J. Phys. Chem. 1993,97, 7174-7177
7174
ARTICLES Long Path FTIR Spectroscopic Study of the Reactions of CF30 Radicals with Alkenes J. Chen, T. Zhu, V. Young, and H.Niki' Centre for Atmospheric Chemistry and Department of Chemistry, York University, 4700 Keele Street, North York. Ontario, Canada M3J 1P3 Received: January 28, 1993; In Final Form: April 16, 1993
The reactions of CF3O radicals with alkenes have been studied using the long path FTIR method in the visible (A 1 400 nm) photolysis of mixtures containing CF3N0, NO, and alkene (in milliTorr range) in 700 Torr of O2/N2 at 297 f 2 K. Kinetic and spectroscopic evidence has been obtained for the occurrence of the addition reaction (a) CF3O' >C=C< adduct, and the subsequent reactions involving the adducts, for several alkenes. Using the reaction (b) CF30* N O CF2O FNO as the reference reaction, values for the relative rate constants kJkb have been determined to be 0.7 f 0.2(u), 0.9 f 0.2(u), 1.1 f 0.3(u), and 1.7 f 0.4(u) for CH2=CH2, CH3CH=CHz, trans-CH3CH=CHCH3, and (CH3)2C=C(CH3)2, respectively. Using the value of the rate constant for reaction b recently reported by Bavilacqua et aL4 (kb = (2 f 1) X 10-l1 cm3 molecule-' s-l), the values of the rate constants ka have been calculated to be (1.4 f 0.8) X 10-11, (1.8 f 1.O) X lo-'', (2.2 f 1.3) X lo-'', and (3.4 f 1.9) X lo-" cm3 molecule-' s-l for CH2=CH2, CH3CH2=CH2, trans-CH3CH=CHCH3, and (CH3)2C=C(CH&, respectively.
+
-+ -
Introduction The trifluoromethoxy radical (CF3O') is well-recognized as an important intermediate in the atmospheric degradation of hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) containing a CF3 group,l and thus, there is considerable interest in its possible subsequent reactions with various atmosphericspecies. The reactionsof CF3O' withNO and with alkanes (C2H6 and C3H8), Le., reactions 1 and 2, have recently been studied in this laboratory by the long path FTIR m e t h ~ d . ~ . ~ CF30'
+ NO
-
-
CF20
+ FNO
(1)
+
abstraction from these alkenes by CF30 were detected, and the addition of CF30 to asymmetrically substituted alkenes was exclusively to the terminal carbon of the double bond, i.e., antiMarkovnikov addition. In the present study, we have attempted to establish the kinetics and mechanisms for the CF,O radical reactions of four representative alkenes (CHz=CH2, CH3CH=CH2, trans-CHpCH4HCH3, and ( C H ~ ) Z C ~ ( C H & ) using the long path FTIR method at 297 f 2 K and in the presence of 700-Torr Oz/Nz mixtures.
Experimental Section
The FTIR facility and the general experimental procedures have been described in detail elsewhere.9 Briefly, a 180-m-path (RH = alkanes) (2) CF30' R H CF,OH R' length IR cell (30-cm diameter, 2-m length, 140-Lvolume, Pyrex cylinder with multipass internal gold-coated White cell mirrors) Values for the relative rate constantskl/k2 (at 298 K) have been surrounded by 26 visible fluorescent lamps (GE F40CW, X 1 determined to be 1104, 50 f 15(u), and 20 f 5(u) for CH4, 400 nm) was used as a photochemical reactor. IR spectra were C2H6, and C3H8, respectively. Quite recently, Bevilacqua et al. recorded in the frequency range 500-3700 cm-l using a Gehave determined the absolute value of the rate constant kl as (2 coated KBr beam splitter and a liquid He cooled Cu-Gedetector. f 1) x 10-11cm3 molecule-1 s-1, using a low-pressure flow reactor The interferometer was operated at I / 16-cm-I resolution, and 16 coupled to a chemical ionization mass ~pectrometer.~ This value co-added interferograms were recorded in 90 s. of kl combined with our relative values of kllk2 yields k2 values As in our previous studies of the CF30radical reactions of NO o f l 2 x lO-l5,(4f3) X 10-13,and(l fO.6)X 10-~~cm3molecule-~ and alkane^,^^^ the visible (X 1 400 nm) photolysis of CFSNO, s-' for CH4, C2H6, and C3H8, respectively. These values of k2 reaction 3, was employed as a convenient source of CF3 radicals. are remarkably similar to those for the corresponding HO radical In the presence of 0 2 and NO, CF30radicals are produced from reactions (Le., 6.5 X 2.4 X lW3, and 1.1 X lo-'* cm3 CF3 radicals via reactions 4 and 5:1J0 molecule-1 s-l at 298 K for CH4, C2H6, and C3H8, re~pectively)~ but are approximately 5 orders of magnitude greater than those C F 3 N 0 hv (A 1 400 nm) CF,' + N O (3) for the corresponding CH30 radical reactions.6 The above observations strongly suggest that the CF30radical and the HO radical may behave similarly toward alkenes (i.e., CF,' + 0, (+M) CF,OO' (+M) (4) fast addition reaction and electrophilic trends in reactivity).' Previously,Chen and Kochi8studied CF30radical reactions with C F 3 0 0 ' N O CF30' NO2 (5) alkenes in CC12F2 solutions at temperatures between -50 and -130 OC using ESR. These authors reported that the CF30 radicals, which were generated photochemically from CF300CF3, Photochemical experiments were carried out with mixtures added to the alkenes, forming CF30CH2cHz,CF30CH2cHCH3, containingCF3N0, NO, and alkene (CHflH2, CH$H=CHz, and CF3OCH&(CH3)2 radicals from ethene, propene, and trans-CH3CH=CHCH3, or (CH&C=C(CH&) in 700 Torr isobutene, respectively. No products corresponding to hydrogen of synthetic air at 297 h 2 K. Reactantswere present at milliTorr
+
+
+
+
0022-3654/93/2091-7 114904.00/0
-
0 1993 American Chemical Society
-
+
Reactions of CF30 Radicals with Alkenes
The Journal of Physical Chemistry, Vol. 97, No. 28, I993 7175
(A) Before irradiation
(B) After 4 min Irradiation
X
I
\
NO
CH2sCH2
I
2000
1500
1000 I l X (cm.1)
Figure 1. IR absorbance spectra in the frequency region 6OC-2000 cm-1 recorded in the photolysis of a mixture containing CF3NO (4 ppm), NO (3 ppm), and CHFCH~ (9 ppm) in 700 Torr of air: (A) before irradiation;(B) after 4-min irradiation;(C) difference spectrum [(B) (A)] scale expanded by X5.
partial pressures (1 mTorr = 1.3 ppm). IR spectra were typically recorded after every 60-s irradiation. NO (99%), CH2=CH2 (>99.5%), CH3CH-CH2 (>99%), trans-CH3CH=CHCH3 (>99%), and (CH3)2C=C(CH3)2 (>99%) from Aldrich were used as received. Purification and gas-handling procedures for CF3NO (>95%, PCR) have been described elsewhere.,
occurred, could account for at most 3% of the CF30 radicals consumed,based on our detection limit for CF3OH. Thus, CF30 radicals reacted with CH2=CH2 predominantly via the addition reaction 6. Also, when CH2PCHz was replaced with CH3CH=CH2, rrans-CH&H==CHCH3, or (CH3)2C=C(CH3)2 in the above photolysis experiment, no CF30H was detected as a reaction product. Thus, additionof the CF30 radical to the double bond predominates over H atom abstraction for higher alkenes as well. This observationis similar to that observed by Chen and Koch? in their low-temperature liquid-phase reactions. Among the photolytic products identified from the spectral data shown in Figure 1 were CF20, FNO,HONO, CH20, NO2, and CF30N02. Several broad bands in the range 1200-1300 cm-l and in the vicinity of 1760cm-I, labeled as X in Figure lC, belong to reaction product(s) which have thus far not been identified positively. These products were stable during aging for 30 min in the dark. The set of bands between 1100 and 1400 cm-1 are located in the frequencyregion of C-F and C-0 stretches, similar to those seen for CF3NO and CF30H,2.3and the band at 1760cm-l is consistent with a C 4 ~ t r e t c h .Not ~ shown in this figure are bands at approximately 2900 cm-I, in the frequency region of aldehydic C-H stretches. The unidentified products are suspected to be CFSOCH~CHO and/or CF30CHO formed via reactions steps analogous to those involved in the HO radical initiated oxidation of alkenesin the presenceof N0,9i.e., reactions 8 and 9
followed by reaction 10 CF,OCH,CH,O'
+ CH,=CH2
+ CH2=CH,
-
-
+
CF30CH,-CH,'
CF,OH
+ CH,-CH'
CF,OCH,OO'
(7)
CF,OCH,O'
CF,OCH,CH,O'
CF,OCH,CHO
-
CF,OCH,'
+ + -
+ 0, (+M)
(6)
In our earlier study of C F 3 0 alkane reactions, the molecular product CF30H arising from the H atom abstraction reaction was observed.3 CF30H exhibits several sharp peaks at 1119, 1183,1361,1400, and 3665 cm-1, and its concentration has been determined using our long path FTIR system with a detection limit of 10.01ppm (1 ppm = 0.77 mTorr). Therefore, we initially expected to detect CF30H as a reaction product of the C F 3 0 alkene reactions. Displayed in Figure 1 are typical IR spectral data obtained in the photolysis of mixtures containing CF3N0, NO, and CHFCH~ in the ppm (or mTorr) range in 700 Torr of air. Spectra A and B of Figure 1 were recorded before and after 4-min irradiation, respectively, of a mixture containing CF3NO (4 ppm), NO (3 ppm), and CH,=CHz (9 ppm). Figure 1C correspondsto the differencespectrum [ (B) - (A)], scaleexpanded by X5. Prior to irradiation, the reactant mixtures were kept in the reactor in the dark for 15 min. During this period, no observable change in the reactant concentrations occurred (cf. Figure 1A). After 4-min irradiation, about 7% of the reactant CFsNO was consumed. However, no IR bands attributable to CF3OH were detected. The H atom abstraction channel, if it
+
CF30CH,'
-
-
CF,OCH,CH,O'
Product Study. The first step taken toward mechanistic understanding was to quantify the two possible channels, e.g., reactions 6 and 7, for ethene.
CF,O'
+ 0,
and/or by reactions 11-14:
Results and Discussion
CF,O'
+ NO
CF,OCH,CH,OO'
NO
0,
+ NO,
+ HOO'
+ CH,O
CF,OCH,OO' (+M) CF,OCH,O'
+
+ NO,
CF30CH0 H O C
(9)
(10)
(1 1) (12)
(13) (14)
The relative importance of the competing 02 reaction and unimolecular dissociation channels for the CF30CH2CH20 radical, Le., reactions 10 and 11, remains unknown. However, it should be noted that in the case of the HO' alkene reactions the resulting alkoxy radicals are known to mainly undergo unimolecular di~sociation.~ Also, it is clear that the oxy radical formed in the CF30*+ (CH3)2C=C(CH3)2reaction can undergo only unimolecular dissociationbecause of the absence of an CY-H atom. In any case,it was observed that each alkene yielded similar but different unidentified product(s) X,as evidenced by slight differences in shape and frequency of bands in the IR spectra. In addition, the formation of HO radicals via the HOO' + NO HO' NO2 reaction in these mixtures led to carbonylproducts expected from the HO' + alkene reactions as well as to the formationofHONOvia thereactionHO'+NO(+M)+HONO
+
-
+
(+M) .5*'1 On the basis of their ESR study, Chen and Koch? suggested that the CF30group interacts with the nearby radical site on the free radical adduct and that this interaction might lead to unimoleculardecompositionor to isomerizationto an oxy radical, reactions 15 and 16.
Chen et al.
7176 The Journal of Physical Chemistry, Vol. 97, No. 28, I993
*
OC Fo
OCF3
. \
I
>C-C
