The Journal of Physical ChemlsrIy. Val. 83,No. 24, 1979 3005
0-Atom Reactions with Methyl- and Ethylamlnas
Identification of Reactive Routes in the Reactions of Oxygen Atoms with Methylamine, Dimethylamine, Trlmethylamine, Ethylamine, Diethylamine, and Triethylamine Irene R. Slagle, Joseph
F. Dudich, and David Gutman”
Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 606 16 (Received May 31, 1979) Publication costs assisted by the Illlnois Institute of Technology
The gaseous reactions of oxygen atoms with mono-, di-, and trimethylamine as well as with mono-, di-, and triethylamine at ambient temperature were studied in a cross-jet reactor coupled to a photoionization mass spectrometer. The products detected and mechanistic information obtained when (CD3)2NHwas used as a reactant indicate that these reactions proceed by electrophilic addition to form an energy-rich amine N-oxide followed by rearrangements and ultimate decompositions into one or more of the following groups of products: 0 + R1R2R3N(R = H, CH3, C,H,) (1)RlR2N + OH (where R3 = H) or R1R2R3/N+ OH (where RS # H), (2) RIRiN + 1120, (3) RlR2N0 + RB,and RIRzNOH + C2H4(if more than one R group is C2Hb). Details of the experiments and special data reduction procedures are presented, and a general mechanism for the reactions of 0 atoms with aliphatic amines is discussed.
-
Introduction Due to the electrophilic character of the oxygen atom, its reactions with molecules containing nonbonding or T electrons generally proceed wholly or at least to a significant extent through a long-lived complex. Because atomic oxygen is divalent, the complex may contain over 100 kcal/mol of internal energy if two new covalent bonds are made to the attacking oxygen. In the absence of collisions, such an energy-rich adduct can undergo extensive intramolecular rearrangements and decompose via several different r ~ u t e s . l -Knowledge ~ of these intramolecular processes and of the decomposition pathways provides a deeper understanding of the factors which govern the reactivity of oxygen atoms. As part of our continuing interest in these processes we have now completed the first direct study of the mechanisms of the oxygen-atom reaction with aliphatic amines. The results of this study are reported here. Although the rate constants for several 0 amine reactions have now been m e a ~ u r e d , ~the J studies which obtained them produced no direct and little indirect information on the identity of their reactive pathways. It is not unreasonable to presume a priori that these reactions proceed at least partially via addition to form an excited amine N-oxide intermediate:
+
(1)
I
The new bond formed between nitrogen and oxygen would provide the adduct with over 70 kcal/mol of internal energy, enough to facilitate rearrangements and ultimate decomposition8 This initial step in the mechanism was suggested for the 0 (CHJ3N reaction by Atkinson and Pitts based on their observation that this reaction has a negative activation energy.7 The result,s we report here include the direct identification of reactive routes of the 0-atom reactions with six amines: mono-, di-, and trimethylamine (hereafter referred to as MMA, DMA, and TMA), and the three possible ethylamines (to be analogously referred to as MEA, DEA, and TEA). In addition details of the intramolecular rearrangements which occur in one of these reactions, 0 DMA, were also obtained, and they are included in this
+
+
0022-3654/79/2083-3065$0 1,0010
report, All reactions were studied in a cross-jet reactor and products were detected and identified with a photoionization mass spectrometer. A preliminary report presenting most of our findings on the 0 DMA reaction has been p~blished,~
+
Experimental Section Basically the experiment consists of studying the reaction in crossed jets containing the reactants. Those products produced at the intersection of the two reactant beams which are scattered in a particular direction are detected with a photoionization mass spectrometer specifically designed for these studies. The reactor configuration virtually eliminates secondary and tertiary reactions which can produce products which would interfere with the identification of the initial products and the use of photoionization maSs spectrometry eliminates ionization of background gases and generally suppresses significant dissociative ionization of species which could obscure product ion signals or generate false product ion signals, respectively. Because of the electron-donating properties of amines and nitrogen-centered free radicals, fragmentation of their ions is common even at very low ionizing energies. In these studies of 0 + amine react,ions, we encountered extensive fragmentation of product ions for the first time in spite of using monochromatic ionizing radiation below 11.6 eV. This section. describes how “apparent products” (genuine products or their fragment ions produced in the mass spectrometer) were established, and the next section discusses how the “genuineness” of each apparent product was determined. Apparatus. The new rotatable cross-jet reactor used in this study is shown in Figure 1. Details of this reactor, its performance, and the vacuum system in which it is housed have been reported elsewhere,1° so only the most important features will be repeated here. The reactants are formed into uncollimated, undiscriminated, ambienttemperature gas beams which intersect at 90” about 1.5 mm from each beam source. Typical beam fluxes are 6.6 X s-l for the 0-atom containing beam (formed by passing a 13% 02-87% He gas mixture at 2 torr through a microwave discharge and containing about 3% 0 atoms) s for the various amine beams. The and 1.6 X background gases in the vacuum chamber containing the reactor were typically a t a pressure near 9 x torr 0 1979 American Chemical Society
3086
Slagie, Dudich, and Gutman
The Journal of phvsical Chemistry, Vol. 83,No. 24, 1979
ROTATABLE CROSS-JET REACTOR
established. This systematic procedure is conducted by using each of the five photoionization energies. A second search for products was conducted by using a second fuel beam inlet 12 mm above the horizontal plane containing the 0-atom beam. No products should be produced as the two reactant beams do not intersect in this configuration. All product ion signals found when the two beams intersect are absent or are insignificant when they do not, indicating that the detected products are not produced to anv simificant extent outside the beam intersection volume. The relative magnitudes of all the product ion signals were measured as a function of reactant beam density to test for products originating from secondary reactions. Each beam density was alternately reduced hy a factor of -2 and the net product ion signals remeasured. Those established apparent products whose ion signals diminished hy the same fraction (a factor of -2) are the primary products of the 0 + amine reaction or their fragment ions. In some cases (particularly in the 0 + MMA reaction) the ion signals were too low to allow us to perform these tests with adequate accuracy. The established apparent products which passed the pressure test and other likely primary products of the 0 amine reactions are presented in Table I. For each reaction studied, confirmed apparent products whose ion signals were below 2% of that of the maximum apparent product ion signal are not included in Table I because it was not possible tb establish their ultimate origin. A few experiments were performed with selectively deuterated DMA, (CD&NH, t o identify the formulas of certain products, t o search for HDO as a product (for this special search photoionization a t 16.7 eV was used), and to answer certain mechanistic questions. Product sampling along the 0-atom containing beam and midway between the two reactant beams produced apparent product ion signals of the same relative magnitude indicating that selective product scattering does not occur under the experimental conditions of this study. "
-
0
,,,
I
Flgure 1.
,
:-.
, , i , -
Drawing of rotatable cross-Jet reactor. CROSS-JET ASSEMBLY (Side View)
PHOTON BEAM lx-sectm)
a CHOPPlNG WHEEL
FWre 2. Schematic drawing of crms-iet reactor, mass spectrometer, and vacuum chamber.
during experiments and consisted largely of helium. Those products and scattered reactants emerging from the intersection volume formed by the two reactant gas jets which are moving toward a skimmer located in the wall between the reaction chamber and the mass spectrometer chamber are modulated by a slotted-tooth chopping wheel before they enter the hole in the skimmer (Figure 2). The collimated and modulated beam then passes through the open ion source of the special photoionization mass spectrometer. Ion signals are detected by using either lock-in amplification or synchronous ion counting. Details of the photoionization mass spectrometer and its light sources have also been previously reported? Identification of Apparent Products. Using the lock-in amplifier, we located major apparent products hy recording mass scans with five available photon energies between 8.6 and 11.6 eV." A t each photon energy three scans are taken: one with both reactant beams on, one with the discharge which generates the 0 atoms tumed off, and one with the fuel beam off. Mass peaks present in the fmst scan hut absent or very small in the second or third are classified as apparent products. A systematic procedure with synchronous ion counting is employed to establish the presence or absence of all possible products. An apparent product is considered established if its net ion signal (i.e., ion count with both reactant beams on minus both ion counts when only one reactant is used) is present with 99% confidence (23 standard deviations of the net ion signal). Those absent or deteded with lower confidence limits are not considered
I
+
Discussion The identification of the reactive routes of the 0 + amine reaction depends on the ability to determine which apparent reaction products are genuine and which are fragment ions. Since the decisions regarding the origin of the detected apparent products involve considerations based more on the type of ion detected than the reaction under study, the discussion which follows is arranged around classes of reactive routes, beginning with those which produce the products of highest mass (e.g., adduct formation) and proceeding through those which produce products of decreasing mass where fragmentation becomes an increasingly important consideration in the interpretation of the data. Mechanistic details of each type of route are included in the subsections which follow, and a general mechanism for 0 + amine reactions is proposed a t the end of the discussion. Adduct Formation. The O.amine adduct was detected in three of the six reactions studied. In all cases the adduct ion signal was relatively minor in relation to those of other products. However, it was not dissimilar in relative magnitude to adduct ion signals detected in comparable studies of 0-atom reactions with olefins and sulfides where an addition mechanism is generally believed to predominate.a4 Since the adduct ion cannot be produced hy fragmentation of any other product ion, these results clearly indicate that the 0 + amine reactions proceed to a significant extent (if not entirely) via an addition route whose beginning is
The Journal of Physical Chemistry, Vol. 83,
0-Atom Reactions with Methyl- and Ethylamines
TABLE I: Apparent Products Observed in 0
+ Alkylamine Reactions likely origin product
identification
mass no.a
No. 24, 1979 3067
fragment of product ion
min energy lamp to produce ion (eV)
0 t CH,NH, Reaction Xb Xe (8.6) 30 CH,N Xb H (10.2) 47 CH ,NO Apparent products were sought but not established a t the following mass numbers: 1 4 , 15, 16, 18, 29, 31, 32, 33, 44, 45, 46, 58 0 t (CH,),NH Reaction X H 15m CH3 0 (9.6) NH,' t C,H, C,H6N' 18 (MI "4 unknown H C,H3 or HCN 27 m 28 m CH,N C2H6N' -+ CH,N' t CH, €I C,H6N' -+ C,H,N+ t H, 0 42 M C3,N X H 43 M C A N X Xe 44 M C2H6N X 0 61 m C2H,N0 Apparent products were sought but not established at the following mass numbers: 14, 17, 19, 29, 30, 31, 32, 33, 34, 46, 47, 59, 60 --f
19 18 49
Additional Apparent Products from the 0 HDO Xh Xb CD 3 xb CD,NOH
+ (CD3),NH Reaction
Ne ( - 16.7)
H C1 (8.9-9.2)
0 t (CH,),N Reaction 1 5 (MI 18 M 30 M 42 M 43 m 44 m 57 m 58 M Apparent products were 60, 61, 73, 74, 75
CH3
X
C,H,N' -+ NH,' t C,H, C,H,N+ CH,N' t C,H, C,H,N+ + C2H,N+ + CH, C3H,N+ +C,H,N' t CH, C,H6N0 -+ CH,NO' + CH, C,H,N' -+ C,H,N' t H
"?
CH,N C,H,N
-+
C2H5N
C2H6N
0 Xe H
0
Ar (11.6) C,H,N Xe C,H.N X s t u g h t but not established a t the following mass numbers: 14, 16, 17, 29, 31, 32, 33, 41, 45, 46,
0
18 (MI 43 M
H H
- C2H,NH, Reaction C,H,N+
4"
C2HJ
-+
NH,' t C,H,
X
c1
44 hl C2H6N xb Xe Apparent products were sought but not established at the following mass numbers: 14, 15, 16, 19, 27, 28, 29, 30, 31, 32, 33, 34, 42, 46, 47, 59, 60, 61
0 29 M 42 m 43 m 44 (MI 46 m 56 m 57 m 61 m 71 M 72 M 89 m Apparent products were 58, 59, 60, 75, 87, 88
C,H. c;H,, C,H,N, or C,H,O C,H,, C,H5N, or C,H,O
+ (C,H,),NH
Reaction
XC
C?H6N
C,H,O or CH,NO C3H6N
unknown unknown C,H,,N+ -+ C,H,N' t C,H, unknown C,H,,N' -+ C,H6N' + CH, unknown
c1 0
H Xe H
c1
H C,H9 or C3H,N C,H,NO X 0 CP9N X Xe C'lHIrJN X Xe C,H,,NO XC 0 sought but not established at the following mass numbers: 14, 15, 18, 28, 30, 31, 32, 33, 45, 47,
0 + (C,H,),N Reactiond X C1 _. 29 M C,H, 89 M C;H;,NO X H 99 ( M ) X Xe 100 M X Xe Additional apparent products were established a t the following mass numbers: 28, 42, 44, 56, 60, 70, 71, 72, 73, 74, 84, 88 Apparent products were sought but not established at the following mass numbers: 15, 18, 27, 30, 31, 32, 33, 34, 35, 43, 45, 46, 47, 57, 58, 59, 61, 69, 7 5 , 86, 87, 94, 97, 98, 102, 115, 116, 117 a ( M ) indicates the largest apparent product ion signal; M, the major ion signal ( > 10% (M)); m, the minor ion signal ( < l o % Pressure test not possible with these products. Nonlinear dependence on pressure of reactant beam. Only con(M)). firmed products listed.
represented by reaction 1. The failure to detect the 0. amine adduct (indeed, the failure to detect any product) does not necessarily indicate its absence but can be a result of interference from trace impurities in the fuel sample, extremely low concentrations of products due to a low rate constant for the reaction under study, or interference from ion signals produced by the amine and its fragment ions.
The 0-amine adduct is created with enough internal energy to decompose prior to arrival at the mass spectrometer ion source.8 Its detection is a consequence of stabilizing collisions undergone by a small fraction of the hot adducts in the intersection region of the two gas jets. In the 0 + DEA reaction, adduct ion signals were large enough to monitor, along with those of other products, as
3068
The Journal of Physical Chemistry, Vol. 83,No. 24, 1979
a function of reduced beam density. The ion signal had a higher order dependence on beam density, indicating the essential role of secondary collisions to produce stable adducts. H-Atom Abstraction. In all the reactions studied, we detected major ion signals from the apparent product of mass one less than that of the amine (M - 1). The corresponding ion could not result from fragmentation of any products of higher mass. (The apparent adduct concentration and the different beam density dependence of its ion signal preclude the adduct as the source of these signals.) In addition we detected major apparent product ion signals from likely fragmentation products of the (M - 1)+ ion (Table I). For example, in the 0 DMA reaction three apparent product ions (NH4', C2H4N+,and CH2N+)are all likely products of the decomposition of C2HsN', (M l)+.12J3 The ion NH4+was the largest of all the apparent ion signals, C2H4N+was also a major apparent product, and CH2N+yielded a minor ion signal. (The contributions of these fragmentation pathways in the 0 f DMA reaction can be seen in the mass scans in Figure 1 of our preliminary reportJg It is apparent from these results that the H-atom abstraction route is an important one in all the 0 + amine reactions studied and may in fact be generally the dominant reactive pathway. There are indications that the abstraction process follows addition in at least two of the 0 + amine reactions studied. In our study of the 0 + (CD3)2NHreaction, the fragment ion ND4+was abundant but ND3H+ was absent indicating that the precursor ion was exclusively C2D6N+. This result indicates that the abstraction route removed exclusively the H atom originally attached to nitrogen, leaving untouched the six deuterium atoms on the methyl groups of DMA. It is known that primary and secondary amine N-oxides rearrange easily to hydroxylamines by H-atom migration.14 The exclusive formation of C2DsN can best be explained by the sequence reaction 1followed by the steps
+
[
0-
CD,N,HCD,
I**[ I*
+=
study where detection sensitivity was very low due to the relatively low rate constant for this reaction. It is our opinion that this ion is almost certainly a product ion (except in the 0 + TMA reaction) and its detection indicates the presence of the important reactive route 0 + amine azomethine + H 2 0 (3) The only possible fragmentation source of (M - 2)+ is [(M - 1)']* (M - 2)+ + H (4) a high-energy process involving an even-electron ion producing two odd-electron products. The necessary energy for reaction 4 may be available even a t the low ionizing energies used to detect (M - 2). However, we have no way of establishing this likelihood since the ionization potentials (IP) of the amino radicals (M - 1) detected in this study are not well established. All the amino radicals were detectable with the lowest ionizing energy available to us, 8.6 eV (xenon lamp), indicating that their ionization potentials are below this value. There are two reasons for believing that (M - 2)+ ions are not produced by a fragmentation process. First, the ratio of the two ion signals (M - 2)+/(M - 1)' changed very little with ionizing energy over the range 8.6-10.2 eV in the study of the 0 DEA and 0 TEA reactions (these were the only two reactions where both of these products could be detected by using ionizing radiation as low as 8.6 eV). This invariance strongly suggests that one is not the fragment of the other. Finally, in our study of the 0 + (CD3)2NHreaction we detected HDO as a product establishing, at least in one case, the presence of the second product of reaction 3. The removal of two hydrogen atoms from a molecule by oxygen in a gaseous reaction was first directly observed by Foner and Hudson in their study of the 0 hydrazine reaction using a similar cross-jet reactor.16 The mechanism involved removal of one H atom from each nitrogen, a likely mechanism being
-
+
+
+
+
r H (CD,),N.
+ OH*
(2)
Direct abstraction would most likely remove both H atoms and D atoms, since C-H and N-H bond strengths are virtually identical in DMA.I5 Although direct abstraction might favor removal of the hydrogen bonded to the nitrogen atom due to zero-point energy differences in this partially deuterated compound, it is unlikely that direct abstraction of deuterium would be completely unobservable, particularly since the deuterium atoms are six times more abundant than the H atoms in (CD3)2HN. The fact that the 0 + TMA reaction has a negative activation energy has been cited as very strong evidence that this reaction proceeds predominantly by an addition mechanism.' Since we have observed that the (M - 1)+ product ion and its likely fragment ions also dominate the apparent product ion spectrum of this reaction, abstraction of an H atom is also probably the major route of this reaction. These two observations taken together suggest that H-atom removal from TMA also follows an initial addition step. The H-atom abstraction route in the reactions involving tertiary amines would have to proceed via a different rearrangement due to the absence of a labile H atom attached to nitrogen. A possible mechanism is suggested at the end of this section. H 2 0 Formation. In virtually all of our studies of 0 + amine reactions the major apparent product ion (M - 2)+ was detected. The single exception was the 0 + MMA
I
O -- H .l *
0 -I H2NNH,-+l -_N-N+, \ l i
OH
CD,NCDj
Slagle, Dudich, and Gutman
+
[
H1 OH I
]*
-+
H-N-N-H
An analogous mechanism can be proposed for the 0-atom reactions with the primary and secondary amines (Figure 3). The energy-rich hydroxylamine formed by rearrangement of the N-oxide expels H 2 0 while forming an azomethine group. Under quite different conditions amines substituted on the nitrogen by anionic leaving groups X are known to eliminate HX easily to form C=N b0nds.l' This mechanism for expelling H 2 0 is again not applicable to the reactions involving tertiary amines. There is some evidence that this reactive route may not be a significant one in the 0 TMA reaction. In our study of this reaction, we observed the (M - 2)+ apparent product ion only when using the highest ionizing energy available, 11.6 eV. All the other (M - 2)+ ions were detected a t significantly lower ionizing energies which suggests that the (M - 2)+ ion signal in the 0 + TMA study originates from the fragmentation process, reaction 4. A possible explanation for the major H 2 0 forming route in the 0 + TEA reaction is included in the discussion of a general mechanism at the end of this section. Cope Elimination. In the study of the 0 + TEA reaction, the (M - 12)+ion was a major apparent product ion, and in the 0 DEA reaction it was a minor product ion. It was not observed in any of the other reactions. This ion
+
+
The Journal of Physical Chemisfry, Vol. 83,No. 24, 1979 3069
0-Atom Reactions with Methyl- and Ethylamines
IT. 0 - A T O M PRIMARY A N D S E C O N D A R Y A M I N E S
r
RFACTIONS WITH
TERTIARY AMINES : ( C H A N AND (C2Hd3N
l$
0 R-N-R
,,
WCH~R')
R': h,CH31
*t"[
Flgure 3. Proposed general mechanism for the oxygenatom reactions with primary and secondary aliphatic amines.
cannot be produced by fragmentation of any products of higher mass [(M - 1)+or (M - 2)+] and therefore it must be a genuine product ion. The (M - 12) product is the hydroxylamine most probably formed by the elimination of C2H4from the excited N-oxide. This route, the wellknown Cope elimination, is the principle thermal decomposition route for tertiary amine N-oxides containing hydrogens in the position.18 Among the three 0 + ethylamine reactions, this route is of decreasing importance as the number of ethyl groups on the amine decreases. This is probably due to the concomitant increasing importance of the competing H-atom migration from N to 0 which would block the Cope elimination and which would lead to other reactive routes. R Loss from Adduct. In all the studies of the reactions involving secondary or tertiary amines, the apparent product ion R+ (CH3+or C2H5+)was detected. These ions were detectable with resonance lamps whose ionizing energies were just above the ionization potentials of these alkyl radicals. For this reason (and because fragmentation of N-centered products would generally leave the charge on the N-centered fragment and not on the R group), we can conclude that these alkyl ions are genuine product ions. This reactive route (0 atom in, R out) was searched for but not found in the studies involving primary amines. From the relative magnitudes of the R+ ion signals and those recorded for the (M - 1)' and (M - 2)' products in the same reaction, it appears as though the route involving R loss from the adduct increases in importance with increasing complexity of the amine. This is particularly apparent when comparing data from the 0 + DMA and 0 + TMA studies obtained at 10.2-eV ionizing energy. In the former study, the CH3+ ion signal is small, less than that of any other products and virtually all of the fragment ions. In the latter study, it is comparable to that of other products and larger than that recorded for any of the product fragment ions. A possible explanation for this changing importance is offered later in the discussion. Possible Other Reactive Routes. Although many other apparent products were detected in these studies, particularly in the investigations of the reactions involving TMA, DEA, and TEA, no additional ones could be established as being genuine products because of the inability of excluding their originating from possible fragmentation processes. For example, some apparent products formed half of a reaction pair in which the complement was a species
OH
R-$-C.HR'
I*+
OH
R-A-CHR'
,R
Flgure 4. Proposed mechanism for the oxygen-atom reactions with trimethyl- and triethylamine.
which is readily detectable in our photoionization apparatus but which was not found in the 0 + amine reaction studied. In other cases the complement had an unreasonable formula (e.g., CH,O). In Table I we have listed some possible fragmentation reactions involving confirmed products of higher mass which could account for the production of apparent product ions of lower mass. In the 0 TEA reaction, a total of 16 apparent products was detected, but only four are established as likely products. General Mechanism. The results of our study of four 0-atom reactions with primary and secondary amines have sufficient similarities in the identity and importance of reactive routes to permit us to suggest a general mechanism for these reactions which we feel could be used for predicting reactive routes for other reactions in this class. The mechanism is shown in Figure 3. All routes begin with addition to form an energy-rich amine N-oxide. The next step is H-atom migration to the 0 atom, either from the nitrogen or via a Cope elimination process, the former being the preferred choice. The excited hydroxylamine can decompose by three possible routes: OH loss, H 2 0loss, or R loss, the first being by far the most important pathway. R loss from the excited amine N-oxide should also be considered possible. Our results on the reactions of 0 atoms with tertiary amines are more difficult to understand because more unusual rearrangements must be postulated to produce the products OH and H20. There are enough differences between the results obtained in the 0 TMA and TEA studies to indicate that structural differences play a larger role in determining the relative importance of the several open reactive routes in this subgroup of 0 amine reactions. We therefore cannot suggest a general mechanism for the 0-atom reactions with tertiary amines, A mechanism can, however, be offered to account for the major results observed in our 0 + TMA and TEA studies (Figure 4). Following addition to form a tertiary amine N-oxide with 60-70 kcal of internal energy, H-atom migration to the 0 atom can occur, as can loss of an alkyl radical. Where H-atom migration is relatively facile (by Cope elimination or by (Y elimination of a secondary hydrogen) rearrangement predominates (0 + TEA reaction). Where H-atom migration is difficult (due to the absence of P hydrogens or secondary hydrogens) R loss becomes a very important route (0 + TMA reaction). The somewhat unusual ylid produced by the migration of an a hydrogen to the oxygen can subsequently decompose via expulsion of OH, HzO, OF R through transition states
+
+
+
3070
The Journal of Physical Chemistty, Vol. 83, No. 24, 1979
analogous to those shown in Figure 3 for the decomposition of the hydroxylamines formed in the reactions involving primary and secondary amines. This heuristic model offers explanations for both the unusually great importance of the R-loss route in the 0 + TMA reaction and the fact that the H20loss route was not observed in the same reaction. The latter route would require the loss of two primary hydrogens in sequential steps, each of which involves a competition with a second pathway which is probably energetically favored. These six studies have begun to reveal details of the mechanism of 0 amine reactions under essentially collision-free conditions following the formation of an energy-rich adduct. Recognizing that an excited amine N-oxide is the first intermediate in this reaction, we have shown in this study that amine N-oxides with 60-70 kcal/mol of internal energy decompose not only along the path of lowest free-energy increase, but to a very great extent by other routes which have not been observed before.
+
Acknowledgment. The authors gratefully acknowledge the financial support of the National Science Foundation.
References and Notes (1) R. J. CvetanoviE, Adv. fhotochem., 1, 115 (1963). (2) R. E. Huie and J. T. Herron, "Progress in Reaction Kinetics", Vol. 8, Part 1, K. R. Jennings and R. B. Kundall, Ed., Pergammon Press, Oxford, 1975. (3) J. R. Kanofsky and D. Gutman, Chem. Phys. Lett., 15, 236 (1972); J. R. Kanofsky, D. Lucas, and D. Gutman, Symp. (Int.) Combust., [ f r o c . ] , 14th, 1972, 285 (1973).
Martin et al. (4) I. R. Slagle, R. E. Graham, and D. Gutman, Inf. J. Chem. Kinel., 8, 451 (1976). (5) J. R. Kanofsky, D. Lucas, F. Pruss, and D. Gutman, J. Phys. Chem., 78, 311 (1974). (6) K. Kirchner, N. Merget, and C. Schmidt, Chem. Ing. Techn., 46, 661 (1974). (7) R. Atkinson and J. N. Pitts, J . Chem. Phys., 68, 911 (1978). (8) Internal energy calculated assuming that the heat of formatlon of R,NOH is the same as that of monomethylamine (-12.0 kcal/mol) which was obtained from S. W. Benson, F. R. Cruickshank, D. M. Golden, G. R. Haugen, H. E. O'Neal, A. S. Rodgers, R. Shaw, and R. Walsh, Chem. Rev., 69, 279 (1969). (9) I.R. Slagle, J. F. Dudich, and D. Gutman, Chem. Phys. Lett., 61, 620 (1979). (10) R. E. Graham and D. Gutman in "Dynamic Mass Spectrometry", Vol. 5, D. Price and J. F. J. Todd, Ed., Heyden, London, 1978, p 156. (11) The lamp gases and their resonance energies (in eV) are Xe (8.Q CI (9.1), 0 (9.6), H (10.2), and Ar (11.6). (12) M. S.Foster and J. L. Beauchamp, J . Am. Chem. Soc., 94, 2425 (1972). 13) G. Hvistendahl and K. Undheim, Org. Mass. Spectrom.,3,821 (1970). 14) P. A. S.Smith, "The Chemistry of Open-Chain Organic Nitrogen Compounds", Vol. 11, W. A. Benjamin, New York, 1966, Chapter 8. 15) In DMA the C-N bond strength is 94.6 kcal/mol while the C-H bond strength is estimated to be 90.6 kcal/mol. Bond strengths are calculated from the known heats of formation and an estimation of AHACH,NHCH2) = 34 kcal/mol. S.W. Benson, "Thermochemical Kinetics", 2nd ed, Wiley, New York, 1976. 16) S.N. Foner and R. L. Hudson, J . Chem. Phys., 49, 3724 (1968); 53, 4377 (1970). Confirmation of these resuk reported by M. Gehring, K. Hoyermann, H. Gg. Wagner, and J. Wolfrum, Ber. Bunsenges. Phys. Chem., 73, 956 (1969). (17) S.Dayagl and Y. Degani, "The Chemistry of the Carbon-Nitrogen Double Bond", S.Patai, Ed., Interscience, New York, 1970, Chapter 2. (18) A. C. Cope and E. R. Trumbull, "Organic Reactions", Vol. 11, R. Adams, V. Boekelheide, T. L. Cairns, A. C. Cope, D. Y. Curtin and C. Niemann, Ed., Wiley, New York, 1960, Chapter 5.
Evaluation of the Olefinic Double Bond Influence in the Unimolecular Homogeneous Gas Phase Elimination of Alkenyl Acetates Ignaclo Marfin, Jose A. Hernhdez A.,+ Alexandra Rotlnov, and Gabriel Chuchanl" Centro de Gdmica, Instltuto Venezolano de Investigaciones Cienthas, Apartado 1827, Caracas, Venezuela (Received February 13, 1979; Revised Manuscript Received August 20, 1979) Publication cost assisted by Instituto Venezolano de Investigaciones Ciendficas
The rate coefficients for gas-phase pyrolyses of five alkenyl acetates have been measured in a static system over the temperature range 240-420 "C and pressure range 44-282 mmHg. The rate coefficientsare expressed by the following Arrhenius equations: for 3-buten-1-yl acetate, log h(s-') = (13.20 & 0.17) - (200.8 f 2.1) kJ mol-l (2.303RT)-'; for 4-penten-1-yl acetate, log k(s-') = (12.81 i0.36) - (204.0 f 4.5) k J mol-1 (2.303RT)-l; for 5-hexen-1-ylacetate, log k(s-') = (12.43 f 0.14) - (197.5 & 1.8)kJ mol-' (2.303RT)-'; for 1-penten-4-ylacetate, log k(s-l) = (12.34 f 0.25) - (178.2 f 2.9) kJ mol-l (2.303RT)-'; and for 2-methyl-4-penten-2-ylacetate, log k(s-') = (13.59 f 0.30) - (169.9 f 2.9) k J mol-' (2.303RT)-'. Steric acceleration seems to be a factor which slightly enhances the elimination when the CH2=CH substituent is interposed by at least three methylene groups with respect to the Cm-0 bond of the ester. However, the vinyl substituent adjacent to the carbon of ethyl acetate causes an appreciable increase in the rate of pyrolysis. The result is adequately explained on the basis of allylic weakening of the C6-H bond. Several arguments are presented to indicate that neighboring double bond group participation is improbable. Such a consideration comes from the fact that the transition state of ester pyrolysis is not very polar.
Introduction There are few works describing the fact that the olefinic double bond in the alcoholic part of acetates appears to anchimerically assist the molecular elimination of esters +Visitingscientist, Faculty of Medicine, Universidad Central de Venezuela, Caracas 0022-3654/79/2083-3070$01 .OO/O
in the gas The gas-phase pyrolysis of cyclohexen-1-ylethyl acetatel giving 1-vinylcyclohexene and spiro[2.51oct-4-ene (reaction 1) has suggested that the production of the spire compound may arise by the formation of a carbonium ion. This positively charged carbon atom is then stabilized by neighboring olefinic double bond participation; notwithstanding, the temperature depen0 1979 American
Chemical Society
