Chemical Ionization Mass Spectra of M ononitroarenes

Alex. G. Harrison and R. Krishna Mohan Rao Kallury Department of Chemistry, University of Toronto, Toronto, Canada MSS 1Al

The H, and C& chemical ionization mass spectra of a selection of substituted nitrobenzenes have been determined. It is shown that reduction of the nitro group to the amine is favoured by high source temperatures and the presence of water in the ion source. The H2 chemical ionization mass spectra are much more useful for distinguishing between isomeric compounds than the CH, CI mass spectra because of the more extensive fragmentation. For ortho substituents bearing a labile hydrogen abundant [MH- H,O]' fragments are observed. When the substituent is electron-releasing both ortho and para substituted nitrobenzenes show abundant [MH-OH]+ fragment ions while meta substituted compounds show abundant loss of NO and NO, from [MH]+. The latter fragmentation is interpreted in terms of protonation para to the substituent or ortho to the nitro function, while the first two fragmentation routes arise from protonation at the nitro group. When the substituent is electron-attracting the chemical ionization mass spectra of isomers are very similar except for the H,O loss reaction for ortho compounds.

The electron impact (EI) spectra of aromatic nitro compounds have been extensively studied and the major features are well known,'-' particularly the pronounced effects of ortho sub~titution.'-~ On the other hand, the effect of a nitro group on the chemical ionization (CI) mass spectra of aromatic compounds has received much less attention," with the CI mass spectra of explosives1171z and biomedical metabolites13 forming the bulk of such studies. Two recent s t u d i e ~ l ~ , ' ~ reported the reduction of nitroaromatics to the corresponding amine in the ion source in the presence of a reagent gas. The present study of the H, and CH, CI mass spectra of mononitroarenes has been undertaken to explore the utility of CI in distinguish- ing between isomers, which has not always been possi- ble by EI, and to determine, if possible, the site of protonation in bifunctional aromatic molecules con- taining the nitro group. In the light of the recent observation of nitro group reduction particular atten- tion has been paid to this aspect of the chemical ionization of the compounds studied.

Reduction of nitro group under CI conditions

Maquestiau et all4 and Shannon et al.,15 in indepen- dent studies, reported conclusive evidence that aroma- tic nitro compounds can undergo reduction to the corresponding amine under CI conditions and con- cluded that the [MH-30]+ ion observed in the CI mass spectra of nitrorarenes may correspond, at least in part, to the protonated amine rather than arising from NO loss from the protonated nitroarene. In their more detailed study, Shannon et a1." reported that this reduction was observed for H,, CH,, iso-C,H,, and NH3 reagent gases using a variety of instruments, although they reported that there was considerable variation in the abundance of the [MH - 301' ion

signal from instrument to instrument, and from day to day on the same instrument.

We also have observed apparent reduction of the nitro group to the amine using both H2 and CH, as reagent gases. Our studies show that the extent of this reduction is dependent on the ion source temperature and on the presence of water in the system. Figure l(a) shows the CH, CI mass spectrum of m- nitrobenzoic acid at a source temperature of 130°C when the spectrum showed the presence of water in the system. An abundant mlz 138 ([MH-30]+) ion is observed as well as additional fragment ions at mlz 120 and mlz 94. The mlz 138 ion in this case appears to correspond primarily to protonated aminobenzoic acid while the mlz 120 and rnlz 94 ions are the fragment ions observedI6 in the CH, CI mass spec- trum of rn -aminobenzoic acid, corresponding respec- tively to loss of H,O and loss of CO, from the protonated aminobenzoic acid. At 130"C, in an ion source showing no background water (Fig. l(b)), the mlz 138, 120 and 94 ion signals are of much lower intensity, while at a source temperature of 110 "C (Fig. l(c)) only a very low intensity mlz 138 ion signal is observed, probably corresponding to [MH - NO]+ from the nitrobenzoic acid. Similarly, the H, CI mass spectrum of m-nitrobenzoic acid at high source temp- erature (Fig. 2(a)) showed not only the rnlz 138 ion but abundant ion signals at rnlz 120 and rnlz 94, with the latter comprising the base peak. However, at 60 "C source temperature the mlz 120 fragment ion was absent from the spectrum and the mlz 138 and rnlz 94 intensities were much reduced compared with the rnlz 168, protonated nitrobenzoic acid. Under these latter conditions the D2 CI mass spectrum showed only the mass shifts expected for fragmentation of the [MD]+ ion of the nitrobenzoic acid. In particular, the mlz 138 ion shifts only to m/z 139 ([MD - NO]+) with n o further shift to higher masses as would be expected if the nitro group was reduced to the amine (and as

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284 ORGANIC MASS SPECTROMETRY, VOL. 15, NO. 6, 1980 @ Heyden & Son Ltd, 1980

CHEMICAL IONIZATION MASS SPECTRA OF MONONITROARENES

138 151

t 94 138 F.

40

80

40

- 139

- -

I23 -

95

40

was observed at higher source temperatures). Similarly the m/z 94 ion shifts to m/z 95 ([MD - NO - CO,]').

Similar results were observed for the other nitro compounds; however, the nitrobenzoic acid systems were particularly suitable for detailed study in view of the characteristic fragmentation pathways of the pro- tonated aminobenzoic acids.16 In summary, the extent of reduction of the nitro group depends strongly on the ion source temperature and on the presence of water in the ion source. (The role of water in the reduction of the nitro group in the mass spectrometric ion source was suggested first by Beynon et a1.l') In the work of Shannon et all5 D,O was added to the reagent gas and- ion source temperatures of 150- 200°C were used, conditions which we find ideal for nitro group reduction. In the present study water has been rigorously excluded and the spectra have been obtained at the lowest possible ion source tempera- ture, usually less than 120 "C. Under these conditions reduction of the nitro group has been essentially com- pletely eliminated and the [MH - 301' ion signals ob- served correspond, in fact, to loss of NO from the protonated nitrorarene.

H, CI mass spectra

The H2 CI mass spectra showed more abundant fragment ions which proved more useful in disting- uishing between isomers. Consequently the major part of the discussion will focus on the H, CI mass spectra which are summarized in Table 1 for the nitroarenes studied. Several characteristic fragmentation routes are apparent from these results.

[MH+]-+ [MH- H,O]+. Nitroarenes with an ortho substituent bearing a hydrogen show loss of H,O from the protonated molecule with this peak comprising the base peak in several o-nitroarenes. This fragmentation mode is absent for the related meta and para isomers, except for the nitrobenzoic acids and nitrobenzyl al-

cohols where HzO loss is observed for the meta and para isomers, but is of lesser importance than for the ortho isomers. The H, CI mass spectra of benzyl alcohols and benzoic acids show16 loss of H,O from [MH]+ regardless of the substituent position (although H 2 0 loss is enhanced for some ortho substituted ben- zoic acids) and the H,O loss observed in the present study for the meta and para nitro derivatives undoub- tedly reflects protonation at the carboxyl group of the acid and the hydroxyl group of the alcohol respec- tively.

The H,O loss observed for the ortho compounds is analogous to the OH loss from [MI" observedLg in the EI mass spectra and is indicative of protonation at the nitro group followed by [l, 41 elimination of H,O (Scheme 1). It should be noted that the ion formed in this manner from o-nitrobenzaldehyde appears to be unstable and fragments further by loss of NO to form the base peak at m/z 104. For o-nitrobenzoic acid and o-nitrobenzyl alcohol it is not clear which oxygen is lost as H 2 0 ; in both cases it is probably the hydroxylic oxygen, forming, respectively, the o-nitrobenzoyl ion and the o-nitrobenzyl ion (Scheme 2).

[MHI+-+ [MH-OH]"+OH. Although all the nit- roarenes showed peaks corresponding to loss of OH from the [MH]' ion, this fragment was particularly pronounced for those nitro compounds with an electron-releasing substituent ortho or para to the nitro function. This fragmentation reaction can be rationalized in terms of protonation at the nitro group with the [MH-OH]' fragment being stabilized by electron donation through the mesomeric effect of the substituent. Such stabilization is not possible for the meta isomers, nor does it provide significant stabiliza- tion when the substituents are electron-attracting. For those ortho substituents bearing an active hydrogen loss of H,O competes with loss of OH, with the result that the [MH-OH]' fragment is less intense for the ortho isomer than for the para isomer. This is not the case, however, when the substituents are halogens, where the ortho isomer shows [MH- OH]+ intensities

m /I

Figure 2. H, and D, CI mass spectra of m-nitrobenzoic acid. (a) H,, source temp. 160°C; (b) H,, source temp. 60°C; (c) D,, source temp. 60°C.

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A. G . HARRISON AND R. K. M. R. KALLURY

Table 1. H2 CI mass spectra of substituted nitrobenzenes"

Subetituent [MH]+ [MH-OH]" [MH-H,OI+ [MH-NO]+' [MH-HNOI+ [MH-NOJ+' [MH-HNO.I' Other

o - C H ~

P - C H ~ 0-OH

rn -CH,

m -OH p-OH 0-NH,

P-NH, 0-F

rn -NH,

rn -F P -F 0-CI rn -CI p-CI o-Br rn -Br p-Br

rn -CHO 0-CHO

p-CHO 0-COOH rn -COOH p-COOH o-CHZOH m -CH,OH p-CH,OH 2-NH2-5-CI 4-N H2-3-CI

18.7 2.8 15.5 3.9 29.0 19.7 17.2 11.9 31.2 2.0 22.0 52.4 12.4 21.4 14.8 2.2 10.0 58.8 32.9 25.2 63.5 3.9 52.4 29.8 21.3 50.8 35.2 13.0 47.1 25.2 23.4 33.4 26.3 8.3 42.5 22.7 10.1 1.5 66.4 1 .o 52.4 3.7

65.1 3.1 52.1 1 .o 10.8 6.2 16.0 5.2 33.8 10.8 33.9 15.6 20.5 62.1

- 1.7

2.2 - -

47.7 - -

31.0 - - - - - - - - - - - 5.5 - -

84.0 4.6 1.6

54.2 11.0 9.8

24.4 -

22.6 17.8 10.7

14.5 3.1 1 .o

13.0 3.5

15.1 10.1 4.2 6.1

16.7 6.1

13.0 19.7 8.1 1 .o 7.3 6.8 1.7

13.7 12.0 9.8

23.9 18.9

-

- -

2.2 3.2 7.0 3.3 1.2 5.8 1.9 6.2 9.4

10.5 - - 6.6 2.1 3.8 4.3 2.1 3.4 5.6

10.0 8.9 - - - - 4.3 3.7 - -

27.5 32.4 21.6 9.1

39.2 13.1 13.6 25.5 7.4 5.0

17.8 13.6 15.2 31.2 16.9 17.4 34.6 20.0 2.0 1.3 3.7 3.4 6.8 4.7 4.3 7.9 6.8

19.0 17.4

12.7 27.2 11.9 -

2.1 6.2

36.7- 5.4

2.3 - - - 1.8 1 .o 4.3 6.6 1.7

19.2 3.3 6.3

4.0 2.6 3.8

16.7 6.1 4.1

-

-

[931+(11.3)

[93j+(7.0)

[991+(3.01 [991+(2.5) [991+(1.9) [ 1431+ (4.0) [ 143N2.4) [1431+(1.7) [1041+(50.5) [941+ (8.0) [941+(9.4) [941+(2.5) [941+(2.6) [94]+ ( 14.6)

a Intensities expressed as % of total additive ionization.

as large as the para isomer (F substituent) or greater (C1 or Br substituent).

Loss of NO and/or NO, horn [MH]+. Although loss of NO, HNO, NO, and HNO, are fragmentation reac- tions common to all nitroarenes studied, the impor- tance of these fragmentation routes is much greater for nitroarenes containing an electron-releasing sub- stituent in the meta position than for the correspond- ing ortho and para isomers. While this might be attri- buted to the lack of other favourable fragmentation channels, discussed above, an alternative explanation is that the electron-releasing substituent favours pro- tonation, in part, para to the substituent and ortho to the nitro group giving the intermediate a (Scheme 3), the resonance form of which b favours nucleophilic attack of the nitro oxygen on the positive site leading to the nitrite which subsequently fragments by loss of NO or HNO. Alternatively, cleavage of the C-N bond with H migration leads to loss of NO, or HNO, as observed. In support of this proposal we note that in the D2 CI of rn-nitroaniline the ratio [MD-

Scheme 1

acx2-0Hd acxz+ -k H,O X z = O o r H 2

\ + \ N-OH NO2 I I 0

Scheme 2

HNO2]+/[MD-DNO2]+= 1.0. For the same com- pound [MD - OH]+/[MD - OD]' I 0.7, suggesting that a significant fraction of the hydroxyl loss reaction also originates from the ring-protonated species in which the added proton equilibrates with one ring hydrogen. Such a ring protonation reaction ortho to the nitro group is not favoured when electron- releasing substituents are ortho or para to the nitro substituents or for any nitrobenzenes containing electron-attracting substituents; consequently the abundance of fragment ions originating by loss of NO and/or NO, is less for these compounds.

As a result of these characteristic fragmentation reactions nitroarenes containing electron-releasing substituents are readily identifiable from their H, CJ mass spectra under conditions where nitro group re- duction does not occur. ortho Isomers where the sub- stituent bears a labile hydrogen are readily identified from the [MH -H20]+ fragment ion observed, while the para isomer is distinguished from the ortho isomer

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CHEMICAL IONIZATION MASS SPECTRA OF MONONITROARENES

a \

x T

X

/O

H H N N 0

Scheme 3

by the abundant [MH-OH]+ fragment and the ab- sence of the [MH-H,O]' fragment. meta Isomers are distinguished by the absence of the [MH- H,O]+ frag- ment, the low abundance of the [MH- OH]' fragment and the greater abundance of the fragment ions arising from loss of NO and NO, from [MH]+. The distinction among the isomeric halonitrobenzenes, particularly the fluoro derivatives, is not readily achieved unless one has all three isomers available for comparison, although for the chloro and bromo derivatives the relative [MH - OH]+/[MH]+ and [MH - NO,]+/[MH]+ ratios should permit a distinction between the three isomers. For those nitroarenes containing electron- attracting substituents the differences in the CI mass spectra of isomers are small, except for the ortho isomers where the loss of H,O is observed.

The effect of disubstitution in the nitroarene was examined briefly by determining the CI of 2-amino-5- chloronitrobenzene and 4-amino-3-chloronitro- benzene, whose H2 CI mass spectra are presented as the last two entries in Table 1. It is clear that the amino substituent plays the dominant role in deter- mining the fragmentation of [MH'] with the o-NH, group of the former compound resulting in predomin- ant loss of H,O and the p-NH, substituent of the latter compound promoting OH loss as the dominant fragmentation route. This behaviour resembles that of the aminonitrobenzenes rather than the m - chloronitrobenzene.

Finally, it should be noted that the H2 CI mass spectra of the halonitrobenzenes showed n o fragment ions corresponding to loss of halogen from [MH]+, either as the hydrogen halide or as the halogen atom, but rather that the fragmentation involved entirely the nitro group. This is in contrast to the H, CI of haloaromatics containing electron-releasing sub- stituents'8-20 where fragmentation of the protonated molecule involves primarily loss of hydrogen halide or a halogen atom. In this respect the nitro group has a pronounced effect on the CI behaviour of haloaroma- tics.

NO

X

+ NO,

+ HNO,

CH, CI of nitrorarenes

In the CH, CI of the nitroarenes studied in the present work the base peak corresponded to [MH]+ except for o-nitrophenol, o-nitrobenzyl alcohol and o-nitrobenzoic acid where [MH - H,O]+ represented the base peak. The remaining ortho substituted com- pounds which showed [MH - H,O]+ fragment ions in their H, CI mass spectra also showed the same frag- ment in their CH, CI mass spectra, but of lower intensity than [MH]'. Apart from these fragments low intensity peaks (1-10% of base peak) corresponding to loss of OH, NO and NO, were observed. However, the intensities were too low to correlate with structural features and the H, CI mass spectra proved much more useful in structure elucidation.

EXPERIMENTAL

The CI mass spectra were recorded on a Dupont 21-490 mass spectrometer equipped with a high pres- sure source at source temperatures 5120°C. The reagent gas pressures were -0.5 Torr (H, and D,) and -0.3 Torr (CH,). The instrument was carefully moni- tored for the presence of H,O background. Solid samples were introduced using a direct insertion probe while liquids were introduced from a heated inlet system.

Most of the samples used were commercial samples purified by recrystallization or distillation. Extreme care was taken to avoid moisture. o- Bromonitrobenzene was obtained by nitration of bromobenzene with red fuming nitric acid,21 while o-nitrobenzyl alcohol was made by reducing o- nitrobenzaldehyde with sodium borohydride by stan- dard procedures.22

Acknowledgement The authors are indebted to the Natural Sciences and Engineering Research Council of Canada for financial support.

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A. G. HARRISON AND R. K. M. R. KALLURY

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(1976).

3168 (1979).

Received 22 January 1980; accepted 14 March 1980

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