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N-alkyl-N'-alkoxydiazene-N-oxides (AADOs) are thermally quite stable [i] but, in con- trast to aliphatic azocompounds, nitrocompounds, and alkyi nitrites of ...
KINETICS OF THERMAL DECOMPOSITION OF N-ALKYLN'-METHOXYDIAZENE-N-OXIDES

IN THE GAS PHASE UDC 541.127:542.921.4: 547.235.5

I. N. Zyuzin, D. B. Lempert, and G. N. Nechiporenko

N-alkyl-N'-alkoxydiazene-N-oxides (AADOs) are thermally quite stable [i] but, in contrast to aliphatic azocompounds, nitrocompounds, and alkyi nitrites of similar structure which have been studied in detail, there is no information on the kinetics and mechanism of the thermal decomposition of AADOs. In the present work we have studied the kinetics of the gasphase decomposition of N-R-N'-methoxydiazene-N-oxides with R = Me (I), t-Bu (II), and t-BuCH 2 (III) with the object of comparing AADOS with other classes of compounds and determining the influence of the N-alkyl group on the rate and mechanism of the reaction. EXPERIMENTAL The AADOs(I)-(III)'were prepared by the method of [2]. After twice distilling in vacuum, the impurity level detected by GLC did not exceed 0.2%. The kinetics of decomposition wave determined in sealed vessels of 150-180 ml capacity having a sickle-shaped membrane. The temperature was maintained by an air thermostat to an accuracy of • 0.2~ Kinetic curves of pressure increase were processed on a microcomputer using the method of "quickest fall" [3] for a first-order reaction: P = P - ( P - P0)exp(-kt). The rate constant k, initial pressure P0, and final pressure P were optimized. The heating-up time was(determined graphically (2-3 min; in experiments wi~h glass packing, 12-24 min) and allowed for in the calculations. Recording P0 and P , an accurate value for k was obtained by the method of least squares. Points were used in the calculations as far as a conversion n = (P -- P0)/P~ -- P0) = TABLE i. Experimental Conditions and Rate Constants for GasPhase Decomposition Compound '

~eN+=NO.~le (I) I O-

T, ~ 270.0 28010 29217 299.8 299,8 299,8 3i0.0 319,5 3302

-BuN+=NO3Ie (II) 1

O-

t-BuCH:N+=NO3Ie (III) I

O-

Po, torr

h, sec "!

P~/Po

0,88 0,90 0,87 0,88 15,5 0,87

6.60. i0 -6 iA34. t0 -5 3.78.10 - s 6,93. t0 -5 8,20.10 -5 8,40. i0 - s

2,80 * 2,80 * 2,64 2.69 2,83 4,08

0.88 0,89 0,92

1.532.i0 -4 2.89.i0 -4 5,69. i0 -r

2,84 2,98 2,72 2,84 2,48 2,78 2.90 2,97 2,85 2,82 2,64

s/v, c m - i t

.J

349 325 275 279 258 t86+ NO t96 267 2~ 250

i60.3 170,4 170,4 i70.4 t70.4 t80.7 t90,0 200.5

231 42 t12 236 2i6 2ii 204 224

0,97 0,88 i,0 0.95 16.7 0,95 0.88 0,9s

2,74-i0 -5 6,35. i0 -5 6,44-10 -5 6.75- l0 -5 6.43. t0 -5 t,7i. 10 -4 3,78. t0 -4 8,90.10 -4

270A

277 237 223 230 219

0,87 0.8~ 15.5 0.9C 0,9C

7,09. t0 -6 3,06. i0 -5 4,16. t0 -5 7.60-10 -5 1,507.10 -4

288;0 288.0 299.7 309.3

3,74 * 3.80 3,53 3,73 3,66

"Experiments at low conversions, values of P /P0 calculated as average of experimental at two temperatur=es. Institute of Chemical Physics, Academy of Sciences of the USSR, Chernogolovka Branch. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1506-1509, July, 1988. Original article submitted February 18, 1987o

0568-5230/88/3707-1329512,50

9 1989 Plenum Publishing Corporation

!329

TABLE 2.

Yield of Decomposition Products of AADOs

Compound

T~

Yield, mole/mole decomposing compound

~

N~+CO 0,46, CH~ 0,t8, C02 0,03, N~O 0,48, MeOH 0,56 Ns 0,22, CO 0,t5, CH~ 0,26, CO~ 0,08, N20 0,63, MeOH 0,56

280.0

0,42

319,5

1,0

(1)+NO (i:1,05)

299,8

0,89

Ns t,06. CO 0,06, CH4 0,05, COs 0,i0, NzO 0,33, NO 0,06

(ID

i90,0

~2

N: 0,005, N20 0,95, Me2C=CH2 0,98,

200,5

i,0

299,7

0,95

309,3

0,99

(I)

(IID

TABLE 3.

MeOH 0,70 N2 0,02, N20 0,99, Me~C==CH2 t,0 N2+CO 0,33. CH~ 0,05, CO, 0,04, N20 0,60, MeOH 0.57 N2+CO 0,35, CHi 0,06, C02 0,04, N~O 0,64

Arrhenius Parameters ,,,,, ,,,,

Compound (I} (If) (Ill)

MeNO~ (IV) i! ] ] .t-BuN02 (V) MeONO (VI) [6] .MeN~NMe (VII) [7]

T, ~ 270-330 i60-200 270-310 360-390 250-300 170-200 254-303

log (A/sec'*) E, kl / mole i4.5• i3,3• i4.6• 14.3 13.6 15.8• 17,3

204-,-5 t48• 206• 227 t79 t76• 232

Correlation coefficient T

0.99976 0.99987 0.99996

0.8. In experiments at lower temperatures, when the conversion H did not exceed 0.4, the value of P was calculated from the average yield of gaseous products (P /Po) in experiments at two temperatures and fixed for the calculations. The results are set out in Table i. In all the experiments, the requirements for a first-order reaction were fulfilled with a correlation coefficient r > 0.9998. The rate of decomposition changed littlewhen the ratio of surface area to volume S/V was increased by a factor of 18 (kwith packing/k for (I) = 1.18 at 299.8~ for (II) = 0.95 at 170.4~ for (Ill) = 1.36 at 288~ which is evidence for a homogeneous reaction. Decomposition of (I) was not inhibited by NO (kNo/k = 1.21 at 299.8~ i.e. there is essentially no chain process. The rate of thermal decomposition of (II) was independent of P0 (at 170.4~ kp~=44:kp0=11=:kp0=23 ~ = 0.94:0.95:1). The results of experiments with packing with NO, and with P0 < 200 tort, were not used in calculating the Arrhenius parameters. The gaseous products of the decomposition were analyzed on an LKhM-8MD chromatograph with katharometer detector, helium carrier gas (i00 ml/min), copper column (6 m • 5 m m), sorb-i (0.2-0.3 mm), column temperature 90, 35, and --60~ Methanol was determined by GLC using a flame ionization detector, nitrogen carrier gas (30 ml/min), steel column (2 m x 3 m m), 15% Carbowax-20 M on Chromosorb W-HMDS (80-100 mesh) column temperature 450C, internal standard THF. Analytical results are set out in Table 2. RESULTS AND DISCUSSION The close adherence to a first-order equation and the homogeneous nature of the decomposition of (I)-(III) provide the basis for assuming a monomolecular limiting stage in the reaction. Support for this is provided by the observation that the rate of decomposition of (II) is independent of the initial pressure and that chain processes are absent from the decomposition of (I). The Arrhenius parameters for the gasphase thermal decomposition of (I)(III) are given in Table 3, the error being within a 95Z confidence limit. Literature data for the monomolecular thermal decomposition of nitromethane (IV), 2-methyl-2-nitropropane (V), methyl nitrite (VI), and azomethane (VII) are given in the same table. Methane is formed in the decomposition of (I), which is evidence in favor of the formation of radicals in the course of the reaction. Possible reactions of radicals with the starting material (I) do not make a significant contribution to the overall process. This follows

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from the fact that NO has no inhibitory action on the decomposition of (I) (cf [4 and 7]). The primary step is probably the rupture of the N-OMe bond (reaction (i)). (I)

(i) >

MeN+=N. + . 0Me

-* N2, CO, CH4, CO~, N~O, MeOH

0I- __, CHs.+N.oO__ I '. L_~!__ Me. +-O--N=NOMe [__~s)

--Me.+ N20 + .OMe

The activation energy of the gasphase decomposition of (I) is significantly higher than that for the homolytic breakdown of methyl nitrite (VI) (Table 3). This is probably connected with the greater strength of the N-OMe bond in (I) on account of p--, conjugation in the N+(O-)--~NO group [8]. It is noted that the preexponential factors for the decomposition of (I) and the breakdown of (IV) into Me" and NO 2 are very close (Table 3). By analogy, one can suppose that the primary rupture is also of the Me N bond in compound (I) (reaction (2)), since the left-hand part of the molecule of (I) is isoelectronic with nitromethane. However, in addition to the considerable difference in activation energies in the decomposition of (I) and (IV), the agreement of the activation parameters for the decomposition of (I) and (III) is contradicted by such a mechanism and replacement of R = Me by the more bulky R = Me~CCH 2 would have an effect on the preexponential factor in the case of R-N bond rupture [9]. It taneous tor for tically

was necessary in advance to exclude from consideration the mechanism involving simulrupture of Me-N and N-OMe bonds (reaction (3)). The fact that the preexvonential facthe decomposition of (I) is several orders of magnitude less than for azomethane pracexcludes such a mechanism anyway [9].

When MeN in (I) is replaced by t-Bu N there is a sharp reduction in the activation parameters of the gasphase thermal decomposition (log (A/sec -I) by 1.2 and E by 56 kJ/moie), which points to a change in the reaction mechanism. There is a parallel here with the gasphase decomposition of aliphatic nitrocompounds (Table 3): (IV) breaks down via a homolytic rupture of the C-N bond, and (V) via a five-membered cyclic intermediate [4, 5]. An almost quantitative yield of isobutylene and N20, taken in conjunction with the preexponentia! factors, which is characteristic for a cyclic intermediate state on rupture of ~ert-alkyl substituents [9], leads one to suggest the following mechanism for the decomposition of (II):

|

[ 9

Me~C---N

+/O-%,N_OMc I ~-,

(m- iHocj [ \ \xH//" The o t h e r

five-membered

Me~C=CH2 + O---N=N+--OMe

'I_N20 +

H

intermediate FMe2

is

less

likely

since

it

Me0H

supposes

high

yields

o f N2:

N--OMe 7

L

|

(II)--X-~ [H2C"/"

XO-

I -~ Me=C=CH2 + HO--N=N--OMe

A mechanism involving a six-membered ring is much less probable: OMemO

N~N.

\C /

(U) --•

-

H2C.--H

""O--Me

-~ Me~C=CH2 ~- N~_O-c MeOH

J

Such a mechanism was proposed for the decomposition of 2-nitro- and 2-difluoroamino-2-(N'fluorodiazene-N-oxido)propane [i0] which are similar in structure to AADOs. However, a Z-con1331

figuration is characteristic for the N+(O-)=N-O group [8] and this excludes such a six-membered intermediate; the barrier to Z ~ E isomerization for compounds with an N=N bond is, as a rule, higher than the activation energy for the decomposition of (II) [ii, 12]. For further verification of the mechanism of the thermal decomposition of (II), including the intramolecular splitting off of a B-hydrogen atom, we studied the decomposition of (III) which has no B-hydrogen. The activation energies for the breakdown of (I) and (III) are practically the same from which one can assume their decomposition by the same mechanism; this supports the proposed mechanism for the decomposition of (II). The higher yield of gaseous products in the decomposition of (III) in comparison with (I) (P~/P0 in Table i) evidently arises from the decomposition of the neopentyl radical, the presence of methane in the products of decomposition of (III) being evidence of this. CONCLUSIONS The kinetics of the thermal decomposition of N-methyl-, N-t-butyl-, and N-neopentyl-N'methoxydiazene-N-oxides have been studied in the gas phase. The reactions are first order with activation parameters: E 204, 148, and 206 kJ/mole, log(A/sec -I) 14.5, 13.3, and 14.6, respectively. A mechanism has been proposed for the decomposition of the MeN- and t-BuCH2Nderivatives by the primary homolytic rupture of the N-OMe bond, and in the case of the t-BuNanalog, via a five-membered cyclic transition state with migration of a 8-H to the N atom. LITERATURE CITED i. 2. 3. 4. 5. 6. 7. 8. .

10. 11. 12.

M. V. George, R. W. Kierstead, and G. F. Wright, Can. J. Chem., 37, 679 (1957). I. N. Zyuzin and D. B. Lempert, Izv. Akad. Nauk SSSR, Set. Khim., 831 (1985). D. L. Marquard, J. STAM, ii, 431 (1963). V. V. Dubikhin, G. M. Nazin, and G. B. Manelis, Izv. Akad. Nauk SSSR, Set. Khim., 1339 (1971). V. V. Dubikhin, O. M. Nazin, D. N. Sokolov, and O. B. Manelis, Izv. kkad. Nauk SSSR, iSer. Khim., 1416 (1971). Lo Batt, R. T. Mibne, and R. D. McCulloch, Int. J. Chem. Kinet., 9, 567 (1977). W. Forst, J. Chem. Phys., 44, 2349 (1966). L~ O. Atovmyan, N. I. Golovina, and I. N. Zyuzin, Izv. Akad. Nauk SSSR, Ser. Khim., 1309 (1987). G. M. Nazin, Usp. Khim., 41, 1537 (1972). V. N. Grebennikov, G. B. Manelis, G. M. Nazin, et al, Izv. Akad. Nauk SSSR, Ser. Khim., 1721 (1984). P. S. Engel, Chem. Revs., 80, 99 (1980). J. W. Lown, S. M. S. Chauchan, R. R. Koganty, and A.-M. Sapse, J. Am. Chem. Soc., 106,

6401 (1984).

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