The Mass Spectrometry of Esters of Phosphorous and Phosphonic Acids J. L. OCCOLOWITZ and G. 1. WHITE Department o f Supply, Australian Defence Scientific Service, Defence Standards laboratories, Maribyrnong, Victoria, Australia
b The mass spectra of trialkyl phosphites show ions involving considerable hydrogen rearrangement as well as ions formed by simple bond fission. Trimethyl and triphenyl phosphite give fewer hydrogen rearrangement ions than triethyl phosphite and higher homologs. Comparison of the spectra with other organophosphorus esters indicates that hydrogen rearrangement in the trialkyl phosphites takes place both to the phosphorus atom and to the oxygen atoms tc, give ions containing pentavalent phosphorus. The spectra of some dialkyl phosphonates are recorded and the m/e value for the base peak diffetentiates these compounds from the isomeric trialkyl phosphites.
T
HE MASS SPECTRA of pentavalent phosphorus esters have been investigated by Quayle (4) us>ngtrialkyl, arylalkyl and triaryl phosphates, and by Harless ( I ) using dialkyl hydrogenphosphonates. The spectra of both classes of compounds showed a large proportion of ions arising from hydrogen migration, a phenomenon originally observed by hIcLafferty ( 2 ) in triethyl phosphate. These rearrangement ions involve migration of hydrogen from
alkyl ester groups to the phosphorusoxygen skeleton. Quayle (4) assigned the phosphorus atom in the ions a quadrivalent state with the positive charge residing on the phosphorus atom and hydrogen migration to the oxygen atoms, while Harlesa (1) preferred to write the structure with pentavalent phosphorus and hydrogen migration to the phosphorus atom. The purpose of this work was to examine some esters of trivalent phosphorus and some isomerically related phosphonates. EXPERIMENTAL
Spectra were determined on an Atlas Model CH4 mass spectrometer, 60degree sector, 20-em. radius, using magnetic scanning. The samples n-ere volatilized in a n enamel-lined container at 65" C. Ionizing electron energy was 70 e.v. and ion source temperature 250" C. With the exception of triethyl phosphite which was a redistilled commercial sample, esters were prepared in these laboratories. RESULTS
Trialkyl Phosphites. Trimethyl phosphite (Figure 1) gives fewer rearrangement ions t h a n the higher molecular weight members of the series, and
its spectrum varies much more during scanning and with age of sample. Possible impurities are dimethyl hydrogenphosphonate arising from hydrolysis and trimethyl phosphate which arises from oxidation. The spectra of these compounds (1, 3 ) show that interference due to these impurities would be most noticeable a t m,'e 110, 80, and 79. To minimize this interference the spectrum recorded is for a sample freshly distilled in an inert atmosphere and scanned \Tithin 5 minutes of introduction to the mass spectrometer. I t is doubtful whether m,le 110 arises from the phosphite. I t most probably results from contributions by the heavy isotope peak of mle 109, the molecular ion of dimethyl hydrogen phosphonate and the (51-30) + fragmentation ion from trimethyl phosphate. .\part from the ion types common to all members of the series shown in Table I, trimethyl Ilhosphite gives the ion PO(OCH3)2+ (m,le 109) because of the loss of one alkyl group wit'hout rea,rrangement, and the rearrangement ions (mle 94), H P ( 0 H ) HP(OCH&+ (OCH3)+ (mie 80), and HP(OCH3)+ ( m ' e 63). The mass doublet 0-CH, raises the possibility of alternate formulas for the ions m/e 94,93,80, 79, and
loor--90
2w
80-
k 60 Z
> u
(CALC.=O. 054)
70 -
50 40
t-
4 30
-
-
POC'',
-J
d
I
20-
lo
t
2 0 30 40 50 60 70 80 90 100 110 I 2 0 M A S S NUMBER Figure 1. Mass spectrum of trimethyl phosphite
H5
i 78
79
J
L
80
NOMINAL
M A S S NUMBER Partial spectrum of trimethyl phosphite and
Figure 2. monodeuterobenzene.
(Leak rates changed during scan)
VOL. 35, NO. 9, AUGUST 1963
1179
I
I
MASS Figure 3.
NUMBER of triethyl phosphite
Accurate mass measurement has confirmed the formulas assigned. The technique is illustrated in Figure 2 which shows a partial spectrum of a mixture of mono-deuterobenzene and trimethyl phosphite. Here the measured separation of 0.057 MU for the PO&&W a s D doublet agrees well with the calculated 0.058 MU. The POr- 12CsHsD doublet has a separation of 0.090 MU. For other trialkyl phosphites (Figures 3-6) the dissociation scheme is shown in Table I. I t differs from that for trialkyl phosphates (4) in that the base peak occurs a t lower m/e, and the carbon skeletons of two alkyl groups are lost as entities. KO ions with fragmentary alkyl groups except HPO(OH)(OX) are found. Triphenyl Phosphite. T h e triphenyl phosphite spectrum (Figure 7) shows fewer ions containing phosphorus, most peaks being assignable t o the aromatic nucleus and oxygen. Ions containing phosphorus are the parent ion, m/e 217 [P(Oc&H,)Z], and theion a t m/e 199 (POC12HB). The base peak occurs a t m/e 94 (C&OH), other peaks a t m/e 77 (C6&) and m/e 65 (C~HH,OH less CO) and m/e 66 (C&OH less CHO). 63.
Table 1.
HP(0R)zOH P(0R)z HP( OR)(OH)z HPO(OH)(OX) P(0R)OH HP(OH)r
%z)):
124
MASS NUMBER Mass spectrum of tri-n-propyl phosphite
The iona at m/e 152 and 154 are assigned the biphenylene and biphenyl skeletal structures, respectively, and the ion a t m/e 170 corresponds to diphenyl ether. For the ion m/e 199 we write the structure (I) P=Of
0
2p=o+
(11)
&-Q
The structure (11) would then be an alternative to that suggested by Quayle (4) for the m/e 215 ion in the triphenyl phosphate spectrum. Esters of Phosphonic Acids. The or alkyl) occurs in group R‘-P(R’=H all of the ions from the dialkyl phosphonates (Table I1 and Figures 8-10) and illustrates the stability of this group under electron bombardment. The base peak in all of the spectra corresponds to the ion R’--P(OH)*, that is to R’+82. Thus the dialkyl alkylphosphonates may
be distinguished from the isomeric trialkyl phosphites which give base peaks a t lower values. At the same time, from the m/e value of the base peak the mass of the alkyl group directly attached to the phosphorus atom ia readily obtained. Comparison of the spectrum of triisopropyl phosphite (Figure 5) with diisopropyl hydrogenphosphonate (Figure 8) shows little similarity in the cracking patterns for ions with m/e greater than 83. The phosphite spectrum gives a medium strength ion a t m/e 166 (parent for di-isopropyl hydrogenphosphonate) but this cannot have the phosphonate structure because of the difference in spectral patterns. It is probably the ion HO-P(OCaH& formed by migration of hydrogen to the oxygen. The stability of the ion a t m/e 83 in dialkyl hydrogenphosphonates and also in trialkylphosphites indicates that in the latter there must have been a hydrogen migration to the phosphorus atom, so that the same ion is produced from both species. This is illustrated in the partial equations, which also show the stable analogous ion from dialkyl alkylphosphonates.
Dissociation of Esters of Phosphorous Acid, P(OR)a
RECHI m/e R I (R0):P
Figure 4.
Mass spectrum
36
R=C&Hr m/e R I
R=nC:H, m/e R I
R4C:H.r m/c R I
166 139 121 111
36 52 37 76
93 83 82 65
46 76 94 100
208 0 . 5 11 167 149 10 21 125 8 109s 14 107 83 100 34 82 27 65
4 208 5 167 8 149 14 125 1090 49 47 107 48 83 82 100 65 30
... ... 93 100 ... ... ... ... ... ... 41 79 ... ... ... ...
... ...
t
R-CdHo m/e 250 195 177 138 123‘ 121 83 82 65
+R’-P-OH
RI 2 9 5 25 4 6 100 5 5
‘OH Dialkyl Alkyl Phosphonate
ANALYTICAL CHEMISTRY
+ R‘)
+OH
F-Pq]’
+H-P-OH // ‘OH
Dialkyl Hydrogen Phosphonate 1 180
m/e (82
m/e 83
-2
1%
-qe
0
z
co 0 W
4
0 4
,
,
,
,
,
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ii
AIISN31NI 3AIlVl3tl VOL 35, NO. 9, AUGUST 1963
W
ii
1181
Trialkyl Phosphite nile 53 R # CHB I n contrast with the spectrum of trin-propyl phosphite where the ion m/e 45 occurs with low intensity, the spectra of tri-isopropyl phosphite, di-isopropyl hydrogenphosphonate and di-isopropyl methylphosphonate show rearrangement ions of high intensity at mle 45. This high intensity ion a t m/e 45 for di-isopropyl hydrogenphosphonate was not reported by Harless ( I ) .
ion m/e 53 (H4P08)in the dialkyl hydrogenphosphonates and trialkyl phos~ ~ h i t ecan s be written in a number of forms. The suggestion by blcLafferty (4) that the ion H4P04in the phosphates is stabilized by resonance between four equivalent structures involving pentavalent phosphorus would account for the stability of the ions R’H3POa (R’=H, dialkyl hydrogenphosphonates and trialkyl phosphites; R’= alkyl, dialkyl alkylphosphonates; R’=OH, trialkyl phosphates), found with strong intensity in all of the organophosphorus esters reported so far. This ion wvoiild then lie represrntrd
In the case of the trialkyl phosphatse a fourth form participates in the resonance hybrid. Similar structures involving pentavalent phosphorus and trivalent oxygen could be written for the ions HP(OR),(OH) and HP(OR)(OH)z found in the trialkyl phosphite spectra. Ions of the type POH(OR), P(OH)2, and P(OR)Z containing trivalent phosphorus ran be represented
\
0-R R
/
DISCUSSION
‘\ =
0L-R
n k y l or TI
0-H
The structure of the ion m/e 99 (Hc POc) in the trialkyl pho.qphates and the LITERATURE CITED
Table II.
Dissociation of Esters of Phosphonic Acids, R’PO(OR)2
R’gH : RiCxHi nz/e RI R’PO(0R )2 R’PO(OR)(OX) R’P(OR)(OH )2 R’PO(OR)(OH) R‘PO(OX)(OH) R’PO(0R) R’P(0H)a R‘PO(0H)
I66 151a
125
124 109b
...
83
65
