P. SMITH AND S. CARBONE
6174 [CONTRIBUTION FROX
THE
VOl. 81
DEPARTMENT O F CHEMISTRY, P U R D U E UNIVERSITY]
The Kinetics of the Thermal Decomposition of 2,2 '-Azo-bis-isobutyronitriIe'" BY P. SMITH'^
AND
S. CARBONE
RECEIVED MARCH28, 1959 The extent of formation of dirnethyl-N-(Z-cyano-Z-propyl)-ketenimine,DKI, and the first-order rate constants for the decomposition of D K I and 2,2'-azoo-bis-isobutyronitrile, AZO, have been estimated from a kinetic study of the breakdown of AZO in benzene solution at 60-SO
.
Introduction The object of this work was to compare the extent of formation of DKI during the thermal decomposition of AZO with that found in a parallel study2 of the photolysis of AZO in the same solvent a t 25'. To this end the thermal breakdown of AZO was followed a t 60-80" via the nitrogen evolution and spectrophotometric methods.3 The mechanism of this reaction, neglecting formation of side product^,^ can be written
[DKI] = klVIAZO]a(e-W - e-k4ut)/(kaU - ki) (5) (R[DKI])o = kiV[AZO]o (6) [DKI],,x = V [ X Z O J O ( ~ I / ( ~ P~ V=) )VIAZO]oe-kdUtlnnr (7) (ki - kaU)tmm = I n ( k l / ( k , C ) ) (8)
(2)
where U , V and /3 are (1 - (1 - p ) (1 - q ) - (fik3)/ ( h f k d ) , ((1- b ) ( l - 6 ) ( b k d / ( k z + k a ) ) and k4U/ (k4U - k l ) , respectively.6 Applying (8) to the previous experimental data a ~ a i l a b l e indicates ~,~ that k4U/kl is ca. 0.7-1.0 a t 70-100° in toluene solution. This near equality of kl and k4U was confirmed in the present investigation and used to give a way of estimating krU and V via the approximate equation
2R*+DKI, -Ri[R.] = k3[R.I2 (3) D K I +2pR. (1 - p)gTSK 4-(1 - p ) ( 1 - q)DKI, R[R.] = ~PKI[DKI] (4)
derivable from ( 5 ) providing certain conditions hold (see Appendix).
AZO
+N2 + 2bR. + (1 - b)cTSS f (1
-b
)(1
- c)DKI,
- R [ A Z O ] = ~ L [ X Z O ](1)
2R*+TSN,- R [ R . ] = kz[R.l2
+
+
where R. and TSN are the 2-cyano-2-propyl radical and tetramethylsuccinonitrile, respectively. Equations 1 and 4 give the net results of the primary breakdown, via free radical and molecular routes, of AZO and DKI, respectively. In each case the products are those that emerge from the reaction cage, taken to be the region of primary and geminate destruction processes, e.g., of those AZO molecules that decompose in the primary act fractions b, (I - b ) c and (1 - b j ( 1 c) appear as R., TSN and DKI, respectively. To emphasize the possibility of cage reconibination of R. radicals in (4), kd is defined in terms of the appearance rate of ICO groups, respectively. It seems reasonable to expect R. radicals to form some dimethylketenazine, (CH3),via N-K coupling C=C=N--N=C=C(CH3)2, as well as TSN and DKI. The fact that none has been found may be because it is unstable under the employed reaction conditions. Appendix Derivation of Equation 9.-Equation written kdU
+ In (/DKI]/(kiVIAZO]ot))
+
=
x2/24 G 0.005 (x/2
In (1
+ y)
