THE PHOTOLYSIS OF AZOISOPKOPANE1

ABSTRACT

Azoisopropane has been photolyzed by 3660 A radiation over the temperature range 30-120" C. The effect of pressure indicates an excited molecule rr~echar~ism. Excited molecules which decompose give nitrogen and isopropyl radicals; the latter either corl~bine, disproportionate, or react with azoisopropar~e. The activation energy difference between the two reactions

C3111. + C3H?.N:N.C3H7 -+ C ~ H B + CBHB.N:N.CJHI 2C3Hi.-3 CsH14

has been found to be 6.5 f 0.5 lical. per mole. The difference in activation energy between the disproportionatio~~ a ~ ~ d

combination reactions is rendered ambiguous by the possibility of CJH~.N:N existing a t the lower temperatures; but this is certainly s~nall . 'l'he ratio of the rates of the two reactions is 0.5 a t room temperature.

INTRODUCTION

The photolysis of azoisopropane has not been previously reported in the litera- ture although tlie thermal decomposition has been studied by Ranisperger (10). ?'his author did not ailalyze the gaseous reaction products and did not propose any free radical mechanism for the decomposition. 1 t seeins reasonable to suppose that the initial reaction in deconlposition is to form nitrogen and isopropyl radicals, these radicals then would react with themselves aiid the remaining azoisopropane.

Weininger and Rice (12) have shown tha t a molecule of azoethane on ab- sorbing a q u a n t ~ ~ m of radiation is raised to an excited state having a lifetime reasonably long conipared to the time between collisions with a normal molecule and hence a proportion of the radiation absorbed by the azoethane is dissipated without reaction. The quantum yield of nitrogen formation is therefore pressure dependent, and although recent work on the photolysis of azomethane (3) has shown that this effect does not occur with the lower homologue it should appear in the pliotolysis of azoisopropane which has a greater number of internal degrees of freedom.

The object of this research has been therefore to investigate tlie disproportion- ation and combination reactions of isopropyl radicals, the rate of reaction of isopropyl with azoisopropane, and to determine whether an excited molecule is produced in the photolysis oE azoisopropane.

EXPERIMENTAL

The azoisopropane was prepared after the method of Lochte, Noyes, and Bailey (6). I t was purified by vacuum fractio~latiori and stored in a darliened trap attached to the apparatus by means-of a mercury cutoff. About 0.2 per cent of C6HI4 was always present in the stored azoisopropane which, i f allowed to remain, would cause an appreciable error in the CSH14 analysis of runs of low conversion. The procedure used in filling the reaction cell therefore entailed con-

Manuscript receivrd January 6, 1955. Contribution fro~n the Division of Pure Chemistry, National Research Coz~ncil, Ottawa,

Canada. Issued as N.R.C. No. 2958. National Research Council of Canada Postdoctorate Fellow 1950-52.

377

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F B

IRM

ING

HA

M o

n 11

/14/

14Fo

r pe

rson

al u

se o

nly.

densing a salnple of azoisopropane into the analysis section, removing the residual C6Hll, and then expanding illto tlie filling section of the apparatus. The pressul-e was measured with a mercury manometer, and the cell then closed off for irradi- ation. The reaction cell was 10 cm. in length, with a volume of 196 cc., and was almost completely filled by the light beam.

The a~ialysis section consisted of a modified Ward still as described by LeRoy (5), a Toepler pump, and R4cLeod gauge. 4 trap ahead of the Ward still main- tained a t - 75OC. removed the bulk of the u~lcllanged azoisopropane.

The mercury vapor lamp used was the Hanovia 100 watt Alpine burner, its bean1 collinlated with two lenses and two stops. The absorption maximum for azoisopropa~le is a t 3600 A so that the 3660 A mercury line was used for the photolysis and was isolated by using two Corning 5860 filters A selenium photo- cell connected dii-ectly to a sensitive inicroam~neter was placed at the back of the reaction cell to measure the transmitted intensity.

The aluminum block furnace had quartz windows at each end to cut down cooling by convection and the temperature varied only by about 1°C. during a 12 hr. run. The voltage a t the furnace and lamp was maintained constant by sing a Sorenson voltage stabilizer.

Results

The absorptio~l coefficient of azoisopropane for 3660 A radiation was found to increase appreciably with temperature. Curves of pressure against log trans- mission were therefore determined for the temperature a t which arbitrary quan t~~n l yields were measured. In this way tlie absorption occurring during a run was 1;1lown by measuring the pressure of azoisoproparle in the cell.

The analytical procedure following a run was as follows. Most of the unreacted azoisopropane and condensable products was condensed

in a side tube with liquid nitrogen before the cell was opened to the analytical section. The remaining conde~isables were removed by leaving the cell open to the Warcl still, cooled with lirluid nitrogen for two to three h o ~ ~ r s , and then the resiclual nitrogen was pu~nped into the gas burette. Mass spectrometer analysis of a sample of this gas confirmed that it was pure nitrogen.

On warnling the Ward still to - 135OC. the propane and propylene were brought off together and, after the volume and pressure had been measured, a sample of the mixture was a~lalyzecl by absorption of the propylene onto a bead of fused ~nercuric acetate (9).

By plotting a vapor pressure curve a t low temperatures for carefully purified azoisopropane and comparing it with that for 2,3-dimethyl butane, the expected C6 hl-drocarbori produced from the combination of two isopropyl radicals, it appeared that a separatiorl of the two con~pounds could be obtained a t a maxi- mum vapor pressure of about 5 X 10P Innl. By holding the Warcl still a t - 85OC. and using the preliminary trap described above, it .was possible to separate the CtjHll hydrocarbon and measure its volume and pressure; the spread of the points in Figs. 1 and 2 probably represent the analytical error i11 the C6H14 estimation. The cracking pattern of this hydrocarbon in the mass spectrometer was used to identify i t with an Buthentic specimen of 2,3-dimethyl butane.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F B

IRM

ING

HA

M o

n 11

/14/

14Fo

r pe

rson

al u

se o

nly.

DURIIABZ AND STEACIE: PHOTOLYSIS OF AZOISOPROPANE 379

A material balance of N2= +(C3Hs+ CaH6) + C6H14 was obtained with standard deviation of f 5% aboiit 100% balance, except for one rill1 where the low pressure made analysis difficult. In three runs it was found that all the C6H14 had not been removed before the runs were started so that the correct amount was calculated by difference from the nitrogen and propane plus propy- lene analyses. The rate of nitrogen productioil was found to be directly pro- '

portional to the incident intensity of the radiation for a constailt azoisopropane concentration.

The following reaction scheme is proposecl to account for the variation i l l

products with varying radiation intensity, absorption, and concentration of azoisopropane. AP refers to the normal molecule of azoisopropane whilst APz is the excited state.

CaH7.N :N.C3H7 ?, APL APZ + N2+ 2 iso-C3H7 [ I ]

A P + A P Z + 2 A P [21 2 SO-C3H7 + C3Hij+ C3He [3 I

-+ csH14 [4 I SO-C3H7+ AP + C3Hs+ C~HG.N:N.C~H? [5 I

I t is seen by inspection of the above scheme that

so that the rate of combination of isopropyl radicals compared with the dispro- portionation rate can be obtained from the propylene to C6H14 ratio. By assuming a constant steady state concentration of isopropyl the following two equations, analogous to those obtained in the photolysis of diethyl ketone (4), canbe applied

ARCS ks [AP] -- - - -- - R C E H ~ ~ k4' ' ~ ~ 6 1 1 1 4 '

Rc3 represents the rate of fornlation of C3Hc+ C3He and ARC, is (C3HR- C3H6). Rc6H14 is expressed in molecu~es/sec./cc. of cell volume and the concentratioil of azoisopropane in molecules/cc.

The large radical appearing in reaction [5] appears to be relatively stable and probably disappears by dirnerization. I t does not seem to remove isoprop)~l radicals to form C6H13.N:N.C3H7 because a good material balance is obtained. Also it does not appear to react with an isopropyl radical to give C6H14 and C3H6. Such a reaction as this would cause R c ~ H ~ ~ to increase with temperature through the C3H6.N:N.C3H7 concentration increasing with temperature whereas Rc6H14 actually decreases with temperature. Similarly the reaction

C3H6.N:N.C3H7 + C3H6+ N2+ C3H7 is not important as C3H6 does not increase with temperature.

Table I shows the results obtained by varying concentration of azoisopropane, radiation intensity, and temperature, and they have been used in Figs. 1 and 2

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F B

IRM

ING

HA

M o

n 11

/14/

14Fo

r pe

rson

al u

se o

nly.

TABLE I PHOTOLYSIS O F AZOISOPROP.4NE

I I RN molecules per sec.

per cc. of reaction volume

RCGHI~ molecules, CC.? set.-I x 10-12

IAPI R c G H ~ ~ ~

moleculest cc.+ sec.4 x 10-12

IAPI molecules,

cc.-l x 10-l8

Incident intensity, arbitrary

units

to obtain k&/k4' by plotting A R c ~ / R c ~ H ~ ~ against [ A P ] / R c ~ H ~ ~ . The logarithms of the least squares values of ks/kr' have been plotted against the reciprocal temperatures in order to evaluate E5- 3E4; from Fig. 3 this activation energy difference is calculated by the method of least squares to be 6.5 =t 0.5 kcal. per mole and the ratio of the steric factors ~ 5 / ~ 4 ' is 5 X 10-9 6.5 kcal. would be a minimum value for the activation energy involved in the abstraction of a tertiary hydrogen atom from azoisopropane as it is possible that E4 is not zero. Trotman- Dickenson, Birchard, and Steacie (11) obtained values of 7.6 and 7.8 kcal. for the abstraction of tertiary hydrogen atoms from isobutane and 2,3-dimethyl butane by methyl radicals which are similar reactions to the above. The re- action involving an isopropyl radical would be expected to be slightly higher in energy due to the fornlation in this reaction of the weaker secondary C-I3 bond of propane. The ratio of the steric factors in the methyl radical work was also

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F B

IRM

ING

HA

M o

n 11

/14/

14Fo

r pe

rson

al u

se o

nly.

D U l W A M A N D STE.4CIE: PHOTOLYSIS OF AZOISOPROPANE 381

( A PI - R' Iz c61(,4

FIG. 2. A R ~ ~ / R ~ ~ ~ ~ ~ plotted against [AP] /R~ ,~ , ,% at 82" and 122°C.

of the order of Kutschke, Wijnen, and Steacie (4) obtained an activation energy of 7.5 kcal. for the reaction of an ethyl radical with diethyl ketone, while Ivin and Steacie (2) found 6.2 kcal. for the similar reaction between an ethyl radical and mercury dimethyl. The difference between these reactions and reaction [5] is that a secondary C-H bond is broken con~pared with a tertiary in azoisopropane, whilst a primary is formed in ethane and a secondary in propane. As the activation energies are in reasonable agreement the primary

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F B

IRM

ING

HA

M o

n 11

/14/

14Fo

r pe

rson

al u

se o

nly.

382 CdiVADI,th: JOURN.4L OF CHEdIISTH17. I'OL. 31

2 . 5 0 3.0 0 3.50 x lo- '

I I T

FIG. 3. Arrhenius plot of k5/k4+.

and secondnrj- C-H bond dissociation energy difference must be practically equal to the secondary and tertiary difference in ethane and propane. From a study of the photolysis of di-n-propyl ketone, Masson (7) has found the activa- tion energy needed for removal of a secondary hydrogen atom from di-n-propyl ketone by the n-propyl radical to be 6.5 kcal.; in this case a primary C-H bond is fornled in the reaction.

Equation [8] should give a straight line passing through the origin but positive intercepts are obtained on extrapolating the 30°C. and 62°C. graphs to zero value of [ A P ] / R C ~ H ~ ~ ~ . This might be due to the presence of the radical C3H7N:N. a t lower temperatures which would give C3H8 + C3Hs probably Illore readily than C6H14 by the following reaction.

C3H7 + C3H7.N:N. + Nz + C3Hg + C3Hs [9] I t is also possible that a t low pressures the diffusion of radicals to the walls becomes important. Table I1 shows the variation of k3/k4 with increasing temperature. The values in the first column have been obtained by averaging those for a series of nlns a t each temperature and those in the second column have been calculated from graphs of equation [7] correcting for the intercepts

TABLE I1 VARIATION OF RATE CONSTANTS WITH

TEMPERATURE

I I

Temperature, ka/kd k3/k4 from average lnterccpts

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F B

IRM

ING

HA

M o

n 11

/14/

14Fo

r pe

rson

al u

se o

nly.

D L'IIH:III A N D STEACIE: PHOZ'OLYSIS OF AZOISOPROP.~lAIE 383

appearing in Figs. 1 and 2, after the manner of Kutschke, Wijnen, and Steacie (4). I t is interesting to note that the value of ka/ka obtained a t 30°C. agrees with that of 0.5 obtained by Blacet and Calvert (1) during an investigation of the photolysis of isobutyraldehyde a t room teinperati~re. A similar result, namely 0.4 was estimated by NIoore and Taylor (8) from the ratio of propane to hexane fornlation during the mercury photosensitized hydrogenation of propylene a t room temperature. The sonlewhat lower ratio of 0.15 has been obtained in the photolysis of di-?z-propyl ketone by Masson (7) and is probably related to the different configiirations of the normal and isopropyl raclicals.

The value of k3/k4 appears to fall off with increasing temperature but the effect is relatively small and in view of uncertainty of explanation of intercepts is perhaps not worth discussing in detail. I t is, however, possible that the presence of C3H7.N :N. may be a complication. The activation energy difference between disproportionatio~~ and recombination is almost certainly zero and the value of 0.5 for ks/ke a steric effect.

The production of an excited molecule of azoisopropane in the primary step is indicated by the way the rate of nitrogen formation drops off with increasing azoisopropane concentration as -is required by reaction [2 ] . An arbitrary primary process quantum yield 4, has been calculated by dividing the rate of nitrogen production, corrected for variations in incident intensity, by the percentage absorption and making 4, equal to 1 a t the extrapolatecl value of zero azoiso- propane concentration. In Fig. 4, 4, has bee11 plotted against concentration for

0.50 1.00 1.50 1 MOLECULES ttr' ( A P I

FIG. 4. Plot of 9, against azoisopropa~le concentration.

two temperatures showing a s~llall increase in qua~ltunl yield with temperature a t higher azoisopropane concentrations.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F B

IRM

ING

HA

M o

n 11

/14/

14Fo

r pe

rson

al u

se o

nly.

381 CANADIAN J O U R N A L OF CHEMISTRY. 1'OL. 31

REFERENCES

1. BLACET, F. E. and CALVERT, J. G. J. Am. hem. Soc. 73: 661. 1951. 2. IVIN, K. J . and STEACIE, E. W. R. Proc. Roy. Soc. (London), A, 208: 25. 1951. 3. JONES, M. H. and STEACIE, E. W. R. J. Chem. Phys. In press. 4. I~UTSCHKE, K. O., WIJNEN, M. H. J., and STEACIE, E. W. R. J. Am. Chem. Soc. 74: 711.

1 R.53 5. LEG~D. J. Can. J. Research, B, 28: 492. 1950. 6. LOCHTE, H. L., NOYES, W. A,, and BAILEY, J . R. J. Am. Chem. Soc. 11: 255G. 1922. 7. MASSON. C. R. T. Am. Chem. Soc. 19: 4731. 1952. 8. MOO RE,'^. J. aria TAYLOR, H. S. J . Chem. Phys. 8: 501. 1940. 9. PYKE, R., I ~ A H N , A,, and LEROY, D. J. Ind. Eng. Chem., Anal. Ed. 19: 65. 1947.

10. RABISPERGER, H. C. J. Am. Chem. Soc. 50: 714. 1928. 11. TROTMAN-DICKENSON, A. F., BIRCHARD, J . R., and STEACIE, E. W. R. J. Chem. Phys.

19: 163. 1951. 12. ~VEININGER, J . L. and RICE, 0. I<. 12lst National Am. Chem. Soc. Meeting, March, 1952.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F B

IRM

ING

HA

M o

n 11

/14/

14Fo

r pe

rson

al u

se o

nly.