J, Phys. Chem. 1982, 86, 3431-3435
3431
Chain Decomposition of Aqueous Triethanolamine Harold A. Schwarz oeperhnent of Chemlsby, erookheven National Laboratory. Upton, New York 11973 ( R e c e M : February 3, 1982; I n Final F m : March 29, 1982)
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A radiation-induced chain decomposition of aqueous triethanolamine into acetaldehyde and diethanolamine is reported. Chain lengths over lo00 have been observed, depending on pH, concentration, and radiation intensity. The chain propagation steps include OH group migration in the 2-hydroxy-l-(diethanolamino)ethylradical and NR2 migration in l-hydroxy-2-(diethanolamine)ethylradical, each producing a 2-hydroxy-2-(diethanolaminelethyl radical. Free-radical spectra and rate constants are given. Studies of diethanolamine and diethylethanolamine solutions gave similar free-radical spectra but much shorter chains.
Introduction Triethanolamine (TEOA) was used recently as a reactant for hydroxyl radicals in a pulse radiolytic study of tris(2,2’-bipyridine)rhodium(III) solutions.’ The interpretation of the results required some information on the radiation chemistry of TEOA solutions in the absence of Rh(II1) complexes. A remarkable chain decomposition of TEOA was found in the course of the latter studies, and the results are reported here. Experimental Section The hydrochloride salts of triethanolamine, N(C2H40H)3, and diethylethanolamine, (C2H5)2NC2H40H, were prepared by passing dry HC1 through a 10% solution of the amine in 1-propanol. The precipitate was recrystallized from 1-propanol ((C2H6)2NC2H40H-HC1) or 90% ethanol-10% water (TEOA hydrochloride). Solutions of the salts were prepared with “Milli Q” water, and the pH was adjusted with 1M solutions of Apache 99.999% NaOH or with recrystallized Na3P04. The solutions were deaerated by bubbling with argon or N20. Fisher prepurified diethanolamine, HN(C2H40H)2,was used as the free base and pH adjusted with HC1 because the hydrochloride salt cannot be prepared in the usual way. Acetaldehyde was determined by spectroscopic measurement of the 2,4-dinitrophenylhydra~one.~ Diethanolamine was determined by addition of sufficient HC1 to neutralize the solution followed by potentiometric titration with NaOH using a pH meter. Glycolaldehyde was determined as the 2,4dinitrophenylhydrmne by a method insensitive to a~etaldehyde.~ Hydrogen was determined by gas chromatography. The dissolved gases were stripped from a 10-cm3sample after irradiation and analyzed at 50 “C on a 2-m molecular sieve 5A column using argon as carrier gas. The method was calibrated by similar treatment of measured volumes of H,-saturated water. Irradiations were performed in three s°Co sources at intensities of 3.3 X 10l6,0.51 X 10l6,and 0.0063 X 10l6eV g-’ s-l, and a t intensities up to 2 X 1019eV g-ls-l with a 2-Mev electron beam from a Van de Graaff accelerator. Pulse radiolysis studies were also performed with the Van de Graaff electron beam. The light path was either 2 or 6 cm. The analyzing light was produced by a deuterium arc below 350 nm or by quartz iodine or Xe arc at longer wavelength. The D2lamp could be pulsed to increase the intensity a factor of 8 for times shorter than 10 ms. (1) G. M. Brown, S.-F.Chan, C. Creutz, H.A. Schwarz, and N. Sutin, J. Am. Chem. SOC.,101,7638 (1979). (2) W. A. Seddon and A. 0. Allen, J . Phys. Chem., 71, 1914 (1967). (3) M. Ahmad, M. H. Awan, and D. Mohammed, J. Chem. SOC.B, 945 (1968).
0022-3654/82/2086-3431$01.25/0
Results The production of acetaldehyde and diethanolamine by y radiolysis of TEOA solutions is shown in Figure 1. The initial yield, corresponding to the dashed line, is 930 molecules per 100 eV absorbed in the solution. The yield of all radicals available to initiate the reaction is 6.3 per 100 eV, so the decomposition is a chain with a chain length of 148 at this concentration, pH, and intensity. The acetaldehyde was identified by formation of dimethone derivatives from the products and from authentic acetaldehyde. Both samples melted in a 0.2 “C range at 142 “C. The diethanolamine product was identified qualitatively by paper chromatography and also by the pK of the product 8.90 found during the potentiometric titration. The pICs of TEOA and of diethanolamine were measured in separate titrations to be 7.81 and 8.86, corrected to zero ionic strength. For comparison, literature values6 are 7.86 and 8.89. It may be seen from Figure 1 that acetaldehyde and diethanolamine are produced in equal yields within a few percent. The overall reaction is (R C2H40H) NR3
radicale
CH3CHO
+ HNRz
At doses lower than those shown in Figure 1,a small induction period, from 0 to 2 X 10l5eV g-l, is found. This is likely due to impurity O2consumption and corresponds to &lo-’ M O2 remaining after deaeration. Reproducibility of the data is about 35%. If the hydrochloride salt of TEOA was prepared without recrystallization, then yields were perhaps 10% lower, but multiple recrystallization was no better than a single one. Some work was done with the sulfate salt, and results were essentially the same. The intensity and pH dependences of the reaction are shown in Figure 2. The data can be expressed as [NR3J/G(CH3CHO)= a[OH-] + BPI2 where a and b are reasonably independent of pH (f25% for a and f20% for b in the range studied). Thus, the chain is carried by the free base, NR3, and not by the protonated form, NHR3+. The form of the equation requires chain termination reactions both first and second order in radical concentration. The first-order termination rate dominates at the lowest intensity and is proportional to hydroxide ion concentration. This is not the effect of an impurity in the NaOH because the same result is obtained when the pH is adjusted with recrystallized Na3P04. (4) J. Gasparic in “Paper Chromatography”, I. M. Hais and K. Macek,
Eds.,Academic Press, New York, 1963, p 420.
(5) G. Douheret and J. C. Pariaud, J. Chim. Phys. Phys.-Chim.Biol., 59, 1013 (1962).
0 1982 American Chemical Society
Schwarz
The Journal of Wysical Chemlsby, Vol. 86, No. 17, 1982
3432 IO r
I
I
I
I
I
termination 2111 I11 + OH-
-
products
R,NCH(O-)CH, IV
x + IV
I11
2X
0
0
I
I
2
4
I
I
6 8 DOSE , eV/g x ~ 0 - t 7
I
I
10
12
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Figure 1. Production of acetaldehyde (a)in N20-saturated solutions and acetaldehyde ((3)and diethanolamine (0)In arysaturated se lutbns of 0.04 M TEOA at pH 8.24 and at 0.51 X 10 eV g-' s-'. The initial yield (dashed line) is 930 molecules per 100 eV. I
I
I
I
I
I
X
(8)
+ H20
(9)
(10)
products
products
In this mechanism ea;, OH, and H all quantitatively initiate the chain and reaction 7 is the sole source of the observed products. It is assumed that radicals I11 and X recombine randomly so that total radical concentration increases with the square root of the intensity. Actually, radical I11 is usually the dominant radical, so this assumption is not critical. The participation of I11 as a chain carrier introduces the PI2 yield term, while the inertness of X towmd TEOA leads to an intensity-independent term. The natures of X and the termination products are not known. The kinetic equation predicted by this mechanism, assuming appropriate steady states is
6 10
0
x 5
where G R is 6.3 radicals per 100 eV and is the total radical yield, Le., the sum of e?-, OH, and H. G R I is the rate of production of radicals in units of M s-'. Equation 11 is 0 - 3 of the same form as observed in Figure 2. The collection 9 of constants k,d(,/k, is 65 and (2ka)1/2/k6is 0.44 M1/zs'/~. 10 Initiation Reactions. Water radiolysis produces eaq-, 5 2 OH, and H in the ratios 0.44:0.46:0.10. Nitrous oxide may I be used in reaction 2 to exchange ea; for OH in a nearly diffusion-limited process. It is shown in Figure 1that the yield of the chain, and hence the yield of radicals capable of initiating the chain, is the same in Ar-saturated solutions as in N20 solutions; hence, the chain is initiated equally well by OH and e., Other work at lower doses, not shown Flgure 2. Intensity and pH dependence of the chain de"position of TEOA in Npsaturated solution: (e)pH 8.3, total TEOA concenin Figure 1, supports this conclusion and gives the ratio tration of 0.04 M; same, but pH adjusted with MaPo,; (0)pH of yields in NzO-saturated solution to Ar-saturated solution 8.3, TEOA concentration of 0.08 M; (0) pH 8.0; m pH 7.9; (A)pH as 1.05 f 0.05. 6.9. Initiation by eaq-is through the sequence of reactions 2 and 313. The rate of reaction 2 was measured in pulse The reaction will be discussed in terms of the following radiolysis experiments by following the disappearance of mechanism: e, - absorption at 460 nm. The value found for k2, 1.6 X initiation lde M-' s-l at 25 OC, is reasonable in comparison with other weak acids.6 The rate constant for reaction 3H was not HzO eaq-, OH, H, (and Hz, H202, H+) measured but may be predicted with reasonable confidence to be about 1.5-3 times the rate constant for reaction of H20 eaq-+ NzO N2 + OH H with ethylene glycol,' depending on the number of ab(1) stractable H atoms, or 4 X lo7 M-' s-l. eaq- HNR3+ H + NR3 (2) The fact that hydrogen atoms react with NR3 by hydrogen abstraction (reaction 3H) was demonstrated by (OH, H) + NR3 measuring the H2 produced. There are three sources of + RzNCHzCHOH + RzNCHCHzOH + (H20,Hz) Hz in the radiolysis of argon-saturated solution: from eaqI I1 through reactions 2 and 3, from H via reaction 3, and from ( 3 ~ ~ 3 0 ~ )molecular hydrogen from water radiolysis. The s u m of the last two, GH2 + GH,is about 1.0 and is the expected yield propagation in NzO-saturated solution. The observed yield in 0.02M I R,NCH(OH)CH, (4) N20, 0.04 M TEOA at pH 8 was 0.89, somewhat less than I11 1.0 because of small effects of N20 on the radical yields. I1 111 (5) 0
I 0 4 10 I
u
--
-
-
+
-
-
111 + NR3
-
I1 + CH&H(OH)NR2
CH,CH(OH)NR2
-
CHSCHO + HNRz
(6) (7)
(6) J. Jortner, M. Ottolenghi, J. &bani, and G. Stein, J.Chem. Phys., 37, 2488 (1962). (7) P. Neta, R. W. Fessenden, and R. H. Schuler, J.Phys. Chem., 75,
1664 (1971).
The Journal of Physical Chemistry. Vol. 86, No. 17, 1982 3433
Chain Decomposition of Aqueous Triethanolamine T A B L E I: Hydrogen Yields from Argon-Saturated 0.04 M TEOA Solutions
DH
eV
obsd
calcda
6.0 6.0 6.0 8.0
1.97 9.9 39.5 0.99 1.97
3.62 3.45 3.02 2.79 2.21
3.60 3.44 3.00 2.80 2.42
8.0
I
i -I
J
Equation 13, G H t~ HH = 1.0, Geaq-= 2.65.
In Ar-saturated solutions the H2 yields were strongly dose dependent, particularly at pH 8 and above. This effect is due to competition of the product acetaldehyde for ea; e-a,
+ CH3CH0
Hz0
CH,CHOH
(12)
Reactions 2, 3, and 12 lead to
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I
G(H2)
10-17( dose),
a
I
kdHNR3'1 At low dose, acetaldehyde growth is linear with time, so integration gives
X
.... O
200
000
.*....*.s*
e..:, 0000
300
400
500
WAVELENGTH, nm
Flgm 3. Pulse radiolysis spectrum obtained from N,O-saturated TEOA solutions at pH 8.0. The TEOA concentration was 2 X lo4 M below and M above 300 nm. Spectrum (0) obtained by extrapolation to middle of pulse, which decays into spectrum (0)with 2.3 X IO5 s-' rate constant. Observed spectra were difference spectra which were converted to those shown by adding on the extinction coefficients for TEOA at pH 8 (negligible above 240 nm, 400 at 215 nm).
while there was no observable reaction of SCN with HNR3+ (k < 3 x 108). The absorption spectrum of the products of reaction 3oH, where D is the dose and a = k12G(CH3CHO)/(k2[H~+]). i.e., the first spectrum seen in the pulse radiolysis of The rate constant k12has been measureda to be 3.5 X lo9 N20-saturated TEOA solutions, is shown in Figure 3. A M-' s-l, k2 is 1.6 X los M-l s-l as shown earlier, Gvery similar spectrum (but not identical) is found in Ar(CH,CHO) can be estimated at the appropriate pH from saturated solutions. However, both spectra decay into the Figure 2, and [HNR3+]can be calculated from the pH. spectrum of 111,also given in Figure 3, with rate constants Consequently G(H2)can be independently predicted. The within 2% of each other. The difference between the N20 predicted and observed H2 yields are given in Table I. and Ar spectra of reaction 3 products is that the former The agreement confirms reactions 2 and 3Has initiation is 7% smaller than the latter in the 275-285-nm region and steps in the absence of N20. As a further point of interest, is 15% larger at 410 nm. These differences are readily at the higher doses used in Figure 1 sufficient CH3CH0 explained as differing proportions of I and I1 produced in was produced that 85% of ea; reacted with it. Again, since the two cases, with I1 rapidly changing to I11 by reaction the yield is the same in N20 and Ar solution, the ethanol 5 (k > lo6 s-l). Hydrogen abstraction by H is slower than radicals produced in reaction 12 must also initiate the by OH, so H atoms would be expected to be more selective. chain. If it is assumed that H atoms produce only I, then the The rate constant for reaction 30Hwas measured by difference in the spectra can be accounted for by OH competing TEOA with SCN- for OH radicals. forming 82% I and 18% 11. The other extreme can be found by assuming that all of the absorption at 430 nm, OH + SCN- SCN (14) for example, attributed in Figure 3 to I is really due to SCN + SCN- (SCN)p (15) contamination of the spectrum by III. This method gives 23% of I11 (and hence of I1 as a precursor) produced by The (SCN)2- radical absorbs strongly at 480 nm. The OH reaction. The two cases are covered by a yield of 20 competition with TEOA gives f 5 % for the production of I1 in reaction 30w C h i n Termination. The principal termination reaction ~ T E O A / ~=~ ([SCN-I/[TEOAI)L~O/A Q - 11 at higher intensities is second order in radical concentrawhere kTEoA is total reaction with TEOA, [TEOA] is the tion (reaction 8). The spectrum of radical I11 disappears sum of NR, and HNR3+,and A. and A are the absorbance by second-order kinetics below pH 8 when followed at 230 several microseconds after the pulse in the absence and or 410 nm, as shown in Figure 4. The rate constant is 2.5 the presence of TEOA. There was a linear increase in X lo8 M-l s-l. The addition of 0.02 M NaC104resulted in apparent kmoA/k14 thus calculated with [TEOA]/ [SCN-I, an apparent increase of about 6% in k,. If the recombining most likely due to the subsequent reaction of SCN radical radical were charged, then ka would vary as exp[2.4 ~ l / ~ / ( l with TEOA. If k14 and kI5are takens as 1.1 X 1O'O and 6.6 + P ' / ~ ) ]where p is the ionic strength. A 30% increase in X lo9 M-l s-l, then k,, was found to be 8 X los M-' s-l ka between 5 X and 0.02 M ionic strength would be and the rate constant for reaction with HNR3+was 2 X expected. Thus, the radical is neutral. lo9 M-' s-l. The rate constant for SCN oxidation of NR3 The decay kinetics of the radical become more comcould be calculated from the variation of apparent plicated at pH greater than 8.5. A pseudo-first-order kTEOA/k14 with [TEOA]/[SCN] and was 1 X lo9 M-l s-l change with a rate constant of approximately 10a[OH-] is mixed with the second-order decay. The system is ap(8) S. Gordon, E. J. Hart, M. S. Matheson, J. Ftabani, and J. K. parently approaching an equilibrium with a basic form, Thomas, DUCUSS. Faraday SOC.,36, 193 (1963). as in reaction 9. The slowness of approach to equilibrium (9)L. M. Dorfman and G. E. Adams, Natl. Stand. Ref. Data Ser. made accurate measurement of the spectrum of the basic (U.S., Natl. Bur. Stand.), No.46 (1973).
--
~~~
~
3434
The Journal of Physical Chemlstty, Vol. 86, No. 17, 1982 140r
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