JOURNAL O F T H E AMERICAN CHEMICAL SOCIETY Registered i n U.S . Patent Ofice.
@ Copyright, 1964, by the American Chemical Society
DECEMBER 4, 1964
VOLUME86, NUMBER 23
PHYSICAL AND INORGANIC CHEMISTRY [CONTRIBUTIONFROM
THE
DEPARTMENT O F CHEMISTRY, RENSSELAER POLYTECHNIC INSTITUTE, TROY, NEW YORK]
Kinetics and Mechanism of the Free-Radical Addition of Trifluoroacetonitrile to Ethylene' BY N. A. GACAND G. J. JANZ RECEIVED JUNE 19, 1964 A kinetic study is reported for the free-radical addition of CFsCN to CHzCH2 at 442' and atmospheric pressure. The reaction is an example of the general class of telomerization reactions. An average chain length of about 300 is observed when kinetic control is such that CF3CH2CHzCN is the predominant product. The cross-combination ratio for the CF3 and C F ~ C H Z C Htermination Z reactions is 2.2 0.5. The recombination of CF3 radicals does not become the exclusively controlling termination process until the C F 3 C S :CH2CH2ratio approaches 105: 1.O. The limiting form of the rate equation is found to be ~ [ C F I C H Z C H ~ C N=] /38.7 ~ ~ X 10-3[C2H4][CF3CN]'/2.
+
Introduction Thermally initiated addition reactions of CF3CN with monoolefins are being investigated in this laborat ~ r y . ~ For - ~ example, the formation of 4,4,4-trifluorobutyronitrile (CF3CH2CH2CN), with some 6,6,6trifluorocapronitrile (CF3(CH2)&N) in smaller amounts, is 0bserved~7~ when CF3CN and C2H4 are heated a t 400' and normal pressures. The rate of reaction is clearly diminished by addition of small amounts of an inhibitor, NO (see ref. 3 ) , and promoted by an initiator, (CH2)20 (see Discussion). From a limited study of the temperature dependence of the rate for this system, in the range 350-450', an estimate of the energy of activation for the formation of CF3CH2CH2CN has been a d v a r ~ c e d . ~The over-all rate was approximately 3 / 2 order, but the kinetic data were insufficient to characterize the details of the reaction mechanism. The appearance of the 2 : 1 stoichiometric compounds in the products is suggestive of a gas-phase, free-radical, telomerization process. The gas chromatographic results showed that the formation of CF3(CH2CH2)2CNrelative to CF3CH2CHgCN decreased as the C2H4: CF3CN reactant ratio decreased. This was understood as a competition between reactions such as CFsCH2CH2
+
+ CFsCN +CFs + CFgCH2CHzCK
C F P C H ~ C H ~CH2CH2 +CFa( C H Z C H ~+ ) ~ telomers
The generalized mechanism for this class of reactionsfi (1) Abstracted in part from the thesis submitted by N. A. G. in partial fulfillment of t h e requirements for t h e Ph.D. degree, Rensselaer Polytechnic Institute. Jan., 1964. (2) G. J. Janz and J . J . S t r a t t a . J . Org. Chem., 16, 2169 (1961). (3) J. J. S t r a t t a , Ph.D. Thesis, Rensselaer Polytechnic Institute, Troy, N . Y., 1961. (4) W. J. Leahy, Ph.D. Thesis, Rensselaer Polytechnic Institute, Troy, N. Y.,1961. ( 5 ) N. A. Gac and G. J . Janz, J. A m . Chem. Soc.. in press. (6) C. Walling, "Free Radicals in Solution," John Wiley and Sons. Inc., New York. Pi. Y., 1957, Chapter 6.
leads to quite complex differential rate expressions. With the exception of the kinetic studies with cII'3I and ethylene by Bell,' data for such thermally initiated processes a t moderately high temperatures and normal pressures appear almost nonexistent. The present communication reports the results of an investigation of the kinetics and mechanism of the CF3CN-CH2CH2 reaction a t 440' and atmospheric pressures.
Experimental The reactants, CF3CX (95Yc minimum purity, Peninsular Chemresearch, Inc.), C2H1 (9970 minimum purity, Matheson Co., Inc.), and ethylene oxide (99.7%, minimum purity, Matheson Co., Inc.), were degassed and vacuum transferred to suitable small metal cylinders for use as required. The CFaCT*; was further redistilled a t low temperatures prior to use. Apparatus and Procedure .-The design of the experimental assembly and vacuum-transfer manifold is shown in Fig. 1. The reaction flask (1 1.) was conditioned* by a 24-hr. ethylene pyrolysis at 480'. T o premix the reactants a larger flask ( 5 1.) and magnetically coupled stirring device was adopted. Homogeneity of the gaseous reactant mixtures was cross-checked with gas chromatographic analysis. T h e reaction furnace (11 in. i.d. X 22 in.) could be used simply as an air thermostated zone ( f O . l O ) , or with a molten salt bath about the reaction flask to further improve the heat transfer. T h e latter technique was used in the free-radical sensitization studies. With the reactant mixture introduced t o the hot zone, time and pressure readings ( 1 0 . 0 5 mm.) were made to gain the data for the kinetic analysis. The time average for such pressure readings was 20-30 sec.; all times were measured with an electric stopwatch. Two pressure and time readings were made every 100 sec. in the first 1000-sec. period of each kinetic experiment. Readings were continued a t less frequent intervals for the duration of these experiments (-30 min.). The flask contents were "rapid-quenched" at liquid N2temperatures a t the completion of an experiment. Gaseous compounds were separated in the convetitional manner from the liquid products by vacuum-transfer techniques and stored iri calibrated flasks (Fig. 1) until aliquots were required for analysis (gas chromatography ( 2 5 " )and infrared spectroscopy). The liquid compounds were weighed, and ali(7) T. S . Bell, J. Chem. S o c , 4973 (1961). (8) W D. Walters, "Technique of Organic Chemistry," Yo1 V I I I , Interscience Publishers, New Y o r k , S . Y . , 1963, Chapter V, p. 235.
5069
5060
N. A . GACA N D G . J . J A N Z
Peak
1
Mo
0.016 0.0028 CpH4CY
0.008 ,210 = 0 . 1 8 5 Identity =
2 0.015 0.013 CpHjCN
Vol. 86
TABLE I 3 0,0024 0.029 CF~CH~)~CFI
quots were removed for analytical attention by gas chromatography (Beckman GC-2 chromatograph; di-n-decyl phthalate on C-22 firebrick, 160').
Results It was apparent from the mechanism advanced (see Discussion) t h a t an experimental investigation Of the free-radical mechanism would be possible most readily if the studies were limited to the region of [CF3C N ] >> [CzH4], i . e . , low values for the mole ratio of ethylene to CF3CN. To define the necessary limits for this ratio, the composition of the product was investigated as a function of the ratio CzH4: CFBCN,hereafter designated by J f , It was found that the rate of formation of CFS(CHzCHzjzC5 to cF3cH.1CHzCN was directly ProPortional to -11 as odd be predicted in a telomeric process.' Results for two such experiments, for high and low values of :If, are shown in Table I ; the identity of the compounds and amounts (relative to CF3CH2CH2CNtaken arbi-
a
0.0025 0 028 CFa(CH2)dCS
The kinetic data and results are shown in Tables 11 and 111 for fixed initial pressures of C F ~ C Nand CzH4, respectively, in the reactant mixtures. The runs a t constant initial CF3CN pressures in Table I1 were made initially to test the order of the reaction with respect to ethylene, The pressure-time data in the initial periods appeared to be pseudo-first order, but the first-order rate constants were found to vary with ethylene pressure. They are numerically equal to the pseudo-kl divided by [cF,cN]'Y~ given in Table 11, Limits of error are shown, from a complete analysis, and the correction of the results to a common temperature was possible using the previously reported value3 for E,,, (=A5 kcal. mole-'). The data and results for these mixtures with ethylene oxide present, as a means of varying the initiation rate,Y-lI are TABLE I1
DATAASD RESULTS FOR EXPERIMESTS WITH COSSTAST OF C F I C S INITIALPRESSCRES a,
T,
Q
4 1.0 1.0 CF3CH?CHpCIi
Expt. A-15 A-16 A-2 A-1 A-6 A-7 A-9 A-10 A-8 A-3 A-4 A-I1 A-I2 A-17 A-18 A-19 A-5 A-14 A~13 a
OC. 441.2 441.7 442.9 442.0 443.7 442 5 441 9 442 4 442 6 441 9 442 4 442 9 442.0 4403 437 4 440.4 442 8 440 8 440 8
mm. CFaCN 361.1 361.4 359.5 329.9 359.7 360 5 360.9 360 0 361.1 360.3 359.9 358.7 362 4 3864 362 6 364.3 360 0 360 9 360 4
b, M ' a mm. (CHtCHd CHiCHt CFaCS) 4 7 4.7 8.8 8.1 9.4 9.4 11.7 11.7 15.8 17.5 17.5 28.6 28 9 43.7 44 6 45.6 48.9 51 9 57.6
Initial values, i . e . , for M
0 013 0,013 0.0245 0,0245 0.026 0.026 0.0325 0.0325 0 0439 0.0485 0.0483 0,0820 0 0820 0113 0 123 0 125 0 136 0.144 0.160 =
-___ x 1-0' 12 1. 4
15 =t 2 14 zk 5 16 =t2 1 7 . 6 10 . 2 23 zk 1 8 . 7 1. 0 . 3 10.4 z 0 . 6 7.5 = 0 3 6 9 0.4 7 3 10 4 5 3 1. 0 . 1 5 3 1. 0 2 4.12zk005 3 62 0 06 3 81 0.06 3 24 = 0 06 2 99 z 0 . 0 8 2.97 =t 0 . 0 8
+
* *
M0.
T
T U
k i l t , (mo1es:'l
=
)
442'",' sec. -1
13 = 4 1 5 ~ 2 14 * 4 16 + 2 17.6 3zO.2 23 1 8 7 1.0.3 1 0 . 2 10 . 6 7 . 3 i- 0 . 3 6 9 0.4 7 2 1.0.4 5 1 i0 . 1 5 3 1 0 2 4 4 2 x 0 0 5 4 44 z 0 06 4 09 = t o 06 3 13 0 08 3 16 3z 0 08 3 . 1 3 i - 0 08
+
+
*
ka/,T normalized to T
=
442 O.
Fig. 1.-Manometric assembly for gas-phase kinetic studies a t moderately high temperatures and normal pressures. Legend : I A ) 1-1. water-calibrated reactor with thermocouple well; ( B ) .1()r; SaNO2-T'!;, SaS03-53$1 K S 0 3 molten salt b a t h ; ( C ) reactor furnace; ( D ) 10-mm. o.d. Hg-in-glass reactor manometer; ( E ) product recover)- t r a p ; ( F ) 5-1. water-calibrated mixer; ( G ) 6-pule .klnico magnet stirrer assembly; ( H ) 50-ml. mixer transfer t r a p ; ( I ) 50-ml. mixer isolation t r a p ; ( J ) 10-mm. o.d. Hg-in-glass mixer manometer; (I()connection to McLeod vacuum gauge; ( L ) connection t o product accumulation system manifold: ( h f , M') 14/53 manifold inlets; ( 0 )2-1. water-calibrated storage bulb; ( P ) 1-1. water-calibrated expansion bulb; ( Q ) 12-mm. o.d. Hg-in-glass manifold manometer. ~~
trarily as 1.0) in the "reaction crude" are indicated in Table I. Formation of "side" products in amounts greater than 3 mole relative to the 1 : 1 addition product was thus readily minimized by using values of M'' 5 0 . 2 . In the preceding study, the identity of the peaks was confirmed by the techniques of n.m.r. spectroscopy, as well as gas chromatography, infrared Spectra, and physical properties @Oiling point and refractive index).
shown in Table IV. The rate increases are 10- and 14.4-fold, respectively, with 1.28 and 2.72% sensitizer present, relative to the unsensitized rate, This corresponds to a rate increase exactly proportional to [(CH2)201''". Gas chromatographic analysis of t h e reaction "crudes" showed that the relative amounts of products were substantially the same in the sensitized and unsensitized experiments.
Discussion The preceding results have the features that characterize telomerization reactions with the difference that CF3CN does not participate as would be forecast by analogy with CC13CK,6i e . , as . F and 'CF2CN but as ' C F 3 and ' C N . X simple molecular reaction for the formation of CF3CH2CHpCN.z.e. CFsCN
+ CHzCH:,
--f
CFsCHzCH2CS
~~~~
(9) E. W . 12. Steacie. ".4tomic a n d Free Radical Keactions," V o l . I , 2nd E d . , Reinhold Publishing C o r p . , S e w York. N. Y., 1 9 5 4 , p. 229. (10) A. Rlaccoll. "Technique of Organic Chemistry." Vol. V I I I , P a r t 1, Interscience ~ u b l i s h e ~ 1nc s , , s e w Y ~ r k s. . Y., 1961, Chapter x. (11) S. W. Benson. J . C h e m . P h y s 4 0 , 105 (1964).
FREE-RADICAL ADDITION OF TRIFLUOROACETONITRILE
Dec. 5 , 1964
TABLE I11 DATAAND RESULTS FOR EXPERIMENTS WITH INITIAL PRESSURES OF CzHa b,
T, 'C.
B-2 B-l B-5 B-6 A-16 A-15
441.7 441.9 441.1 440.8 441.7 441.2
5.0 5.0 5.0 5 0 4.7 4.7
629.1 556.8 410.6 410.2 361.4 361.1
C-l C-2 C-3 A-5 A-14
442.2 441.9 442.7 442.8 440.8
52.7 51.6 49.7 48.9 51.9
486.9 477.9 493.5 360 0 360.9
D-8 D-5 D-4 A-I3 D-7 D-6
441.2 442 6 441.0 440.8 441.4 441.4
60.0
435.5 400.4 400.3 360.4 310.2 312.4
-X
b
18 16 13 14 15 12
+ CFsCHzCHz. +CFsCHzCH2CF: (7) ka CF:CHzCHz. + CF:CHzCHz. +CF:( CHzCHz)*CF:(8) .CF:
sec.-'
10-
i1 i1 11 k 1 k2 i4
19 11 15 f 1 13 i 1 14 i 1 15 1 2 13 i 4
50.8 a m
=
0 108 0.108 0.109 0.136 0.144
4.2 z t * O . l 3.8iO.l 4 . 2 0 10 . 0 8 3.24 k 0.06 2.99 i0.08
4 . 2 i0 . 1 3 . 8 k O l 4 . 2 0 10 . 0 8 3 . 1 3 i0.06 3.16 i O . 0 8
3.0 i0.2 3 . 2 6 10 . 0 6 2 . 8 6 j; 0 . 0 8 2.97 i0.08 2 35 i 0 . 0 7 2 . 4 4 =t0 . 0 7
3.1 1 0 . 3 3.17 1 0 . 0 6 2.99 kO.09 3.13 i 0 . 0 8 2.41 f0.07 2.51 10.07
b = 58.0 mm. 57.9 57.9 57.6 57.5 57.8
0.139 0,145 0.145 0.160 0.185 0.185
a Initial values, L e . , for M 442'.
= N o .
*
k3lZT
normalized t o T
=
A-18 E-1 E-2
T,
.@ (CZHI/
OC.
CFsCN)
437.2 437.0 438.2
a, mm.
0 5.2 11.0
1.25 12.5 18.0
is clearly ruled out since the reaction is not first order with respect to ethylene. Moreover, ethylene oxide sensitization, nitric oxide inhibition, and the formation of "side" products favors a free-radical mechanism. The following mechanism may be advanced in accord with the product composition a t low ratios of M. ( i ) initiation kla
+ C F ~ C N2CFEX* + C F ~ C N
CF~CN
(la, 2a)
+ CHzCH2
kib
CF:ChT*
+ CH2CHz
(lb, 2b)
kzb
ks
CF:Ch'*
+ 'CF:
+ 'CiY
(3)
Steps lb and 2b are negligible12-15 under conditions where telomerization is minimized. It follows also that under such reaction conditions the initiation process may be simply expressed as ki
CFICS --+
k;[CFsCN]
( i i ) propagation ,CF: 4- CHzCH2
kr
+CFzCHzCH2.
+ CFICN --+ CF:CHzCHzCK
(4)
kk
CFiCHzCHz.
(5)
M. Volpe and H. S. Johnston, J . A m . Chem. Soc., 78, 3903 (1956). H. F. Cordes and H. S Johnston, ibid., 76,4264 (1954). D. J. Wilson and H . S. Johnston, ibid., 7 6 , 5763 (1953). H. S. Johnston, ibid., 76,1567 (1953). (16) S. W. Benson, J . Chem. P h y s . , '22, 46 (1954): (12) (13) (14) (15)
= ki[CF3CN]
- 2kg[.CF3I2 -
2k7[.CF3][CF3CH2CH2.] - 2ks[CF3CH2CHz.l2 (9) with the assumption of a long-chain mechanism, ;.e. kg[CF3CH2CH2.]
Ro
[ C F C N ] (10)
d [CF~CHZCHZCN] = dt
= -
ki"'k4
[CHzCHZ][CF3CN]'/'
The relatively low yields of the termination products relative to CF~CHZCH~CN qualitatively confirm that the long-chain assumption is approached a t sufficiently low M values. The above equation (11) transforms to dx/dt = [ ( b - X)(U - x)]"' X
+ PM + a1W')Ii/'
(12)
in terms of partial pressures, where a and b are the initial pressures of the reactants, and x is the pressure a t time t of the product CF~CHZCHZCN, and the parameters a, 6, and y are given by cy = (2ksk4'/k5')(kik4')-', = (2k&/k~)(kikr)-', and y = 2k~(kik4')-'. Inspection of (12) shows that for the experimental conditions minimizing telomerization, i e . , [CF3CN] >> [CPHI],a three-halves-order rate law is predicted dx/dt = y - ' / ' [ b -
CFI and . CN
and that the rate of initiation, R i , is accurately described16by the first-order process Ri
0
[(Y
krs
CF:C?;
CH2.,i.e.
that the over-all rate is
Init. rate, b , ~ ( c H ~ ) ~ min. o , sec.-l mm. mm. X 102
326.6 4 4 . 6 358.3 4 4 . 1 350.9 4 3 . 2
0.123 0.123 0,123
The nitrile abstraction reactions such as step 5, although somewhat unusual, have been previously postulated. 1 7 s 1 * While it is recognized that both the CF3 and CN radicals may participate in the chain-initiation steps, the results of related studies with propylene as the olefin5 are strong support for the viewpoint that attack by CF3 is the important step in the chain-initiation process. It can be readily shown from the steady-state condition for the formation of .CF3 and CF3CH2-
kq[.CF3][CHzCHz] =
TABLE IV ETHYLENE OXIDESENSITIZED EXPERIMENTS
NO.
(6)
ki
b = 5.0 a m . 0.0080 0.0090 0.012 0.012 0,013 0.013
ks + .CF: ---+ CFzFe
.CF:
T = 442°",b
k v / , , (moles/l.)-'/2
mm. mm. (CHzCHr/ CHICHZ CFaCN CFsCN)
Expt:
(iii) termination CONSTANT
Ta
MO
a,
5061
X][U
- x]~"
(13)
This result is also predicted if the initiation process, and steps 4, 5, and 6 only are considered in the derivation of the rate equation; under these conditions, i e . , as M -+ 0, the termination rate is the ,CF3 recombination, i.e., step 6. A limiting three-halves-order rate expression is also predicted for the case when [CF3CN]
