22
The Journal of Physical Chemistry, Vol. 82, No. 7, 1978
R. F. Anderson and D. Schulte-Frohlinde
@actionsInduced by Hydroxyl Radical Attack on Acetylene in Aqueous Solution. A Pulse Radiolysis Study Robert F. Andersont and Dietrich Schulle-Frohllnde" Institut fur Strahlenchemie im Max-Planck-Institut fur Kohlenforschung, 0-4330 Mulheim a.d. Ruhr, West Germany (Received July8, 1977) Publication costs assisted by the Institut fur Strahienchemie im Max-Pianck-Institut fur Kohlenforschung
The rate constants for the reaction of hydrated electron (2.0 f 0.2 X lo7 M-' s-l1, H atom (2.3 X lo9 M-' 1, and .OH radical (4.5 f 0.3 X lo9 M-' s-l) with acetylene have been measured in aqueous solution. The addition product (A) of .OH radical with acetylene is converted to the formylmethyl radical by hydroxyl ions (h = 1.5 f 0.3 X 1O1O M-l s-'). Therefore A is assumed to be the enol form of the formylmethyl radical. A disappears bimolecularly (2k7 = 6.4 X lo9 M-'s-l ) at pH values less than 7 and a resulting intermediate has a lifetime of several minutes. Furthermore A adds to acetylene (h, = 8.8 f 0.9 X lo6 M-l s-l) to form species B which also reacts with acetylene to give C ( k 3 = 5.0 f 0.5 X lo6 M-' s-l ). C does not react further with acetylene, but decays away bimolecularly (2h4 =: 2.5 X lo9 M-' 9-l) to form product(s). The formylmethyl radical is at least 10, times less reactive toward acetylene than radical A. The consumption of acetylene is approximately three molecules per .OH radical.
Introduction Although several investigations on the reactions of hydroxyl radicals with acetylene in the gas phase have been undertaken, reactions in aqueous solution have received little attention: a steady-state 6oCoy-radiolytic study,' some pulse radiolysis work on the Fe2+-acetylenesystem,2 an investigation using Fenton's reagent,3 and a related experiment on acetylenedicarboxylic acid using ESRS4 These studies show that the hydroxyl radical adds rapidly to the acetylenic bond, The ESR experiment revealed that only the keto form of the .0H-C2(C02-)2 adduct could be seen. The present work was undertaken to study the keto-enol rearrangement and other reactions of the P-hydroxyvinyl radical in the presence of acetylene. Also of interest is the study of postulated chain reactions1 leading to polymerization. Experimental Section For pulse radiolysis experiments a Van de Graaff generator (3.0 MeV, maximum beam current 0.2 A, 0.42.0-ps pulse length) was used. Details of the associated optical measurement circuit have been publi~hed.~ Optical path lengths of 2 or 6 cm were used and all spectra are shown normalized to a path length of 1 cm. Single doses of between 100 rd and 6.0 krd were delivered to fresh Solutions. Steady-state radiolysis experiments were carried out using a 10 kCi 6oCoy source (dose rates 2.54 X 1017and 1.04 X eV 8-l min-l). Dosimetry was determined using an oxygen saturated M KCNS solution or a modified Fricke dosimeter instead of M FeS04, without NaCl, bubbled with oxygen instead of air) with G(0H) = 2.8 and G(FelI1) = 15.6, respectively. Triply distilled water from successive alkaline permanganate, acidic dichromate, and finally from a silica still was used throughout. The pH of the solutions was adjusted when required using perchloric acid or freshly prepared sodium hydroxide solution following deaeration of the sample. Present address: C.R.C.Gray Laboratory, Mount Vernon Hospital, Northwood, Middlesex HA6 2RN, England.
Acetylene gas (Linde AG, Krefeld) supplied in acetone free cylinders was successively washed through sulfuric acid (2 M) and triply distilled water before use. Nitrous oxide and argon were deoxygenated to 14 ppm of O2 by passing through a heated column of Oxysorb (MesserGriesheim, Frankfurt) when required. Samples were prepared by saturating water with acetylene/nitrous oxide mixtures (unless otherwise stated) prepared by using two Sho-Rate valves (Brooks Instrument, SA, Fribourg, Switzerland). The concentration of each gas in solution was calculated using literature solubility data6 from their respective flow rates and specific gravities assuming that Charles law is obeyed. Low concentrations of acetylene were obtained by syringe dilution techniques. Acetylene concentrations were measured on a PerkinElmer 900 gas chromatograph using a FI detector. The column of length 3 m and inner diameter 2.68 mm was packed with Porapak Type Q mesh 100-120 (Waters Associates Inc., Framingham, Mass.) and heated to 140 "C. Samples were degassed into an evacuated bulb from which 0.1-mL analysis samples were injected into the column using argon as the carrier gas. Peak areas were measured by an on-line computer. As the geometry of each sample vessel was slightly different, the acetylene content of each sample vessel for a fixed C2H2/Aror C2H2/N20mixture flow rate was first determined and then the sample was resaturated before irradiation. Succinaldehyde was determined using a specific colorimetric methods7 Results
I. Reaction Rates with eaq-,.OH, and .H.The reaction rate of ea; with acetylene was determined from the first-order decay of the eaq- absorption at 700 nm in C2H2/Armixtures. The first-order rate constant for ea; decay increased linearly with acetylene concentration over M [CZH,] for both 200- and 400-rd the range 1.0-2.5 X dose pulses giving a calculated value k(e,- + C2H2) = 2.0 0.2 x 107 M-1 s-1. Competition between acetylene and substrates was used to determine rate constants of OH and H with acetylene. The decrease in initial absorption at 500 nm of the (SCN)L radical at various [SCN-]/[C,H,] ratios in N2O bubbled
0022-365417812082-0022$01.0010 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978 23
Hydroxyl Radical Attack on Acetylene in Solution
TABLE I: Reaction Rate Constants of e&, *OH, and .H with Acetylene k(e,- + C,H,) = 2.0 c 0.2 x l o 7 M-' s-I k(.OH + C,H,) = 4.5 * 0.3 X l o 9 M - l s - l k(.H + C,H,) 2.3 X l o 9 M-'s-'
e
;:::; $ OOOL
0" 0002 0001 200 220 240 260 280 300 320 nm Figure 1. Transient spectra at high acetylene concentration and low dose (1200 rd), normalized to 100-rd dose. 1.2 X lo-' M [C2H2](nitrous oxide added): (0)pH 2-6 immediate absorption after 1-ps pulse; (0) pH 4.0, 150 ps (corrected for slow decay): (A) pH 3.25, 150 ps. 3.6 X lo-' M [CZHP](nitrous oxide added): (X) pH 5.7, 20 ps (corrected for 260-nm peak absorption), (0) 150 ps (corrected for slow decay), (+) 40 ms. 1.2 X lo-' [CzHP](argon added): ( 0 )pH 2.0, 200 ps (corrected for slow decay).
Figure 2. The effect of acetylene concentration on the rate of transient appearance at 290 nm [(O) pH 4.0, (x) pH 6.01 and at 260 nm [(I) pH 4.0, (m) pH 6.01 for 5200 rd doses. The values for 290 nm are determined from exponential growth curves and the values for 260 nm are calculated as described In the text.
firsborder kinetic plots and the rate increases with [C2H2]. As B absorbs a negligible amount at 260 nm the percentage of C present at any given time can be determined from the proportion of the maximum absorbance at 260 nm. At 300 nm the decay of B can be observed. This decay rate corresponds approximately to the rate of buildup of the 260-nm band of C. Precise measurements a t this wavesolutions gave the value of kl(.OH + C2H2) = 4.50 f 0.3 length could not be carried out due to too small absorption X lo9 M-l s-l taking h(.OH SCN-) = 1.1X 1010 M-l s - ~ . ~ differences between B and C. However, on subtracting the Competition with ferrocyanide was also carried out using calculated absorbance due to C from the measured total the decrease in absorption at 420 nm. The value of h(-OH absorbance a t 290 nm at various times it is seen that B C2H2)= 4.35 f 0.3 X lo9 M-l s- found by taking k(.OH decays into C. [Fe(CN)6]4-)= 9.3 X log M-' s-18 is in agreement with Plots of optical density at 260 nm vs, time are sigmoidal the SCN- competition results. type curves consistent with the observed absorbance being Competition with phenol was used to obtain an estimate associated with the second of two consecutive reactions. of the reaction rate of .H with acetylene. In acidic (pH The rate of. buildup of C increases with [C2H2]. The 1) argon bubbled solution the absorption due to the concentration of C at time t is given by the equation eOH-phenol adduct is rapidly destroyed at 330 nm9 leaving only the absorption due to the .H adduct. The decrease in the transient absorption a t 330 nm for increasing [C,H2]/[phenol] ratios gave a calculated value of k(.H C2H2)= 2.3 X lo9 M-l s-l taking k(-H phenol) = 1.8 X where A. is the concentration of A formed by the pulse and 109 M-1 s-l 9 h i and k3/ are first order rate constants. A t time t' when 11. Measurements a t High [C2H2]and Low Dose. The C,' = Ao/2 the equation reduces to absorption spectrum present immediately following a 1-ps, a e - k , t - e-odz,'t' 1-krd pulse to 1.2 X lo-' M [C2H2]at neutral pH is shown 0.5 = as species A in Figure 1,and is assigned to the CHCHOH a-1 adduct, the /3-hydroxyvinyl radical. There is no absorption maximum for A measurable since it is probably located where k3/ = aki. The parameter a was varied for sets of below 200 nm. The decrease in the immediate absorption h i and t' a t -25 "C to determine k3'. From Figure 2 it of A could not be followed due to swamping by the growing is seen that both k i and k i are dependent on [C2H2], in of strongly absorbing transients. Doses of 5200 rd were giving k2 = 8.8 f 0.8 X lo6 ML1sdl and k3 = 5.0 f 0.5 X required for the optical density of the grow in spectra to lo6 M-ls-l. be proportional to dose. Two distinct steps could be seen, Further information on this consecutive reaction was first a buildup with absorption below 230 nm and a obtained by studying the temperature dependence of k2 maximum a t 295 nm. This absorption is due to species and k3. The activation energies, E,, for both reactions are B. The rate of the buildup of species B depends on calculated from Figure 3 as 25.1 kJ mol-l. The enthalpy acetylene concentration. The formation of the absorption of activation for the formation of the transition state, LW, maximum a t 295 nm is followed by an increase of abis calculated by the equation AH* = E, - RT. At 25 OC sorption with a strong maximum at 260 nm with a shoulder AH* becomes 22.6 kJ mol-l. The entropy of activation, a t 330 nm. This absorption is due to species C. Spectra AS*,can now be calculated using the Eyring equation. At of B and C are shown in Figure 1. Species B is formed 25 "C he and k3 are 8.43 X lo6 and 4.25 X lo6 M-l s-l, approximately two times faster than species C. The rate respectively, so that the AS* values become -207 and -213 of the buildup of species C also increases with acetylene J K-I mol-'. These changes in AS" are large and in the concentration. range accounted for by the entropy change which occurs The kinetics of the buildup of B can be conveniently when two molecules of such size combine.1° studied in the 290-nm region as the absorption of both B The decay of C followed good second-order kinetic plots and C are equal. The buildup of B followed at 290 nm gave of l/(Dt - D,) VS. t , A t 25 "C 2k4/(tC- e p p ) = 2.47 x 10'
+
+ +
+
+
,
I
24
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
R. F. Anderson and D. Schulte-Frohlinde .0.14
24
.0.12 .0.10
23
.0.08 .0.06
Y
-C 22
e x
2
-B
s
._
c1
I o~T-)
4-
a
0
Flgure 3. Temperature dependence of (i) first-order rate constants for the appearance of the transient species absorbing at 290 (H)and 260 (ii) second-order rate constants for the disappearance of the nm (0); transient species absorbing at 260 nm (V).
I
220 2iO
2b
280 300 320nm
Flgure 5. Transient spectra at low acetylene concentration and high dose; ("6 krd) normalized to 6.0-krd dose; 2-ys pulse length; 1.0 X M [CzH2]: (X) pH 4.0, 100 1 s ; (A) pH 8.7, 100 ps; ( * ) pH 9.8, 2 ys; (0)pH 2.0, argon added.
QI
0
-
s
.-
4-
a
0
0.002
0.001
200 220 240 260 280 300 320nm Flgure 4. Transient spectra 5500-rd dose and high pH, normalized to 500-rd dose: (X) 5.0 X M [C2Hz] pH 9.8, 20 1s; (0)1.0 X M [C H2] pH 9.8, 20 ys; 0 2.9 X M [C2H2]pH 9.8, 20 ps; A 1 X 10' M [CPHz]pH 13.1, immediate absorbance. The insert shows the optical density at 210 nm, 1.0 X M [C2H2b:(X) 1.5 X M [OH-]; (0) 6.1 X 10- M [OH-]. M [OH-]; (0)2.1 X
cm s-l. Using 60:c = 11.5 X lo3 M-l cm-l and E~~~~ = 2.6 X lo3 M-l cm-l the bimolecular rate constant becomes 2k4 = 2.5 X lo9 M-l s-l. The temperature dependence of 2k4 (Figure 3) gave E, = 17.4 kJ mol-l and AS*= -130.2 J K-l mol-l. These values are consistent with the occurrence of two molecules combining, however, it is unlikely that a product with six conjugated bonds is formed since no absorption a t high wavelengths could be detected. It should be added that on a long time scale (rlp N 120 ys) a sigmoidal buildup of optical absorption at 330 nm is observed. The reactions occurring on this timescale were not studied. 111. Reaction of Hydroxyl Ion with the 0-Hydroxyvinyl Radical (A). The effect of hydroxyl ions on the rearrangement of A was studied. Below pH 6 the absorption spectra of A, B, and C are independent of pH and [C2Hz] when low doses are used. On using 1.0 X M [CZH,] and 0.5-krd dose, the absorption spectra of the transients were observed to progressively change with increasing pH from pH 8 to 10 to a new transient, species E (Figure 4). Above pH 10 the spectrum of E is present immediately after the pulse indicating that both the -OH and SO-lead to species E under these conditions. The spectrum of E equals that reported for the formylmethyl radical.ll The reaction rate of OH- with A was measured between pH 9.4
and 10 (Figure 3a), and the bimolecular rate constant is kS(OH- + HOC2HZ.) = 1.5 f 0.3 X 1O1O M-' s-l. That a competition between OH- and CzHzfor A occurs is seen from Figure 5 where the resulting spectra measured 20 ps after the pulse changed markedly with [C2Hz]at pH 9.8. Also when using a fixed [C2H2]of 1.2 X M and increasing [OH-] the measured absorbance at 260 nm attributed to species C was reduced to half its maximum value at an [OH-] of 5 X lo4 M. From this result it can be estimated that k6 is of the order 1O1O M-l s-l. No change in conductance was observed under experimental conditions in which species A, B, and C are formed in the pH range 3.3-9. Also no conducting species at high pH (>9) and low [C2Hz]were observed implying that the lifetime of the deprotonated P-hydroxyvinyl radical (D) is very short (reactions 5 and 6) H Ho \I C=C. I H A k
DtH,OA
t OH-&[ k-s
o:C=C!H]-t H
H,O
(5)
D
0 \\
C-CH,.
t OH-
(6)
HI
E
This can be explained by assuming reaction 6 to be much faster than reaction 5. IV. Reaction of the Formylmethyl Radical with Acetylene. To check whether the formylmethyl radical can add to acetylene its spectrum was first produced by pulsing a 1.0 X 10-1M ethylene glycol solution at pH 10.3.11 Then in a second experiment 2 X M [CZHZ] was added. Over 90% of the hydroxyl radicals are scavenged by the ethylene glycol and the observed spectra and decay kinetics of the formylmethyl radical produced in the two experiments were similar for 1- and 2-krd doses. Although it cannot be concluded with certainty that the formylmethyl radical does not add to acetylene, calculations based on the bimolecular rate of disappearance of the formylmethyl radical show it to be at least lo2 less reactive than the P-hydroxyvinyl radical toward acetylene. V. Measurements at Low [C2Hz]and High Dose. The bimolecular reaction of the P-hydroxyvinyl radical in
The Journal of Physical Chemistty, Vol. 82, No. 1, 1978 25
Hydroxyl Radical Attack on Acetylene in Solution
TABLE 11: Succinaldehyde Production (G Values r0.05) 1.0 x M [C,H,] 1.2 x lO-'M [C,H,] Dose rate, eV g-' min-I pH 7 pH 10 pH 11 p H 7 pH 1 0 pH 11
Steady State
2.54 x 1017 1.04 x lo1*
0.67 0.97
9.36 x 1 0 2 3 1.12 x 1025 0.68
0.51 0.96 Pulse 1.43 1.30 1.67
0.07
0.08 0.08
0.16 0.29
0.1
0.45 1.40
0.52
TABLE 111: Consumption of Acetylene ([C,H,]= 1.0 x lO-'M) PH 2 (C,H,/Ar) 7 (C,H,/N,O) 11 (C,H,/N,O)
G(-C,H,) 16* 2 15t 3 13 * 2
200 220 240 260 280 300 320 nm
slightly acidic conditions was studied using 1.0 X M [C2H2]and a 6.0-krd dose. A long lived transient (several minutes) with absorption maximum at 240 nm was formed by second-order reaction kinetics, Figure 5. Plotting 1/(D, - Dt) - l / ( D J vs. t gave a value for 2k7/qi = 2.0 X lo5 cm s-l, The absorbing transient is presumably the dimer of A, species F, which slowly rearranges to succinaldehyde (reaction 7 ) . HO 2 H
\
I
H
c;;c.
/
HO
\
k
-3
H
I
/
c=c\
HI
H H
/
(7)
c=c
\
OH
F The apparent cF240 from Figure 5 is 8.1 X lo3 M-l cm-l, however since the measured G(succinaldehyde) at this pH, [CzH2],and dose is 0.7 (Table 11),the actual q Z 4 O is likely to be 4 times higher, assuming that any species formed on the disproportion of A does not absorb a t 240 nm, giving 2k7 = 2.0 X lo5 X 8.1 X lo3 X 4 = 6.5 X lo9 M-l s-'. When the pH is raised the absorption spectrum of the transient changes to one which is similar to that of the formylmethyl radical. The measured decay rate of this transient formed at pH 9.8 followed second-order kinetics with 2k/c = 3.4 X loe cm s-l. Using a value tE300= 260 M-l cm-l a value of 2k = 9.0 X lo8 M-l s-l in accordance with the previously reported decay rate of the formylmethyl radical'l is obtained. No conductance associated with the formation of dimer F could be found within the measured range of pH 3.4-8.5 using 1.0 X M [C2H2]and a 6.0-krd dose. Using an AC method for long time measurements within the pH range 5.0-8.5, a slow buildup in conductance independent of the absorption spectrum of F was observed. This rate of buildup, 3.5 X 10-1s-l, was independent of pH and may be associated with the hydrolysis of a product (possibly ketene) formed on the disproportionation of A. VI. Succinaldehyde Yields and Acetylene Loss. The increased succinaldehyde yields with increased pH and dose rate (Table 11) are consistent with the assumptions of reactions 5 and 7. The relative proportions of succinaldehyde arising from the @-hydroxyvinyland formylmethyl radicals cannot be determined. The loss of acetylene was measured under steady-state radiolysis conditions in acidic, neutral, and basic solutions (Table 111). Both a high [C2H2]and dose had to be used to effect measurable percentage changes. It is seen that for every radical produced approximately three acetylene molecules are consumed. At pH 11 most of the phydroxyvinyl radicals are converted to formylmethyl radicals in preference to further reaction with acetylene. A
Figure 6. Transients produced by OH and H atom attacks at 200 ps after pulse: ( X ) transient induced by OH radical attack, 200 ps; (0) transient induced by H atom attack, 200 ps; both corrected for slow decay. The H spectrum was obtained at pH 2 by correcting for the OH adduct absorption.
However G(-C2H2) is still high and indicates that the formylmethyl radical can add to an acetylene molecule. These results show that no long radical chain polymerization occurs in the acetylene system. The previously reported precipitation on irraditionl could only be reproduced in acidic solution (pH 1 2 ) and could be associated with the coagulation or condensation of small product molecules or the cross linking of radical species. The reaction of the .H-C2H2 adduct with acetylene has not been fully investigated in this study. However sep, from reactions arating out the spectrum at 200 ~ sarising initiated by .H at pH 2 from that arising from .OH reacting at pH 7 a different spectrum is obtained and is shown in Figure 6. The spectrum resembles that reported for the cyclohexadienyl radical.12
Discussion I. High [CzH2]and Low Dose. The analysis of the kinetic data obtained by optical measurements shows the following consecutive reaction scheme to occur: A
*OH t C,H,
-+
+ C,H,
B
A
--c
B t C,H2 -+ C
(3) (4)
c t c - P
At a [C2H2]of
M reaction 1 reaches completion in
