TRIETHYL PHOSPHITE
Jan., 1957
TO
account differences in irradiation procedure, it is possible to compute the relative F-center concentration from irradiation time using the data of Mador, Wallis, Williams and Herman.I5 Then, if the rate of light emission is first order, the ratio of the rate to F-center concentration should be constant. This is shown to be the case for sodium chloride in Table I. The above-mentioned investigators a,lso found a linear increase in surface F-center concentration with irradiation time for lithium fluoride; Fig. 3 shows a similar linear relationship of the rate with irradiation time in these experiments. TABLE I Ratio of the rate of light emission, A., to the F-center concentration COin the first 0.2 mm. of the surface layer for sodium chloride strained a t G-l.%/min. Irradiationtiine, Inin.
5 10 20 GO
i./Co X 101s (arbitrary units) 6
= 6%
1.2 1.3
1.4 0.96
€
= 8%
€
= 10%
0.68 0.79 .84 .75
0.91 0.96 1.0 0.90
(15) I. L. Mador, R. F. Wallis, M. C. Williams and R. C. Herman, Phus. Rev., 96, 617 (1954).
DIETHYLETHYL PHOSPHONlTE
89
Mechanism 3, in which the rate-determining step is the excitation of an electron from an F-center to the conduction band by a thermal spike, is the only one of those proposed which can account for all the experimental observations. This process depends directly on the strain rate rather than the strain, and predicts that the luminescence would decay rapidly on interruption of compression. Also, since the probability of exciting an electron from the Fcenter to the conduction band depends on the thermal spike rather than on the average temperature of the crystal, the temperature coefficient of the excitation process should be quite small. This mechanism leads directly to a spectrum of emitted light containing two peaks. The excited electron in the conduction band can combine with either a vacancy or a halogen atom to give an F-center or a negative ion, thereby giving off light a t two different energies. . Acknowledgment.-The authors wish to express gratitude to Dr. R. A. Lad for many valuable suggestions made during the course of this investigation.
RATES OF ISOMERIZATION OF TRIETHYL PHOSPHITE TO DIETHYL ETHYL PHOSPHONATE IN THE PRESENCE OF ETHYL IODIDE BY A. F. ISBELL, G. M. WATSONAND R. E. ZERWEKH,JR. Department of Chemistry, The Agricultural and Mechanical College of Texas, College Station, Texas Received July 87, 1066
Rates of the isomerization reactions have been determined by continuous density measurements a t 90, 100 and 110’ over a concentration range from 10 to 56 mole yo ethyl iodide. The initial G0-70yO of the reaction was found to be zero order with respect to triethyl phosphite and first order with respect to ethyl iodide. Initial specific reaction rate constants have been expressed as functions of concentration. The over-all composition-time curves resemble in shape those for an autocatalytic process. The experimental results over the entire temperature composition range studied have been found to be satisfactorily expressed in terms of concentrations by means of a single equation proposed much earlier by Zawidzki and Staronka.‘
The reaction investigated is commonly indicated by the over-all equation
+
P(0CzHs)a f CzHd --f ~zHsP(O)(OCzHs)z CIHSI . The general reaction was first described by Mi-
chaelis and Kaehne2 and was studied extensively by Arbuzov3 who showed the general nature of the reaction and proposed a possible mechanism. The general mechanism proposed by Arbuzov postulates two steps for the reaction: the formation of a phosphonium halide intermediate followed by its decomposition into the products
+
P(0R)s R’X +R’P(OR)s+XR’P(OR),+X- +R’P(O)(OR)z RX
+
Intermediates, which are condensation products of triphenyl phosphite with alkyl halides, have been isolated by Arbuzovs and by other^.^ They have (1) J. Zawidzki and W. Staronka, Anzsiger Alcad. W t k . Kvalcau A . , 319 (1915); Abhand. Abad. Wkss. Krakau, 66, 101 (1915): via C. A., 11, 2294 (1917). (2) A. Michaelis and R. Kaehne, Ber., 81, 1048 (1898). (3) A. E. Arbuzov, J . Russ. P h y s . Chem. Soc., 42, 395 (1910); 88, 161,293,687 (1906); Ber., 38, 1171 (1905). (4) 8.R. Landauer and H. N. Rydon, J. Chem. 800.. 2224 (1953).
the expected empirical formulas and have been assigned structural formulas corresponding to phosphonium halides. Landauer and Rydon,* however, have indicated that these phosphonium halides are rather unusual since they precipitate silver iodide quite slowly from silver nitrate solution^.^ In reactions involving trialkyl esters of phosphorous acid, no intermediate has been isolated, presumably due to their very transient existence and low stability under experimental conditions. I n the absence of a better alternative mechanism, more recent investigatorsa apparently agree that the reaction follows the general course proposed by Arbuzov. Kinetic measurements on this reaction had been performed by Zawidski and Staronka’ and by Staronka.6 The original publication of Zawidski ( 5 ) Since this reaction w&b carried out in a solution of ethanol, with which triphenyl phosphite methiodide was ohown t o reaot rapidly to form ethyl iodide, the slow precipitation of AgI may neither prove nor disprove the ionic character of the methiodide. (6) W. Staronka, Roczniki Chin.,7 , 42 (1927); via C. A . , a%, 1264 (1928).
90
A. F. ISBELL, G. M. WATSONAND
and Staronkal has not been available. They made measurements at 85 and 9,50, and agreed with Arbuzov’s mechanism. They postulated that the speed of the reaction was autocatalytically accelerated by the diethyl ethylphosphonate product. A rate equa,tion was presented, which was claimed to incorporate in a, quantitative manner all the data observed. This equation, after slight modification, has also been tested successfully in the present work. fhronka6 later postulated that the shape Of the composition-time curves for the isomerization reaction were caused by changes of the reaction mediuni rather than by autocatalysis. Pudovik’ has discussed the kinetics of this reaction. He also supports Arbuzov’s idea of a phosphonium halide intermediate and believes the intermediate to be of an ionic character. Kosolapoff8 observed rates of reactions between triethyl phosphite and n-butyl bromide by measuring the quantities of ethyl bromide which distilled from the reaction mixture. The shapes of the curves obtained suggested that the reactions were of a complex nature and no numerical rate constants were presented. The present investigation may be regarded as an extension of the work of Zawidski and Staronka.1 The experimental work, hotvever, was completed before any reference to the older work had been found. Experimental Materials*-The reagents used were triethyl Phosphite and ethyl iodide. The triethyl phosphite used was a Victor Chemical product. Purification of this reagent was accomplished by refluxing the triethyl phosphite with metallic sodium for 3 to 4 hours i n vacuo to remove diethyl phosphonat,e. The material was then distilled twice a t reduced pressure (48-50 mm.), and the distillate collected between 75 and 76”. The triethyl phosphite was stored under a dry nit,rogen gas atmosphere in black bottles. The ethyl iodide was prepared, purified and stored according to approved and well established procedures.9 Apparatus and Procedure.-The apparatus consisted of a specially designed reaction flask, a stirrer, a calibrlted glass bob suspended from an analytical balance by means of 34 gage silver wire and a constant temperature bath. The reaction flask was designed as a combination and H tube constructed of glass tubing and standard tapered ground glass joints. A stirrer, made from 6 mm. glass rod was introduced vertically into the reaction mixture through a gastight stirrer bearing placed on the upper end of one of the legs of the reactjon flask. The calibrated glass bob, immersed in the reaction mixture, was suspended from an analytical balance permanently mounted directly above the other leg of the reaction flask. The suspension wire passed through a reflux condenser directly attached to the reaction flask. The flask was enclosed in an oil-filled thermostat held t t constant temperature with variations less than
u
+O.l
.
To perform an experiment, a weighed amount of triethyl phosphite was introduced into the reaction flask which had been flushed previously with dry nitrogen. Adequate time was allowed for the temperature of the ester to attain thermal equilibrium. At this time a weighed quantity of ethyl iodide was introduced quickly. The starting time of the reaction was taken as the time a t which one-half of the ethyl iodide had drained into the reaction flask. The complete drainage time was usually less than 45 seconds. The initial (7) A. N. Pudovik, Doklady Akad. Naulc. S J S R , 84, 519 (1952); uin C. A . , 47, 3227 (1953). ( 8 ) G. M. Kosolrtpoff, J . Am. Chem. Soc., 66, 109 (1944).
(9) A. H. Blatt, “Organic Syntheses,” coil. voi. 11, j o h n Wileyand Co., New York, N. Y . , 1943,p. 390.
R.E. ZERWEKH,JR.
Vol. 61
temperature of the mixture showed a tendency to rise due to exothermic effects. This had to be compensated by adding the iodide a t a lower temperature, designing the reaction flasks with a large surface, and using efficient stirring. The specific volumes of the reacting mixture were determined periodically by observing the variation in weight.of the calibrated bob immersed in the solution. The reaction mixture was stirred continuously except during the brief periods required to obtain density values. The number of weighings performed was dependent upon the rate of reaction. Usually weighings were taken a t approximately equal intervals during the first two half-lives of the reaction, and less frequently during the third and fourth half-life. After the fourth half-life the weighings ,were continued, however, until no further changes could be detected over a period of an hour.
Discussion of Results As mentioned Previously, the rates of reaction were followed by measuring the changes in specific volume of the reacting mixture. During the initial phases of the investigation, it was observed that the specific volumes a t 30” of mixtures of triethyl Phosphite and diethyl ethyl phosphonate were linear with the mole fraction of either constituent. The linearity Persisted in the Presence of ethyl iodide throughout the concentration range investigated. I n the case of ternary mixtures, the specific volumes were plotted versus the ratio of the number of moles of diethyl ethYlPhosPhonate to total number of moles of esters. No deviation from linearity greater than 0.05% was detected, even as the concentration of ethyl iodide was increased to 50 mole %. The fraction of triethyl phosphite converted to diethyl ethylphosphonate was calculated from the relation a=-
vo - vt vo - v,
Where vt and vmrefer to the specific volumes of the reacting mixture a t times zero, t and “infinite.” The initial specificvolume could not be measured directly due to incomplet>emixing and rapidity of reaction. The value of Vo, however, could be determined satiSffi,CtorilYby extrapolation to zero time of the function log (vt - V m ) The specific volumes were measured to 0.0001 cc, and mere internally consistent to better than 0.001 cc. This constitutes an estimated average uncertainty in the determination of a notl greater than & 10% of the indicated value. ~ ~ ~measurenlents ~ ~ mere iperformed ~ at~ loo and l1Oo. The are summarized in Table T where rounded values of the fractional lives of the reaction, concentrations and temperaturesare t,abulated. ~ ~experimental ~ i ~ are also shown graphically in Fig. 1. At the same temperature, the degree of duplication a t similar concentrations and fractional life appears to be within It loyo. The appearance Of the composition-time curves is rather striking and several observations can be made by simple inspection. During the first Goyo of the reaction, the reactionrate appears to be of zero order with respect to the concentration of ester and first, order with respect to the Concentration of ethyl iodide. If these observations are approximat el^ correct, there should be a linear correlation between the periods of half-life and the parame-
vo,
e
~
~
TRIETHYL PHOSPHITE TO DIETHYL ETHYL PHOSPHONATE
Jan., 1957
91
TABLE I FRACTIONAL LIFE PERIODS OF ISOMERIZATION REACTION (MINUTES) Temp.,
Mole % ' CzHd
90
56.0 50.0 50.0 37.5 25.2 24.9 37.6 37.5 30.0 25.0 25.0 12.5 24.8 20.0 15.0 10.0
OC.
looo
lloo
0.10
0.20
Fraction of ester isomerized 0.30 0.40 0.50
14
33 41 44 64 120 105 30 30 43 54 52 128 27 40 51 90
52 62 67 96 167 158 45 45 64 78 78 191 41 57 75 126
19 21 30 62 51 15 15 22 28 26 65 14 22 27 44
ter ( n ~ ~ l nwhere ,) n ~ ,denotes , the initial number of moles of triethyl phosphite and nc the moles of ethyl iodide. This correlation is shown in Fig. 2. Specific reaction rate constants for the initial 60% of the reaction as functions of the initial concentrations of the reactants are readily expressed by a general equation of the form
72 85 89 129 212 212 60 60 85 103 103 255 54 73 99 160 I
I
-
92 107 112 161 257 255 75 75 106 128 129 317 66 89 124 194 I
I
0.60
0.70
112 129 135 192 304 318 90 89 127 152 155
136 158 158
108 105 148 177 182
82 106 148 227
96 124 173 264
I
LEGEND' 0 24 90 O C , 5 6 mole % C2H51
-
I
I
-
A 9 - 9 0 a C , 5 0 mole % G2H51
where b and m are constants related to the intercepts and slopes of the linear correlations given in Fig. 2. Table I1 lists the numerical values of the particular constants. TABLE I1 INITIAL SPECIFICREACTION RATE CPNSTANT (NUMERICAL VALUESOF CONSTANTS FOR EQ. I ) Temp.,
min. m,
b.9
min.
OC.
90 100 110
58 0 14
157 87.6 42.5
No particular theoretical significance has been attached to the above equation other than that it represents a convenient and concise means of expressing the experimental results for the first 60% of the reaction. The average deviation between the calculated and experimental values of CY is less than f10% of the experimental value. Regardless of the actual mechanism of the reaction, it was considered of interest to attempt to derive a rate equation that would give the over-all shape of the experimental time-composition curves. Vsing a simple model where a reactant A changes to a product B in the presence of a, catalyst C and assuming the product to be a.lso a catalyst, one can write the scheme kl A+C-+AC
320
I
I
200
2 40
2 00
-2 160 I20
80
40
k2
AC+B+C k8
A+B--,2B
Assuming ICZ
>> kl
and a steady-state concentra-
Fig. 2.-Period
of half-life as a function of initial concentration ratio.
92
A. F. ISBELL, G. M. WATSONAND R. E. ZERWEKH,JR.
Vol. 61
tion of the intermediate AC, rate equation (2)
is derived easily
Equation 2 has the necessary properties to give, upon integration, a vs. t curves of shape similar to the experimental, provided the proper values of kl and k3 are used. The authors, however, did not test eq. 2 as a t this stage they found the abstract of the work of Zawidski and Staronka' performed some 40 years earlier at 85 and 95" on tthe same system. These investigators proposed a remarkably similar equation
+
where n' = n ~ , / ( 0 . 2 ~ A O 0.185 no)is a"numerica1 factor,'' to correlate their experimental findings. ilccording to the abstract, "by means of this equation not only are the results of the velocity measurements exactly represented but also all the related phenomena are quantitatively expressed by it." Equation 3 as written appeared to the writers to be dimensionally inconsistent, perhaps due to a typographical error in the abstract. The equation was modified and rewritten as
Fig. 4.-Progress of isomerization reaction.
agreement is quite consistent with the degree of experimental precision. Specific reaction rate constants were obtained from the slopes of curves similar to those in Fig. 3. Some typical results are summarized in Table 111. TABLE111 SPECIFIC REACTION RATECONSTANTS FROM EQ.5 Temp., OC.
90
100
and integrated to obtain 110
Equation 5 was tested graphically by plotting In (1 n'a)/(l - a) vs. t from the experimental results. Typical curves are shown on Fig. 3 for four experiments at 100". It may be observed that the
+
Fig. 3. (IO) Even though eq. 2 and 4 are similar, they are not identical. Although attempts were made to derive eq. 4 from theoretical considerations, we were unable to do so.
Mole % ethyl iodide
50.0 37.5 25.0 37.5 25.0 12.5 24.8 20.0 15.0 10.0
k (10+8)
(n./nno)
n'
(min. -1)
1.000
2.60 3.21 3.83 3.21 3.83 4.32 3.79 4.06 4.30 4.53
4.04 4.00 4.04 8.75 8.32 8.29 16.4 16.1 16.1 15.9
.600
.333 .600 * 333 .I43 ,330 .250 .176 .111
The tabulated results show surprisingly good correlation a t the three temperatures studied and throughout the entire concentration range investigated. Furthermore, eq. 5 holds not only for initial rates but throughout the entire life of the reaction as shown in Fig. 4. The temperature dependence of the rates of reaction corresponds to a process with an energy of activation of approximately 19600 cal. Zawidski and Staronka obtained a '(temperature coefficient of the velocity constant" to be 2.168 for their experiments at 95 and 85". The ratios of the average k's obtained in this investigation are 2.10 at 10090" and 1.91 a t 110-100", which are in good agreement with the earlier value. Although one of the objectives of kinetic studies of this kind is to arrive a t the mechanism of the reaction, we do not believe that sufficient experimental data have been collected to justify such an attempt. However, additional investigations of this type should reveal eventually the true course of the reaction.
c
