Kinetics and Mechanism of the Thermal Gas-Phase Reaction between NO, and

Trichloroethene at 303-362.2 K

J. CZARNOWSKI Instituto de lnvestigaciones Fisicoquimicas Tebricas y Aplicadas, Casilla de Correo 16,

Sucursal4, 1900 La Plata, Argentina

Abstract

The kinetics of the gas-phase reaction between NO2 and trichloroethene has been investi- gated in the temperature range 303-362.2 K. The pressure of NO2 was varied betwen 5.1 and 48.7 torr and that of trichloroethene between 7.3 and 69.5 torr. The reaction was homogeneous. Two products were formed: nitrosyl chloride, ClNO, and glyoxyloxyl chloride, HC[OlC[O]Cl, which was identified by its infrared spectrum and its molecular weight determined by chroma- tography. The rate of consumption of the reactants was independent of the total pressure and can be represented by a second-order reaction:

-d[NOz]/dt = -2d[CHClCClzl/dt = h[NOzl [CHClCClZ]

The following mechanism was proposed to explain the experimental results:

(1)

(2)

NO2 + CHClCClz - OzNCHClCClz

OzNCHClCCIz - NO2 + CHClCClz

(3)

(4)

0 .. , CH- CClz R, . . 02NCHClCC12 +

O N ' ' ' Cl

0 . . . CH -ClC, .. 0 ' . A . . R, + NOz ,

ON...C1 C l . . N O

(5) A - HC[O]C[O]Cl + 2ClNO

The following expression was obtained for k :

k = 2klk3/[k~ + k3] = 4.07 2 1.2 X 10' exp[-8800 2 800 cal mol-'/RT]l mol-' 5 - l . 0 1992

John Wiley & Sons, Inc.

Introduction

The reactions of NO2 with halogenated alkenes have been subject of sev- eral studies [l-41. These works were generally undertaken for preparative purposes, providing the evidence that NOz may act as nitrating or oxidizing agent.

The production of 0 2 NCF&[ O]F, 0 2 NCFHC[O]Cl, CFzClC[ N02]C1C[ OIC1, and CF3C[NO2]ClC[O]Cl has been reported as result of the respective nitra- tions of CFzCF2 [51, CFHCClz [61, CF2C1CC1=CCl2, and CF3CC1=CClz [71. Other authors have detected the formation of XNO, where X = F or C1, in

International Journal of Chemical Kinetics, Vol. 24, 679-687 (1992) 0 1992 John Wiley & Sons, Inc. CCC 0538-8066/92/070679-09$04.00

680 CZARNOWSKI

addition to that of 02NCF2C[O]F or 02NCF2C[O]C1, in the reactions of NOz with CF2CF2 [8,91, CF2CFCl 1101, and CF2CC12 [ill.

The elimination of XNO and the formation of acyl halides can be ascribed to the concominant weakening of the C - X bond when C - 0 bond is form- ing and to the lower bond dissociation energy of C - N bond compared with the C -0 bond [121.

Detailed kinetic and mechanistic studies were made only for the additions of NOz to CF2CFz [91 and CF2CC12 1111. In both reactions, the consumption rates of NOz were found to be first-order with respect to each reactant, NOz, and alkene. The corresponding preexponential factors of the rate constant were very low, 1.3 X lo4 and 3.16 X lo6 1 mol-' s-', respectively.

There are not available data on the addition of NOz to chlorinated alkenes. In this work the thermal reaction of NO2 with trichloroethene, CHC1CClZ, is studied.

Experimental

The experiments were performed in a grease-free static system, allowing pressure measurements at constant volume and temperature, connected to a high-vacuum line and equipped with two reaction vessels: a spherical bulb of 180 cm3 and another one of similar dimension filled with small pieces of quartz tubing. The corresponding surface-to-volume ratios were 0.75 and 4.7 cm-'. Pressure measurements were made with a quartz spiral gauge. A Lauda thermostat maintained the temperature within 50.1 K.

The products were identified by their infrared spectra. The concentration of reactants and that of the products in the reaction mixtures, were deter- mined using infrared calibration curves for the conversion of the absorption intensities into pressures. The infrared spectra were recorded on a Perkin Elmer 325 spectrometer, using a 10 cm cell with sodium chloride windows.

A Gow-Mac 625 gas chromatograph, using 5% SE-30 on CG column and Nz as a carring gas, was used to determine molecular weight. This chro- matograph is provided with a density balance detector, which gives the mo- lecular weight of an unknown compound using the formula M = [K A/PV] + M,, where P and V are the pressure and the volume of the gas, K is the chromatograph constant, A is the peak area of the unknown compound, and M, is the molecular weight of the carrier gas [131. The chromatograph constant, K, was determined using pure CF301?

The reactants were commercial products. Trichloroethene was purified by several trap-to-trap distillations on vacuum line, retaining each fraction dis- tilling between 266 and 271 K. NO was eliminated from NOz by a series of freeze-pump-thaw cycles in presence of Oz until disappearance of the blue color of N2O3. Finally degassed NOz was purified by fractional condensation using the fraction that distilled between 193 and 233 K. Nz was bubbled through 98% analytical pure grade HzS04 and passed slowly through a Pyrex coil at liquid air temperature.

Experiments and Results

Only the data of the experiments made in the temperature range of 303- 362.2 K, were considered to postulate the mechanism and to determine the

THERMAL CAS-PHASE REACTION 681

kinetic parameters. The pressure of NOz was varied between 5.1 and 48.7 torr and that of CHCICClz between 7.3 and 69.5 torr. Some experiments were made in presence of added Nz as well as in the reactor vessel with larger surface-to-volume ratio.

Within the temperature range studied, the reaction proceeded without pressure change. Two products were formed: ClNO, identified by its known infrared spectrum [14] and glyoxyloxyl chloride, HC[O]C[O]Cl. The prepra- tion of the latter compound has been reported by other authors [151, includ- ing its identification with dinitrophenylhydrazine. In this work HC[O]C[O]Cl was identified by its infrared band frequencies and its molecular weight determined by chromatography U61.

To identify HC[O]C[O]Cl, it was necessary to obtain a greater amount of this compound. For this purpose, the reaction temperature was risen to 416 K and 10 high conversion batch preparative were carried out, varying the initial pressure of NOz between 101 and 122 torr and that of CHCICClz between 58 and 68 torr.

At 416 K, the reaction proceeded with a pressure incerase of 0.02 torr per min and the formation of CO and HC1 was detected in addition to the main products. The separation of the products was performed by vacuum frac- tional condensation. CO, HC1, and ClNO distilled together at 173 K. The fraction volatile between 173 and 228 K consisted of the nonconsumed reac- tants. The fractins of all preparative left at 228 K were collected together and gas infrared spectra of this residue performed. Bands at 625, 735, 781, 828, 855, 984, 1028, 1353, 1770, 1777, 1783, and 2852 cm-' have been ob- served. The following structural groups were identified by their correspond- ing frequencies: C1-C=O by 625 cm-', C1-C by 735, 781, 828, and 855 cm-', C-C by 984 and 1028 cm-', C-H by 1353 and 2852 cm-', and C=O by 1770, 1777, and 1783 cm-I, pointing out that the structure of the residue remaining at 228 K is HC[O]C[O]Cl.

To confirm its identity, the molecular weight of the residue was deter- mined by chromatography. The value obtained was 90 ? 6. The theoretical molecular weight of HC[O]C[O]Cl is 92.5.

The chromatography also established the purity of HC[O]C[O]Cl, as only one peak appeared in the chromatograms.

To determine the concentrations of the products and reactants in the re- action mixtures, the infrared calibration curves were made using the pure compounds, thereby allowing the conversion of the absorption intensities at 735, 1806, 2912, and 3097 cm-' into pressures of HC[O]C[O]Cl, ClNO, NOz, and CHC1CCl2, respectively. The corresponding absorption coefficients [to base 101 were: 4.1 X 0.03, 1.4 X and 1.25 X torr-' cm-l.

To obtain the yields of the products with respect to the reactants con- sumed, several high conversion runs were performed within 303-362 K, in- creasing the reaction times to 7-34 h. Two infrared spectra of each reaction mixture were carried out for computing the concentrations of the products and the nonconsumed reactants.

The data of 11 experiments are summarized in the Table I, where At is the reaction time, indices i and f signify initial and final, respectively, and Nz04 is the content. of Nz04, calculated for each pressure of NOz + Nz04, using the corresponding equilibrium constant [17].

TA

BL

E I.

Dat

a of

11

expe

rim

ents

.

Nr

T/K

A

t/h

At/m

in

CzC

13H

,/tor

r C

pC13

Hf/t

orr

N02

,/to

rr

NZ

O4,

/tor

r N

02r/

torr

N

204f

/tor

r H

C(O

)C(O

)Cl/t

orr

ClN

O/to

rr

a B

15

303

34

-

54.8

16

.5

0.41

7 15

.4

0.36

4 0.

6 1.

0 17

32

3 21

-

52.8

29

.0

0.31

6 26

.2

0.25

8 1.

5 2.

8 20

34

3 31

-

52.8

10

.6

0.01

2 8.

1 0.

007

1.4

2.6

21

343

33

-

7.3

6.6

29.6

0.

096

28.3

0.

088

0.6

1.2

22

343

32

-

12.1

11

.3

28.3

0.

088

26.7

0.

078

1.0

1.8

64'

343.

1 -

301.

2 60

.8

19.0

0.

04

18.2

0.

037

0.3

0.8

26

362

20

-

7.8

6.4

49.0

1.

3 2.

8 27

36

2 20

-

59.0

11

.4

0.00

5 8.

4 0.

003

1.7

3.3

28

362

7 -

30.5

29

.6

30.4

0.

036

28.9

0.

032

0.8

1.6

29

362

7 -

30.0

29

.2

29.1

0.

033

27.4

0.

029

0.9

1.6

61b

343.

1 -

332.

8 61

.6

19.4

0.

041

18.2

0.

037

0.7

1.2

a

'Thi

s co

ncen

trat

ion

was

not

det

erm

ined

. bT

his e

xper

imen

t was

mad

e in

pre

senc

e of

572

.6 to

rr o

f N

O

'Thi

s ex

peri

men

t was

mad

e w

ith

S/V

= 4

.7 c

m-'

THERMAL CAS-PHASE REACTION 683

From these data it can be deduced that: CHClCClz consumed =

HC[O]C[O]Cl NOz consumed = ClNO = 2 CHCICClz consumed. The consumption of NOz and CHClCC12 can therefore be determined

knowing the corresponding amount of ClNO formed. The reaction is homogeneous and its rate independent of the total pres-

sure. It can be represented by the overall reaction: 2 NO2 + CHClCC12 = 2

The consumption rate of NOs can be represented by a first-order reaction ClNO + HN[O]C[O]Cl.

with respect to each reactant, NOz and CHC1CClZ:

-d[NOz]/dt = -Zd[CHClCClz]/dt = k[N02] [CHClCClZ] I

In the experiments made to determine the temperature dependence of the rate constant, h, only the concentration of ClNO was determined by infra- red spectroscopy.

Due to the low conversion of NOs in the temperature range studied, the contribution of NO2 from the dissociation of Nz04 did not affect signifi- cantly the values of the amounts of NO2 consumed by the reaction, allowing to calculate the rate constant h by using the expression k t = 2[2b - a] -' . ln[b - 0.5x]a([a - x]b)-'; which is the integrated form of the equation dx/dt = k[a - x][b - 05x1, where a and b are the initial pressures of NOs and CHCICC12, respectively, x is the concentration of ClNO, and t is the re- action time. Figure 1 shows two plots of [2b - a] -'ln[b - 0.5x]a([a - x]b)-' vs. reaction time at 323.1 and 343.1 K. The straight lines indicate that the reaction is first order with respect to each reactant NO2 and CHClCC12 [18].

The calculated rate constants of 29 experiments are represented in the Table 11.

The following mean values of the rate constants at different tempera- tures are: k[303 K] = 1.86 ? 0.3 X 1 mol-' s-', h[323.1 K] = 4.74 ? 0.5 X 1 mol-' s-l, and h[362.2 K] = 2.0 2 0.6 X 1 molF's-'.

The temperature dependence of the experimental rate constant k can be expressed in the Arrhenius form [Fig. 21:

1 mol-' s.-l, h[343.1 K] = 9.84 t 2.2 X

h = 4.07 ? 1.2 x 10' exp [-8800 2 800 cal mol-'/RT]l mol-' s-'.

m

-lk

8 - 0T=323.1 'K

T = 343.1 'K 6

4 -

2 -

1

t /min

Figure 1. with respect to each reactant concentration, at 323.1 and 343.1 K.

Second-order plots of the reaction of NO2 with trichloroethene, first-order

684 CZARNOWSKI

TABLE 11. Rate constants of 29 experiments at different temperatures.

Nr T/K At/min C2C13H,/torr NOz,/torr CINO/torr k X 106/torr-’ min-’

55 303.0 401.5 54.2 16.3 0.2 0.568 56 303.0 638.9 25.2 10.5 0.1 0.595 57 303.0 361.5 50.2 10.5 0.1 0.528 58 303.0 564.1 52.5 5.1 0.08 0.534 59 303.0 240.3 60.9 8.9 0.08 0.617 60 303.0 481.1 61.2 10.0 0.2 0.687

48 323.1 182.2 53.7 29.8 0.4 1.38 49 323.1 360.4 57.5 41.6 1.1 1.30 50 323.1 360.8 51.7 29.9 0.8 1.46 51 323.1 435.3 50.8 16.0 0.5 1.44 52 323.1 332.8 22.7 29.8 0.3 1.34 53 323.1 120.0 64.3 18.7 0.2 1.39 54 323.1 182.1 69.5 15.6 0.3 1.54

40 343.1 127.4 33.4 28.8 0.4 3.30 41 343.1 364.3 31.2 29.9 0.8 2.40 42 343.1 241.9 29.5 29.9 0.6 2.86 43 343.1 414.0 55.8 5.1 0.3 2.63 44 343.1 421.6 52.6 10.6 0.6 2.64 45 343.1 482.0 7.4 29.6 0.3 2.89 46 343.1 424.6 12.3 28.1 0.4 2.77 47 343.1 209.0 54.5 30.6 0.9 2.63 61” 343.1 332.8 61.6 19.4 1.2 3.13 64b 343.1 301.2 60.8 19.0 0.8 2.36

34 362.2 260.75 20.7 15.4 0.5 6.15 35 362.2 301.65 7.3 48.7 0.7 6.74 36 362.2 293.2 58.2 11.3 0.8 4.32 37 362.2 121.2 30.1 28.8 0.6 5.80 38 362.2 240.6 30.4 30.4 0.9 4.14 39 362.2 362.9 29.9 28.8 1.4 4.65

= 0.59 2 0.1

k = 1.41 -t 0.15

k = 2.76 2 0.6

- k = 5.3 f 1.5

“This experiment was made in presence of 572.6 torr of Nz. bThis experiment was made in the reactor with S/V = 4.7 cm-’.

Discussion

To explain the experimental results the following mechanism is postulated:

NO2 + CHClCClz OzNCHClCClz

OzNCHClCClz - NO2 + CHClCClz

0 . . . CH-CC1Z (3) OzNCHClCClz - R,

ON.. . Cl

0 .. . CH-ClC . . . O (4) R, + NO2 - : : A

ON.. . Cl Cl ..No A - HC[O]C[O]Cl + 2ClNO

THERMAL CAS-PHASE REACTION

-3.5

685

-

ul c 'I -2.51

E I

I

2.5 3.0 3.5

I1O3/T) /K- '

Figure 2. Arrhenius plot of k.

The primary reaction pass must be the addition of NO2 to the double bound of alkene, forming nitro radicals [9,11,19-211. These nitro radicals may redissociate or rearrange to give the nitrite radicals R,.

The reversible addition of NO2 to the double bond of alkenes has been es- tablished by other authors [19,211, as well as the nitro-nitrite rearrange- ment [221.

The configuration of the radical R, must favor the addition of the electron- deficient oxygen atom of NO2 to the group - CClZ, forming the dinitrite ad- duct, which decomposes into HC[O]C[O]Cl and 2 ClNO. In general the nitro compounds XN02 are stable, but the nitrite compounds XONO tend to break up to XO and NO [231. The formation of nitro-nitrite adduct was proposed to explain the formation of ClNO and OZNCF2C[O]C1 as products of the reac- tion of NOz with CFzCClz [lll.

The configuration of the dinitrite adduct was postulated by analogy with the mechanism proposed by other authors to explain the formation of COFz and FNO in the reaction of radicals CF3 with NO2 [241:

FX . . . O CF3+NOz- i ' -COFz+FNO

F . . . N O

The application of a similar mechanism to the reaction of NO2 with radi- cals 02NCFzCFz can explain the formation of OZNCF~C[O]F and FNO as the main products of the reaction of NOz with CFZCF, [91.

The configuration of the dinitrite adduct may be rationalized in terms of the competition for carbon atom electrons between the electron-deficient oxygen atom of NOz and the electrophilic chlorine atom. The elimina- tion of ClNO can thus be ascribed to the concomitant weakening of the C - C1 bond when C - 0 bond is forming and to the lower bond dissociation energy of C - N bond compared with the C - 0 bond 1121. This assump- tion is confirmed by the formation of OZNCF2C[O]C1, 02NCHFC[O]C1,

686 CZARNOWSKI

CF2C1C[N02]C1C[O]C1, and CFsC[N02]C1C[O]C1 as result of the nitrations of CF2CClz [lll, CHFCC12 [61, CF2ClCCl=CC12, and CF&C1=CClz 171, respectively.

It is well known that the formation of C - 0 bond in oxy perhalogenated methyl radicals weakens the C -C1 bond leading to the rapid detachment of the chlorine atoms and to the corresponding carbonyl halides. Similar be- havior was observed for the radical CFzClCC120, which decomposes into CF,ClC[O]Cl and chlorine atom 1251.

The reaction is homogeneous and independent of the total pressure. Its ex- perimental rate for the consumption of NO, is well represented by a first- order reaction with respect to each reactant, NO,, and CHClCC12. The preexponential factor A, equal to 4.07 5 1.2 x 10' 1 mol-' s-', has an abnor- mally low value for a rate constant of an homogeneous reaction, pointing out that A is a relation between different preexponential factors.

The first-order reaction with respect to each reactant, NO2 and alkene, and very low preexponential factors, 1.3 x lo4 and 3.16 x lo6 1 mol-' s-l, have been reported for the homogeneous additions of NO2 to CFZCF, [9] and CFzCClz 1111, respectively.

Applying the steady state approximation method to the mechanism, the following equation is obtained for the consumption of NOz:

-d[NOJ/dt = -2d[CHClCClz]/dt = 2hik3/[h2 + h3] [NO,] [CHClCCl,] I1

Comparing eq. I1 with eq. I, that: h = 2hlh3/[h2 + ha] = 4.07 -+ 1.2 X 10' exp[-8800 ? 800 cal mol-'/RT] 1 mol-' s-'.

Assuming that the preexponential factor A, is equal to lo7 1 mol-' s ', A2 is equal to l O I 4 s-l andA3 is equal to lo9 s-', the value of 10, 1 mol-' s-' is obtained for A1A3/[A2 + A3]. The value of 10" 5-l was estimated for the preexponential factor for isomerization of alkoxy radicals [26].

Acknowledgment

This work was financially supported by the Consejo Nacional de Inves- tigaciones Cientificas y Thcnicas and the Comisi6n de Investigaciones Cien- tificas de la Provincia de Buenos Aires.

The author wishes to thank Dr. M. I. Florit for helpful comments.

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Received May 14, 1991 Accepted November 5, 1991