CHARLES L. KIBBYAND G.B. KISTUKOWSKY
126
Photochemical Decomposition of Diazoethane
by Charles L. Kibby and G. B. Kistiakowsky Gibs Chemical Laboratory, HarWTd U n h w d y , Cambridge, Maaaachuaetts (Reeeived Seflember 9, 1966)
Incomplete diazoethane photolysis in essentially monochromatic radiation of the 4360-A. wave length gives only Nz, c2H4, C2Hz Hz, and butene-2 as products. The ratio CZH4/ CzHzis linearly dependent on pressure in agreement with previous work of Frey, but the yield of butenes (about 1%) is lower than his and the truns/Cis ratio is constant at 1.2. Radiation of the 25QO-A. region produces on incomplete decomposition mainly ethylene 0.044P0(torr) and butene-2 and acetylene in amounts given by (Ca&)/(C2HZ) = 0.5 in about 10% yields which are highly irreproducible and appear not to depend on the pressure of diazoethane in the 0.2 to 50 torr range. The ratio of trans- to cis-butene-2 is reproducibly 1.2. About 3% of the products consist of saturated hydrocarbons, propylene and butadiene. Addition of oxygen completely eliminates these minor products and greatly accelerates the decomposition of diazoethane, but has no direct effect on the acetylene, ethylene, and butene-2 yield. With the 4360-A. radiation oxygen has no effect on the course or extent of the reaction. It is pointed out that the mechanism proposed by Frey CHaCHNz h~ +CHaCH N2
+
+
+
+ CHaCH + CHsCHNa +C& + Na CHaCH +C2H4* C2H4*
+ M +C& + M
CzH4* +CzHz
+ Hz
does not account for all of the present data and that the real mechanism must be more complex. Reaction of oxygen is attributed to the formation in small yields of a tripletstate ethylidene and/or radicals by the 2500-A. radiation, with a chain reaction resulting that involves many molecules of diazoethane.
Introduction Studies of the properties of ethylidene have now been reported utilizing the photochemical decompositions of
diazoethane112 (DAE) , methylketene, and methyldiazirine.4 However, there has been relatively little information obtained with these systems compared to that now available for methylene. Only with diazoethane photolysis at 4358 studied by Frey12 has the production of ethylidene been made very probable, and even with this system very little reactivity was observed with ol&ns, presumably due to the rapid isomerization to ethylene. The main objective of this work was to extend Frey's observations with DAE to other wave lengths,
w.,
The Journal o j Phyisieol C h i s t l . y
especially to the region around 2500 A. where the radiation probably excites the molecules to a different electronic state than the absorption band in the visible part of the spectrum.' The other objective was so to modify the analytical procedure that experiments could be made with only partial photolysis of diazoethane in order to study more quantitatively the
(1) R. IC, Brinton and D. H. Volman, J . Chem. Phys., 19, 1394 (1961). (2) H.M.Frey, J . Chem. SOC.,2293 (1962). (3) D.P.Chong and G. B. Kistiakowsky, J . Phys. Chem., 68, 1793 (1964). (4) H.M.f i e y and I. D. R. Stevens, J . C h m . SOC.,1700 (1966).
PHOTOCHEMICAL DECOMPOSITION OF DIAZOETHANE
127
formation of butenes. In this objective we were not wholly successful, as will be discussed later.
tor equipped with 2000-7000-8. grating and three seB of matched slits. Slits providing half band widths of 200 8. (at 4358 and 3660 B.) and 100 8. (at 2500 8.)were used. The f 3.5 light beam emerging from the monochromator was collimated before passing through the cell. A front surface coated plane mirror was placed after the cell to increase the light intensity in the cell. Pressure measurements were made with a Wallace-Tiernan diaphragm-type differential pressure gauge, 0-50-torr range, or a mercury manometer with a cathetometer. Analyses of the product mixtures were performed using a Perkin-Elmer Model 810 dual column gas chromatograph, equipped with dual flame and dual thermal conductivity detectors, and recording potentiometer with a disk integrator. The grease-free inlet system utilized Teflon needle-valve stopcocks and an Aerograph XA-204 six-way gas sampling valve. Unfortunately, neither this nor especially the valve supplied originally with the instrument was completely vacuum tight. Four columns were used for the analyses, a 9-m. column of 33% dimethylsulfolane on 60-80 mesh Chromosorb P, operated at 35O, a 2.5-m. column of 1.4% di-n-decyl phthalate on 60-80 mesh Alcoa Type F-1 activated alumina, operated at 75O, a 3.5-m. column of 20% UCON 50 LB-550X polyethylene glycol on 60-80 mesh Chromosorb P, operated at 100', and a l-m. column of 60-80 mesh Type 13X molecular sieve, operated at -20'. Flow rates of 10-35 cc./min. of helium were used. Reaction products were identified by relative retention times obtained by comparison with known samples run under identical conditions or comparison with values quoted in the literature. The relative sensitivities were determined in calibration runs using pure samples and accurately prepared mixtures. The flame detector response was linear over the range of concentrations obtained in the photolysis runs.
Experimental Section Diazoethane was prepared from N-nitroso-N-ethylurethan in a procedure modified slightly from that described by Wilds and Meader.5 The urethan, prepared as described5 and purified by vacuum distillation, was added from a syringe to a vigorously stirred solution of potassium hydroxide in ethylene glycol. A stream of dry nitrogen was used to sweep the diazoethane evolved through a series of five cold traps; the temperatures of the traps were 0, -25, -196, -196, and -196O, respectively. The first two traps removed solvent vapor, the third condensed most of the diazoethane, and the last two were to exclude water vapor from the third trap. In most preparations 1 ml. of urethan, 1 g. of potassium hydroxide, and 10 ml. of ethylene glycol were used. As previously ~ o t e d ,yields ~ were highest when the urethan addition was rapid (5 min. sufficed). A room temperature water bath was used as the reaction was very exothermic. The product was purified by several trap-to-trap distillations in vacuo. The only impurity, ca. 0.5-1.0 yo,was identilied as isoprene by its retention t,imes in analyses using a gas chromatograph. The product was stored in a light-shielded vessel at - 196'. Diazoethane-2,2,243 was prepared in an identical The manner from N-nitroso-N-ethylurethan-2,2,2-ds. urethan was prepared ' starting with ethylamine2,2,24 (minimum stated isotopic purity 98% 73) obtained from Volk Isotopes Co. Diazoethane removal from the reaction product mixtures was effected by condensing it in a cold finger with liquid nitrogen, just above a layer of frozen propionic acid. The condensate was allowed to contact the acid by very slowly lowering the liquid nitrogen level. With diazoethane alone or in mixtures prepared with the approximate composition of the photolysis product mixtures, the diazoethane was quantitatively removed, yielding only nitrogen and less than 0.1% butenes as volatile products. Photolyses were performed in a cylindrical 34-cc. quartz reaction cell at 25-30'. The 2-cm. i d . cell had a path length of 10 cm. and a dead volume of 2.6 cc. It was closed off with a Teflon needle valve which led to a grease-free working section of the vacuum system in which the diazoethane was handled. The cell could be evacuated to to 10" torr. The source image of an Osram HBO 500 W shorbarc, highpressure mercury lamp was focused on the entrance slit of a Bausch and Lomb high-intensity monochroma-
Results In the radiation of the 4360-A. region, the reaction was carried in most runs to about 5070 decomposition and gave results only partially consistent with those reported by Frey. The main products, besides nitrogen, are ethylene, acetylene, and hydrogen. The latter was identified but generally not quantitatively measured and therefore the equivalence of its yield with that of acetylene is an assumption. As the lower line of Figure 1 shows, the relative yields are well represented by the equation (6) A, L. Wilds and A. L. Meader, Jr., J. Org. Chem., 13,763 (1948).
Volume 70,Number 1 Januaru 1066
CHARLES L. KIBBYAND G. B. KISTIAKOWSKY
128
(4
CzH4/CzHz = 0.98Po although a slightly better fit is obtained with CzH4/CzHz
=
0.98Po
+3
(b)
where Po is the initial pressure of diazoethane in torr. Frey gives 0.86 for the slope in runs in which decomposition was essentially complete. The difference is probably within combined experimental errors in determining the absolute magnitude of the C2H4/CzHz ratio. I
I
I
,
i
3'
140 120
for the ratio of C2H4to C2H2. If one assumes this ratio to be linearly dependent on initial pressure of diazoethane, the slope would be 0.16 torr-'. The yield of butenes was somewhat higher than at 4358 B., the c4/cZ ratio being about 0.04, but the transto cis-butene-2 ratio was unchanged at 1.2. Propylene formation was definitely observed, its ratio to C2H4 CZHZbeing between 0.01 and 0.015. Traces (0.020.06% each) of CH4, C2H6, n-C4Hlo,and butadiene were found. Extensive series of runs with the 2500-A. radiation showed that the amount decomposed in 3-hr. exposures first rose linearly with pressure (about 50% decomposition) and then levelled off at about 15-20 torr. Since the extinction coefficients of diazoethane are quite high in this spectral region,' these findings indicate a quantum yield independent of pressure. An experiment with interposed heated mercury vapor filter gave the same results as described below, indicating that they are not due to a photosensitized reaction. The upper line of Figure 1 shows that the C2H4/CzH2ratio is again linearly dependent on pressure and extrapolates to a finite intercept
+
CzHd/CzHz = 0.5 '0
20 30 1'4 TIAL PRESSUSE TOPR
40
;5
Figure 1. The ethy1ene:acetylene yield ratios in radiation of two wave lengths, plotted against initial pressure of diazoeJhane: V and scale on the right, data obtained with 4358-A. radiation; 0 and scale on the left, data obtained with 2500-A. radiation; data obtained after complete decomposition with 2500-A. radiation.
4,
Small yields of cis- and trans-butene-2 were observed, both with CH3CHNz and CD3CHN2,varying from 0.005 to 0.02 of the yield of Cz hydrocarbons without any obvious trend with initial pressure or extent of decomposition in the 10-50 torr range. The average relacive yield in ten runs was 0.010 (*0.007). Frey's data would predict a yield given by t,he equation C4Hs/(C2H4 C2HJ = 0.013 0.00045P0, that is, about 0.026 for the midpoint of our pressure range. The trans-butene-2/cis-butene-2 ratio observed was consistently 1.2 while Frey's data require a ratio given by the equation translcis = 1.0 0.025P0, that is, 1.75 for the midpoint of our pressure range. A trace (about 0.1%) of propylene was noted and no other products. Only a few runs were made with the radiation of the 3660-A. region because even a 24-hr. exposure gave only 24% decomposition due to the very low extinction coefficient of diazoethane. The analyses of runs which were made at 35 torr initial pressure gave 5.7
+
+ O.O44Po
(4
This equation applies to runs in which the extent of decomposition varied from 30 to 50%. Somewhat lower slope, 0.034 torr-', is indicated by a limited number of runs in which decomposition was nearly complete. Figure 2 gives the values of these ratios in runs with 2500-A%..radiation in which several other gases were added to 20 torr diazoethane. It will be noted that all of the gases tried are less effective than
i
I I
d
12-
I
+
+
The Journal of Physical Chemistry
I
200
I
I
400 600 ADDED GAS PRESSURE, TORR
I
800
Figure 2. Effect of other gases added to 20 torr of diaeoethane on ethylene: acetylene ratios with 2500-A. radiation: 0 , He; 0, Nz; 0, cis-butene-2; A, propylene. Line marked 1 shows the effect of increasing DAE pressure on this ratio.
IC
PHOTOCHEMICAL DECOMPOSITION OF DIAZOETRANE
129
Table I : The Yield of Butenes under Several Conditions CtHd(CrHr -IC2H4)
C4Hs/DAEi
CdHs/DAEf
0.10 0.09 0.10 0.22 0.14 0.33 0.62
0.010 0.009 0.005 0.004 0.004 0.019 0.012
0.005 0.0045 0.005 0.004 0.006 0.008 0.012
0.010 0.009 >0.10 >0.11 >O. 14 0.015 >O. 62
0.40 0.65 0.78 0.021 0.79 2.19 0.72 1.27 0.60 1.00
0.040 0.18 0.13 0.028 0.013 0.149 0.018 0.098 0.110 0.143
0.040
>O. 40
0.033 0.039 0.028 0.079 0.055 0.018 0.025 0.030 0.050
0.65 0.056 >>o. 02 0.40 0,081 >0.72 0.034 0.60 0.125
A,
A.
DAEi
DAEf
0 2i
4360 4360 4360 4360 4360 4360 4360
205 20“ 20“ 56“ 25b 406 50b
10 10
