W. H. MCFADDEN, L. E. BOGGS, AND R. G. BUTTERY
3516
Specific Rearrangements in the Mass Spectra of Butyl Hexanoates and Similar Aliphatic Esters
by W. H. McFadden, Lois E. Boggs, and R. G. Buttery Weatern Regional Research Laboratory,l Albany, California 94710
(Received May 19, 1966)
The mass spectra of esters with a t least two carbons in the alcohol moiety and a t least four carbons in the acid chain have ion peaks a t masses 60 and 61. The relatively complex rearrangement mechanism leading to formation of these ions has been investigated using various deuterated butyl hexanoates and similar esters. The data appear to show that the first step in this reaction is transfer of one hydrogen from the y position of the acid group to an oxygen, followed by rupture of the a-P bond. This daughter fragment then undergoes a further hydrogen rearrangement in which one or two hydrogens (for masses 60 or 61) are transferred from the alcohol moiety to the oxygens and the resulting intermediate complex breaks a t the carbon-oxygen bond to give the ions CtH4O2f (60) and C2H502+ (61). The data also showed that rearrangement ions which occur a t masses 73, 87, etc. are formed by similar consecutive reactions. The ion fragment at mass 101 in the spectrum of butyl hexanoate was shown to be partly due to a simple bond break and partly due to a rearrangement process. I n the case of the simple bond break, a selective exchange of hydrogen apparently occurs between the butyl and hexanoate groups prior to formation of COOC4H9+ in approximately two-thirds of the events.
Introduction Various rearrangement ions are observed in the mass spectra of aliphatic esters. Of considerable interest is a rearrangement reaction that yields ions a t masses 60 and 61 from esters with two or more carbons in the alcohol moiety and four or more straight-chain carbons in the acid moiety. This reaction appears to be sterically controlled on both sides and requires transfer of two and three hydrogens, respectively, for formation of the mass 60 and 61 ions. Sharkey and co-workers suggested that the mass 60 rearrangement ion contained two hydrogens from the alcohol moietyJ2analogous to the well-known rearrangement reaction which forms ions a t masses 47, 61, etc., from formates, acetates, etc. However, this speculation was not supported by labeling experiments. Conventional mass analysis of a-methyl esters has est,ablished that the a position of the acid is retained in the ion decomposition,a and high resolution mass analysis has established that two oxygens are involved, as expectedn4 To obtain a further understanding of this system, several deuterated n-butyl n-hexanoates and The Journal of P h y s i d Chemistry
similar esters have been synthesized and the mass spectra have been obtained.
Results and Discussion A . Rearrangement Ions at Masses 60 and 61. 1. Hydrogen Transfer from the Acid Moiety. The mass spectra of ethyl butyrate-Zd2 show that the ion current due to masses 60 and 61 shifts to 62 and 63 in the spectrum of the deuterated species, which indicates that the a carbon of the acid moiety remains intact (Table I). This conclusion is substantiated by data obtained previously on several a-methyl esters whose mass spectra showed peaks a t masses 74 and 75 rather than 60 and (1) A laboratory of the Western Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture. (2) A. G. Sharkey, Jr., J. L. Shults, and R. A. Friedel, Anal. Chem., 31, 87 (1959). (3) ASTM Committee E-14 File of Uncertified Mass Spectra, Chairman, A. H. Struck, Perkin-Elmer Corp., Norwalk, Conn., 1958 to date. (4) J. H. Beynon, R. A. Saunders, and A. E. Williams, Anal. Chem., 33, 221 (1961).
3517
SPECIFICREARRANGEMENTS IN THE MASSSPECTRA OF BUTYLHEXANOATES
TEbie I: Participation of Hydrogen from Acid Moiety in Formation of Ions at Masses 60 and 61 (Per Cent of Total Ion Current)
Ethyl butanoate Ethyl butanoate-2-d2 Ethyl butanoate-Z-dz, 3 4 , 4d3" Butyl hexanoate Butyl hexanoate-3-& Butyl hexanoate-4&
4.7 0.05 0.1
1.8 0.5
0.4
0.0 4.5 0.3
3.9
1.1 0.9 2.8
...
...
0.06 0.8
0.02
3.2 0.9
0.0
1.7 4.1
1.6
0.1
6.6 6.7 (6.6) 5.0 4.2 4.5
0.02
These data obtained on Bendix time-of-flight mass spectrometer and normalized to be comparable to the other ethyl butanoate data.
Table 11: Participation of Hydrogen from the Alcohol Moiety in Formation of Ions a t Mssses 60 and 61 (Per Cent of Total Ion Current)
-
m/e
Ethyl butanoate Ethyl-1-d2 butanoate Ethyl-2-da butanoate Butyl hexanoate Butyl-I-& hexanoate Butyl-242 hexanoate Butyl-ds hexanoate Hexyl hexanoate Hexyl-3-d2 hexanoate Hexyl-4d2 hexanoate a
60
61
62
63
ZI
4.7 4.2 1.2 3.4 3.3
1.8 1.4 3.4 1.1
0.04 0.9 0.8 I.. 0.2 0.4 0.8 0.1 0.3 0.7
0.03 0.1
6.6 6.6 6.0 4.5 4.8 4.4 (4.4)" 4.1 4.3 4.0
2.5 0.4
2.0 1.6 1.6
Sample run with different focus conditions.
1.3 1.5 2.6 2.0 2.4 1.7
0.6
... 0.02 0.03 0.6
... 0.03 0.4
% Labeled position (mass 60)
Random selection
11% 73%
40 % 60%
11% 24%
22% 22%
24%
15% 15%
20%
ZI was normalized to be 4.4%.
61.3 The mass spectra of n-butyl n-hexanoate-3-dz, and n-butyl n-hexanoate-4-dz establish that the 4 position of the acid group (y-hydrogen) predominates in formation of these ions and that little contribution comes from the other hydrogen positions of the acid. Thus, as seen in Table I, deuterium on the 3 position (butyl hexanoate-34) was not involved (less than 573 but deuterium on the 4 position is involved about 75% or more. A similar conclusion can be deduced from data presented by Biemann and Dickelman for ethyl butanoate-3-dz and ethyl b ~ t a n o a t e - 4 - d ~ . ~The * ~ mass indicates spectrum of ethyl butanoate-2,2,3,3,4,4,4-d, that essentially only one hydrogen is transferred from the acid moiety in forming this ion. Mass peaks at 60 and 61 are shifted to 63 and 64 due to retention of the two a-deuteriums and transfer of the one deuterium from the y position. 2. Hydrogen Transfer from the Alcohol Moiety. Transfer of hydrogen from the alcohol moiety was studied first with ethyl-1-dz butanoate and eth~l-2-d~
butanoate. The data for these compounds show (Table 11) that the bulk of the hydrogen transferred in forming the mass 60 ion was from position 2. Assignment of approximate values for CH&OOH3+ and CH2COOHD+ at mass 61 permits a simple calculation which gives transfer of one hydrogen a 73% probability from position 2 and 11%from position 1. (Failure to add to 100% may be attributed to isotope effects, or to H-D exchanges. These will be discussed in later sections.) These values are close to those obtained by Godbole and Kebarle for transfer of one hydrogen (deuterium) from the ethyl group of deuterated ethyl acetate to form the ion CH3COOHor CH3COOD.' Data pertinent to transfer of the second hydrogen ( 5 ) K. Biemann, "Mass Spectrometry. Organic Chemical Applications,'' McGraw-Hill Book Co., Inc., New York, N. Y.,1962,p 121. (6) T. E. Dickelman, M.S. Thesis, Massachusetts Institute of Technology, 1962. (7) E. W. Godbole and P. Kebarle, Trans. Faruduu SOC., 58, 1897 (1962).
Volume 70, Number 11
November 1966
3518
from the alcohol moiety of ethyl butanoate to form the ion CHZCOOH3f are not so easily analyzed, but it appears that a statistical selection of the hydrogens at positions 1and 2 is taking place. Thus, in the spectrum of ethyl-1-dz butanoate, approximately one-half of the ion current due to this species appears at mass 61 and one-half at mass 62. I n the spectrum of ethyl-2-da butanoate, due to the predominant transfer of a first deuterium from position 2, this ion fragment occurs about one-half a t mass 62 and one-half at mass 63. This also was consistent with the data obtained by Godbole and Kebarle for transfer of a second hydrogen. Further information on the transfer of hydrogen from the alcohol portion was obtained from the spectra of butyl-l-dz hexanoate, butyl-2-dz hexanoate, hexyl-&& hexanoate, hexyl-4-dzhexanoate, and butyl-1,1,2,2,3,3,4,4,4-d9 hexanoate (Table 11). Again, deuteration in the 1 position had only a small influence on the mass 60 rearrangement ion (about 11% probability). However, the considerable transfer from the 2 position observed in the spectrum of eth~l-2-d~ butanoate is not apparent in the spectrum of butyl-2-dz hexanoate. Instead, a transfer probability of 24% is noted, as compared with a statistical value of 22% if all hydrogens (deuteriums) were equivalent. Similarly, the 3 and 4 positions of the hexyl hexanoates show no selective transfer but rather, an approximate 20% probability of transfer from either position. The spectrum of butyl-dg hexanoate essentially confirms that only one deuterium is transferred from the alcohol in forming the mass 60 ion. However, the ioncurrent values observed at masses 62 and 63 require some rationalization. The value 0.8% total ion current at mass 62 is possibly due to an exchange involving the three hydrogens from the acid moiety. This is consistent with the fact that the nine hydrogens (deuteriums) of the alcohol group were observed to exchange rapidly, and that this process may occur in part through transfer back and forth to the oxygens. The value of 2.6% total ion current at mass 61 (plus the 0.2-0.3% at mass 60 attributed to the butyl-d8 impurity) indicates that such an exchange process is not predominant but may influence 10-20% of the events. The decreased value observed at mass 63 for transfer of two deuteriums is also consistent with this suggestion. Transfer of deuterium and hydrogen to give mass 62, equivalent to the mass 61 ions, is approximately statistical (low for butyl-l-dz hexanoate, high for hexyl-4-dz hexanoate), and transfer of two deuteriums to give mass 63 is insignificant (at the expected 1-2% probability level). 3. Suggested Mechanism. The data suggest that the first step in the formation of the mass 60 and 61 reThe Journal of Physical Chemistry
W. H. MCFADDEN, L. E. BOGGS, AND R. G. BUTTERY
arrangement ions is transfer of a y-hydrogen from the acid portion with subsequent cleavage of the CY-pcarbon bonds. The ion thus formed is one of the well known ester rearrangement ions (mass 74 for methyl esters, 88 for ethyl esters, etc.). This daughter ion subsequently transfers a hydrogen from the alcoholic portion to (presumably) the other oxygen, and may then lose the remainder of the hydrocarbon group by cleavage of the oxygen-carbon bond thus forming the mass 60 ion. Alternatively, after transfer of the first hydrogen from the alcohol portion, a second hydrogen may be transferred to form the mass 61 group on subsequent decomposition. Thus
nile =60
+ m/e=61
Although the data for the ethyl esters show steric selectivity for the first alcohol hydrogen transferred, no significant selectivity for the hydrogens of carbon 2 is indicated for butyl or hexyl esters. I n addition, the second hydrogen transferred from the alcohol is statistically selected from all possibilities. This could mean that no steric preference occurs in the initial hydrogen transfer (as has been suggested for amines and ethers819),but it is more probable that hydrogen-deuterium exchanges are taking place. As is well known, with fragment ions or unsaturated parent ions, the rate of hydrogen-deuterium exchange is often comparable to, or exceeds, the rate of the decomposition reactions,"J-l3 and the failure to establish a precise mecha(8) C. Djerassi and C. Fenselau, J . Am. Chem. Soc., 87, 5751 (1965). (9) C.Djerassi and C. Fenselau, ibid., 87, 5747 (1965). (10) W.A. Bryce and P. Kebarle, Can. J. Chem., 34, 1249 (1956).
SPECIFIC REARRANGEMENTS IN THE RilAss SPECTRA OF BUTYLHEXANOATES
nism is generally attributed to H-D exchanges in which positional identity is lost. B. Rearrangement Ions at Masses 73, 87, etc. 1. C3H502+, Mass 73. Other rearrangement processes in esters lead to ions with two oxygens at masses 73, 87, etc., which may be formed by a mechanism similar to that which forms the mass 60, 61 ions. It is true that ions such as C3HjOZ+,which occur a t mass 73 from ethyl esters, may be attributed in some cases to a simple bond break, e.g., R-COOC2H5+ --t COOCzHs+ (mass 73). However, in many instances this proves to be a subordinate process. Data which summarize the distribution of deuterium from various species in the C3H602ion are given in Table 111. The acid portion of the molecule was studied using ethyl butanoate-2-dZl butyl hexanoate-3-dz1 and butyl hexanoate-4-d2. For the a- and @-labeledspecies (ethyl butanoate-2-dz and butyl hexanoate-3-d2), the ion current shifts from mass 73 to mass 75 indicating that these CH2groups are retained in formation of the ion. Furthermore, the y-hydrogens are not involved as is evident from the spectrum of butyl hexanoate-4-d2.
Table 111: Distribution of Deuterium Observed in the Formation of the Mass 73 Ion, CsHb02, from Various Esters (Per Cent of Total Ion Current) m/e---
Ethyl butanoate Ethyl butanoate-2-d Butyl hexanoate Butyl hexanoate-3-dz Butyl hexanoate-4-d Butyl-d9 hexanoate Butyl-1-dz hexanoate Butyl-2-dz hexanoate Hexyl hexanoate Hexyl-3-dz hexanoate Hexyl-4d2 hexanoate
73
74
75
ZI
4.1 0.6 2.3 0.1 2.2 0.1 1.9 1.6 1.6 1.4 1.2
0.2 0.5 0.3 0.3 0.3 2.5 0.7 0.8 0.2 0.5 0.5
0.02 2.9 C.2 1.5 0.2 0.7 0.2 0.2 0.2 0.1 0.2
4.3 4.0 2.8 1.9 2.7 3.3" 2.8 2.6 2.0 2.0 1.9
' This sample run with different focus conditions. ZZ may not necessarily relate to other butyl hexanoates.
The four esters labeled in the 1, 2, 3, or 4 positions of the alcohol show between 16 and 24y0 involvement of that particular position in transferring one hydrogen. The statistical selection would be 22% for the butyl esters and 18% for the hexyl esters, so again it is concluded that there is essentially a random selection of hydrogen from the various alcohol positions, most likely due to multiple exchange phenomena. The indicated mechanism for formation of the mass 73 ion involves a break of the @-y carbon-carbon bond,
3519
transfer of one hydrogen from the alcohol portion, and loss of the remainder of the alcohol moiety. Thus RCH&H2COOC4Hg+ +
(mass 73) The suggested sequence of reactions, namely breaking of the P-y bond prior to rearrangement, etc., can only be surmised in the absence of the observation of metastable ions to prove a particular sequence. Again, however, the random selection of hydrogen from the alcohol portion supports the suggestion that a fragment ion is an intermediate. 2. CJY702+ and C*H902+, Masses 87 and 89. Ion currents due to the fragments C ~ H ~ O and Z + C4H902+ are observed at masses 87 and 89 in the spectrum of butyl hexanoate but, unfortunately, the deuterated species give only a partial understanding of the mechanism of formation and certain features are not revealed. Thus, deuterium on the one or two position of the butyl group (b and c in Figure 1) is modestly involved in formation of C4H7O2+(mass 87 shifts to 88 about 25%). For the fully deuterated butyl group, 87 quantitatively shifts to mass 88 (f in Figure 1). Transfer of one hydrogen from the butyl portion is indicated, possibly by a mechanism analogous to that suggested for the mass 73 rearrangement fragment. Further justification for this analogy is obtained from the spectrum of butyl hexanoate-4-dz (e in Figure 1) in which the mass 87 ion has shifted to mass 89 indicating involvement of the y-carbon and presumably the a- and ,&carbons as well. However, the deuteriums of butyl hexanoate-3-dz exchange considerably, so that the suggested mechanism, namely breaking of the y-6 bond, transfer of hydrogen from the butyl group, and expulsion of the depleted butyl, is not as clearly defined as for formation of the mass 73 ion via a break of the @-y bond. A few features on the formation of the mass 89 ion may be deduced from the data in Figure 1. In f, the mass 89 ion shifts to mass 91 indicating that two hydrogens from the butyl group are retained. One of these appears to be selected from the 2 position of the alcohol (85y0transfer), but the 1 position is not in(11) W.H.McFadden, J. Phys. Chem., 67, 1074 (1963). (12) A. B. King, ibid., 68, 1409 (1964). (13) D. P. Stevenson and C. D. Wagner, J. Chem. Phys., 19, 11 (1951).
Volume 70. Number 11
November 1966
3520
W. H. MCFADDEN, L. E. BOGGS,AND R. G. BUTTERY
CSH,,C0OC4 H
l:ilL .6
87 89 91
C5H 9- 302-COOC, H
r
C5 HllCOOC4H7- lo2 (b)
IlLL87 89 91
CSHl,COOC4H,-
r
202
.(E)
ILL 87 89 91.
C5 H9-402-COOC4Hp
CsH1,COOC,09 I
rn
CI
$ r
87 89 91
m/e
87 89
m/e
91
CsH,-4
D2-COOC4Hg
CsHl,COOC4Dp
87 89 91
m/e
Figure 1. Ion intensities observed corresponding to the formation of mass 87 and 89 ions, C4H102 and C4&02, from deuterated butyl hexanoates.
volved (less than 9%). Furthermore, the quantitative shift to mass 91 in d and e indicates that the ion contains the p- and y-carbons of the acid group. Although a CH20 group or other groups may be expelled from the middle of a m o l e ~ u l e - i o n ,it~ is ~ ~unlikely ~~ that CH2 would be eliminated via a cyclic intermediate, so that the a-carbon is also presumed to be contained in the mass 89 ions. Thus, to form the ion around a CH2CH2CH2CO0nucleus (mass 86) requires transfer of the two indicated butyl hydrogens plus one additional hydrogen from the 6 or E position of the acid. These two positions were not labeled, SO that further detail is not known. However, it is interesting to note that if this hydrogen is transferred from the 6- or e-carbon to one of the oxygens, then a seven or eight-membered ring intermediate would be involved. 3. C5H902+,Mass 101. Ion current due to the CsH902+species could arise from butyl hexanoate by the simple bond break R-COOR+ -+ GOOR+, or from a rearrangement mechanism similar to that observed for the mass 75 and 87 ions. The data from the deuterated esters indicate that both mechanisms are operating in a ratio of approximately 3 : 1 favoring the simple bond break. However, it is also apparent that even the so-called simple bond-break reaction involves an unexpected hydrogen-deuterium exchange prior to fragmentation. The data for the per cent of ion current, due to CsH902+,or deuterated equivalents, are given in Figure 2. A solid line is used to represent that amount formed by the simple bond break, and a dotted line is used to represent the fraction formed by the rearrangement reactions. As is noted in b or c (alcohol deuterated on the 1 or 2 position), about 70% of the ion current appears a t mass 103 indicating the reaction RCThe Journal of Physical Chemistry
C5H9-3D2-COOC4H ,
Figure 2. Ion intensities observed corresponding to the formation of mass 101, CsHl102 from deuterated butyl hexanoates. The dotted lines represent the ion current due to a rearrangement reaction; the solid lines represent that due to a simple bond-break reaction.
OORf -P R and COOR+. However, in f (butyl group fully deuterated), the mass peak should shift to 110. I n actuality, both 109 and 110 are observed, and the 109 peak is twice the intensity of the 110. Since the butyl-& species was not greater than lo%, this rather unexpected event indicates that a single hydrogendeuterium exchange is probable between the butyl and the hexanoate prior to the simple bond-break reaction. The data for the butyl hexanoates deuterated on the acid moiety (Figure 2, d, e) give further evidence that about 30% of the CsH902+ ion is formed by a rearrangement involving loss of the terminal methyl and transfer of a hydrogen from the butyl group. I n f, an ion peak is observed at mass 102 indicating a clean transfer of one butyl hydrogen. I n e, however (acid deuterated on position 4), a significant fraction of the simple bond break ion current (RCOORf -P R COOR+) has shifted from mass 101 to 102. This appears to correspond to the mass 109 ion peak unexpectedly observed in f , and it is concluded that the hydrogen-deuterium exchange apparently involves one of the y-hydrogens of the acid. Further understanding of this unusual exchange would require specifically deuterated species not currently available. C. Other Fragmentation Characteristics. 1. The Olefin I o n ( R M i n u s H ) + f r o m the Alcohol Moiety. One of the more important ion fragments observed in the mass spectra of esters is the olefin ion formed from the
+
~
(14) W. H. McFadden, K. L. Stevens, S. Meyerson, G. S. Karabatsos, and C. E. Orzech, Jr., J . Phys. Chem., 69, 1742 (1965). (15) D.R. Black, W. H. McFadden, and J. W. Corse, ibid., 68, 1237 (1964).
SPECIFICREARRANGEMENTS IN THE MASSSPECTRA OF BUTYL HEXANOATES
alcohol portion by transfer of a hydrogen to oxygen and subsequent frachre of the 0-C bond. This reaction is often regarded as formation of a neutral acid molecule and an olefin ion, and the ion fragment is diagnostically important in qualitative identification work. The mechanism for the formation of this olefin ion has recently been the focus of considerable attention. Labeling experiments with butyl propanoate16and with pent.yl acetate and hexyl acetate" have established that for these molecules, approximately 55% of the migrating hydrogen is from the p position and 45% from the y position. Formally, this would yield 1-alkenyl or 2-alkenyl ions, presumably through six or sevenmembered ring intermediates that transfer hydrogen to the carbonyl oxygen. In the present experiments, certain data indicate a complexity greater than was revealed by the three simple examples cited. Of modest concern is the confirmation that about %lo% of the hydrogen transfer may arise from the a position (butyl-1-d2 butanoate). This fact was noted, but glossed over, in the previous reports. The spectrum of butyl-2-dz butanoate confirmed a 55% involvement of the p position, but butyl242 hexanoate indicated only about 30% migration from the p position. The spectrum of hexyl-4-dz acetate previously reported showed no transfer from the y position. In sharp contrast, the spectrum of hexyl4-dz hexanoate obtained here indicated that the y position was involved about 26%. The data for the hexyl hexanoates showing this controversial point are given in Table IV. Table IV : Per Cent of Total Ion Current in the Mass Spectrum of Hexyl Hexanoates Due to Six Carbon Ion Fragments Hexyl-
Hexyl-
Hexyl
Mass
do
3-dr
4-dr
82 83 84 85 86 87 88
0.1 0.4
0.0 0.1 0.4 2.2 6.1 1.1
L:
9.9
0.0 0.1 0.3 3.2 5.1 1.3 0.1 10.1
7.9
1.3 0.2
9.9
More significant data occur in the mass spectrum of the perdeuterated ester, butyl-& hexanoate. I n this mass spectrum, the C4 ion fragments from the butyl group should all be of even mass, after correction for the C1* isotope contribution and the per cent of the ds species. The upper limit of this latter parameter was determined from the ratio of the ion currents at masses
352 1
125 and 124, corresponding to the rearrangement ion normally observed at mass 116 (CH2COOC4Hg.H+). The possibility of H-D exchange between butyl and hexanoate moieties was ignored, and more important, the deuterated equivalent of the ion peak at mass 115, normally about 10% of the 116 peak, was not considered as a possible contributor at mass 124. Thus, ignoring the alleviating factors, a value of 9.1% was obtained as the upper limit for butyl-ds hexanoate. The corrected data obtained for the C4H, ions from butyl hexanoate and butyl-& hexanoate are presented in Table V. It is noted that for the simple bond break reaction, C5H~&OO-C4Dg + C4D9+,approximately 6% acquire a hydrogen from the hexanoate portion. For the rearrangement reaction, C5HllCOO-CD&DC2D5+ 4 C4DS+, approximately 10.5% acquire a hydrogen. Clearly, the existence of H-D exchanges between the butyl and hexanoate are indicated. However, it is not apparent whether the oxygens serve as an intermediate bridge or whether the exchange occurs --+
Table V: Distribution of Hydrogen and Deuterium among the CIH, Ion Fragments from Butyl-& Hexanoate. Sum of Ion Current from Mass 53 to 66 Equal to 100% Mass
53 54 55 56 57 258-61 62 63 64 65 66
Formula
% 1:
% I,
do
d0
3.6 1.2
16.4 56.4 22.4
i
47.4 butyl 1 . 2 group 18.8)
a All data corrected for Cis. The d9 butyl section has been corrected for 9.1% ds (maximum possible).
directly from alkyl group to alkyl group. These exchanges would possibly be related to the anomalous observation discussed in the previous section, namely that for the simple bond break leading to COOC4Hg+ (mass lOl), the spectrum of the dg compound showed a shift of 35% to mass 110 and 65% to mass 109. In a previous study, Djerassi and Fenselau'B were unable to account for 22% of the hydrogens in the rearrangements reaction which produces the benzoic acid (16) C. Djerassi and C . Fenselau, J. Am. Chern. Sac., 87, 5756
(1965).
(17) W. Bene and K. Biemann, ibid., 86, 2375 (1964).
Volume 70,Number 11
November 1966
3522
W. H. MCFADDEN, L. E. BOGGS, AND R. G. BUTTERY
ion from butyl benzoate. This difference was attributed to an excessive isotope effect, but it is more probable that similar H-D exchanges were occurring between the aromatic ring and butyl group. 3. The Ion Formed by Loss of Mass 43. It has been well established that during fragmentation of long chain fatty acid esters (c18, etc.), a rearrangement occurs in which the chain forms a cyclic intermediate, transfers a hydrogen to the a-,p-, or y-carbon, and then expels the a-,0-, and y-carbons as a C3H7 group (mass 43).18 In some instances, this information has been extrapolated to smaller esters (C5,C,) and silyl derivatives. Some of the data obtained in this work show that such an extrapolation is not warranted. From the data of Table VI, it is apparent that approximately one-half of the ion current due to loss of mass 43 is due to loss of C3H7 from the butyl chain. ;\lore important, it is observed that when the acid is deuterated in the p position (butyl hexanoate-3-d$, the loss of mass 43 does not shift to loss of mass 45, thus ruling out a cyclic intermediate. This is confirmed from the observation that in the spectrum of butyl hexanoate4 - d ~the ~ loss is shifted to mass 45, confirming the occurrence of a simple bond break.
Table VI: Ion Current Due to Loss of Propyl Fragments in the Mass Spectra of Deuterated Butyl Hexanoates
Butyl hexanoate Butyl-1-dz hexanoate Butyl-2-& hexanoate Butyl hexanoate-3-d~ Butyl hexanoate-4-dz
Loss 43
Loss 45
0.35 0.31 0.17 0.20 0.17
... ... 0.18
... 0.14
Experimental Section A. Sample Preparation. Deuteration of the specific positions labeled for this study was accomplished by the following reactions. Ethanol-ldz and n-Butanol-ld2 RCOCl
(1) LiAlD4
RCDzOH
Ethanol-2-d3, n-Butanol-2d2, Hexanol-4-dZ RCOCl
(1) LiAlH4
(R
=
CH3, CaH7)
n-Hexanol-3d2, n-
RCHZOH
(R = CD3, CzHsCDz, CaH&D2CH2, CzH&DzCzHd) Perdeuterio-butanol CD3CD2CDzCOOH
LiAlD4 HzO
The Journal of Physical Chemistry
CDaCDzCDzCDzOH
n-But yric A cid-2d2
5
CH~CHZ!CH(COOC~H,)~
-
PI06
CH3CHzCD(COONa)2 D CHsCHzCDzCOOD
A
(-Con)
P~
CH3CH&D(COOD)2
n-Hexanoic Acid-4d2 CH3CH2CD2CH20H
HBr
CH3CH2CD2CH2Br
CHz(CO0Et)z
CH3CHzCDzCH&H(COOEt)2
A
(-
cod
CH3CH2CD2CH2CHzCOOH n-Hexanoic Acid-3dz CH3CHzCHZCDzOH
PBrr
a8
C H ~ C H Z C H ~ C D above ZB~
CH~CHzCHzCDzCH2COOH The acid chlorides were prepared from the corresponding acid using thionyl chloride. l9 CD3COOH was obtained commercially. Esters deuterated on the alcohol group were synthesized from the alcohol and the appropriate acid chloride. Esters deuterated on the acid group were synthesized by refluxing Ihe free acid and the appropriate alcohol in benzene using ptoluenesulfonic acid catalyst. Isotopic purity was determined by inspection of the mass spectra. With the exception of the perdeuteriobutyl-dg hexanoate, the compounds were better than 95% isotopically pure. No corrections were made for the isotopic impurities or for the natural isotopic species, but the conclusions are based on effects that exceed these minor contributions. For perdeuteriobutyl-d9 hexanoate, a maximum estimate of 9.1% was used for the d8 species. This estimation could be significantly reduced by factors discussed in the text, but a higher value is exceedingly improbable. A value of over 20% of the d8 compound would have been required to eliminate the conclusion that CdD7Hf was formed by H-D exchanges. B. Mass Spectrometry.20 Most mass spectra were obtained on a CEC 21-110B mass spectrometer operating with resolution set a t about 8000. The ion source (18) N. Dinh-Nguyen, R. Ryhage, S. Stallberg-Stenhagen, and E. Stenhagen, Arkiv Kenzi, 18, 393 (1961). (19) I. H.Gilman and A. H. Blatt, Ed., "Organic Synthesis," Coll. Vol., John Wiley and Sons, Inc., London, 1941, p 147. (20) Reference to a company or product name does not imply approval or recommendation of the product by the U. S. Department of Agriculture to the exclusion of others that may be suitable.
ZINCOXIDE-PHOTOSENSITIZED PHOTOLYSIS OF LEADCHLORIDE
was maintained a t 150’. Where possible, samples were run with identical focusing conditions. All CH,-0 doublets were separated, thus permitting hydrocarbon, monooxygenated, and dioxygenated peaks to be reported individually.
3523
A few spectra were obtained on aBendix time-of-flight mass spectrometer, Model 12. One example is reported in Table I. The data were normalized to be consistent with those data from the CEC mass spectrometer.
Zinc Oxide-Photosensitized Photolysis of Lead Chloride
by W. C. Tennant Chemistry Division, Department of Scientific and Industrial Research, Wellington, New Zealand (Received May 1 1 , 1966)
Lead chloride (PbC12) is decomposed by ultraviolet light at wavelengths below 3200 A. I n wetted mixtures with ZnO, this reaction is effected by light of wavelength 3660 A. The extent of decomposition is strongly dependent on the fluorescence type of the ZnO. A mechanism which is similar in many respects to the dye-photosensitized decomposition of silver halides is proposed.
Introduction The action of ZnO solid particles in bringing about the photochemical decomposition of certain inorganic salts in spectral regions where they are not normally photosensitive has long been known.’ The studies of Goodeve2 and of Goodeve and Kitchener3 showed that the primary process in such reactions is the absorption of energy by ZnO, but the mechanism of its subsequent transfer to the decomposing molecule is not known. This is indeed true for many photosensitized reactions in s01ids.~ Early attempts to provide an explanation of ZnOphotosensitized reactions were made by Baur and Perret5 and Perret.6 Thus the decompositions of AgN03 and of HgClz in irradiated aqueous suspensions with ZnO were explained in terms of an “intramolecular electrolysis,’’ metal being deposited a t the ZnO ‘‘cathode” and oxygen liberated a t the “anode” following the absorption of energy by the sensitizer. While such schemes satisfactorily account for the simultaneous oxidations and reductions which occur in these reactions, they fail to account for the nonstoichiometric ratios of oxidized and reduced products. More
seriously, the schemes fail to differentiate between the photolytic reactions and the purely chemical reactions which ZnO undergoes with solutions of heavy metal salts resulting in most cases in the format,ionof sparingly soluble basic compounds.’ I n this paper, the chemical and photochemical reactions of ZnO-PbClz in aqueous suspensions and wetted powder mixtures are discussed. Irradiation of such mixtures by ultraviolet light of wavelength 3660 A result,s in the decomposition of lead chloride. Lead chloride itself has long been known to be photochemically active a t shorter wavelengths.8 From a (1) For review, see M. Burton and G. K. Rollefson, “Photochemistry and the Mechanism of Chemical Reactions,” Prentice-Hall Inc., New York, N. Y., 1939. (2) C. F. Goodeve, Trans. Faraday SOC.,3 3 , 340 (1937). (3) C. F. Goodeve and J. A. Kitchener, ibid., 34, 902 (1938). (4) J. Bourdon, J. Phys. Chem., 69, 705 (1965). ( 5 ) E. Baur and A. Perret, Helv. Chim. Acta, 7 , 910 (1924). (6) A. Perret, J . Chim. Phys., 2 3 , 97 (1926). (7) J. W. Mellor, “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. IV, Longmans Green and Co., London, 1923. (8) C. Renz, Z. Anorg. Allgem. Chem., 116, 62 (1921).
Volume 70, Number 11 November 1966
