J. Phys. Chem. 1986, 90, 173-178 reaction between nitrobenzene and its anion radical since the reduction potentials for nitrobenzene and p-nitrobenzyl bromide are expected to be very similar. Reaction 7 cannot be monitored spectrophotometrically since the absorption spectra of both anion radicals are very similar. However, the product anion radical undergoes rapid dehalogenation which in this case was found to be much more rapid than reaction 7 . By monitoring the dehalogenation (through the absorption of the product benzyl radical) in the presence of excess nitrobenzene and varying concentrations of p-nitrobenzyl bromide we can determine, therefore, the second-order rate constant for reaction 7. The value is found to be 1.9 X lo6 M-' s-I, slightly higher than that determined by ESR line broadening for the self-exchange of C 6 H 5 N 0 p C6H5NO2 ( k 105 M-' s-l).23 The activation energy for reaction 7 (Figure 4) is found to be only 6.0 kcal/mol, i.e., about half the value found for the dehalogenation reaction. The activation energy for this bimolecular reaction is higher than that of the nearly diffusioncontrolled24 radical-radical reaction (reaction 3) only by 1.4 kcal/mol, which is sufficient to slow the reaction only by a factor
of 15. Assuming equal diffusion rates for the two reactions, reaction 7 is also slowed by a probability factor of about 0.05. Both the increased activation energy and low probability factor may be related to a geometrical reorientation required upon the reduction of A r N 0 2 to form the more rigidly planar A r N O p . Acknowledgment. This work was supported by the Office of Basic Energy Sciences of the US. Department of Energy and by the Australian Research Grants Committee. Registry No. (p-No2C6H4CH2c1)-., 34509-98-3; (oNO&H,CH2CI)-., 7426 1-76-0;@-NO2C6H,CHCICH,)-., 83966-31-8; @-NO,C6H,CHCICH2CH3)-., 98799-24-7; @-NO&H4CCI(CH,)C(CH,),)-., 89727-62-8; @-N02C6H4CH2Br)-.,345 12-14-6; @N02C6H4CHBrCH3)-.,84024-96-4; @-N02C6H4CHBrC(CH,)3)-., 89727-63-9;(o-ClC6H4C(0)CH3)-., 68225-75-2;(o-BrC,H,C(O)CH,)-., 77510-37-3; @-BrC6H4C(0)CH3)-.,34473-43-3; (p-IC6H,C(0)CH,)-., 775 10-38-4;(p-BrC,H,CHO)-., 77510-40-8;(m-CIC6H4CN)-.,6827 192-1; p-N02C6H4CH2*, 19019-93-3; PhNOZ-., 12169-65-2; pNO2C6H4CH2Br,100-11-8; l-(bromomethyl)-2-methyl-4-nitrobenzene radical anion, 98799-25-8; 2-(bromomethyl)-l-methyl-3-nitrobenzene radical anion, 98799-26-9;o-xylene, 95-47-6;4-nitro-o-xylene,99-51-4; 3-nitro-o-xylene, 83-41-0; l-(bromomethyl)-2-methyl-4-nitrobenzene, 22162-14-7; 2-(bromomethyl)-l-methyl-4-nitrobenzene,98799-27-0; 2-(bromomethyl)-l-methyl-3-nitrobenzene,77378-54-2; I-(bromomethyl)-2-methyl-3-nitrobenzene,39053-40-2.
+
-
173
(23) Layloff, T.;Miller, T.; Adams, R. N.; Fah, H.; Horsfield, A,; Proctor, W. Nature (London) 1965, 205, 382. (24) Hagemann, R. J.; Schwarz, H. A. J . Phys. Chem. 1967, 71, 2694. Huggenberger, C.;Fischer, H. Helu. Chim. Acta 1981, 64, 358.
Rate Constants and Mechanisms for the Reaction of OH (OD) Radicals with Acetylene, Propyne, and 2-Butyne in Air at 297 f 2 K Shiro Hatakeyama,* Nobuaki Washida, and Hajime Akimoto Division of Atmospheric Environment, The National Institute for Environmental Studies, Ibaraki 305, Japan (Received: June 17, 1985)
OH (OD) radical initiated photooxidation of acetylene, propyne, and 2-butyne was studied at atmospheric pressure at 297 f 2 K. The rate constants were determined to be (8.8 & 2.0) X lO-I3, (5.71 0.18) X 10-I2, and (3.01 & 0.28) X lo-" cm3 molecule-' s-l for acetylene, propyne, and 2-butyne, respectively, by the competitive reaction method using cyclohexane = (7.57 0.05) X lo-'* cm3 molecule-' s-'1. The major ultimate products are as a reference compound [k~OH+cyclohexane) a-dicarbonyl compounds, Le., glyoxal from acetylene, methylglyoxal from propyne, and biacetyl from 2-butyne, as well as formic acid from acetylene and propyne and acetic acid from 2-butyne. On the basis of product analyses the reaction of OH with alkynes was deduced to proceed via addition resulting in the formation of hydroxyvinyl radicals, which further react with O2 to give carboxylic acid + RCO or a-dicarbonyl compounds.
*
*
Introduction Although OH radical initiated oxidation of alkynes is important from the point of view of both atmospheric' and combustion chemistry?~~ kinetic studies of this reaction under atmospheric conditions are still very limited and reaction mechanisms have not been well studied. As for the reaction mechanism, the initial step of the reaction of OH radical with acetylene has been thought to be C2H2 O H CIH HzO (1) C2H2 OH C2H20 + H in earlier investigations related to combustion ~ h e m i s t r y . ~ - ~ Kanofsky et aLs detected C 2 H 2 0from the reaction of O H with acetylene as well as propyne under crossed molecular beam conditions using a photoionization-mass spectrometer. On the other hand, the rate constant for this reaction has recently been reportedG8 to be pressure dependent, e.g., varying from -3 X
+ +
+
+
+
(1) Atkinson, R.; Darnall, K. R.; Lloyd, A. C.; Winer, A. M.; Pitts, J. N., Jr. Adv. Photochem. 1979, 11, 375. (2) Jachimowsky, C.J. Combust. Name 1977, 29, 5 5 . (3) Levy, J. M. Combust. Home 1982, 46, 7. (4) Breen, J. E.;Glass, G. P.Inr. J . Chem. Kinet. 1970, 3, 145. (5) Kanofsky, J. R.; Lucas, D.; Pruss, F.;Gutman, D. J . Phys. Chem. 1974, 78, 311.
0022-3654/86/2090-0173$01.50/0
cm3 molecule-' s-' in the low-pressure region (120 torr) to the high-pressure limit value of 7 X cm3 molecule-' s-I at 2700 torr of Ar at room temperature. From this evidence, the reaction of OH radicals with C2H2at room temperature has been proposed6-* to proceed via the initial addition followed by stabilization. C2H2 + OH + C2H20H* C2H20H*
+M
+
C2H2OH
+M
(3) (4)
While the possibility of direct channels such as reaction 2 has not been excluded, Perry and Williamson* suggested that such channels are minor under atmospheric conditions. A similar situation was pointed out for propyne by Atkinson and Aschmann: who reported the rate constants for the reaction of OH with a series of alkynes under atmospheric conditions. In order to assess the atmospheric fate of alkynes emitted by rate constants and fossil fuel burning lo and biomass (6) Perry, R.A,; Atkinson, R.; Pitts, J. N., Jr. J . Chem. Phys. 1978, 67, 5517. ( 7 ) Michael, J. V.; Nava, D. F.; Borkowski, R. P.; Payne, W. A,; Stief, L. J. J . Chem. Phys. 1980, 73, 6108. (8) Perry, R. A,; Williamson, D. Chem. Phys. Lett. 1982, 93, 331. (9) Atkinson, R.; Aschmann, S.M. In?. J . Chem. Kinet. 1984, 16, 259.
0 1986 American Chemical Society
174
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986
Hatakeyama et al.
mechanisms for the O H radical initiated oxidation reactions of acetylene and propyne were investigated in the present study.
Experimental Section O H (OD) radicals were generated in two ways. One was by the photolysis of hydrogen peroxide (H202) at 254 nm using germicidal lamps (Toshiba GL-15 and GL-10) as the light source, which was mainly employed for the kinetic studies. Acetylene (6.3-13.0 mtorr), propyne (7.1-14.5 mtorr), or 2-butyne (1 1.4-13.4 mtorr)-H202 (>90%, 100 mtorr) in 1 atm of purified air was irradiated at 297 f 2 K in an 11-L cylindrical quartz vessel equipped with multireflection mirrors for FT-IR analyses. For product analysis studies, the photolysis of alkyl nitrites in = 360 nm) air with black light lamps (300 IX I400 nm, A,, was employed to generate OH: R C H 2 0 N 0 + hv RCH20 + NO (5)
0.2[
I
I
05
10
-
+
+0 2 HO, + NO
RCH2O
+
RCHO
+
+ HOz
+ NO2
OH
+ OH
products
(9)
thus giving the relative rate constant by the following equation. k8
k9 = In
Figure 1. Plots of In ([a~etylene]~/[acetylene],)against In ([cyclohe~ane]~/[cyclohexane],)at 297 f 2 K. Open symbols represent the data obtained by use of four germicidal lamps. Closed symbols represent the data obtained by use of eight germicidal lamps. 1 01
0
I
1
05
1 .o
/
1.5
In([cyclo hexanel~/~cyclohexanelt) Figure 2.
Plots of In ([pr~pyne]~/[propyne],) against In ([cyclohe~ane]~/[cyclohexane],)at 297 f 2 K. Open symbols represent the data obtained by use of four germicidal lamps. Closed symbols represent the data obtained by use of eight germicidal lamps.
-
h
2
0
02
0. L
0.6
In(kyclohexanelo/[cyclohexanelt) Plots of In ([2-butyneIo/[2-butyne],)against In [cyclohe~ane]~/[cyclohexane],)at 297 f 2 K. Open symbols represent the data obtained by use of four germicidal lamps. Closed symbols represent the data obtained by use of eight germicidal lamps. Figure 3.
Here, k8 and k9 are the rate constants for reactions 8 and 9, respectively, [alkyneIo and [cyclohe~ane]~ are the initial concentration of the alkyne and cyclohexane, respectively, and [alkyne], and [cyclohexane], are the corresponding concentrations at time t . Plots of In ([alkyne],/[alkyne],) against In [(cyclohexane],/ [cyclohexane],) were made (Figures 1-3) for acetylene, propyne, and 2-butyne, respectively. All the data obtained from several runs are plotted together including the runs with a different number of lamps. From the slopes calculated by using the least-squares method, the rate ratios k 8 / k 9were determined to be 0.1 16 0.025, 0.754 0.019, and 3.98 f 0.35 for acetylene, propyne, and 2-butyne, respectively. Errors were calculated from a 95% confidence limit of the slope of the lines through the points on the plots; for acetylene 10% of the error of spectral reading is included. Second-order rate constants for the reactions of OH with acetylene, propyne, and 2-butyne were calculated from these ratios by using the rate constant k9 of (7.57 A 0.05) X lo-'* cm3 molecule-' s-I.'~ They are listed in Table I together with several
{el/1n { cyclohexane], :~yclohexane]~\ ( l o )
(10) Rudolf, J.; Ehhalt, D. H. J. Geophys. Res. 1981, 86, 11959. Greenberg, J. P.;Zimmerman, P. R.: Heidt, L.: Pollock, W. J . Geophys. Res. 1984, 89, 1350. (12) Greenberg, J. P.; Zimmerman, P. R. J . Geophys. Res. 1984,89, 4767. ( 1 1)
In ( [cyc lohexane10 /[cycle hexanelt )
(7)
Results Measurements of Rate Constants. Irradiation of acetylene, propyne, or 2-butyne with H 2 0 2and cyclohexane in air at 254 nm was carried out at 297 f 2 K. Cyclohexane was used as the reference compound for the competitive reaction method. The reactions taking place in these systems are alkyne + O H products (8) cyclohexane
15
(6)
In these experiments deuterated alkyl nitrites (CD,ONO, C2D50NO)were mainly used in order to distinguish the products of alkyne oxidation from the secondary products of the reactions of alkoxy1 radicals produced in reaction 5. Typically, acetylene (-40 mtorr), propyne (-15 mtorr), or 2-butyne (-13 mtorr)-alkyl nitrite (-30 mtorr) in purified air was irradiated at 297 f 2 K in the quartz vessel. Additional experiments with added NO were also performed for both acetylene and propyne. For the purpose of checking for the products of secondary reactions, O H (OD) radical reactions of glyoxal and methylglyoxal and photolyses of these compounds were also carried out. Concentrations of reactants and products were monitored by means of a long-path FT-IR. The path length was 40 m. Spectra were obtained by using 32 scans (scanning time was 1 min) at a resolution of 1 cm-I. The absorptivities (base 10, torr-' m-I at 30 "C) used were as follows: acetylene, 0.289 (3318 cm-I, peak to valley); propyne, 0.182 (3345 cm-I); 2-butyne, 7.91 X (2977 cm-', peak to valley); glyoxal, 0.316 (2824 cm-'); methylglyoxal, 0.350 (2836 cm-I); biacetyl, 0.492 (1 116 cm-I); formic acid, 1.24 (1 105 cm-l, peak to valley); acetic acid, 1.05 (1 178 cm-I); PAN, 1.83 (1160 cm-I); cyclohexane, 3.31 (2934 cm-I). Acetylene and propyne (both from Matheson) were used without further purification. Monomeric glyoxal and methylglyoxal were prepared by distilling the polymer-like material (concentrated by evaporating a commercial solution of glyoxal (Wako) or methylglyoxal (Aldrich) under reduced pressure and mixed with molecular sieves to remove water) under vacuum and were stored at 77 K. Biacetyl was from Tokyo Kasei. Alkyl nitrites were prepared by the nitrosation of the corresponding alcohol, stored at 77 K, and used after trap-to-trap distillation. H 2 0 2 (90%) was used after concentration by a few minutes of pumping.
-
0
*
*
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 175
Reaction of OH Radicals with Alkynes TABLE I: Rate Constants for the Reaction of OH Radicals with a Series of Alkynes alkyne acetylene
ProPyne 2-butyne
cm3 molecule-'
*
0.88 0.20 0.88 f 0.14 0.679 f 0.0706 0.776 f 0.073' 0.675 f 0.070" 0.83 f 0.06 0.83 f 0.08" 5.71 f 0.18 6.21 f 0.31 30.1 f 2.8
s-l
ref this work 9 6 7 8 19 20 this work 9 this work
*Or
t
HCOOH HCOOD
High-pressure limit value.
Time/ min
Figure 5. Time profile of propyne and products in propyne (37.5 mtorr)-CD30N0 (31.3 mtorr)-pure air (1 atm) irradiated system.
The observed yield of a-dicarbonyl compounds and carboxylic acids should be corrected for reaction with OH radical^'^-^' and phot~lysis,'~ on the basis of following reactions
OH
+ alkynes
--
Y[a-dicarbonyl compound]
+ Y'[carboxylic
OH + a-dicarbonyl compound
-
I
I
10 Time/ min
Figure 4. Time profile of acetylene and products in acetylene (37.5 mtorr)-CD30N0 (31.3 mtorr)-pure air (1 atm) irradiated system.
literature values. The rate constant for 2-butyne has not been reported previously. Product Analyses. OH (OD) reaction with acetylene gave formic acid and glyoxal as the major products. C O was also produced, but no quantitative measurement was made. Ketene was under the detection limit, which implies that the yield of ketene under atmospheric conditions does not exceed 0.5% of the consumed acetylene. Figure 4 shows a typical time profile of acetylene and products in a CD30NO-acetylene-air irradiated system. The concentration of HCOOD was estimated by using the IR absorption peak at 1773 cm-' l4 and the absorption coefficient of HCOOH for the band at 1776 cm-l. Formaldehyde was not detected. OH (OD) reaction with propyne gave formic acid, methylglyoxal, and PAN (peroxyacetyl nitrate, C H 3 C ( 0 ) O O N 0 2 )in the presence of NO, as the major products. The yield of P A N was nearly equal to that of formic acid at the primary stage of the reaction, which strongly suggests that PAN and formic acid are formed from the same precursor. A later increase of PAN can be ascribed to OH reaction or photolysis of methylg1yo~al.l~ Neither ketene nor methylketene was detected. Acetic acid was also not formed. Figure 5 shows a typical time profile of propyne and products in a CD30NO-propyne-air irradiated system. OH reaction with 2-butyne proceeded nearly quantitatively. The major products are biacetyl and acetic acid. PAN was detected a t nearly the same yield as acetic acid in the presence of NO,.
products
+ carboxylic acid products a-dicarbonyl compound + hv products OH
5
-
acid] (8)
-
(1 1)
(12)
(1 3)
where Y and Y' are the formation yields of the individual a-dicarbonyl compound and carboxylic acid, respectively. On the assumption that the OH radical concentrations are almost constant during the interval of concentration measurements, the average concentration of OH ([OH],,) between times t l and t2 can be calculated from the measured decay of alkynes and the rate constants for alkyne OH determined above.
+
[OHlav =
U/W2- t1)1 In ([alk~nel,,/[alk~nel,,)(14)
Here [alkyne],, and [alkyne],, are the alkyne concentrations at t , and t2, respectively. Thus [ADClf, = [ADClf, exp(-(kll[oHlav ~
Y
~
l
-
~
,
kl,~l[exp(-k8[OHlav(t2
[
~
- tl)l
+ k13)(t2 - tl)) +
-~ k d ~ [OHIav ~ ~ , +~ 8 [ ~ - expk(kll[oHlav + k13) (t2 - tl)ll (15) ~
k
~
[CAI,, = [CAI,, ex~(-(k12[OHlav)(tz- t l ) l + (Y'fl-t2[alkynelrlk8 [OH]av/ (kl2 k,)[OHlav~[exp(-k8[OHlav(t2 - tl)) - exp(-k12[OHlav(t2 - tl))] (16) where Y,,-t,and Y',,-,, are the derived yields for the individual a-dicarbonyl compounds and carboxylic acids over the time period t , to t2. ADC denotes a-dicarbonyl compound and CA means carboxylic acid. From eq 14-16 the corrected yields for a-dicarbonyl compounds and carboxylic acids are given by [ADCl,,CO' = [ADC],,car+ Y,,-,,([alkyne],, - [alkyne],,) [CAI,?
= [CA],,Co'+ Y',,-,,([alkyne],, - [alkyne],,)
(17)
(18)
(15) Plum, C. N.; Sanhueza, E.; Atkinson, R.; Carter, W. P. L.; Pitts, J. N., Jr. Enuiron. Sei. Technol. 1983, 17, 479. (16) Darnall, K. R.; Atkinson, R.; Pitts, J. N., Jr. J. Phys. Chem. 1979, (13) Atkinson, R.; Aschmann, S. M.; Winer, A. M.; Pitts, J. N., Jr. Int. J . Chem. Kinet. 1982, 14, 507. (14) Redington, R. L. J. Mol. Spectrosc. 1977, 65, 171.
83, 1943.
(17) Wine, P. H.; Astalos, R. J.; Mauldin, R. L., 111 J . Phys. Chem. 1985, 89, 2620.
~
~
a
~
/
~
176 The Journal of Physical Chemistry, Vol. 90, No. 1 , 1986 TABLE 11: Yield of Products in OH
Hatakeyama et al.
+ Alkyne Reactions under Atmospheric Conditions" products
reactant
HCOOH
HC=CH CH3CrCH CH3CrCCH3
0.4 f 0.1 0.12 f 0.02
CHqCOOH
RCOCOR' 0.7 f 0.3 (R = R' = H) 0.53 f 0.03 (R = CH3, R' = H) 0.87 f 0.07 (R = R' = CH3)
0 0.12 f 0.01
,
"Errors are 2u (including 10% of the error of spectral reading for acetylene and glyoxal)
,
L
08
I
I
A
I
I
I
I
4
0.2c
-A[C2H21 / m T o r r 4
I
0
I
2
1
3
-AtC2Hzl / m T o r r
I
I
mo
A
I
I
I
I
I
I
6
7
8
0.6
8
I
-
V
s2
0.4-
Y -AI I
I
-r9
8
C3Hbl/ mTor r I
I
0
I
A
-C? A
0
0.2-
0
I
" t 0
1
2
3 4 5 -A[C3H41 / m T o r r
Figure 7. Ratios of the corrected concentrations of products, (Alformic
acid]/A[a-dicarbonyl compound], plotted against the amount of consumed alkyne: (A) acetylene (-40 mtorr)-CD30N0 (-30 mtorr)-NO (0-5.91 mtorr); (B) propyne (- 15 mtorr)-CD30N0 (-30 mtorr)-NO (0-6.25 mtorr). Open symbols denote the data obtained at [NO], = 0. Closed symbols denote the data obtained at [NO], = 1.14-5.91 mtorr for (A) and at [NO], = 1.36-6.25 mtorr for (B).
"/ I 7 2 ' 6
1
I
6
-A[ 2 -C4Hsl
1
1
I
8
10
12
]
/mTorr
Figure 6. Plots of the typical profile of the corrected concentrations of products vs. the amount of consumed alkynes: (A) HCOCHO and
+
HCOOH from acetylene OH; (B) C H 3 C O C H 0and HCOOH from propyne OH; (c) CH3COCOCH3 and CH3COOH from 2-butyne. Open symbols denote a-dicarbonyl compounds. Closed symbols denote carboxylic acids.
+
where [ c o m p o ~ n d ]means ~ ~ ' the concentration of the compound at time t after correction for reaction with OH and photolysis. Rate constants k , , are from ref 15 and 16. Values of kI3 were estimated by the reported ratios of photolysis rates for glyoxal, methylglyoxal, and biacetyl to the rate for NO2 photoly~is,'~ and the photolysis rate for NO2 was measured in the present work. and 1.15 X Estimated values were 2.55 X loe5, 6.05 X s-l for glyoxal, methylglyoxal, and biacetyl, respectively. The rate constant for k I 2is from ref 17 for HCOOH. As for CH3COOH no rate constant is available for the reaction with OH. Therefore, two rate constants, 4.62 X 1O-I3 cm3 molecule-' s-' (the value for cm3 molecule-' s-I (the value for HCOOHI') and 1.8 X CH3COOCH3'*),were tentatively used for calculation. These two rate constants gave no significant difference in the corrected-yield calculation of CH3COOH. The corrected concentrations of each product are plotted vs. the amounts of consumed alkynes in Figure 6. The yields of a-dicarbonyl compounds and carboxylic (18) Campbell, I. M.; Parkinson,
P.E. Chem. Phys. Lett.
1978, 53, 385.
acids derived from least-squares analyses of these data are listed in Table 11. The effect of NO, on the reaction pathway was checked for both acetylene and propyne by using C D 3 0 N 0 as a source of OD radical. Ratios of the corrected product concentrations, A[formic acid]/A[a-dicarbonyl compound], are plotted in Figure 7 against the extent of the consumption of the alkynes. The presence of up to 6 mtorr of N O gave no signficant change in the total yield of products and the product ratios.
Discussion Rate Constants f o r OH + Alkynes under Atmospheric Conditions. Studies on the reactions of alkynes with OH under atmospheric conditions are very limited. Atkinson and Aschmann9 reported the rate constants for acetylene, propyne, and 1-butyne. C H 3 0 N 0 photolysis was used as the source of OH radicals. The rate constant for acetylene is in excellent agreement with that reported by them. The rate constant for propyne determined in the present study also agrees with theirs. Recently, Wahner and ZetzschIg reported the measurement of the rate constant for O H + C2H2at high pressure (up to 770 torr) employing a direct technique (laser photolysis-UV laser absorption). Their values at 749 torr and (8.1 are (8.3 f 0.6) X cm3 molecule-' 0.7) X cm3 molecule-' s-I at 771 torr, which agree well
*
(19) Wahner, A.; Zetzsch, C. Ber. Bunsenges. Phys. Chem. 1985,89, 3 2 3 .
The Journal of Physical Chemistry, Vol. 90, No. 1 , 1986 177
Reaction of OH Radicals with Alkynes
t OD
RC-CR'
R
-
C=C
-
\OD
R
-13
1
,
,
9
10
11
I
Open circles are from ref 9 . Closed circles represent the data obtained in this work.
with the rate constant determined in this study. Schmidt et aL20 also reported the rate constant of -8 X lo-', cm3 molecule-' s-l at about 1 atm using the laser photolysis-laser fluorescence technique. All the rate constants measured under atmospheric conditions including those reported by Atkinson and Aschmann9 are semilog plotted against the ionization potential of alkynes in Figure 8. It was pointed out that logarithms of rate constants for some reaction and ionization potentials of reactants show a linear correlation if the reaction proceeds via a d d i t i ~ n . ~ ' The - ~ ~ plot gives a good straight line. It indicates that the reaction of O H with alkynes proceeds via the path on which the charge-transfer surface has an important contribution and the transition state is somewhat polarized. Thus, the addition mechanism is strongly supported. The Mechanism for the Reaction of OH (OD) with Alkynes. OH (OD) reaction with alkyne gave formic acid and an a-dicarbonyl compound (glyoxal, methylglyoxal, and biacetyl from acetylene, propyne, and 2-butyne, respectively) in the presence of oxygen. Ketenes were not produced, which indicates that the direct route depicted as eq 2 is negligible under atmospheric conditions. It is concluded that formic acid is a primary product of OH acetylene since formaldehyde, which is usually a precursor of formic acid in photooxidation systems, was not detected during the OH acetylene reaction or the photolysis of glyoxal2s and the OH-initiated oxidation of glyoxal also yields no formic acid. Formic acid from propyne and acetic acid from 2-butyne are concluded to be primary products for similar reasons. PAN was observed clearly from propyne and 2-butyne in the presence of NO,, which is strong evidence for the existence of the acetyl radical (CH,CO), when the following reactions are taken into consideration. CH3CO 0 2 CH,C(O)OO (19) CH,C(O)OO NO2 CH,C(O)OON02 (20)
+
+
+ +
-
+
By analogy, H C O is presumed to be produced in the reaction of acetylene with OH. All the product analyses also strongly support the addition mechanism, which was first proposed on the basis of the pressure dependence of the rate constant for O H acetylene.6-8 The following pathways are proposed here as the mechanism for the OH (OD) radical initiated oxidation of alkynes.
\OD
Subsequent reactions of 2 should give carboxylic acid, a-dicarbonyl compounds, and RCO radicals. As for the mechanism to produce a carboxylic acid, the route similar to that proposed as the pathway of the reaction of vinyl can be considered. Recently, Gutman and radical with 0226,27 c o - ~ o r k e r detected s~~ formation of HCHO and H C O as the sole products of the reaction of the vinyl radical with molecular oxygen, and they explained the reaction pathway by the mechanism originally proposed by Baldwin and Walker26as follows.
I PJeV
Figure 8. Log (rate constant) for OH + alkyne reaction vs. ionization potential of alkynes: 1 , acetylene: 2, propyne: 3, 1-butyne; 4, 2-butyne.
Z
2
1
lo
C
/ 0-0
t 02
CHz=CH
-
-
H\
H-C.-F-H
li il O...Q
HCHO
+
HCO (22)
Since the intermediate 1 is an OH-substituted vinyl radical, a similar reaction to eq 22 can be expected to take place in the OH-alkyne-O2 reactions. Thus, the carboxylic acid forming step is suggested to be
2
-
R
\
c'-I,oD
7
RCO t R'COOD
(23)
Ii iI o.....o
Acetic acid was not detected as a product of the O H + propyne reaction. This fact implies that O H (OD) radical adds to the terminal carbon atom of the triple bond quite selectively in reaction 21. Teddar and Walton28reported that CF, radical adds to the terminal carbon of propyne preferentially (the orientation ratio is 1:0.2). Since both CF3 and OH are electrophilic radicals, their tendencies for the position of addition were expected to be similar. The results agree with this expectation. In the case of OD acetylene or propyne, HCOOH (in addition to HCOOD) was also produced as an apparently primary product. Several explanations can be made for this phenomenon. The secondary reactions of HCO or C H 3 C 0radicals can produce OH, and it produces HCOOH. The proton-exchange reaction (HCOOD H 2 0 HCOOH HDO), perhaps on the wall, should have some contribution. In addition, O H regeneration from the [OD-alkyne] adduct can also take place. Recently, Tully and Goldsmith29observed the OD formation from O H + C3D6 reactions and supposed a rapid H-D scrambling within the [OHC3D6]adduct and a redissociation of OD. Similar H-D scrambling within [OH-alkene] adducts was also reported by Slagle et aL30 O D production from OH C2D2reactions reported by Schmidt et aL20 may be explained at least in part by such a reaction. As for the pathway to form a-dicarbonyl compounds, the following reaction can explain their formation in the presence of NO,.
+
+
-
+
+
r
+
1
H
0
OD HCOCHO (24)
The formation of a-dicarbonyl compounds was observed even in (20) Schmidt, V.; Zhu,G. Y.; Becker, K. H.: Fink, E. H. Ber. Bunsenges. Phys. Chem. 1985, 89, 321. (21) Cvetanovic, R. J. Adu. Photochem. 1963, 1 , 115. (22) Atkinson, R.; Pitts, J. N., Jr. J . Chem. Phys. 1977, 67, 2492. (23) Gaffney, J. S.; Levine, S. Z. Int. J . Chem. Kinet. 1979, 11, 1197. (24) (a) Lenhardt, T. M.; McDade, C. E.; Bayes, K . D. J. Chem. Phys. 1980, 72, 304. (b) Ruiz, R. P.: Bayes, K. D. J . Phys. Chem. 1984, 88, 2592. (c) Paltenghi, R.: Ogryzlo, E. A.; Bayes, K. D. J. Phys. Chem. 1984, 88, 2595. (25) Recently, Langford and Moore (J. Chem. Phys. 1984, 80, 421 1 ) showed that the photolysis of glyoxal by 308-nm light gives 2HC0 as a direct product at a quantum yield of 0.4 h 0.2.
(26) Baldwin, R. R.; Walker, R. W. Symp. (Znt.) Combust., [Proc.],18 1981, 18, 819.
(27) (a) Park, J.-Y.; Heaven, M. C.; Gutman, D. Chem. Phys. Lett. 1984, 104, 469. (b) Slagle, I. R.; Park, J.-Y.: Heaven, M. C.; Gutman, D. J. Am. Chem. SOC.1984, 106,4356. (28) Teddar, J. M.; Walton, J. C. Tetrahedron 1980, 36, 701. (29) Tully, F. P.; Goldsmith, J. E. M. Chem. Phys. Leu. 1985, 116, 345. (30) Slagle, I. R.; Gilbert, J. R.: Graham, R. E.; Gutman, D. Int. J. Chem. Kinet., Symp. 1 1975, 317.
J. Phys. Chem. 1986, 90, 178-181
178
the absence of NO, when H202was used as a precursor for OH, which clearly indicates that N O is not required to form a-dicarbonyl compounds from 2. Thus, the reaction step to form a-dicarbonyl compounds is proposed to be 2 RCOCOR' OD (R, R' = H or CH3) (25)
-
+
Additional evidence was obtained from the effect of N O on the product ratio. If reaction 24 takes place, the branching factors for the two possible reactions of 2 (reactions 23 and 24) should depend upon the initial concentration of NO. As is shown in Figure 7, however, the product ratio A[formic acid]/A[a-dicarbonyl compound] shows no dependence on the initial concentration of NO. This fact is consistent with the suggestion of Slagle et al.27bthat the adduct formed in the C2H3 + O2reaction has an extremely short lifetime and that it decomposes readily by the channel leading to the products. We can conclude that the lifetime of 2 is also very short. Recently, Schmidt et aL20observed the regeneration of O H from OH C2H2reaction directly by means of the laser fluorescence technique. Thus, reaction 25 was strongly supported. At the same
+
-
time they observed the production of C H 2 C H 0 radicals. Therefore, C H 2 C H 0 O2 HCOCHO O H may be, as they proposed, an additional pathway to form glyoxal. From the yields of products listed in Table I1 the branching fractions for 2 to react by routes 23 and 25 are estimated to be 0.4 f 0.1 and 0.7 f 0.3, respectively, for acetylene. Other reaction pathways such as reactions 1 and 2 are negligible under atmospheric conditions. For propyne the branching fractions for reactions 23 and 25 are 0.12 f 0.02 and 0.53 f 0.03, respectively. In this case a possibility that some additional reaction channel is involved cannot be wholly excluded at present. For 2-butyne the branching fractions for reactions 23 and 25 are 0.12 f 0.01 and 0.87 f 0.07, respectively. Other reaction pathways seem insignificant in this case, too.
+
+
Acknowledgment. We are grateful to Dr. H. Bandow and Mr. H. Takagi for supplying the glyoxal and methylglyoxal they purified. Registry No. HC=CH, 74-86-2; CH,C=CH, 74-99-7;CH3C=CCH,, 503-17-3;OH, 3352-57-6.
Thermal Decomposition of Energetic Materials. 8. Evidence of an Oscillating Process during the High-Rate Thermolysis of Hydroxylammonium Nitrate, and Comments on the Interionic Interactions J. T. Cronin and T. B. Brill* Department of Chemistry, University of Delaware, Newark, Delaware 19716 (Received: June 17, 1985)
Relatively dry, solid hydroxylammonium nitrate (HAN), (NH30H)N03,was examined by IR spectroscopy from 170 K to well above its decomposition point. The modes in the 2600-2950-cm-' range are assigned to -NH3+ combination tones, rather than to v(0H) as was recently proposed for aqueous HAN solutions. Frequency-to-distance correlations for solid HAN cast doubt on the viability of using v(NH/OH) fundamentals to determine accurate interionic distances in HAN. Several of the thermolysis products of HAN (HN03, NzO, and NO2) were quantified in real time by rapid-scan FTIR and were found to oscillate when solid HAN and aqueous HAN solutions were pyrolyzed at a high rate (1 30 K sd) with pressures 2 100 psi of Ar. The onset of the oscillation is delayed by increasing the initial H 2 0 content. The period of oscillation at constant pressure increases with the heating rate. Some comments on the reactions that may lead to the decomposition products of HAN are made based on these findings.
Introduction Extremely concentrated aqueous hydroxylammonium nitrate (HAN), ( N H 3 0 H ) N 0 3 ,and aliphatic ammonium nitrates are combustible at elevated temperature and pressure making them interesting as chemical propellants.' Many of the fundamental structural and decomposition questions about H A N and H A N solutions remain to be answered. In this paper the vibrational mode assignments and their relationship to interionic interactions in H A N have been reexamined in an attempt to clarify several contradictory aspects. Rapid-scan FTIR spectroscopy (RSFTIR) characterizes real-time, high-heating rate thermolysis of H A N and H A N solutions and has provided an intriguing view into the dynamics of the process. The most interesting observation is the temporal oscillations that take place among the product concentrations at elevated pressure. Experimental Section Samples of solid and aqueous H A N and HAN-d4 were generously supplied by R. A. Fifer and N . Klein, BRL, Aberdeen, MD. Multiple freeze-thaw cycles under vacuum were used in an effort to obtain H20-free solid HAN. The percent deuteration (1) For a review see Klein, N. BRL-TR-2641, Ballistics Research Laboratory, Aberdeen Proving Ground, MD, Feb 1985.
0022-3654/86/2090-0178$01.50/0
of HAN-d, was enhanced by the addition of D 2 0 before the freeze-thaw cycles. The calibrated pyrolysis cell, spectral methods, and data reduction for high-rate thermal decomposition at specific pressures and heating rates using RSFTIR are fully described elsewhere.2 Ramp heating occurs until the final filament temperature (T,) is reached. Tf is then maintained for the remainder of the 10-s data collection. The Nicolet 60SX FTIR spectrometer with an MCT detector was set for 10 scans s-l, 2 scans per file, 4-cm-' resolution. Approximately 1 mg of sample was placed on the nichrome filament by a micropipet as necessary for the solutions. This procedure was carried out under an Ar blanket. The cell was then closed and the Ar pressure adjusted to the desired level. Thermolysis was conducted at 2, 15, 50, 100, 200, 500, and 1000 psi (1 psi = 6.9 KPa). The heating rates were 130 f 10 K s-l and the final filament temperature ranged from 600 K at 2 psi to 500 K at 1000 psi. These experiments were repeated a number of times to perfect the technique and because an occasional erratic result was obtained. While the details of the product concentrations vary slightly from run to run, the e3sential features are reproducible and accurate in all cases. Erratic behavior seemed to occur when the sample droplet size was allowed to vary too ~
(2) Oyumi, Y.; Brill, T. B. Combust Flume 1985, 62, 213.
0 1986 American Chemical Society
