REACTIONS OF METHYL RADICALS WITH SUBSTRATES
Nov. 5 , 19%
[CONTRIBUTIONS FROM THE CHEMISTRY
DEPARTMENT, STATE UNIVERSITY
OF
5537
NEWYORK, COLLEGE
OF FORESTRY]
Reactions of Methyl Radicals with Substrates Acting as Hydrogen Donors and as Methyl Radical Acceptors BY R. P. BUCKLEY, F. LEAVITT AND M. SZWARC RECEIVED JUNE 13, 1956 It is shown t h a t the mechanism proposed by Levy and Szwarc, describing reactions of methyl radicals with solvents which are hydrogen donors and olefinic or aromatic compounds which are methyl radical acceptors, can be extended to systems in which either of these two components act in a dual fashion: as a hydrogen donor and as a methyl radical acceptor. Furthermore, it is shown that for acceptors of methyl radicals which are simultaneously hydrogen atom donors, the equation derived by Levy and Szwarc does not lead to a ratio of rate constants of addition to an olefin and of abstraction of a hydrogen from a solvent (this ratio is denoted by k 2 / k l ) , but to a quantity (kz/kt),,, which varies with the molar ratio of olefin and solvent. It is shown that (k2/k1)-lexp is a linear function of molar fraction of the olefin/molar fraction of the solvent. The intercept of such a straight line gives the “true” value of kz/kl and its slope the relative rates of hydrogen abstraction from and methyl radical addition to the olefin. These deductions are confirmed by experiments, and the significance of the derived rate constants is further discussed.
it was shown that methyl I n a series of radicals, generated in a solution composed of an aliphatic hydrocarbon HS and an aromatic or olefinic compound A, undergo two competing reactions. They abstract hydrogen atoms from the solvent HS
illustrate the conclusions on suitably chosen examples. Systems Involving Solvents Which Are Acceptors of Methyl Radicals.-In our studies methyl radicals are generated by the decomposition of acetyl peroxide. It was shown previously that the decomposition of a dilute solution of acetyl peroxide ki in a non-polar solvent produces one methyl radical CHs H.S + CH, Sa (1) for each molecule of carbon dioxide formed in the or they add to the aromatic or olefinic molecules A reaction.6 Moreover, if the solvent is an aliphatic kz hydrocarbon then all the methyl radicals formed by CHa f A ---+ A.CHs (2) the decomposition yield either methane or ethanej6 If the stationary concentration of radicals is main- i.e., the ratio (CH, 2C2H,)/COz, is essentially tained a t a sufficiently low level, then reactions 1 unity (see e.g., Table I in ref. 1). However, if the and 2 account for all the methyl radicals which dif- decomposition takes place in toluene solution, then fused out into solution. Under these conditions the ratio (CHI ~CZH~)/C is Osubstantially ~ less the ratio of the rate constants k z / k l is given by the than unity, e.g., its average value calculated from simple e x p r e s ~ i o n ~ . ~ ten experiments carried out a t 65’ was found to be 0.858 0.010. Since this ratio is not affected by k 2 / k 1 = { (A,CHaformed)/(CHc formed)] X X ~ X A the concentration of the peroxide, if the latter is M , one concludes that the “missing” in which XHSand X A denote the mole fractions of less than the solvent HS and of the aromatic or olefinic com- methyl radicals are not lost in reaction 3 pound A, (CH, formed), the amount of methane CsH6.CH2 CHI. --f CeHs.CHz.CHa (3) formed in the reaction, and (ACH, formed), the but that their disappearance is due to the reaction amount of methyl radicals consumed by reaction 2. The primary product of reaction 2, i.e., ACH3, is analogous to reaction 2, namely CH, f CoHa.CH3 -----f adduct (4) itself a radical and therefore is not the final product of the over-all process. Under experimental condi- Assuming that all the “missing” methyl radicals tions chosen in those investigations, A.CH, disap- are consumed by reaction 4 we calculate k4/kl as pears by a reaction involving either another radical (1 - (CHI 2C2H,)/C0,)/(CH4/C0~). Since A C H 3 or a solvent radical S.. Therefore, the (CH4/C02) was found to be 0.742 0.08 (again the amount of A.CH8 originally formed can be deter- average from the same ten experiments) , hence, mined by measuring the difference in the amounts k4/kl = 0.19, i.e., the probability of the interaction of methane formed in the presence and in the between a methyl radical and a toluene molecule absence of A in experiments which otherwise are leading to a hydrogen atom abstraction is about five identical. times greater than the probability of the addition.’ The kinetic scheme proposed above applies to Let us now consider the effect of reaction 4 on the those systems in which the solvent HS is only the determination of the relative rate constant k2. hydrogen donor, while the aromatic or olefinic com- I n the absence of compound A (Le., in pure toluene pound A acts only as methyl radical acceptor. solution) the amount of methane formed is given by However, the situation is more complex when one or g ( k l / ( k l k4)), where q denotes the difference bethe other component of the solution can act dually: tween the total number of methyl radicals generas a hydrogen donor and as a methyl radical ac- ated by the decomposition and the number dimerceptor. It is the purpose of this communication to ized in a cage recombination. The amount of discuss the kinetic behavior of such systems, and t o ( 5 ) M. Levy and M. Szwarc, ibid., 76, 5981 (1954).
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(1) M.Szwarc, J . Polymer Sci., 16, 367 (1955). (2) M.Levy a n d M. Szwarc, THIS JOURNAL, 77, 1949 (1955). (3) A. Rembaum and M. Szwarc, ibid.. 77, 4468 (1955). (4) F. Leavitt. hl. Levy, M. Szwarc and V. Stannett, ibid., 77,5493 (1955).
( 6 ) It was shown t h a t ethane is formed by a “cage” recombination of methyl radicals, see A. Rembaum and M. Szwarc, THIS J O U R N A L , 77,
3486 (1955). (7) I n this section toluene
i4
sented as:
-+
CHI
+ C‘HrCHs
t h e solvent, and equation 1 is repreCHI CsHo.CH2.
+
methane formed in presence of compound A is given by the expression
The intercept of this line equals the reciprocal of kz! kl and its slope gives k j l k : , i.e., the probability of hydrogen abstraction to the probability of CH:I ( C H I fortlied) = c l [ k l ( Y i m / ( ( k l kl)-Yli5 kJdI1 addition, both reactions referring to molecules A. Hence Such linear relations were tested by investigating (A.CH3 formed) = (CH4 "105t") a series of compounds which are not too powerful methyl radical acceptors and a t the same time = dki/(kl k4)) - q k i ( X i l s / ( ( k 1 kA)XiIs k&\J = q ( k i k r ( S * / (( k l k4)Xfi.: k ? S i j ) X 1 kt k , ) sufficiently good hydrogen donors. Methyl Affinity of Allyl Acetate.-Allyl acetate is Therefore a not too reactive monomer and a self-inhibitor ( A . C H 3 formed)/(CHd formed) = k2(.Y4/{ ( k , k,)YIp}) of its oini polymerization. Studies of the polyand merization of allyl acetate by Bartlett and kn/(k, k4) = { (A.CH3 f o r m e d ) / ( C H , formed)] ( X n ~ / x ' * ) .4ltschul l o denionstrated that the molecular weight of polyallyl acetate is abnormally low, and from One concludes therefore, that the technique applied the kinetics of polymerization they deduced that in studies involving aliphatic hydrocarbons as the termination is due to the abstraction of a hytlrosolvents is also valid when the solvents used are gen atom from tlie niorionier, tlie latter reaction not only hydrogen donors but also methyl radical competing efficiently with the propagation step. acceptors. The relative rate constant kz is given by Our studies of methyl aliiiiities of allyl acetate the same expression, the only difference being in were carried out in iso'iictaiie solution a t 65 and 85". the interpretation of the results, namely, kl k4 in- The results are listctl in Table I and the plot of restead of kl appears in the denominator. ciprocal of (k? 'kdEXI,L ' C Y S ~ L SX L y ; X ~is~shown s in Fig. Levy and Szwarc2 demonstrated that (kl 1. The expected linear relntionship seems to be k4)toluene is about three times greater than ( k l ) l s o o c t a n e well obeyed and the data listed in Table I1 are deRemembering that (k4 l k l ) t o r u e n e is 0.19, we calcu- rived from the intercepts and the slopes of the late (k4)toluene '(k1)isoOCtane to be 0.48. The latter respective straight lines. value is not much greater than (k2)benzene ( k l ) Isooctane, found to be20.29 a t G S O . It was shown TABLE 1 by Heilman* that the rate of addition of methyl ,4I,I.YI, 24CE1.A1.11'L radicals to substituted benzenes is only slightly 1 l i ) l e $7 of t h v 7', *C. monctrnpr CH~~CO~ (kn/ki)exp. enhanced by a substituent. Hence, the pres58. ,5 C, 3 1.02 I) ,;on ent result showing the methyl affinity of tolu0 ,i 2,105 .4"0( ? ) 43.6( ? ) ene to be about 1.G.5 greater than the methyl af55.1 0 ,? 3.0: .R31 finity of benzene seems to be plausible. It is C, :i .5 . 0,i 2 5s 39.7 interesting to mention at this juncture the work of (i,i 7,(1.-) . -.+I;, 33.8 Hey and his schoolg who showed that substituents 45.3 83 1.0" , ,548 enhance only slightly the rate of addition of phenyl .401(?) 49 . O( ? ) S,5 2 , Os? radicals to the substituted benzenes. ,383 34.8 85 3.03 Systems in Which the Olefinic or Aromatic ,335 32.9 85 4.00 Component Acts Also as a Hydrogen Atom Donor. ,280 29.0 85 0.m -Let us consider a system in which molecule -4 ,270 22.6 8,5 8.0.5 adds methyl radicals and also serves as a hydrogen donor. The reactions of methyl radicals with the hf solutioii These experiments were carricd out in solvent molecules and the A niolcculcs are repre- of acetyl perositlc (for 65" runs) and 10-3 solution of the peroside (for 85" runs). CH4/COs i n p r r i i o i i c t a n r is srntcd by the three equations 0.802 for (is" a l i i 1 0 . X O t for 8 5 ' .
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ki + 11s + CH4 + s k:, CH~ + ,4 -+C H I + .I' k2 CII3 + A --+ CH31
CHI
(1)
(5) (2)
+
The (CHI formed) is proportional to ( ~ I X I I S k6X.4), while the (ACH3formed) is proportional to kZXA. Hence ( C H 4 formed)/(.\ CH3 formed) = k L X m / k 2 X 4
+ ks/kZ
After rearrangement this expression acquires the form
The left-hand side represents the reciprocal of the expression previously used in calculation of kq k l , and denoted now as ( k z ' k l ) e x p . We conclude, therefore, that plotting of the reciprocal of ( k ? ' k l ) e x Dversus X A / X H Sshould lead to a straight line. ( 8 ) W HeilInnn, Thesis, Syracuse Univ , New York, 1956 ( 0 ) D II IIeT, P/ n l J Ciiriii Sor 7112 (lq,:?) a n d related Tinper-
TAIXF: I1 AT.I,YLACEI'A'L'E 'l'rnip , oc.
G3 8,5
k ?,'I
