Mass spectra and fragmentation mechanisms of some nitrophenylhydraziks and nitrophenylhydrazones

Transcription

1 Mass spectra and fragmentation mechanisms of some nitrophenylhydraziks and nitrophenylhydrazones F. BENOIT AND J. L. HOLMES Chetnistry Department, University of Ottawa, Ottawa 2, Canada Received April 30, 1969 Can. J. Chem. Downloaded from by on 01/12/18 The mass spectra and fragmentation mechanism of 2- and 4-nitro and 2,4-dinitrophenylhydrazines and their N,N' trideutero analogues are discussed with respect to the operation of an "ortho effect". The mass spectra of nitrophenylhydrazones of cyclohexanone, methyl cycolhexanones, and their deuterated analogues are discussed with reference to the fragmentation of the ketone portion of the molecule. Canadian Journal of Chemistry, 47, 3611 (1969) The mass spectra of aldehyde and ketone derivatives of 2,4-dinitrophenylhydrazine (1-5) and N,N-dimethylhydrazine (4) have been reported previously. The results were mainly interpreted with respect to McLafferty rearrangements (4) and the sequential losses of water and a hydroxyl radical from the molecular ions (6a). Little attention has been given to the fragmentation of the aldehyde or ketone moiety of the molecule. Seibl andvollmin(1) and Seibl(5) noted that the migration of a hydroxyl radical from the ortho-nitro position produced a fragment ion corresponding to the protonatedform of the aldehyde. The operation of an "ortho effect" in aromatic nitro compounds has been considered by Harley-Mason et al. (7). We report here on the fragmentation mechanism of 2- and 4-nitro- and 2.4-dinitrophenylhydrazines and also on cyclohexanone and methylcycloliexanone nitrophenylhydrazones. The fragmentations of the ketone moiety of the molecules are discussed with respect to the possible use of the mass spectra of these derivatives in identifying structural features in new keto compounds. Transitions that are supported by a metastable peak are indicated by an asterisk. Where the deuterated compound has also been investigated, the masses of the deuterated fragment ions are shown in parentheses beside the appropriate species in the fragmentation schemes of the undeuterated compound. It should be emphasized that many of the ion structures presented in the fragmentation schemes are necessarily hypothetical. Results and Discussion 2,4-Dinitrophenyllzydrazine The mass spectrum of 2,4-dinitrophenylhydrazine 1 (Fig. la) may be interpreted by invoking an "ortho effect". The mass spectrum of the N,N' trideutero analogue was obtained (Fig. lb) and results therefrom used to support the fragmentation mechanism shown as Schemes I and 11. In Scheme I, two H atoms are shown as migrating from the hydrazine function to the o-nitro group and the subsequent loss of water produces fragment ion a, mle 180 which further dissociates via the successive losses of N,, NO, and NO to produce fragment ions b, c, and d. The fragment ion d, tnzle 92 then dissociates further to produce the peaks mle 64 and 63 by loss of CO or 'CHO, as is found in the case of aryl oxygen compounds (6b). A fragment ion of some interest in the above sequence is b, mle 152. Upon deuteration of the hydrazine fuilction (Fig. lb), nz/e 152 is shifted to nzle 153 (2% b.p.) and mle 155, e, (1% b.p.) (Scheme 11), the former arising by loss of D,ON, (either as D,O and N, successively or as [D,ON,]) from the molecular ion while the latter arises by loss of NO, from the molecular ion. The nitro group lost is believed to be from the ortho position because e does not show the loss of D,O or 'OD that could be expected from interaction of an ortho-nitro group with the hydrazine function. Fragment ion e then successively loses ND, and NO to yield fragment ions f and g (corresponding to f' and g' in Scheme I). Fragment g' can then undergo the loss of HCN, CO, or HCO' to produce fragments of mle 79,78, and 77 respectively. It should be noted that inle 79 is not shifted upon deuteration while inle 78 is shifted to m/e 79. Thus the intensity of the latter peak is increased sufficiently for it to become base peak while in the undeuterated compound, mle 51 is base peak. para- and ortho-nitrophenylhydrazine The mass spectra of 1 andp-nitrophenylhydrazine 2 are superficially similar, except that in the

2 CANADIAN JOURNAL OF CHEMISTRY. VOL

3 BENOIT AND HOLMES: MASS SPECTRA AND FRAGMENTATION MECHANISMS Can. J. Chem. Downloaded from by on 01/12/18 latter many fragment ions appear at one mass unit higher. For example, peaks at mle 152, 122, 106, and 79 in 1 are observed at mle 153,123,107, and 80 in the mass spectrum of 2 which is shown as Fig. 2n. One peak prominent in Fig. 2a which does not appear strongly in the mass spectrum of 1 is that at nzle 90. In the N,N'-trideuterated analogue of 2, the peaks at mle 107 and 90 are shifted to mle 110 and 91 respectively. These can be explained if the loss of NO, from the molecular ion is followed by ring opening together with SCHEME I Fragmentation rnechanisnl of 2,4-dinitrophenylhydrazine. migration of NH,' to the ortho position, yielding the fragment ions lz and h' shown in Scheme 111. The linear structure h' would be better able to carry two positive charges than the cyclic form and indeed a small peak (0.1%) is found at nzle Fragment h' can lose either HCN or NH, to give ions mle 80 and 90 respectively; the latter pair can undergo conjugate losses as shown, to produce the ion ~nle 63. It is noteworthy that the ion nzle 107 is not the analogue of m/e 106, g' (which contained oxygen) in the mass spectrum

4 3614 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 Fragmentation mechanism of fragment e formed in the mass spectrum of N,N'-&-2,4-dinitrophenylhydrazine. 11' * 1- HCN H H H I l l H2N=C-C=C-CGC-H HC=C-C=C-C=C=N-H + + I 1 of 1. This explains the lower intensities of peaks at mle 78 and 79 in 2; these were found to arise by loss of HCO' or CO from g' in 1. The mass spectrum of o-nitrophenylhydrazine 3 (Fig. 2b) is appreciably different from that of the para isomer. This is due to the "ortho-effect" described in the fragmentation of 1. The ortho isomer sequentially loses H,O and NO from the molecular ion to give fragment ion k as shown in Scheme IV. The loss of N, or NH' from k to produce ions nzle 77 and 90 respectively could occur readily from a cyclic entity. To explain the loss of HCN or CN' from k we again propose ring opening following migration of NH. to the ortho position to produce k'. The latter can sequentially lose CN' and HCN (or vice versa) yieldingmle79,78, and 52. Thus the ortho isomer, whose mass spectrum is superficially different from that of 2, fragments by a closely similar mechanism. SCHEME 111 Fragmentation mechanism of p-nitrophenylhydrazine.

5 BENOIT AND HOLMES: MASS SPECTRA AND FRAGMENTATION MECHANISMS

6 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

7 BENOIT AND HOLMES: MASS SPECTRA AND FRAGMENTATION MECHANISMS Can. J. Chem. Downloaded from by on 01/12/18 The loss of H,02 from the molecular ion is not widely encountered and deserves mention; the transfer of two H atoms to different oxygen atoms on the nitro group with subsequent loss of H,O, could yield the stable entity 1, which can further lose either N, or HCN to produce mle 91 and 92 respectively as shown in Scheme IV. Cyclohexanone 2,4-DNP The mass spectra of ketone 2,4-DNP's show peaks similar to those found in the mass spectrum of 2,4-dinitrophenylhydrazine. Since these fragment ions give little or no information as to the structure of the ketone moiety they will not be discussed further. A prominent feature in the mass spectrum of g- HCN \- CN. M/e 77 (78) + i- CH=CH-CH=CH-CGN HN-C-CH=CH-CH=CH' M/e 78 M/e 79 (80) Fragmentation mechanism of o-nitrophenylhydrazine. cyclohexanone 2,4-DNP (4) (Fig. 3a) is the intense peak at rnle 99 (fragment-in). This peak arises from migration of a hydroxyl group from the ortho-nitro group to the ketone moiety to yield the protonated ketone (Scheme V)'. This is supported by the observation that the mle 99 peak is shifted to mle 100 in the N-deuterated analogue and to mle 103 in the a-d, compound (Fig. 3b). The structure of m is a matter for conjecture but incorporation of the oxygen into a ring (m') is feasible (as shown in Scheme V). Fragment m' is then seen to lose water, yielding mle 81 ; the latter remains in the spectrum of the N-deuterated 'This migration has been independently suggested by Seibl and Vollmin (1) and Seibl (5).

8 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 Can. J. Chem. Downloaded from by on 01/12/18 CH2 \ CH2 ti M/e 96 SCHEME V Fragmentation mechanism of cyclohexanone-2,4-dinitropl1enyll1ydrazone. species while being shifted to nz/e 84 and 85 in the a-cl, compound. This indicates that water loss from m' occurs equally by a 1,2 and 1,3 and/or 1,4 elimination. Fragment 177 corresponds to either "protonatedcyclohexanone" or the "(M-I) ion of cyclohexanol" and could be expected to show cleavages similar to those observed in the mass spectra of cyclohexanone and cyclohexanol. In cyclohexanol the H atom involved in the loss of water was found to come from either the 3 or the 4 carbon position (6c). H20 loss in cyclohexanone involved H atoms from the 2 or 3 carbon positions (64. The suggested mechanism for the loss of water is shown in Scheme V. Fragment m' also loses CH,O and C2H,0. These neutral fragments, corresponding to CH,OH and C2H,0H are believed to be lost by the mechanism shown in SchemeV. The determiningfactor seems to be the carbon position from which the H atom I is transferred. Thus migration of a H atom from the 3 position induces loss of H,O, from the 4 position loss of CH,O and from the 5 position loss of C,H,O. The fragmentation of the deuterated species supports this dissociation mechanism. Migration of a H atom from the 2 position also gives rise to loss of H20 and is observed as the loss of HDO from the a-d, analogue of 177. Even though migration of a H atom from the 4 and 5 positions induces loss of CH,O and C2H,0 respectively, water loss may also occur. All the features described above are present in the mass spectrum of cyclohexanone o-nitrophenylhydrazone (Fig. 4a), but are absent in they-nitro analogue (Fig. 46). The latter fragments differently, via cleavage of the central N-N bond to produce fragment ion n. This species has also been observed in cyclohexanone-n,n-dimethylhydrazone (4).

9 BENOIT AND HOLMES: MASS SPECTRA AND FRAGMENTATION MECHANISMS

10 M / e FIG. 5. Mass spectra of (a) 4-methylcyclohexanone-p-nitrophenylhydrazone and (b) 4-methylcyclohexanone-2,4-dinitrophenylhydrazone,

11 BENOIT AND HOLMES: MASS SPECTRA AND FRAGMENTATION MECHANISMS 3621 Can. J. Chem. Downloaded from by on 01/12/18 Fragment ion n may be represented by many structures; one of these is shown in Scheme V, i.e. with the nitrogen atom incorporated into a ring. Fragment 12 behaves similarly to fragment m; H atom migrations from the various ring positions are associated with losses of HCN, CH,CN, and C,H,CN to yield the species nzle 69, 55, and 41. High resolution measurements on mle 69 and 55 showed that these are C,H,' and C,H,' respectively. The charge appears to remain with the aliphatic fragments rather than with the cyanide entity. That H atom migration from the 2 position is associated with the loss of HCN is shown by the loss of DCN in the a-d, species. The mass spectrum of cyclopentanone 2,4- DNP showed peaks corresponding to fragmentations analogous to those described above for cyclohexanone 2,4-DNP and thus a similar mechanism presumably operates. Methylcyclohexanone Nitrophenylliydrazones The mass spectra of the three isomeric methylcyclohexanone-2,4-dnp's were found to be very similar. The only differences observed lay in the relative intensities of the major peaks. The presence and position of the methyl substituent does not appear to exert any significant influence on the fragmentation of the ketone moiety of the molecule which behaves similarly to cyclohexanone-2,4-dnp. The similarity is not surprising since the three isomeric methylcyclohexanones (6e) and the methylcyclohexanols (6f) are not readily distinguishable by means of their mass spectra. The mass spectra of 4-methylcyclohexanone-2,4-DNP and its p-nitrophenylhydrazone are shown in Figs. 5a and 5b. The substituent position in molecules known to be isomeric could be determined (with some uncertainty) from relative intensities of fragment ions in their mass spectra but these alone would be of little aid in structural determinations of unknown molecules. Experimental All mass spectra were measured on a Hitachi-Perkin Elmer RMU-6D mass spectrometer using the direct sample inlet system. The 2,4-DNP's were run at an inlet temperature of 250 "C while o- and p-nitrophenylhydrazine were run at room temperature. The spectra were measured at electron energy 70 ev. The nitrophenylhydrazones were prepared by reaction of the appropriate nitrophenylhydrazine and the ketone in acid medium with 95 %ethanol as solvent. Deuteration of the hydrazine function was carried out by recrystallization of the sample from a mixture of D,O and dioxane. 1. J. SEIBL~~~J.VBLLMIN. Org. Mass Spec. 1,713 (1968). 2. R. J. C. KLEIPOOL and J. I. HEINS. Nature,. 203, (1964). 3. C. DJERASSI and J. D. SAMPLE. Nature, 298, 1314 (1965). 4. D; GOLDSMITH and C. DJERASSI. J. Org. Chen~. 31, 3661 (1966). 5. J. SEIBL. Helv. Chim. Acta, 50, 263 (1967). 6. H. BUDZIKIEWICZ, C. DJERASSI, and D. H. WILLIAMS. Mass spectrometry of organic compounds. Holden- Dav. Inc.. San Francisco DD.. (a)., (b)., 116., (c)~zl, (dj 145, (e) 149, (f) J. HARLEY-MASON, T. P. TOUBE, and D. H. WILLIAMS. J. Chem. 396 (1966).