t.ested, and were obtained €rom a minimum of four determinations on each base. Urea was titrated photometrically also; however, the values were from 23 to 48% below quantitative recovery and were not of sufficient meaning to be included in Table 111. ACKNOWLEDGMENT
The authors express their appreciation to Research Corporation for financial support of this investigation.
LITERATURE CITED
(1) Bruckenstein, S., J . Am. Chem. SOC. 82,307 (1960). (2) Conant. J.. Werner., T.., Ibid.., 52., 4436 (1930). ' (3) Connors, K., Higuchi, T., A X A L . CHFM.32,93 (1960). (4) Higuchi, T., Connors, K., J. Phys. Chem. 64, 179 (1960). (5) Higuchi, T., Feldman, J., Rehm, C., ANAL.CHEM. 28, 1120 (1956). (6) Higuchi, T., Rehm, C., Barnstein, C., Ibzd., 28,1509 (1956). (7) Hummelstedt, L., Hume, D., Zbid., 32,576 (1960).
.,
(8) Kolling, 0. W., J . Am. Chem. SOC. 79,2717 (1957). (9) Kolling, 0. W., J. Chem. Educ. 35, 452 (1958). (10) Kolling, 0. W., Trans. Kans. Acud. Sci. 63, 67 (1960). (11) Kolling, O., Smith, M., ANAL. CHEM. 31,1876 (1959). (12) Stock, J., Purdy, W., Chemist Analyst 48,22, 50,55 (1959).
RECEIVED for review April 17, 1961. Accepted July 3, 1961.
-
Organophosphites The Effect of Ionizing Electrons on the Relative Abundance of Their Ion Species H. R. HARLESS Research Department, Union Carbide Chemicals Co., South Charleston, W . Va.
b The mass spectra of a series of dialkyl phosphites and the effects of ionizing electrons upon the relative abundance of their ion species have been studied. Anomalous rearrangement phenomena were observed in the ionization-dissociation schemes of dimethyl, diethyl, di-n-propyl, diisopropyl, diallyl, and di-n-butyl phosphites. Severe alkyl chain scission of phosphites occurs in the mass spectrometer with a resultant extreme degree of migration of hydrogen atoms from the alkyl moiety to the electrophilic PO1 entity. Relative intensities of ionized fragments of the various phosphites are correlated with structural features of the parent compounds. This work has led to the assertion that phosphorus(V) is a prevalent state of the element and that the quadricovalent state is not always achieved b y ionizing electrons.
T
BEHAVIOR of organophosphates in the mass spectrometer has been. reported by Quayle (6) in a study of trialkyl, triaryl, and alkyl/aryl phosphoric esters. The triaryl phosphates dissociate and ionize in a classical manner of ordinary bond scission observed in the mass spectra of hydrocarbons (1). Trialkyl and alkyl/ aryl phosphates, however, exhibit a singular mode of rearrangement when bombarded by ionizing electrons in that niany of their ions are formed by the shifting of one or two hydrogen atoms from substituent groups to the strongly electrophilic phosphate central body. The multiple rearrangements of hydrogen atoms in triethyl phosphate have been noted by McLaf-
HE ANOMALOUS
ferty (3), who also studied replacement rearrangements (4, 6) for many types of compounds with electronegative substituents such as ketones, amides, esters, nitriles, acids, phosphates, and sulfites. Since organophosphates exhibit a degree of rearrangement or atomic migration unparalleled by previously reported classes of compounds, it was expected that this mode of ionization would hold true for other types of phosphorus compounds. However, organophosphites display an even more severe mode of rearrangement under the effect of ionizing electrons. To explain the nomenclature for dinlkyl phosphites, the quinquevalent esters of phosphorous acid, our choice of structure is that used by Van Wazer (7). The dinlkyl phosphites are those compounds in which a hydrogen is bonded directly to the phosphorus atom and the molecules do not contain hydroxyls: (RO)zPH(0). This, therefore, is the series of compounds formerly designated as dialkyl hydrogen phosphonates, a term still frequently used by the British. APPARATUS A N D MATERIALS
The mass spectrometer was a General Electric Model G-5 (60' sector, &inch radius instrument, with magnetic scanning). Samples were introduced into the mass spectrometer by glass pipets through a mercury orifice (8). Sample pressures (microns of mercury) were' measured by the CEC Model 23-105 micromanometer (Consolidated Electrodynamics Corp.). The alkyl phosphites were obtained from Hooker Chemical Corp., Niagara Falls, N. Y., and Virginia-Carolina Chemical Corp., 401 E. Main St., Richmond, Va.
DISCUSSION
The mass spectra of simple dialkyl hydrogen phosphites were studied and anomalous rearrangement phenomena were observed. The spectra of dimethyl, diethyl, dipropyl, diisopropyl, diallyl, and dibutyl hydrogen phosphites are shown in Figure 1. Ions of insignificant intensities are not shown. A detailed discussion of the various spectra are given and intercomparisons are made for analogous and for incongruous ionization-dissociations. Dimethyl Phosphite. Dimethyl phosphite is characterized by a classical type of bond fission reported for various hydrocarbons and other classes of organic molecules. Excluding a mass-49 ion and a mass-65 ion, its mode of fragmentation is one of the rupturing of single bonds of carbonoxygen or phosphorus-oxygen, All of the expected ions (CH3+, C&O+, PO+, HPO+, POz+, HP03+, etc.) are observed. A single hydrogm shift from an alkyl group to the phosphorus electrophilic entity is achieved in two cases. A mass49 ion is formed by the migration of a hydrogen from a methyl group to the H P 4 nucleus and a mass-65 ion is arranged by the shifting of one hydrogen to the 0-P(H)=O central group. The full molecule (parent peak) is an abundant ion in the spectrum of dimethyl phosphite; all other homologs exhibit a small to almost insignificant parent peak because of the high probability of bond rupture, a condition which often occurs in large molecules. Diethyl Phosphite. I n addition to the expected simple bond scissions, the diethyl ester spectrum displays VOL. 33, NO. 10, SEPTEMBER 1961
1387
DI-n-PRWL
PHOSPHITE
H
R.n-C$+,
H:a:P:kH H
PARENT-4s
PAREKI-IS
R'CqHg H HOPOH
lo* JH
I
so
I
t
IC JF 90
110"130
140' 190
Y m
Figure 1.
Mass spectra of dimethyl, diethyl, dipropyl, diisopropyl, diallyl, and dibutyl phosphites
ions formed by migration of one or two hydrogcns from the severed alkyl groups to form four different centralbody polyatomic nuclei based on phosphorus. Table I indicates the degree of rearrangement of hydrogens to the basic central ions: P(H)02, P(H)O,, -CH,-O-P(H)02, and CeH~-O-P(€I)Oz. An ion a t mass 83 is the most abundant formation from the electron bombardment of diethyl phosphite and higher diitlkyl phosphite homologs. This ion represents a severe form of hydrogen rearrangement and is also frequently found in mass spectra of phosphates (6, 6). A mechanism for the formation of mass 99 by triethyl phosphate has been given by McLafferty (6).
Table 1.
Phosphorus Skeletal
P(H)Oz
CHs--O-P(H)O, CIH~--O-P(H)OI
1388
An obvious analogous scheme for formation of the mass 83 ion by dialkyl phosphites would be:
+
(C2Hs0)i-HP-of + CzH3 CzHi (HO)zHP=O+H
+
This manner of depicting explicit migrations of individual atoms and the formation of discrete atomic entities has met the needs of the writer for a number of years and is in general use by mass Ppectroscopists. In the discussion of additional homologs, it will be apparent (see Figure 1) that the mass-83 ion is of major importance. Relative intensities (R.I.) of the alkyl and alkoxy ions from dimethyl and diethyl phosphites are indicative of the ease of rupture of respective molecular bonds. The ethyl and ethoxy ions from diethyl phosphite exhibit greater intensities in a ratio of about 2' to 1 to those of the methyl and meth-
Degree of Rearrangement of Diethyl Phosphite Ions
Ion
Mass 64
R.I. 1.7
80
6.2
94
12
109
10
ANALYTICAL CHEMISTRY
Rearrangement Ion
H*P(H)Oz H2 .P(H)Oz H*P(H)Ox H *CH4--P(H)02 Hz CHI--O-P(H)Oz H .C&q-P(H)oz H2 .CtHsq--P( H)On
Mass 65 66 81 82 83 95 96
110 111
R.I. 93 31 10 14 100
10 1.9
8.3
70
oxy ions of dimethyl phosphite, as seen in Figure 1. Dipropyl Phosphite. A marked case of alkyl chain bond scission with a portion of the alkyl group migrating to the basic skeletal body of the molecule is observed in the spectrum of dipropyl phosphite. An ion based skeletal aron a CH2-O-P(II)O2 rangement is formed by the shift of an alkyl hydrogen to this group to give an appreciable mass-95 ion. Additional rearrangement is noted by the formation of mass-96 and -97 ions which exist to a significant degree. An ion of mass 109 (-CH2-CHz0-P(H)Oz.H) is also formed by migration of hydrogen; this formation, however, has an intensity only half that of the 95 (-CH2-O-P(H)02. H) ion. I t is significant that an ion of mass 151 (parent minus methyl) is not formed in the spectrum of the dipropyl esters, but is an appreciable peak in the diisopropyl phosphite spectrum. Other differences in the spectra of the two isomers are noted below. Diisopropyl Phosphite. Alkyl chain fission to form a mass 93 (-CH-0PHO,) ion is not found in the spectrum of the isopropyl ester, and consequently the mass 94 and 95 rearrangement peaks so evident for the n-propyl isomer are absent in the isopropyl spectrum. However, the mass 109 entity, (CH&H-0-PHO2. H), is much more intense for isopropyl than for n-propyl (-CHrCHz-OPH02.H). The greater abundance of
109 ions and absence of 93 ions in the iso-ester illustrates the high probability of rupture of one bond beta to the PO3 nucleus and the much lower probability of scission of both beta bonds. The mass-43 ion (-CHCH&HJ is much more intense (R.I.=37) in the case of the isopropyl ester than with n-propyl mass-43 ion (-CH2CH2CHa, R.I.=3). This and other apparently anomalous phenomena become understandable after study of the spatial atomic arrangement of the two isomeric esters. (The Courtauld atomic models, distributed by Griffin and Tatlock, London, were used in this work.) This is0 C-3 group, by its inherent compactness, achieves a certain degree of free rotation about its bond attachment to the PO, nucleus. However, the n-propyl radical, in its three varying modes of rotation, is subjected to repeated influence of the electronic fields of the PO, central body. Perhaps intramolecular hydrogen bonding in the n-propyl molecule is severe enough to prevent a clean breakway of the - C H ~ C H Z C H ~whole body. At any rate, while the 43 ion is quite small (R.I. = 3) in the n-propyl spectrum, a 41 ion (--CHzCH&H-;R.I.=15) and a mass42 ion (--CHZCH&Hr) (R.I.=7) are more intense. Other ions (masses 65, 82, 107, 125, and 137) illustrate a greater degree of hydrogen migration for the normal C-3 isomer. These various esamples may indicate that the electrophilic nature of the PO, nucleus cannot achieve severe hydrogen migration unless a certain degree of hydrogen bcnding esists. Di-n-butyl Phosphite. As expected, the spcctrum of the dibutyl ester is very similar to t h a t of the dipropyl compound. Some differences are noted: a significant mass-65 ion formed by the dipropyl ester is not as intense in the spectrum of the dibutyl compound; a good mass-94 ion found with dibutyl is not found in the spectrum of the dipropyl ester; the dipropyl compound exhibits an appreciable parent-minus-pro~~osy ion (107), but dibutyl none of a parent-minus-butosy (121) ion. The absolute sensitivities of the base peaks differed widely for the various homologs, therefore correlations of the degree of ionization and comparisons of total ionization among the various spectra mere meaningless. However, some trend can be established in that the number of rearrangement ions, which evolve from each particular phosphorus-oxygen central body, increases with homolog molecular weight. Evidence of severe hydrogen migration in the butyl ester is greater than with the propyl esters; therefore, i t is assumed that the number of electrophilic hydrogen shifts increase with increasing alkyl chain length.
Diallyl Phosphite. The cohesiveness of the allyl group usually prohibits the rearrangement phenomena (2) so frequently found in mass spectra (1). But in the case of diallyl phosphite, one and two hydrogens are attracted to the phosphorus groupings to form rearrangement ions of mass 122 and 123 based on the CH-CHCHt-0-P (H)02 entity. A mass-41 allyl ion is, as expected, the ‘most abundant species in the fragmentation pattern and the allylosy ion (mass 57) is second in intensity. The third level of intensity is represented by a classical mode of ordinary bond fission (the direct loss of an allyl group), but many subsequent ion forms are B result of hydrogen shifts to the electrophilic phosphorous groupings (r’g ’1 ure 1). CONCLUSIONS
Many observed bond scissions of alkyl phosphites are analogous to classical fragmentations observed in the mass spectra of hydrocarbons, but the phosphite skeletal formation is the basis for most of the abundant ions. Many intense ions are formed by the migration of one or more hydrogens from alkyl substituent groups to the estremely electrophilic phosphorous central bodies. This shifting of hydrogens to form major ions was found to be more pronounced in dialkyl phosphites than in any other class of compounds. Specific instances of this shifting to form ions increases with increasing alkyl chain length. It has been assumed by Quayle (6), and this writer (in the past) that phosphorus achieved a quadricovalent state under electron bombardment and that, in the case of the phosphorus esters, all ionic formation could be esplained by directing migratory hydrogens to oxygen atoms :
!
adhere to the PO, central body. But the predominate mass-83 ion must contain four hydrogens. The graphic depiction of the mass-83 ion shown in Figure 1 is offered as one possible arrangement, but it does not completely satisfy all spatial ramifications of the dynamic body. Perhaps it is not sufficient to apply the simple valence-bond theories of single, double, and triple bond states used in organic chemistry to the study of phosphorus compounds. Many questions about the nature and scope of phosphorus bonding to other atoms have been posed by Van Wazer (79,and many other phosphorus chemists and more data and more detailed studies are needed. The almost universal assumptions of the past may not be completely valid and perhaps it is misleading to assume that each individual atom must ,progress to a particular position, there ”tot : remain undisturbed, as implied by mechanistic postulations of rearrangement so adequately discussed by McLafferty (6) and many others. Rather, a more generalized point of view might be taken that electronic attractions of discrete electrophilic bodies acquire the association of available hydrogen atoms or nucleophilic particles to achieve a state of equilibrium; that this is a form of mutual attraction by two entities coming together to form an ionic body; and that such a body can esist for an interim and need not be predicated upon a permanent arrangement exemplified by a typical structural formula. ACKNOWLEDGMENT
The author is indebted to A. E. Gabany, Jr., and P. E. Hastings for obtaining these mass spectra and assisting ”1 their interpretation and to Q. Quick for helpful suggestions, advice: and discussions.
HO- --OH I
(5
H
LITERATURE CITED
Mass-83 ion
However, this present work has led to the belief that phosphorus(V) is a prevalent state. A conventional electronic configuration will not satisfy the phosphorus(V) state for the mass-83 fragments indicated in Figure 1, H X. xx H-xg>P;gX+H XY
xx
:0:
