Nov. 20, 1964
HYDROGEN FORMATION I N ?-IRRADIATED HYDROCARBOKS
Acknowledgments.-The authors are indebted to the Xational Research Council of Canada and to the Alfred
[CONTRIBUTION FROM
THE
4761
P. Sloan Foundation for generous financial support of this work.
RADIATIOX LABORATORY~ A N D THE DEPARTMENT OF CHEMISTRY, UNIVERS~TY OF SOTRE DAME, NOTREDAME,IXDIANA~
The Mechanism of Hydrogen Formation in ?-Irradiated Hydrocarbons BY SAM2 . TOMA A N D WILLIAM H . HAMILL RECEIVED J U S E 21, 1964 The yields of cyclo-C6Hsand cyclo-C5Hlafrom radiolysis of mixtures of c ~ c I o - C ~and H , ~cyclo-c6~IIlZare not linear in cornposition, unlike the vields of the corresponding cycloalkyl radicals. -4dditions of c y c l o - C s D ~to~ cyclo-CsH,? decrease G(H>)and increase G(HD) in constant proportion. Yields of H D and D Pfrom cycloC6DiYand cpclo-CsDln as solutes are much less in ethanol than in alkanes. Radiolysis of 1,4-cyclo-CsH8 as solute in hydrocarbon solutions produces 1,3-cyclo-CsH8, cyclo-CsFI,., arid &He. A'ill of these results are consistent with decomposition of molecular cations following electron return, loss of two hot hydrogen atoms, and formation of C=C and hot hydrogen. Alkane, alkene, and alkadiene produce alkene, diene, and triene, respectively.
Introduction Yields of hydrogen from y -irradiated organic liquids can be classified empirically into fractions according to the way they respond to reagents for H atoms, electrons, or positive-charge exchange. An interesting technique which warrants further application is to add perdeuterated solute, using yields of H D and Dz per unit solute content as evidence for niechanisms of hydrogen format i ~ n . Still ~ another technique depends upon immobilizing and examining electrons, ions, and radicals in rigid media..' All of these techniques have been used in a recently reported study of the mechanisms of hydrogen formation.j The present report describes an extension of this work. Experimental Only procedures differing from those already reported5 will be described here. Materials.-Phillips pure grade 3-methylpentane (3-MP) was FerLe2 through a 6-it. column of silica gel, retaining the firstone-third. Phillips research grade cyclohexane was used as received when v.p.c. boiling point columns were t o be used; otherwise Fisher spectral grade cyclohexane was used. Aldrich 1,4-cyclohexadiene was used as received and stored under refrigeration. -411 samples were carefully outgassed in the cell by refluxing on a vacuum line. 7-Irradiations were performed a t dose rates approximating 2 X IOi8e.v./min. g. Isotopic hydrogen analyses were performed in duplicate on a C E C 21-103A mass spectrometer using a vibrating reed.
Results and Discussion It has been proposed5 t h a t an important contribution to G(H2) in cyclopentane and in cyclohexane from radiolysis a t -20" arises from the elimination of two hot H atoms when the molecular cation recombines with an electron, leaving the corresponding olefin. Each H atom is assumed to abstract hydrogen efficiently to give the corresponding alkyl radicals. I t can be predicted from this mechanism that in mixtures of cyclopentane ( I = 10.53 e.v.) and cyclohexane ( I = 9.88 e.v.) there should be a tendency to transfer positive charge to cyclohexane (to the extent t h a t ground-state molecular (1) T h e Radiation Laboratory of the University of Notre Dame is operThis is ated under contract with the U. S . Atomic Energy Commission document COO-38-342. (2) J . Roberts and W H. Hamill, J . Phrs. Chem., 67, 2446 (1963). (3) W. >I. Jenkinson and P. J. Dyne, C a n . J . Chem., 38, 539 (1960). (4) W . H . Hamill, J . P. Guarino, A I . R.Ronayne, and J. A . Ward, Discussioizs F a v a d a y SOL.,5 6 , 169 (1963) ( 5 ) S . Z . Toma and W. H. Hamill, J . A m . Chem. S O L .86, , 1478 (1964).
ions are involved). Consequently, yields of cyclohexene should increase more than linearly with the concentration of cyclohexane and yields of cyclopentene should concurrently decrease to a comparable extent. The yield of free radicals from reactions of hot H atoms, on the other hand, should be linear in the concentrations of the respective hydrocarbons. The necessary experiments have already been reported, in part, by Muccini and Schuler6 who have measured G(cyclo-C5HgI) and G(cyclo-C8Hl1I) a t 6 X e.v.,'g. in liquid mixtures of these two cycloalkanes containing 0.003 Miodine. They found that these yields "are directly proportional to the electron fraction of the parent material present.'I6 The results from other experiments necessary to test the proposed mechanism of hydrogen formation in these mixtures appear in Table I where any departure from a proportional dependence of yield upon composition is reflected by AG = XGO Gobsd ( X = mole fraction and Go is the yield from the pure component). Yields of olefins measured without added iodine will be somewhat affected by disproportionation, but both olefins were measurable in these systems. At moderately small values of X(cyc1o-C6Hl2), the yield of cyclohexene is disproportionately large and the yield of cyclopentene is correspondingly diminished. Mixtures with added iodine were also examined, corresponding to the conditions of earlier experiments,'j but it was not possible to measure cyclopentene reliably on the v.p.c. column because of interference by cyclopentyl iodide. The yield of cyclohexene, however, is clearly far in excess of a proportional dependence. The results of Table I in conjunction with the earlier measurements6 on yields of cyclopentyl and cyclohexyl radicals support the mechanism which has been proposed. It may be suggested that these facts, the insensitivity of G(H2) to scavengers, the effect of additivies to alter products from cyclohexane and cyclopentane in the ratio AG(C6HI1)/ A G ( C ~ H ~= O )AG(C5Hg)/AG(CjHs) = 2 , and the large H D ID2 ratio in dilute solutions of perdeuterated cyclohexane in hydrocarbons, could all be accounted for by postulating ion-molecule reactions. One difficulty is t h a t no plausible ion-molecule reaction can produce, e . g . , cyclo-C6Hlo 2 cyc1o-C6Hl1as neutral products, while a reaction of the type
+
(6) G. A hluccini and R . H Schuler, J . P h y s . Chem., 64, 1436 (1960)
SAM2 . TOMA A N D WILLIAM H. HAMILL
4762
TABLE I YIELDSOF CYCLOPENTENE ASD CYCLOHEXENE FROM MIXTURES OF CYCLOPENTANE A N D CYCLOHEXASE IRRADIATED AT CU. 20" A N D 10" E . V . / G . X X X = mole (CsHa). (C-CGHI?)' G GO (c-CrHs) (c-CrHlo),
fraction C6Hn
A c
G Go (c-CaHio) (c-C6Hia)
iG
30 Iz present
0
2.67 2.07 1.73 1.32 0
0.092 0. 229 0.464 1
2 2 2 1
67 43 06 43
0 0 47 0.91 1.30 2 40 0.01 M Iz present - 0 36 - 0 33 -0 11
0.04
0.31 0.426 0 660 1.07 1.43 1.56 1.51°
0.114 0.228 0.464 0.722 0.862
1 4
0 0 22 0 55 1.11 2 40
0.25 0.36 0 19
0.06 0.173 0.345 0 70 1.09 1.30
0.25 0.253 0.315 0.37 0.34 0.26
With 0.006 M I,.&
Vol.
xc,
collision cross section is smaller than that for H-I, some enhanced yield of Dz a t low temperature is to be expected from the proposed mechanism which postulates formation of two hydrogen atoms from vicinal carbon atoms. The results do not exclude the possibility t h a t an appreciable part of G(D2)may be due to a truly molecular process. It has been shown that irradiation of cyclohexene and of cyclopentene in alkanes produce conjugated olefins, and that the processes involve ion-electron recombinat i ~ n . The ~ fate of the hydrogen was not examined then. Evidence bearing upon the question of thermal and hot hydrogen atom reactions of olefins can be obtained by examining mixtures of cyclohexene-dlo in cyclohexane. The results in Table I1 suggest that D1 arises in part from direct interaction of CBDlo with radiation, but that H D is mainly due to a nonlinear indirect effect not involving thermal deuterium atoms. The nearly constant ratio AG(H2) G(HD) indicates a predominately substitutional effect in the radiolysis of cycloC6D10 for cyclo-C6H12. To account for this effect we
TABLEI1
-
HYDROGEN YIELDSFROM MIXTURES OF CYCLOHEXEDIENE-dlo I N CYCLOHEXASE AT ca. 20'
X mole fraction L-CsDio X lo*
G(H2
C(HD)
AG(Ht)"/ G(HD)
G(Dd
AG(HC - 0.20/ G(HD)
AND
1.3 X lozoE.v./c.
G(HD)/ X(c-CsDio)
G(Dz)/ X(C-CeDia)
7.1 6.2 42 -1 4 -0.007 0.21 4.0 7 0 6.3 27 -1.0 -0.01 0.27 1.0 3.6* 6 6 6.1 9 0 60 0.03 5.0 2 6 0.44 6.7 6.3 6 0 55 0.044 0.49 8.0 2.2 a Based upon G(H2) = 5.5 for cyc10-C6H12. b For the same concentration of cyclo-CeHlo in cyclo-CeHin, G(Hz) = 3.82 compared to G(hydrogen) = 3.88 in this run.
0.5
CyClo-C6Hi2+
+ 2 CYCIO-C~HIZ
----f
cyclo-CeHio+
+ ~ C Y ~ C O - C+~ 2H2 HII
does not account for the fate of Cyclo-C6Hlo+, nor is i t thermochemically allowed. The facts appear to require decompositions of the type Cyclo-CeHiz +Cyclo-CeHio
+ 2H
Even if the products are in their lowest energy states this reaction is about 6 e.v. endothermic and a highly excited neutral species appears to be required. Evidence has been presented that this activated entity has an ionic precursor.6 A somewhat related problem is the origin of D2 from cyclohexane-dlz in hydrocarbon solvents. Is DZ entirely of molecular origin, or does it arise-at least in part-from thermalization and cage recombination of two D atoms? Parallel runs of 9 mole yo cyclohexaned12 in 3-hZP were performed a t ca. 20 and -196" at a dose of approximately 2 X 1018 e.v./g. Since reliable dosimetry was impossible for the low temperature run it is necessary to use ratios of yields. At ca. 20" the yields in pmoles were 7.59 Hz, 0.884 H D , 0.164 Dz, and 0.334 CyClO-CsDlo; a t - 196", 7.27 Hz, 0.993 H D , 0.207 Dz, and 0.304 CGD10. Since olefin is presumable the conjugate product of Dz by either mechanism we compare D2/:cyclo-C6Dloequal to 0.49 a t ca. 20' and 0.69 at -196". The increased relative yield of Dz a t low temperature can be related to earlier work in which it was shown that considerably more geminate combination between H and I from photolyzed H I occurs at - 19G" than a t 20" in this solvent.' Although the D-D (7) J . R Nash and W H . Hamill. J . A m . Chem SOL.,81, 5974 (1960)
observe that the yield of molecular hydrogen from CeH12 is small on the basis of evidence from cyclo-C6Dlzcyclo-C6Hlz mixtures.3 AG(H2) of Table 11 is due in part t o scavenging of thermal hydrogen atoms by cycloCsDlo, assumed to be complete a t all concentrations used and taken to be constant at Gn(H); in part it is assumed t o be due to charge exchange from c ~ c ~ o - C ~ H I ~ ~ to cyclo-C8Dlowith a dependence on the yield of Go(ion) and a concentration-dependent probability of charge transfer p. The hot H2 yield diminishes by pGo(ion)a, where CY is the yield of hot hydrogen per ion pair for cyclo-CsH12capable of forming hydrogen. The major contribution to H D a t small X(cyclo-C6Dlo) arises from hot deuterium reacting with C6H12, amounting to p . G"(ion)b, where 6 is the yield of hot deuterium from cyclo-C6Dlo per ion pair which is capable of forming hydrogen. Then
or AG(H2) - Go(H) - CY G(HD) P
=
constant
(1)
Letting Go(H) = 0.20 by trial gives the results listed in Table 11. It had been estimated from other evidence that GO(H) = 0.46.5 Despite the oversimplification of the mechanism, it is qualitatively clear that Gn(H) must be rather small. For comparison with the results just considered a single run with 5 mole % cyclohexene-dloin cyclohexene
HYDROGEN FORMATION I N ? - I R R A D I A T E D HYDROCARBONS
Nov. 20. 1964
4763
TABLEI11 ISOTOPIC YIELDSOF HYDROGEN FROM C Y C L O H E X A SAEN-D~ C ~~ YCLOHEXEDIEN UNDER E-~~~ \'ARIOUS CONDITIONS AT 4 X loaoE . v . / G . 7c solute Solvent G(Hz) G(HD) G( Dd G( H D ) / X ( C e ) G( Dz)/X(Ca) 4.73 0.80 3.16 0 237 0.040 Cyclopentane 0.02 MI2 5CyClO-CsDiz 0.017 3.74 0.33 80c/c Cyclopentane, 207, ethanol 4.71 0.187 5CyClO-CeDip 2CyClO-CsDiz Ethanol 4.05 0.0323 0.0023 1.64 0.12 0.0016 3.29 0.080 2.70 0.0648 ZCyclo-CsDio Ethanol Mole
+
?-IRRADIATION Solvent
Mole
%
OF
TABLE IV l,'&CYCLOHEXADIEXEIS HYDROCARBON SOLUTlONS
Temp., OC.
Dose, e.v.:g.
G 0.64 1.2 Unidentifiable 0 . 2 0.2 0.5 1.2
Product
1.3 0.6 0.5 0.2
a Sample volume 0.2 cc. ; product analysis semiquantitative. bleached, then thawed.
* ?-Irradiated sample thawed in dark.
was irradiated to 1.4 X lozo e.v./g. to give G(DJ = 2.2 X G(HD) = 4.0 X and G(H2) = 1.18. Again the evidence is consistent with the assumption of hot hydrogen reaction. The results of a considerable number of experiments are consistent with the hypothesis of positive charge exchange as an important process in liquid systems under a priori acceptable conditions with respect to relative ionization potentials. There is evidence for an important exception to this simple rule : protonation tends to take precedence over charge exchange when both are permitted.2 Ethanol protonates efficiently.8 The results in Table I11 show that adding ethanol to a solution of cyclohexene-dlzin cyclopentane (in which one expects charge transfer to the soluteg) diminishes both G(HD) and G(Dz). The ratio G(Dz)/G(cyclo-CsDlo) is decreased from 0.44 to 0.175 for the first and second runs by adding 20% ethanol. When cyclohexane-dlt was added t o pure ethanol the values of G/X(CeD12) for both H D and D2 were much smaller than had been observed in hydrocarbons with ionization potentials comparable to I = 10.5 e.v. for ethanol. As an example, G/X was found to be 11.5 for H D and 2.6 for Dz in npentane (I = 10.55 e . ~ . ) The . ~ values of G/X (cycloCeDlo) for H D and Dz were correspondingly much less in ethanol than in cyclohexane (cf. Table 11). There is considerable evidence that efficient formation of solute cations can occur in rigid hydrocarbon media but not in polar media (e.g., amines, ethers, and alcohols). In face, even a few per cent of polar additive markedly depresses cation yields. The evidence is consistent with charge exchange from solvent to solute, inhibited by protonating additivies.'O(ll The results in Table I11 suggest the same effect, both for cyclohexane and for cyclohexene. We conclude t h a t molecular cations are precursors of the major part of the hydrogen yield. The residual yields of H D and Dz from perdeuterated ( 8 ) T. F. Moran and W. H . Hamill, J . Chem. P h y s . , 39, 1413 (1963). (9) P. J. Dyne and J. Denhartog, Can. J . C h e m . , 40, 1616 (1962). (10) J. P. Guarino and W. H. Hamill, J . A m . Chem. Soc., 86, 777 (1964). (11) M. Kondo, M. R . Ronayne, J . P. Guarino, and W. H. Hamill, i b i d . , 86, 1297 (1964).
c
0.5 0.2 ?-Irradiated sample optically
hydrocarbons in ethanol are attributed to direct interaction with radiation. y-Irradiation of cyclohexane yields monoolefin and of cyclohexene yields diolefin, and the mechanisms of hydrogen formation are similar. I t is desirable to extend the observations to cyclohexadiene, choosing the 1,4-isomer as a matter of convenience. Except for one run, cyclohexane was the solvent for runs a t room temperature and 3-methylpentane was used a t - 196". The data are summarized in Table IV. The principal organic products were benzene, measured by v.P.c., and 1,3-cyclohexadiene, measured spectrophotometrically on the unopened sample. The extinction coefficient is 1.0 X l o 4 a t Amax 2600 A. while benzene absorbs weakly in this region.lZ At laboratory temperature G(1,3-C6H8) = 0.64 and G(C6Ha) = 1.2 with only ca. 0.5 mole yo solute. An unidentifiable unsaturated product was also present with an estimated G 0.2. In a parallel run with 1,4-cyclohexadiene in cyclohexane-dlz as solvent, there were rather large yields of Hz and H D (see Table IV) by comparison with corresponding experiments using light and heavy cyclohexane. Thus, the extrapolated yields are G(H2)/X(cyc10-C6H8) 2 13 and G(HD)/X(cycloCsH8) 33, suggesting that nonscavengeable hydrogen results from some energetic process such as ion recombination following charge exchange. Since G(H2 0.5HD) is considerably less than G(cyclo-C~H6)it must be concluded that a t least part of the hydrogen yield arises from thermal hydrogen atoms which are scavenged by diolefin. On the other 0.5 is probably not due to thermal hyhand, G(HD) drogen atoms since 1.5 mole% of diolefin should effectively remove them. We suggest that two hot hydrogen atoms are eliminated from positions 3 and 6 of cyclohexadiene, that 20y0 react hot giving H D , 15% thermalize and undergo geminate recombination giving Hz, while the rest diffuse away and are scavenged. Other evidence which must be considered for runs a t -196" is an absorption band with maxima a t 309 and N
+
(12) L. Pickett and E. She5eld, i b i d , 68, 217 (1946).
2. Lvz
4764
AND
S MEIBOOM
316 mp which is attributed to cpclo-CsH7. This assignment rests upon the appearance of a very similar band when benzene in 3-MP was y-irradiated a t - 196". Its formation was suppressed by' addition of M T H F , a scavenger for hydrogen atoms.? The present results suggest that nascent, excited CsH6 and H from 1.1cyclohexadiene undergo cage recombination promoted by the dense glassy state. The free radical C6Hi would have the lower energy 1,3-configuration and it could abstract a hydrogen from the solvent to give 1,3-cyclohexadiene. Finally, when 3-MP containing !).5Yc1,1cyclohexadiene is y-irradiated a t - 196', an absorption band appears a t rmax 770 mp, quite similar to olefin cation bands observed previously, together with the electron band a t Amax 1700 mp. Optically bleaching the latter removes the cation band. The summarized mechanism is
+ e --+ ( c y c l o - C ~ H ~+" 2 H ) + + Hp
cyclo-C6Hs+
(CvClo-csH~* 2 H ) ---+C)-Clo-CsHs C-C6D12
---+
[COSTRIBUTIOS FROM
CsH6
THE
+ HD f
H
i'ol SA ----f
--f
H
+
CJCIO-CBH; H CaH6
+ 2H
+ c ~ c I o - C ~ H+ S c~cIo-C~H~
Cyclo-CaH;
+
IIH
--+ 1 , 3 - c ~ c l 0 - C s H ~
If this mechanism is to account for the results, it would be expected that cage effects will be more pronounced a t low temperature than a t room temperature Parallel runs in 3-LIP (Table I Y ) , followed by thawing and chemical analysis, show that yields of benzene and of 1,3-cyclohexadiene are indeed much depressed a t - 196" This is consistent with earlier measurements of the quantum yields of hydrogen from hydrogen iodide under the same experimental conditions In summary, varied evidence supports the interpretation that the major part of the hydrogen yield from radiolysis of liquid cycloalkane, -alkene, and -alkadiene arises from charge neutralization of the respective molecular cations, producing C=C and two hot hydrogen atoms
BELLTELEPIIOXE LABORATORIES, Isc.,MURRAY HILL, SEW JERSEY 079711
Rate and Mechanism of Proton Exchange in Aqueous Solutions of Phosphate Buffer BY 2 . Lcz
AND
S.XEIBOOM
RECEIVED JVSE 15, 1064 The proton transfer reaction between H?PO4- and H P O < - ? in dilute aqueous solution. has been studied by n . m . r . line-broadening technique. The second-order rate constant is k = 1.45 X l o 9 mole-' I . sec a t 25' I t is shown t h a t this figure is consistent with a diffusion-controlled mechanism in which the proton transfer takes place vzu two solvent water niolecules
Introduction The kinetics of the proton transfer between an acid and its conjugate base in solutions has been studied for a number of buffers. l In this paper we report such measurements in aqueous solutions of phosphate buffer. 'I he n.m.r. technique used for measuring the exchange rates has been described p r e v i ~ u s l y . ' ~ In , ~ the present case the rate of proton exchange. between the various phosphate ions and water is very high, and only a single shzrp proton line, resulting from the averaged-out proton lines of the phosphate buffer and solvent water, is observed. I t is, however, possible to measure this rate indirectly, by measuring its contribution to the rate of proton exchange between solvent water molecules. The latter rate can be measured by using "0-enriched water. References 2 and 3 present a detailed description of the method used. Only those reactions in which the solvent water participates in the exchange process can be measured by this method. Simple examples of such mechanisms are of the type AH
+ OH2
or AH -f OH2
+B
A.%-
+ HOHl-
+ HOH + H B -
(1) (2)
(1) For a review and references see the articles of H. Strehlow and of >I Eigen and L . De Maeyer, in "Technique of Organic Chemistry," Vol. V I I I , Arnold Weissberger, E d . , 2nd Ed., Part 11, Interscience Publishers. Inc., New Y o r k , S . Y . , 1963. ( 2 ) Z Luz and S. Xeiboom, J . Ciiem. P h y s . , 39, 366 (1963). ( 3 ) Z L u z and S . hleiboom, J . Ani. C h e m Soc., 8 6 , 3923 (1963).
where AH symbolizes an acid and B a base. AH and R are not necessarily neutral molecules and the charges in eq. 1 and 2 are meant to indicate differences only. Reactions of type 2, in which the proton transfer takes place through one or more water molecules (which may be thought of as belonging to the solvation shells of the buffer components) have been found to be important in many systems. In reactions 1 and 2 the buffer can be looked upon as catalyzing the proton exchange between water molecules. The rate of the latter process is obtained from the broadening of the proton peak in "0-enriched water. In principle very high rate constants for reactions 1 and 2 can be measured, since the rate of proton exchange in water can be kept within measurable limits by using sufficiently low buffer concentrations.
Results h number of solutions of constant buffer ratio [NaH2POa]/[NazHPO4]= 1.70 and Na2HP04concentrations between about 0.008 and 0.03 -11 were made up. The solvent water was enriched to 0.33 atom yo I7O. The proton relaxation times T I and T 2of these solutions were measured by the spin-echo technique as described before.?b3 From these quantities the average lifetime between successive transfers of a proton in water, 1, T , is calculated using eq. 3 of ref. 3. The result is plotted in Fig. 1 as a function of the square of the buffer concentration. All measurements relate to a temperature of
