NOTES hydrogen chloride, as might be predicted on the basis of the straight-forward reaction mechanism (C3H70)2POH HC1 + C3H70P(OH)z C3H7Cl under conditions of a large excess of phosphite.
+
CLEAVAGEOF
+
Vol. 77
and Ke = [(R0)2POH,HCl]/ [ ( R O ) ~ P O H ] F [ H CE‘I ] so
-d(HCl)F/dt = K K ~ [ ( R O ) ~ P O H ] F [ H‘rC ~ ] TABLE I Since [(RO)nPOH] >> [HCl] DIISOPROPYLPHOSPHITE B Y HYDROGEK CHLORIDE i t is essentially constant and First-order rate constant”
Second-order rate constanta
- d [ H C l ] ~ / d t = K’[HCl]%
Then
Time, hours
x
106
[ H C ~ I T= [(R0)2POH,HCl] -I- [ H C ~ ] F [HC~IT K,[(R~)&’OH]F[HC~]F [HC~IF [HC~]T = [HCI]F(K”)
+
At room temperature (25’) 0 1 2 19 25 140.3 165.3 192
259
... 8.09 2.34 2.18 1.64 1.53 1.48 1.33 Average of six valuesD
... 6.18 1.88 1.79 1.89 1 .85 1.92 1.98 1.89
A t 40” ... ... 20.1 10.9 3 10.9 6.1 5.5 15.7 9.7 6.3 15.3 9.7 I 15.0 9.6 21 11.6 10.5 26 11.2 10.4 28 10.9 10.4 30 10.7 10.6 31 10.5 10.6 Average of eight values’ 10.2 a Calculated from densities of (C3H70)2POHof 0.992 and 0.978, and neglecting volume and weight changes due to HC1 and reaction products. Units of specific rate constants are see.-’ and liter mole-’ see.-’. * Values of K a t times of less than three hours were not averaged since experimental errors show a comparatively great effect during the initial period of reaction. It may also be t h a t attainment of the equilibrium suggested below is a factor. 0 1
-
That the alkyl phosphites form complexes with hydrogen halides is generally accepted ; the hydrogen halides dissolve in dialkyl phosphites with considerable evolution of heat, and are difficult to remove by application of reduced pressure or by sweeping with an inert gas. We observed a heat of solution of 10.7 kcal./mole for the solution of hydrogen chloride in an excess of diisopropyl phosphite a t 25 to 30’. (mole ratio of ca.1/160) If it is assumed that dealkylation of the dialkyl phosphites takes place by attack of hydrogen halide on a phosphite-hydrogen halide complex, second-order kinetics in respect to hydrogen chloride might be expected: Assume
+ HC1 (RO)nPOH.PICl + HC1 d R O P ( 0 H ) n ~ H C+l RC1
(R0)zPOH (RO)ZPOH.HCl
Then, letting [HCl]p and [ H c l ] ~represent the concentrations of free and total hydrogen chloride, respectively, -d[HC‘l!>,’Jt
=
K [ ( K O i i P O H . H C I JHCl]k ]
and - d [ H C l ] ~ / d t = K”’[HCl] 2~
The condition of an excess of dialkyl phosphite in our experiments not only permits simplification in deriving the kinetics for the proposed mechanism, but also tends to minimize the effects of two complicating factors : (1) the consecutive reaction ROP(OH)2
HC1 ROP(OH),.HCl -+ P ( O H ) , + RCI
+ HCl
and ( 2 ) the equilibrium4 (R0)2POH
+ P(OH)3
2ROP(OH)2
Employing the Arrhenius equation and the specific rate constants determined a t 25’ and a t 40°, the energy of activation for the cleavage of diisopropyl phosphite by hydrogen chloride is calculated to be 20.9 kcal. per mole. We wish to thank Dr. G. B. Kistiakowsky for his helpful suggestions. (4) D. H. Chadwick, P. A. Sanguinetti and E. E. Hardy, presented a t t h e 122nd National Meeting of t h e American Chemical Society, Atlantic City, N . J.. Sept., 1952.
ST.Lours, MISSOURI Air Oxidation of Hydrocarbons. IV. The Effects of Varying Solvent and the Mechanism of Uninhibited Chain Termination BY CHARLES E. BOOZER, GEORGE S. HAMMOND, CHESTER E HAMILTON A N D COXRAD PETERSON RECEIVED NOVEMBER 6, 1954
In the previous papers in this series we have shown that in benzene and chlorobenzene the inhibition of cumene and tetralin oxidation can be accommodated by a consistent picture as to the mechanism of the inhibition reactions. In the course of this study we have made other observations, mainly concerned with oxidation in other solvents, which are less complete but are considered to be of sufficient interest to be reported a t the present time. The results include some interesting deviant behavior for which no good explanation can be offered at the present time. Results and Discussion Rates of Uninhibited Oxidation.-The rate of initiated oxidation a t partial pressures of oxygen which are high enough to make the reaction zero order with respect to oxygen and i t 1 the abseiict‘ oi
NOTES
June 20, 1955 added inhibitors is given by the following kinetic law.'
--
= 2uki [AIBN]
'/2[RH] + (uki)'/zks[AIBN] ktl/z
3381
2( CHS)PCOZ.+(CH,)*COgH
AN
I
CN
+ CHz=C-CN + I
CHI
0 2
(3)
Since both of these reactions involve the evolution of oxygen the rate of oxygen uptake during the where decomposition of the initiator in chlorobenzene 2aki[AIBN] = the rate of initiation by azo-bis-isobutyro- was measured and compared with the known rates nitrile of efficient production of radicals. The results are k3 = the rate constant for the reaction of ROZ. with the not highly precise because of the small volume substrate, R H k t = the rate constant for chain termination by bimolecu- changes involved. However, it is found that the lar reaction of two ROz. oxygen uptake exceeds by a small amount the There is no really good information as to the nature rate of nitrogen evolution. Since the efficiency of the chain-terminating step2 SO there is no basis of AIBN decomposition in chlorobenzene is 0.65 for an a priori judgment as to the behavior of it would be expected that an expansion of the total oxidation rates as solvents are varied. The de- gas volume should occur if radical destruction were velopment of methods for the measurement of by way of either reaction 2 or 3. A further indiinitiation rates3 enables us to correct the observed cation of the same fact is found in the observation rates of oxidation of tetralin for variations in the that in the oxidation of the initiator in the presence rate of initiation. When this is done, as is shown of 2,6-di-t-butyl-p-cresoI slightly less oxygen is by the data in Table I, it is found that there is consumed than in the absence of the inhibitor. relatively little variation in the ratio k3/kt'/2. It may be inferred from the nature of the isolated Since k3 is the rate constant for the propagating products of this reaction4 that each reactive fraghydrogen abstraction reaction i t would not be ment produced from the initiator is oxygenated. expected to show any great sensitivity to medium The experimental data are best presented graphieffects unless alkylperoxy radicals are in some way cally as in Fig. 1. complexed with the solvents. Since the ratios of the constants for termination and propagation are nearly constant it is implied that the solvents are not intimately concerned in either the propagation or termination reactions. This is by no means an 2.0 inescapable conclusion and other lines of reasoning, some of which are given below, lead us to believe that judgment on this question should be reserved a t the present time. Similar, but less complete observations, have been made concerning the oxidation of other substrates but are not reported in detail. dt
(1)
r t
TABLE I EFTECTOF SOLVEKT ON THE RATE OF OXIDATION OF TETRALIN AT 62.5' fia/ktl/Z x los, I.'/z mole-'/2 sec.-'/l
Solvent
Chlorobenzene o-Dichlorobenzene Nitrobenzene Nitromethane Carbon tetrachloride Diphenyl ether 66% Trimethylpentane-l7% chlorobenzene
9.5 10.5 10.3 14.2 9.4 9.6 9.4
Oxidation of the Initiator.-The oxidation of the initiator in the absence of a substrate should provide some information concerning the chain-terminating step since there is, presumably, no propagation step. The oxygenation of initiator fragments must produce an ROz. species and these radicals should be destroyed either by reaction with each other or with solvent. Two conceivable reactions for their m u t u d destruction are 2(CH3)zCOz. +(CHs),COOC(CH3),
I
CX
I
CN
/
+
0 2
(2)
cri
(1) P a r t 111, G. S. Hammond, C. E . Boozer. C. R . Hamilton and (1955). (2) I.. Bateman, Quart. Reos., 8, 147 (1954). (3) C,. S . Hammnnd, J. N. Sen and C. E. Boozer, T R IJ ~ O U R X A I . , 7T, 3244 (1955).
1. 9. Sen, THISJOURNAL, 77,3238
40 80 Time, minutes. Fig. 1.-Oxidation of 6.1 X mole of AIBN a t 62.5' in chlorobenzene: 0 , no added inhibitor; 0, in presence of 3 X mole of di-t-butyl-p-cresol; A , nitrogen evolution in absence of oxygen. 0
A large batch of the initiator was oxidized in chlorobenzene without definitive results. A maximum of 4.270 conversion of the radicals produced (4) C.
E. Boozer, G. 3.Hammond, C . E. Hamilton and J . N. Sen.
zbid.. 77, 3233 (1955).
NOTES
3382
etfciently from the initiator3 were converted to titratable peroxidic materials. Since iodometric assays were about twice as high in the presence of hydrochloric acid as in the absence of mineral acid, this material was probably part hydroperoxide and part peroxide. Tetramethylsuccinonitrile was isolated along with an oil which showed both carbonyl (1740 cm.-l), nitrile (2240 cm.-') and olefinic (1680 cm.-I) bands in the infrared. A white, amorphous solid which resembled polymethacrylonitrile and methacrylonitrile-oxygen copolymer also was isolated. A considerable amount of hydrogen cyanide was produced during the reaction. It seems likely that some breakdown of primary products occurs and that the inference of the nature of the reactions which destroy the peroxy radicals will be aided relatively little by a more exhaustive study of the reaction products. It has been difficult for us to visualize any reaction of two (CH3)2C(CN)02.radicals which does not involve any third body Fnd which does not lead to the release of oxygen. I t is, however, possible that reaction 3 occurs and that more oxygen subsequently is fixed by the oxidation of methacrylonitrile. It can be stated rather unequivocally that the oxygen displacement, reaction 2, does not occur to any significant extent. Another possibility, which should be kept in mind, is that the solvents may become involved in the chain termination step. Solvent Effects on Inhibitor Stoichiometry.-In Table I1 the results of a large number of variations in reaction conditions of inhibited oxidations are summarized. The table is organized in such a way as to group the results with a given inhibitor together. ?"E
TABLE 11 EPFECTS O F VARIATION OF SOLVENT A N D SUBSTRATE STOICHIOMETRIC FACTORS OF THE INHIBITORS Inhibitor
Substrate
2,fi-Di-l-butyl-p.cresol
Tetralin Methacrylonitrile I-Me-cyclohexene Methacrylonitrile Cumene Methacrylonitrile Cumene Cumene Tetralin hlethacrylrmitril~ Cumene l'etralin I-Me-cyclohexene Methacrylonitrile Tetralin Cumene Tetralin 3Iethacrylonitrile Tetralin Cumene Tetralin Cumene 3Iethacrylonitrile Methacrylonitrile Cumene Tetralin
4-L-Butylcatechol
Diphenylamine P-Hydroxydiphenylamine N,N,h-',"-Tetramethyl-$-phenylenediamine
Solvent
ON
Stoichiometric factor
CCl, CClr TMP-CBHsCl" CHaN02 CeHaN02 CsHaNOz CClr CsHs CClr CClr TMP-CnHaCI" T3lP-CsHsCl" TMP-CRH~CP CHsNOz CHiNOz CsHsSO: CaHsNO? CsHaN0.l 0-C6H,Cl% CsHaCl CeHaCl CaHaCl CClr CHaKO:! CaHaCl CHaN02
1.98 1 0 2.0 1.0 1.9 2.1 2.06 2.00 2.06 1 .oo 2.0 2 0 1 .Of' 2.0'' 1 , l\,i I 4 1 8 2 li 9 0 ' R
3 r,
2 0 1 33 1 96
i a 2 4
Solvent consisting of S0'3'0 2,2,4-trimetliplpeiiteiie-l and Period o f i r i i l d rrt;irdati
