (from loss of olefin) a t m/e 151 of the oxygen esters. o-Methylamino benzamides move us back in the other direction toward more complex spectra. The ability of the nitrogen atom to support a positive charge permits the appearance of fragments which do not contain the aromatic ring b u t which do contain the amide nitrogen. The amines, whose reaction products were examined, are n-butylamine, din-butylamine, and ethylene diamine. The first two yielded the expected amides and the spectrum of N,N-dibutyl 0-methylaminobenzamide is given in Figure 3. Ethylene diamine yielded mostly the expected diamide, b u t the spectrum was complicated by the pres-
Quantitative Analysis
Z HT. (Div.) 582 449 390 90.5
Mole Yo 40.8 29.5 23.7 6.04
Sensitivity 14.3 15.2 16.5 15.0
VOL 38, NO. 12, NOVEMBER 1966
1737
1007
0
-
80 c
r
.? 60a3
I a2
.-> c
2 40a3
20-
I
0 40
80
II
I 111
80
60
I
I 1
I
II I
I;
100
120
100
120
I
140
CH, Mw 262
=I 20
40
II
60
80
I m le
Figure 3.
Partial mass spectrum
DI-n-butyl o-methylaminobenzarnide
1738
ANALYTICAL CHEMISTRY
I
I
I
200
220
I
i
240
ence of other compounds. It is not known yet whether these were due to side reactions or impurities in the amine. Proposed Reaction Schemes. Evidence derived from mass measurements a t high resolution, from metastable peaks, and from selective deuteration was combined to yield the following reaction schemes to explain the major fragments in the pattern of n-butyl o-methylaminobenzoate. The figures in parentheses refer to the trideuteromethyl species which will be discussed later. Loss of C4Hs from the molecular ion (m/e 207) forms the o-methylaminobenzoic acid ion (m/e 151) with a metastable a t 110.0.
Table II. Exact Mass Measurements o-Methyl-da-hinobenzoate
Deuterated Species Undeuterated species mle Formula
Formula
Calculated 78.0454 106,0403 107.0704 108.0767 118.0626 134.0575 135.0653 136.0716 137.0794 Not measured
+C6I4D -&iHaDO C7HsDzN
77 105 105 116 132 133 134 151 207
c
151 CH,
(El
Another metastable a t 117.1 indicates that the 133 fragment is formed from the 151 acid fragment by the single-step elimination of water. Mass measure-
of the spectrum of a deuterated ester. N-trideuteromethylisatoic anhydride was prepared from the unsubstituted anhydride and trideuteromethyl iodide.
m' = 83.0 (m = 84.8)
ment confirmed the composition of the 133 entity as CsH7N0. The major fraction of the 105 fragment was found t o be C7H7N at high resolution, and a metastable a t 83.0 shows that it is formed from the 133 by expulsion of neutral CO. Loss of
+
-
105 H (107)
It was then treated with n-butanol and KOH and the product was isolated. Its spectrum showed the expected shift of three in mass for those ions still containing the intact N-methyl group. High resolution measurements, however, laid bare some complications.
\-I
H. leads to the 104 (metastable a t 103.0). There was no detectable metastable peak for the transition 207+ + 134+ 71, but it seems likely that the 134 (CsHsNO) is formed directly from the molecular ion by loss of a
