J. Phys. Chem. 1981, 85, 382-388
382
tation effects. Steric inhibition to solvation does explain the fact that ASo of electron transfer to TBCOT is less negative than that to ETCOT. [16]Annulene, BCOT, and NCOT all form fully delocalized anion radicals with delocalization over 16 carbon p orbitals. The enthalpy of electron to the BCOT is the only one of the three that is positive. This is attributed to the -28 kcal/mol that is necessary to flatten out the two ring systems as opposed to the 14 necessary for a single COT moiety.l8 The equilibrium constant for reaction 7
t
n U (18) Anet, F. A. L.; Bock, L. A. J. Am. Chem. SOC.1968, 90,7130.
is too large to be measured, but it is larger than 200. Since the entropy term must be positive, AHo must be less than -4 kcal/mol. This is a reflection of the greater electron affinity of [16]annulene than COT, NCOT, or BCOT. Interestingly, Paquette and co-workerslg found that E (XCOT) - E1,,(COT) measured polarographically in H%PA is negative for methoxycyclooctatetraene, methylcyclooctatetraene, and PCOT. That is, it is more difficult to reduce these substituted COTS than it is for COT. This is in agreement with the fact that we have found Kegto be less than one for TBCOT, ETCOT, and PCOT. The observed amplification effect of solvation upon the electron-transfer enthalpies as opposed to the expected attenuating effect sugests to us that A€ itself may be appreciably effected by solvation. Gas-phase studies on these systems, such as those preformed by Jorden et al.3 should prove invaluable. Acknowledgment. We thank the Nation Science Foundation (Grant No. CDP-8000535) for the purchase of the data aquisition system, which is coupled to our ESR equipment. (19) Paquette, L. A.; Wright, D. C.; Traynor, S. G.; Taggart, D. L.; Ewing, G. D. Tetrahedron 1976, 32, 1885.
Pulse Radiolysis of Aqueous Solutions of Benzyltrialkylammonium Cations. Reactions with the Primary Transients from Water Radiolysis' K. Bobrowsk12 Radiation Laboratory, Universlty of Notre Dame, Notre Dame, Indiana 46556 (Received: Msy 18, 1980; In Final Form: September 19, 1980)
The primary reactions of hydrogen atoms, hydrated electrons, hydroxyl radicals, and oxide radical ions with benzyltrimethylammoniumcations in aqueous solutions have been studied by the technique of pulse radiolysis. Hydrogen atoms react with the rate constant (2.0 f 0.1) X lo9 M-l s-l. The cyclohexadienyl-typeradical decayed in a second-order process with a rate constant 212 = (4.8 f 0.2) X lo8 M-l s-l. Hydrated electrons react with a rate constant (4.3 f 0.2) x lo9 M-l s-l to form benzyl radical. It has been concluded that irradiation of benzyltrimethylammoniumcations results in deamination brought about by hydrated electrons. Hydroxyl radicals react with benzyltrimethylammoniumcation with a rate constant (5.0.f 0.6) X lo9 M-' s-l to form the cyclohexadienyl-type radical HOC6H5CH2N+(CH3)3 (-65%) and C6H5CHN+(CH3)3 (-35%) radicals. The cyclohexadienylradical has a broad absorption maximum at 325 nm (6 = 3300 M-' cm-l). Oxide radical ions react with the rate constant (5.9 f 0.5) X 10' M-' s-l to form C&,C"+(CH3)3 with absorption maxima at 260 and 305 nm. With the increasing chain length of the alkyl substituent (as in the benzyltri-n-butylammonium cation) the reaction of hydrogen abstraction from the alkyl groups becomes more important. Introduction In 1968 Kabi and Clay published a report3 on the effect of y irradiation on deaerated solutions of benzyltrimethylammonium cation. They identified trimethylamine as the main product. Competition experiments involving H,+ N20, and NO3- demonstrated that all three scavengers were effective in reducing the trimethylamine yield. From these studies it was concluded that hydrated electrons were responsible for the cleavage of the benzyl N (1) The research described herein was supported by the Office of Basic Energy Sciences of the Deparment of Energy. This is Document No. NDRL-2135 from the Notre Dame Radiation Laboratory. (2) On leave of absence from the Institute of Nuclear Research, 03-195 Warsaw, Poland. (3) Kabi, A.; Clay, P. G. Radiat. Res. 1968, 34, 680.
bond resulting in formation of trimethylamine. In the case of oxygenated solutions, these authors found that addition of 2-propanol decreased deamination yields. It was assumed that this reduction resulted from the reactions of OH radicals with the methylenic hydrogens. From the G(Me3N)values in the presence and the absence of 2-propanol, it was estimated that approximately 30% of the OH radicals reacted via abstraction of methylenic hydrogens in competition with the addition to the aromatic ring. Aqueous solutions of benzyltrialkylammonium cations have not, as yet, been investigated by pulse radiolysis methods. The purpose of the present work is to use pulse radiolysis to provide direct confirmation of the deamination reaction and to study primary reactions of hydrogen atoms
0022-3654/81/2085-0382$01.00/00 1981 American Chemical Soclety
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The Journal of Physical Chemistry, Vol. 85, No. 4, 198 I
Pulse Radiolysis of Benzyltrialkylammonium Cations
TABLE I: Spectral and Kinetic Characteristics of the Transient Species Formed from Benzyltrimethylammonium Cation rate constant rate constant primary absorpn extinction for formation, for decay, radical species formed max, n m coeff, M - l s-l M-1 s - l M-1 s - l H eaq -
OH
0-
H adduct benzyl radical
322 258 293 304 316 325
OH adduct, PhCHN+(CH,),
Experimental Section The sources of the benzyltrimethylammonium cation were the benzyltrimethylammonium hydroxide obtained from Fluka A.G. and the benzyltrimethylammonium chloride obtained from Knoll A.G. The benzyltri-n-butylammonium chloride was also from Knoll A.G. Alcohols were Baker Analyzed reagents. All other inorganic reagents were Mallinckrodt and Fisher. Water was purified by a Millipore Milli-Q system. Fresh solutions were prepared before each irradiation. They were deoxygenated by bubbling with Nz or NzO. In the latter case the NzO was used in order to remove oxygen and to convert e,; into OH by the reaction NzO + e, N2 OH + OH-. tert-Butyl alcohol and methyl ahohol (0.5 M) were used as OH scavengers. The pHs of the solution were adjusted with NaOH, HC104, NaZHPO4,and NaHZPO4. Irradiations were carried out by using an ARC0 LP-7 linear accelerator supplying 5-11s pulses of 9-MeV electrons. Each pulse produced 2-4 p M of radicals. Dose effects were measured with pulses of up to 50-11s duration. Optical detection and signal averaging were carried out by the computer-controlled pulse radiolysis apparatus described p r e v i ~ u s l y . The ~ ~ ~absorbance measurements were compared to (SCN)z-with a reference value of 47 0006taken for the product of yield and extinction coefficient at 480 nm in NzO-saturated solutions. Optical absorption spectra of the solutions before and after irradiations were recorded on a Cary 219 spectrophotometer. The recorded spectra of the benzyltrialkylammonium cation solutions at different pH indicate that the identity of these cations does not change with pH. The absorption spectra were corrected for the depletion of solute (S) in the wavelength range where the solute absorbs light (A I 300 nm). G(-S) was assumed to be 6.0 (=g(ea -) + g(H) + g(0H)) in NzO-saturated solutions and 3.2 g(e, -) + g(H)) in nitrogen-saturated solutions of benzyltriaiirylammonium cations.
5.9 x
5 , 0 0 0 - 1 1
lo8 1
1.0 x 109 1
1
1
1
1
,
A -
4,000-
+
!=
Results and Discussion Reactions of Hydrogen Atoms with Benzyltrimethylammonium Cations. The reaction of H atoms with the benzyltrimethylammonium cation, PhCHzNC(CH3)3,was studied after removal of most of the OH radicals through reaction with tert-butyl alcohol or methanol at pH 1. The absorption spectra were corrected for the contribution to (4) Patterson, L. K.; Lilie, J. Int. J.Radiat. Phys. Chem. 1974,6,129.
(5) Schuler, R. H.; Neta, P.; Zemel, H.; Fessenden, R. W. J. Am. Chem. SOC.1976,98, 3825. (6) Schuler, R. H.; Patterson, L. K.; Janata, E. J. Phys. Chem., sub-
mitted.
3.3 x 109
12000 2100
and hydroxyl radicals with benzvltrimethvlammonium cation and other benzyltrialkylammoniumcations in order to elucidate the transient spectra and the reaction mechanism.
-
4 . 8 x 109 5 . 8 x 109
1 . 7 x 109
260 305 260 305
PhCHN+(CH,),
2 . 0 x 109 4 . 3 x 109
4300 14400 1750 3400 6300 3300
250
300
350
A-nm
Flgure 1. Absorption spectrum of hydrogen-atom addition to PhCH,N+(CH,), observed in the pulse radiolysis of 1 mM solutions at pH 1 saturated with N, and also containing 0.5 M fert-butyl alcohol. The experimental polnts (0)were corrected for the bleaching of the parent-compound absorption. The dashed line (- - -) represents the corrected spectrum of the initial transient produced in reaction of H atoms with PhCH2N+(CH3),cation. The spectrum was recorded 5 hs after the pulse.
TABLE 11: Percentage o f Transient Formation from OH and 0 Reactions with Benzyltrialkylammonium Cation
benzyltrialkylammonium cation
primary radical
PhCH,N+(CH,),
OH
0PhCH,N+(C,H9),
OH
0-
efficiency of formation, species formed
%
0H.adduct PhCHN+(CH), PhCH,N+( CH,),CH, PhCHN'(CH,), PhCH,N+(CH,),CH, OH. adduct PhCHN+(C,H,), PhCH,N+(C,H9),C,H8 PhCHN+(C,H,), . PhCH,Nt(C,H9),C,H8
65 35 0 100 0 45
4 51 25 75
the absorption of methanol or t-BuOH radical^.^ In both cases, the spectrum observed immediately after the pulse has an absorption maximum around 322 nm (cf. Figure 1 and Tables I and 11). (7) Simic, M.; Neta, P.; Hayon, E. J. Phys. Chem. 1969, 73, 3794.
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The Journal of Physical Chemistry, Vol. 85, No. 4, 1981
I/, 1
, 2
,
3
, 4
, 5
, 6
, 7
,
,
0
9
Bobrowski
I
c[BzMe3Nt] rnM
Flgure 2. Plot of /cob& at X = 322 nm vs. [PhCH,N+(CH,),] in the solutions of benzyltrimethylammonium cation solutions at pH 1, saturated with N,, containing 0.5 M tert-butyl alcohol. Inset: plot of the exponential increase of the 322-nm absorption in the solution containing 9.2 X M PhCH,N+(CH3),.
To observe the formation of the H adduct alone, the OH radicals have to be scavenged. tert-Butyl alcohol ( 5 X 10-1M) was used as a scavenger. The rate of formation of the transient followed a first-order law. From the half-life of the exponential increase of the 322-nm absorption after the pulse and the concentration of PhCH2N+(CHJ3 (in the range 10-4-10-3 M), a rate constant hl = (2.0 f 0.1) X lo9 M-ls-l was calculated for reaction la-c (Figure 2). The fraction of H atoms which reacted with the benzyltrimethylammoniumcation is more than 98% in the case of tert-butyl alcohol and -45% in the case of methanol. The fraction of OH radicals scavenged by alcohols is more than 98%. The molar extinction coefficient t of this transient at the wavelength for maximum absorption, 322 nm, taking G(T) = 0.98g(ea; + H) in tert-butyl alcohol solution and G(T) = 0.45g(ea,H) in the methanol solution, is €322 = 4300 f 200 M-l cm-I (Figure 2). Hydrogen atom may be assumed to react with the benzyltrimethylammonium cation by addition to the aromatic nucleus (reaction la), by the cleavage8of C-N
+
I CHZ
+I
-cH3
CH3
.CH H,C-+N-C
I
I CH3
Hj
bond and formation of the benzyl radical (reaction lb), or by abstraction of methylenic hydrogensgfrom the benzyl (8) Christensen, H. C.; Sehested, K.; Hart, E. J. J. Phys. Chern. 1973, 77,983.
250
300
350
X-nm
Flgure 3. Absorption spectrum observed in the pulse radiolysis of 1 mM PhCH,N+(CH,), solutions at pH 11 saturated with NP,containing 0.5 M CH30H. The experimental points (0)were corrected for the bleaching of the parent-compound absorption. The dashed line (- - -) represents the initial corrected spectrum. The spectra were recorded 2 ps after the pulse.
group (reaction IC). The absorption maximum of the transient is found, where it would be expected for the addition radical, according to the calculation of the bathochromic shifts given by Chutny.lo The magnitudell of its formation rate also indicates that the H atom adds to the benzyltrimethylammonium cation rather than abstracts an H atom from it. The observed transient is therefore the cyclohexadienyl-type radical (reaction la). The H adduct was found to decay according to secondorder kinetics; changing the dose by a factor of 4 altered the half-life of Ph(H)CH2Nf(CHJ3by the same factor. The bimolecular rate constant, 2k, was found to be (4.8 0.2) x 109 M-' s-1. Reaction of Hydrated Electrons with Benzyltrimethylammonium Cations. Pulse radiolysis of nitrogensaturated aqueous solutions of benzyltrimethylammonium cation at pH 11yielded a transient species and products. Figure 3 shows the spectrum observed immediately after the end of the electron pulse in benzyltrimethylamrnonium cation solution. On saturation of this solution with nitrous oxide, which converts ea; into OH radicals, the transient absorption disappeared. Thus, the transient absorption is caused by the reaction of the benzyltrimethylamrnonium cation with eaq-. The transient spectrum corrected for the absorption of CHzOH radicals has four maxima at 258,293,304, and 316 nm with the estimated extinction coefficients of 14 400, 1750, 3400, and 6300 M-' cm-l, respectively, taking g(ea;) = 2.8. This spectrum is identical, within experimental error, with that of benzyl radicals8 Our values for the extinction coefficients and the ratio of the intensities of
*
(9) Christensen, H. C. Int. J. Radiat. Phys. Chem. 1972, 4, 311. (10) Chutny, B.Nature (London) 1967,213, 593. (11) Anbar, M.;Farhataziz; Ross, A. B. Nutl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.) 1975, No. 51.
The Journal of Physical Chemistty, Vol. 85, No. 4, 1981 385
Pulse Radiolysis of Benzyltrialkylammonium Cations
I
I
A-260nm
A
Time
Figure 4. Normalized plots of relative absorbance change in the time monltored (0)near the maximum of the absorption band of the C8H5CH2-radical (260 nrn) and (A) at 600 nrn of the absorption band of the hydrated electron.
the bands at 258 and 316 nm (2.3) are very close to the values of Christensen et a1.8 The transient absorption at 258 and 316 nm disappears in a bimolecular reaction with 2k = (5.8 f 0.9) X lo9 M-l. These results suggest that the transient species produced in the reaction of PhCH2Nt(CH,), cation with hydrated electron is the benzyl radical. The following reactions are presumed to take place in the system:
H 2 0 k- H, ea;, OH, HzOz,Hz, H30+,OH- (2)
eaq-
--
OH
+ CH30H
H
+ CH3OH
+
PhCHzN+(CH3)3
*CHBOH+ H2O
+ Hz
eCH20H
\
-
(3) (4)
PhCH*i(CH3)3
I
PhCH2
(5)
+
N(CH3)3
PhCHz + PhCHz products (6) The reaction of hydrated electron with benzyltrimethylammonium cation may proceed. through the benzyltrimethylammonium radical PhCH2N(CH3),which can decompose later into benzyl radical and tertiary amine; alternatively, the electron may be transferred directly to the benzyl group causing release of the benzyl radical. The tetraphenylphosphonium radical (Ph4P.)was observed in the microsecond pulse radiolysis of the tetraphenylphosphonium cation.12 In order to check these two possibilities, we observed the decay of the hydrated electron simultaneously with the formation of the benzyl radical. As one can see from Figure 4, the two dependences of normalized plots of relative absorbance as monitored at 600 and 258 nm gave symmetrical curves with the same rate constant k = (4.3 f 0.2) X lo9 M-l s-l. We have also checked the spectrum in the nanosecond time scale to find out whether PhCH,N(CH,), radical is sufficiently stable to be observed. The present experiment shows no indication that such an intermediate radical is formed. The spectrum observed was characteristic of the benzyl radical. Although it cannot be ruled out that the benzyltrimethylammoniumradical, PhCH2N(CH3),,might not be sufficiently long-lived to be observed in the nanosecond pulse radiolysis, it seems reasonable to assume that eaq-attacks the benzyl group and causes the cleavage of the C-N bond before the electron is delocalized to form the ammonium radical. (12) Horii, H.; Fujita, S.; Mori, T.; Taniguchi, S. Int. J.Radiat. Phys.
Chem. 1976,8,521.
Recently, formation of benzyl radicals from quaternary compounds was reported', in pulse radiolysis of benzyltriphenylphosphonium chloride in aqueous solution. In both cases the reaction mechanism might be similar to that of the electroreduction of quaternary ammonium compounds."16 The electrolysis of aqueous solutions of these compounds resulted in the cleavage of one of the groups attached to nitrogen. The cleavage appears to be dependent on the nature of the group. Benzyl, cinnamyl, and allyl groups, by virtue of their resonance possibilities, can exist as more stable radicals than alkyl or phenyl groups. This might be a reason that we cannot observe phenyl radicals in the radiolysis of tetraphenylphosphonium cation,12contrary to the case of benzyltrimethylammonium cation and benzyltriphenylphosphonium cation,13where the benzyl radicals are observed. Reactions of OH Radicals with Benzyltrialkylammonium Cations. Benzyltrimethylammonium Cation Solutions. The transient spectrum obtained after pulse radiolysis of 2 X M PhCH2Nt(CH3)3(pH 6.9, N20 saturated) solution is composed of two peaks, one at 310 nm and a second at 325 nm. Under these conditions, the spectrum represents the products of the reaction of OH radicals with the solute with a contribution of -9% from the H-adduct radicals. Aromatic H adducts usually have an absorption similar to that of OH adducts, and no features specific to the H-adduct spectrum are to be expected. The 325-nm absorption is assigned to the OH adduct of benzyltrimethylammonium cation because the reaction product with hydrogen atoms has its maximum near this wavelength (see Figure 1) and a displacement of -10 nm from the cyclohexadienyl radical17itself would be expected. Because the experimental points of the transient spectra represent the difference between the absorption of the product radical and that of the parent compound, we have corrected those points for bleaching of the parent-compound absorption. These "true" spectra of the radicals are shown by the dashed lines. The accuracy of the corrected spectra is less than that of the experimental spectra, since it is limited by the precision of the extinction coefficients and the yields of the radiolytic reactions. After that correction and the further correction for the H adduct, the spectrum is composed of three peaks at 260,310, and 325 nm with extinction coefficients of €260 = 4400, €310 2400, and €326 = 2500 M-' cm-l, respectively (see Figure 5). The extinction coefficient of 2500 M-' cm-l (based on the G O H = 6.0) for the cyclohexadienyl-type radical does not indicate a full yield of radicals, since the typical emax for such radicals is 3000-4000 M-' cm-l.l8 Therefore, it can be concluded (in agreement with the observation of Clay3 et al.) that addition of OH radicals to the aromatic ring competes with abstraction of methylenic hydrogens. The reaction of the OH radicals with benzyltrimethylammonium cation can then be described by reaction 7a-c. OH
+ PhCH2Nt(CH3)35 s P ~ ( O H ) C H ~ N + ( C H , ) ~ (74
5 PhCHN+(CH3), 5 PhCHzN+(CH2)(CH3),
(7b) (74
(13) Horii, H.; Fujita, S.; Mori, T.;Taniguchi, S.Bull. Chem. SOC. Jpn. 1979,52,3099.
(14) Finkelstein, M.; Petersen, R. C.; Ross, S. D.J. Am. Chem. SOC. 1959,8I,2361. (15) Ross, S. D.; Finkelstein, M.; Petersen, R. C. J. Am. Chem. SOC. 1960.82. 1582. - - ,--, ~ - - (16) Mayell, J. S.;Bard, A. J . J. Am. Chem. SOC.1963,85,421. (17) Cercek, B. J. Phys. Chem. 1967,71, 2354. (18)Neta, P. Adu. Phys. Org. Chem. 1976,12, 223.
386
The Journal of Physical Chemistry, Vol. 85, No. 4, 1981
t
E I oO*
-
3000 -
3
+ W
2000
-
1000-.
2 50
300
350
A-nm
Figure 5. Absorption spectrum observed in the pulse radiolysis of 2 X lo4 M PhCH2N+(CH3)3solutions at pH 6.9 saturated with N20. The experimental points (0)were corrected for the bleaching of the parent compound. The dashed line (- - -) represents the corrected spectrum of the initial transients produced in the reaction of OH radicals with PhCH2N+(CHJ3 cation. The spectrum was recorded 5 ps after the pulse.
Reaction 7c can be neglected in accordance with the observation of tetramethylammonium cation solutions where the value -1.6 X lo6 M-l s-l for the direct abstraction of an H atom by OH per methyl group was found.lg The direct abstraction of methylenic hydrogens amounts to -35% of the OH yield calculated on the basis of the 260-nm absorption (-4400 M-' cm-l) and the measured extinction coefficient in alkaline solution (e2@ = 12000 M-' cm-l). This is in excellent agreement with the -30% direct abstraction of an H atom from the benzyl group measured by Clay3 et al. If we assume that the absorption of 2500 units corresponds to the 65% of the extinction coefficient of the OH adduct and to the 35% of the extinction coefficient of radical which is formed by abstraction of methylenic hydrogens (see next paragraph), the corrected extinction coefficient €325 N 3300 M-l cm-l for the OH adduct of benzyltrimethylammonium cation can be calculated from the simple equation -2500 = (0.35)(1000) + (0.65)(€325OH) where lo00 is taken from Figure 6 as €325 of PhCHN+(CH3)3 radical. Since reaction 7a does not predominate over reaction 7b, the observed rate of buildup at 325 nm does not depend upon the rate constant of eq 7a alone.20 From a consideration of the differential equations -d[OH]/dt = d[*Ph(OH)CH2N+(CH3)3] /dt d[PhCHN+(CHJ,] /dt
+
(8)
d[.Ph(OH)CH2N+(CH3)3]/dt = k,a[OHl [PhCHZN+(CH3)31(9) d[PhCHN+(CH,),]/dt = k7b[OH][PhCH2N+(CH& (10) (19) Bobrowski, K., submitted for publication. (20) Greenstock, C. L.; Dunlop, I. J. Am. Chem. SOC. 1973, 95, 6917.
,
250
,
I
,
,
,
300
350
A-nm Figure 6. Absorption spectrum observed in the pulse radiolysis of 5 X lo4 M PhCH,N+(CH,), solutions at pH 14 saturated with N,O. The experimental points (0)were corrected for the bleaching of the parent compound. The dashed line (---) represents the corrected spectrum of the initial transient produced in the reaction of 0-radlcal Ions with PhCH2N+(CHJ3 cation. The spectrum was recorded 10 ps after the pulse.
describing the rate of the decay of OH radicals and the rates of formation of .Ph(OH)CH2N+(CH3)3and PhCHN+(CH3)3,respectively, one can obtain (see Appendix) d[.Ph(OH)CH2N+(CH3)3]/dt = -(k7a + k7b) X [.Ph(OH)CHzN+(CHJ3][PhCHzN+(CH3)3]0+ ~~~[~HIo[P~CHZN+(CH~)~IO (11) d[PhCHN+(CH,),]/dt = -(k7a + k7b) X [PhCHN+(CH3)3][PhCH2N+(CH3)3lo+
~~~[OHI~[P~CH~N'(CH~)~IO (12) where [PhCH2N+(CH3)3]o and [OH], are the concentrations at t = 0. After solving eq 11and 12 one can see that
Equations 13 and 14 show that the "observed" rate of buildup depends upon the sum of the rate constants k7a + k7b, and the value of the relative absorbance at the infinite time is equal to {k7,*~[.Ph(0H)CHzN+(CH3)3] + k,b*e[.PhCHN+(CH3)3])/(k7a+ k7b). The rate constant for OH addition to PhCH2N+(CH3)3is obtained experimentally by multiplying the observed rate constant k7a + k7b by the addition efficiency. The rate constant for methylenic hydrogen abstraction is then the difference between the experimentally measured rate constant and the true calculated addition rate constant. With the observed rate
The Journal of Physical Chemistty, Vol. 85, No. 4, 1981 387
Pulse Radiolysis of Benzyltrialkylammonium Cations
constant for OH reaction 5.0 f 0.6) X lo9 M - ' d and the addition efficiency 65%, the rate constant for OH addition and methylenic hydrogen abstraction are 3.3 X lo9 M-'sd and 1.7 X lo9 M-l s-l, respectively. Benzyltri-n-butylammonium Cation Solutions. It is reasonable to expect that increasing the chain length of the alkyl substituent should increase the possibility of the hydrogen abstraction from the side alkyl groups. Therefore, similar experiments were performed with N20-saturated solution at pH 6.9 with benzyltri-n-butylammonium cation. It is interesting to point out that the reaction of OH radicals with benzyltri-n-butylammonium cation produces a similar transient absorption with the maximum at X = 325 nm, as in the case of benzyltrimethylammonium cation, indicating that OH radicals also add to the aromatic ring. However, the relative absorbance at X = 325 nm in the solution of PhCH2N+(C4H9)3 is -0.7 of the relative absorbance in PhCH2N+(CHJ3solution. This can be due to a significant competition between the addition reaction and the abstraction of the hydrogen atoms from alkyl groups according to reactions 15a-c. If we assume that OH
+ PhCHZN+(C4Hg)3 kl,
__+
.Ph(OH)CH2N+(C4Hg)3
(W
.kPhCHN+(C4H9)3+ H2O
kl5C
+
PhCHzN+(*C4Hs)(C4Hg)2 HzO
05b) (15~)
the abstraction of methylenic hydrogens from PhCH2N+(C4H9)3 by OH radicals is 2 times faster21,22 than that by 0- and k15b = 2.4 X lo8 M-l s-l (see next paragraph), then, from the observed rate constant of the absorbance increase at X = 325 nm (k15 = 6.4 X lo9 M-'s-l ), one. can calculate the percentage of the formation of PhCHN+(C4H9)3 as -4%. After a correction similar to the case of PhCH2N+(CH3)3cation, a relative absorption of 1500 units was found for the hydroxycyclohexadienyltype radical. This value is -45% of the extinction coefficient of the OH adduct to PhCH2N+(CHJ3. It is reasonable to assume that the values of the extinction coefficients for the hydroxycyclohexadienyl radicals of PhCH2N+(CHJ3and PhCH2N+(C4H9)3 are very similar; thus the last value represents fairly accurately 45% of the full yield, considering the known intensity of the absorption of OH adducts. From the addition efficiency and k observed, we have calculated the rate of OH addition to PhCH2N+(C4H9)3 as k15a = 2.9 X lo9 M-ls-l. There is still 51% of the OH radicals not yet accounted for. We have attributed this portion to the radicals formed by abstraction of hydrogen atoms from the butyl groups. Using the same scheme of calculation, we have estimated the rate constant for reaction 15c as klk = 3.3 X lo9 M-ls-l, which is in good accordance with the value obtained from tetra-n-butylammonium cation solution^.'^ The complete scheme is given in reactions 15a-c. In this case, the addition reaction competes with the abstraction of hydrogen atoms from the side alkyl groups. Reactions of 0- Radical Ions with Benzyltrialkylammonium Cations. Benzyltrimethylammonium Cation Solutions. The transient spectrum obtained on pulse radiolysis of 1 M NaOH, 5 X M benzyltrimethylammonium cation, and NzO-saturated solution is shown in Figure 6. The only primary radical reacting with PhCH2N+(CH3)3in this solution is 0-, which is known (21) Neta, P.; Schuler, R. H. J. Phys. Chem. 1975, 79, 1. (22) Farhataziz; Ross, A. B. Natl. Stand. Ref. Data Ser. (U.S., Nutl. Bur. Stand.) 1977 No. 59. (23) Sehested, K.; Holcman, J. Nukleonika 1979, 24, 941.
preferentially to abstract hydrogen from the methyl substituents rather than add to the aromatic ring.24i25 An alternative reaction of 0- with PhCH2N+(CHJ3would be an electron-transfer reaction as suggested for aniline by Neta and SchuleraZ6In this case a radical cation would be produced probably followed by a reaction of the latter with OH- yielding the OH adduct. As the observed spectrum is quite different from the spectrum which is assigned to the OH adduct, we feel confident that this kind of reaction can be excluded in the case of PhCH2N+(CH& Therefore we assigned the spectrum with the maxima at 260 and 305 nm to the product of methylenic hydrogen abstraction. 0- + PhCH2N+(CHJ3 PhCHN+(CH3)3+ OH- (16) We have found that 0- reacts readily with the methyl groups of Me4N+cations, but the presence of the aromatic ring may influence the reactivity of the hydrogen atoms. It was observed in the case of benzyltri-n-butylammonium cation that the calculated rate constant for the abstraction of hydrogen atoms was 1 order of magnitude lower than for tetra-n-butylammoniumcation.lg Therefore we have determined the extinction coefficients at 260 and 305 nm as 12000 and 2100 M-' cm-l, respectively, assuming that all 0- radicals react with PhCH2N+(CH3)3by abstraction of methylenic hydrogens. The rate constant for reaction 16 was determined from the period of the optical change observed at 260 and 305 nm. This formation follows a first-order law and has a rate constant k16 = (5.9 + 0.5) X lo8 M-' s-l, determined a t both 260 and 305 nm. The radicals formed decay in a second-order reaction with 2k = (1.0 f 0.2) X lo9M-l s-l, assuming the decay to be caused by the reaction of two PhCHN+(CH3I3radicals produced in reaction 16. If we take the k16 rate constant and assume that the rate of methylenic hydrogen abstraction by OH radicals is 2 times higher,21322 we obtain a good agreement with the kTb evaluated from our and Clay's3 data in neutral solutions. Benzyltri-n-butylammonium Cation Solutions. It is also reasonable to expect (as in the case of the OH radical reactions) that the increasing of the chain length of the substituent will increase the probability of reaction 17b.
-
N
0- + PhCHzN+(C4H9)3 2PhCHN+(C4H9)3+ OH-
-
(174
kllb
PhCH2Nf(C4Hs)(C4Hg), + OH- (17b) Therefore we have performed experiments with N20-saturated solutions a t pH 14 with benzyltri-n-butylammonium cation. The obtained spectrum is also composed of two peaks, one at X = 260 nm and a second one a t X = 305 nm. However, the calculated extinction coefficients at both wavelengths based on the yield of 0radical ions are approximately 4 times less than in the case of PhCH2N+(CHJ3. Because the aliphatic radicals in which the unpaired electron is separated from the aromatic ring absorb below -250 nm, the absorption at X = 305 nm is caused only by the benzylic radical formed in reaction 17a. On the basis of Sehested et a l . ' data, ~ ~ ~it .is reasonable to assume that extinction coefficients of PhCHN+(CH3)3 and PhCHN+(C,H9)3are equal within f15%. Therefore, we have postulated that there is a significant competition between the abstraction of methylenic hydrogen atoms and hydrogen atoms in alkyl groups and that only -25% of 0- radical ions react by abstraction of methylenic hydro(24) Sehested, K.; Corfitzen, H.; Christensen, H. C.; Hart, E. J. J. Phys. Chem. 1975, 79, 310. (25) Holcman, J.; Sehested, K. J. Phys. Chem. 1977, 81, 1963. (26) Neta, P.; Schuler, R. H. Radiat. Res. 1975, 64, 233.
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The Journal of Physical Chemistry, Vol. 85,No. 4, 1987
gens. The rate constant for reaction 17a was obtained in the same way by multiplying the observed rate constant k17a kI7b (4.6 X lo8 M-ls-') obtained from the kinetics of PhCHN+(C4H9)3 radical buildup at X = 305 nm by the efficiency of its formation. The rate constants for the abstraction of methylenic hydrogens and the hydrogen atoms from butyl groups are equal to 1.2 X lo8 and 3.4 X lo8 M-' s-', respectively.
Bobrowski
+
Conclusions A previous paper3 on y radiolysis of benzyltrimethylammonium cation concluded that the hydrated electrons were responsible for the cleavage of the benzyl N bond resulting in the formation of trimethylamine. The present investigation supports this view in that the benzyl radicals observed on the microsecond time scale can easily result from reaction of ew- with PhCH2N+(CH3)3With hydrogen atom the reaction of PhCH2N+(CH3)3 cation resulted in the formation of a cyclohexadienyl-type radical with the absorption maximum in agreement with the correlation of bathochromic shifts given by Chutny.lo The reactions of OH and 0- radicals with these cations represent more complicated cases. With PhCH2N+(CH3)3 the reaction of OH takes two paths, with formation of the OH adduct form being more important. With the increasing of the chain length of the alkyl substituent, the reaction of hydrogen abstraction from the alkyl groups becomes more and more crucial. However, in the case of 0- the competition between abstraction of methylenic hydrogens and hydrogens from the alkyl groups becomes more noticeable with the increasing of the length of the alkyl substituent. In this case the efficiency of methylenic hydrogen abstraction varies from 100% for PhCH2N+(CH3)3 down to 25% for PhCH2N+(C4Hg)3.
Acknowledgment. I am grateful for helpful discussions with Dr. P. Neta, Dr. N. V. Raghavan, and Dr. D. Behar. Appendix From a consideration of the differential equations -d[OH]/dt = d[.Ph(OH)CHzN+(CH3)3]/dt + [PhCHN+(CHJ,]/dt (A)
d[*Ph(OH)CH2N+(CH3)3] /dt = k7a[OH][PhCH2N+(CHJ31 (B) d[PhCHN+(CHJ,]/dt = kyb[OH][PhCH2N+(CH3)3] (C) one can obtain
Substituting eq E into eq D, one can obtain -d[OH] -- k7a + k7b d[*Ph(OH)CHzN+(CH3)3](F) dt k7a dt Upon integration from time t = 0 to t' = t -[OH],
+ [OH],
k7a
-k k7b
-
X
If one substitutes eq H into eq B, and because PhCH2N+(CH3)3is an excess [PhCH,N+(CH,),] [PhCH2N+(CH3)3]0 one can obtain -d[.Ph(OH)CH2N+(CH3),1 dt ~ ~ ~ [ O H I O [ P ~ C H Z N +- ( C H ~ ) ~ I ~ (k7a + k7b)[.Ph(OH)CH2N+(CH3)31 [P~CH~N+(CHJ~IO (1) Then if we put [*Ph(OH)CH2N+(CH3)3] =x
k7,[0H],[.PhCHZN+(CH3)~], =b -(k7a
+ ~ ~ ~ ) [ . P ~ C H ~ N + ( C=Ha, ) ~ ] O
we obtain the following type of differential equation: dx/dt = ax b
+
After solution x = -(b/a)[l - eat]. Returning to previous symbols, one can obtain [-Ph(OH)CH,N+(CHJJ =
