Effect of ionic dissociation of organic compounds ... - ACS Publications

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Ionic Dissociation

Organic Compounds

of

(

Y

—·

2)/

6

2673

(24)

0.47 and 0.31 at 258 and 214 Equation 3 leads to F nm while eq 24 gives ¥ 0.48 and 0.27. The fraction of C2He molecules produced by reaction 12 is estimated from (1 8/ 2 «) as 0.047 at 254 nm and 0.104 and 214 nm. These data lead us to the conclusion that the photochemistry of ethanethiol involves three primary processes and a set of hot reac=

=

—

J. Phys. Chem. 1972.76:2673-2679. Downloaded from pubs.acs.org by UNIV OF ALBERTA on 07/08/18. For personal use only.

tions.

Examining Figure 1 we find that at 214 nm two electronic states appear to be involved in. absorption. These states have been discussed by Clark, and Simpson18 and characterized as non-Rydberg transitions 'with, the low-energy transition considered to involve promotion of a nonbonding sulfur electron to an antibonding molecular orbital. The higher energy band is described as promotion from a bonding C-S local

orbital to an antibonding H-S orbital. The implications of the differences in these excitations for the dissociation of ethanethiol are not clear particularly since they relate directly to the excitation process and. only indirectly to the dissociation process. From our data the primary processes do vary somewhat in passing from the weak band centered near 230 run into the wing of the band centered at 200 nm. The effect is not strong, however. It will be of considerable interest to investigate the primary processes at 185 ran since Clark and Simpson18 describe this band as a local C-S excitation. Assuming this to be the ease we might expect C-S bond cleavage to predominate at this wavelength. .

(18) L. B.

Clark and W. T. Simpson, J. Chem. Phys., 43, 3666

(1965).

Effect of Ionic Dissociation of Organic Compounds on

Their Rate of Reaction with Hydrogen Atoms1 by P. Neta and Robert H. Schuler* Radiation Research Laboratories, Center for Special Studies and Department of Chemistry, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213 (.Received March 13, 1972)

Publication

costs assisted by

Carnegie-Mellon University and the U. S. Atomic Energy Commission

Further development of the steady-state in situ radiolysis-esr method for the determination of H atom reaction rate constants has enabled measurements to be made in neutral solutions. It is possible, in many cases, to use H2PO4” to convert eaq™ into H atoms at pH 7. With certain organic solutes, however, scavenging of eaq™ by the solute interferes with this conversion and it is necessary to use only the low-residual H atom yield with a corresponding reduction in sensitivity of the measurements. Representative organic solutes have been examined and it is found that only in the cases where acid-base equilibria are involved does the rate constant depend on pH. Typically increases of a factor of ~2-3 are observed for the simple organic acids and amino acids upon conversion of COOH to COO™. Only in the case of oxalic acid was the rate constant found to decrease in going from pH 1 to 7. Deprotonation of the NH3+ group results in an increase in the rate of abstraction from the a position by a factor of -~10 and the rate of addition to aromatic rings by a factor of ~5. Effects in aromatic and heterocyclic systems are, for the most part, relatively small (a factor of 1-3). Formic and barbituric acids are exceptional in that their rate constants for reaction with H atoms increase by two orders of magnitude with an increase in pH over the range of 1-7. The rate constant for reaction of H with OH™ has been measured to be 1.5 X 107 M~' sec™1.

Introduction A steady-state electron spin resonance method for the determination of rate constants for reaction of H atoms in acidic solutions has recently been developed.2 This method is based on the measurement of the decrease caused by the addition of solutes on the esr signals of H atoms observed during continuous irradiation

of aqueous solutions. Rate constants in acidic solutions (mostly at pH 1) have been measured for many (1) Supported in part by the U. S. Atomic Energy Commission. Presented at the 20th Radiation Research Society Meeting, Portland, Ore., May 1972. (2) P. Neta, R. W. Fessenden, and R. H. Schuler, J. Phys. Chem., 75, 1654 (1971).

The Journal of Physical Chemistry, Vol. 76, No. 19, 1972

P. Neta and Robert H.

2674

organic compounds2-4 including a considerable number of compounds of biochemical interest.4 In the latter case, however, it is important to know the rate constants in neutral solution should a biological system be under consideration. Possible changes in rate constants in going from acidic to neutral solutions have been estimated4 by analogy with the pH effects on the reaction of OH radicals but an experimental verification is still lacking. Only compounds which undergo acid-base equilibria are expected to have pH-dependent rate constants. It is the purpose of the present study to investigate the effect of ionic dissociation on the rate constants for reaction of H atoms.

Experimental Section The details of the measurement of the H atom signal by the steady-state in situ radiolysis-esr technique have been described previously.2 In general the intensity of the low-field (inverted) hydrogen line was examined at a continuous dose rate in the range 1019~ 1020 eV g-1 sec-1 and the reduction in this intensity at appropriate solute concentrations was observed. The kinetic experiments and treatment of the data are generally similar to the previous ones2-4 except for adjustment of the pH. All the organic chemicals used were of the same grade as those used previously.2-4 In most of the studies at pH 7 phosphate has been used both to buffer the solution and to convert eaq- to H atoms via the reaction eaq-

+ H2P04-

—

H + HP042-

(1)

which has a rate constant of 7.7 X 106 M~1 sec-1.5 The second pK of phosphoric acid is 7.2 so that slightly more than 50% of the phosphate is in the form of H2PQ4- at pH 7. Buffer solutions made from available samples of K2HP04 and Na2HP04 showed relatively small H atom signals, apparently because of impurities in these samples. The maximum signals in the presence of phosphate were obtained by using Baker Analyzed KH2P04 and adjusting the pH to the desired value by adding either Baker Analyzed KOH or HC104. Experiments at pH 1 showed that addition of 0.1 M of this phosphate had only a small effect on the H atom signals (vide infra) so that complicating effects of impurities are presumably at a minimum. Results and Discussion

In acidic solutions the signal-to-noise ratio of the H atom esr lines is ~ 50: l.2 In neutral solutions, where the yield of residual H atoms is only ~15% as great, a proportionately smaller signal is observed6 and direct measurement of H atom rate constants by the steadystate esr methods is limited to relatively unreactive compounds. One desires, therefore, to include in the irradiation system a reagent that is capable of converting eaq- into H atoms in near neutral solutions. We have explored the use of H2P04- as an H atom The Journal of Physical Chemistry, Vol. 76, No. 19, 1972

Schuler

in the esr experiment and found it to be adequate for this purpose for cases where the rate constant of reaction of the solute with eaq- is not more than 10 times its rate constant for reaction with H atoms. Where the rate constant for reaction with eaq- is greater, either very large corrections have to be made or measurements have to be carried out utilizing only the low yield of residual H atoms. A number of studies have been carried out to examine the properties of the esr experiment at pH values above 1. The rate constants for reaction of a number of representative carboxylic source

acids, amino acids, amines, and heterocyclic compounds

with II atoms have been measured at pH 7 for comparison with the values previously determined at pH 1. In most of these studies the low-field (inverted) H atom line has been examined but in eases where the high-field line was also examined it was found to have comparable intensity. In principle this experiment is based on a competition between chemical reaction of H atoms and decay with time of the noneqtiilibrium population of the H atom electron-nuclear spin levels which gives rise to abnormally intense esr signals. Previously the kinetics was treated on the assumption that this decay involved only a relaxation process and At this writing it now appears that was exponential. spin polarization is produced upon partial reaction of the H atoms.7 Decay of the esr signals will therefore not be purely exponential and the mathematical treatment of the competition kinetics is necessarily somewhat more complex than that given previously. With scavenger present the concentration of H atoms is controlled, to a large extent at least, by reaction with the scavenger and it would appear that the. expressions previously used to intercompare the relative rate constants for different compounds do describe the competitive situation quite well.2 Dependence of H Atom Signal on pH and [W2P04-]. Initially it was necessary to investigate the effect of pH and of phosphate concentration on the esr signals of the H atoms in the absence of scavengers. In neutral, as in acidic solutions,2 the signals observed, under steady-state conditions are found to be proportional to the production, rate of H atoms. The effect of phosphate concentration on the intensity of the H atom signal is shown1 in Figure 1 for both solutions. The relative values in the absence of phosphate are given by the limiting dashed lines in the figure. At pH 1 the observed signal intensity corresponds to an H atom yield of 3.65.® In neutral, solutions the relative signal H. Schuler, J. Amer. Chem. Soc., 94, 1056 (1972). (4) P. Neta and R. H. Schuler, Radial. Res., 47, 612 (1971). (5) M. Anbar and E. J. Hart, Advan. Chem. Ser., No, 81, 79 (1968). (6) P. Neta, R. W. Fessenden, and R. H. Schuler, Nature (London), (3) P. Neta and R.

Phys. Set237, 46 (1972). (7) R. W, Fessenden, private communication. (8) See, e.g., M. Anbar in “Fundamental Processes in Radiation Chemistry,” P. Ausloos, Ed., Interscience, New York, N. Y., 1968, p 651.

Ionic Dissociation

of

Organic Compounds

Figure 1. Effect of phosphate concentration on the relative intensity of the low-field, (inverted) H atom signal: ·, measurements at pH 7; A, measurements at pH 1. The limiting values in the absence of phosphate are given by the dashed lines on the left. At pH 7 this latter value was determined with the use of 2 mM N20 to remove eaq”. The open circles represent the data at pH 7 corrected for the decrease in signal observed at pH 1 upon the addition of phosphate.

which corresponds to the residual H atom yield cannot be determined directly because, in the absence of a buffer, H+ ion produced by the radiolysis builds up and acts as an eaq~' scavenger, artificially increasing the observed H atom signal. One can proceed either by adding a buffer which cannot act as an H atom source or by removing eftq~. As described below nitrous a oxide was used as competing electron scavenger and

signal intensity corresponding to the generally ac0.6s was observed. cepted value of Gh. Addition of phosphate to deoxygenated water increases the H atom signal gradually. With a 1 X 10 "3 M solution the period for reaction of eaq'~ with H2PO4 is ~100 Msec and at the dose rates of these experiments the majority of the electrons disappear by mutual recombination or by reaction with radiolytically produced H2O2. In general, at low concentrations of electron scavenger the distribution of the electrons among the various possible reaction paths is complex and depends critically upon beam current and other experimental details. Increase in the phosphate concentration above 10~3 M results in an increased signal. One, however, never reaches the plateau expected from the signal observed in acidic solutions in the absence of phosphate. A maximum signal of about 60% of this expected value is found in the region of 10”"1 M phosphate. At higher concentrations a drop in signal is observed and this drop is, to some extent, similar to the decrease observed upon the addition of phosphate at pH 1. It is not clear at this point whether this latter effect is the result of scavenging of the H atom by the phosphate itself or by chemical or magnetic impurities in the phosphate. The curve at pH 1 indicates that the rate constant for reactions of H atoms with phosa

=

2675

Figure 2. Effect of pH on the relative intensity of the H atom signal: ·, in the presence of 0.1 M phosphate; ®, in the absence of phosphate; O, in the absence of phosphate but in the presence of 2 mM N20 to remove eaq”. The insert shows the latter data on an expanded scale along with a curve calculated on the assumption that reduction results simply from scavenging of H by OH- with a rate constant of 1.5 X 10’ M”1 sec-1.

M~l sec”"1. For practical purnear the maximum at concentration poses phosphate 0.1 M was chosen for further study of the neutral system. The effect of pH on the H atom signal from 0.1 M phosphate solutions is shown by the solid circles in Figure 2. The three ionization equilibria in phosphoric acid have pK values of 2.12, 7.21, and 12.67. In the pH region 1-4 a decrease in signal results from a decrease in the rate of scavenging by H+ so that, as mentioned above, other terminating reactions become more important. Above pH 4 reaction 1 dominates over direct reaction of eaq~ with H+ and a modest plateau results. The pronounced drop in the region of pH 8 is the result of the conversion of H2PO4”’ into HPO42”" which reacts much more slowly with eaq~ and is ineffective in converting eaq~ into H atoms. The decrease in signal observed at pH >10 (Figure 2) is caused by the reaction of H with OH- and is used to calculate the rate constant for this reaction (see below) The occurrence of this latter reaction, however, effectively limits the range in which the esr experiment is applicable to pH values below 10. Kinetic Experiments in the Presence of Phosphate. The relative rate constants for the reaction of H with different solutes can be obtained from plots of (H0/H 1) vs. concentration where H and H0 are the relative H atom signals per unit current of electron beam observed in the presence and absence of added solute (see ref 2). Ethanol served as a reference and was measured with each set of experiments. The concentration of ethanol at which there was a 50% decrease in signal, i.e., M within (H0/H 1) 1, was found to be 4.4 X 10 ± 10% in all experiments in acid and neutral solutions phoric acid is <

104

a

.

—

—

=

The Journal of Physical Chemistry, Vol, 76, No. 19, 1972

P. Neta and Robert H.

.2676

Table I:

Schuler

Rate Constants for the Reaction of Hydrogen Atoms in Aqueous Solutions kji pKa values

Compound

Ethanol Methanol

7ch at

2.6 1.6 1.04 3.5 7.5 8.4 6.4 2.6 4.6 1.8 3.5 4.1

15.5 15.5 ~15 10.6 3.75 4.75 4.87 4.86 4.83 3.83 4.21, 5,64 1.27, 4.27 2.3, 9.6

Hexanol Hexylamine Formic acid Acetic acid Propionic acid Isobutyric acid Hexanoic acid Glycolic acid Succinic acid Oxalic acid Glycine Valine Aspartic acid Glutamic acid

Histidine Aminoacetonitrile Ascorbic acid Benzoic acid

Aniline Pyridine Uracil Cytosine Adenine Adenosine

Barbituric acid OH-

8

2.3, 9.7

9

1.9, 3.9, 9.7

8

2.1, 4.2, 9.6 1.8, 6.0, 9.2 5.34 4.1, 11.8 4.17 4.6 5.2 9.4, ~12 4.45, 12.2 4.2, 9.8 3.4, 13 4.03

1.7 4.8 6.6 1.1 8.5 4.9 2.2 2.8 9

8.3 1.1 2.0

kh at pH

pH 1°

X 107 X 10s X 10» X 107 X 10s« X 10» X 10* X 107 X 107 X 107 X IQ6 X 10® X 10» X 106 X 10® X 10s X 7' X 10* X 10s X 10s X 10s X 10s X 10s X 107 X 107 X 10= X 107

(2.6 ve 1.0 3.5 1.3 4.2 1.8 5.9 5.3 4.0 1.1 15) and hexylamine (as C6H13NH3+ with pif 10.6) served this purpose. In fact the two alcohols and the hexylamine ail gave the same competition curves in neutral as in acidic solution (as did ethThe rate constants calculated from the data at anol) pH 7 are given in Table I and are seen to be identical with those measured at pH 1. It is seen that where no ionization equilibria are involved there is no apparent dependence of the rate constant on pH. These studies indicate that no difficulty is introduced by using phosphate to convert eaq- to H atoms and thus it becomes possible to make measurements in neutral solution over =

.

([Sji/J-1.

In order to examine the validity of the relative rates obtained with the phosphate system at pH 7 it seemed The Journal of Physical Chemistry, Vol. 76, No. 19, 1970

(9) M.

(1967).

Anbar and P. Neta, Int. J. Appl. Radiat. Isotopes, 18, 493

Ionic Dissociation

of

Organic Compounds

wide range of solutes with approximately the same possible in the more acidic solutions. Measurements on some 20 additional compounds at pH 7 are reported in Table I. In the above it must be noted, however, that the test solutes do not react rapidly with eaq”. In certain cases where eaq"' reacts with the solute much more rapidly than does H, reduction in the H atom signal will result: from scavenging of eaq” by the solute in competition with H2PO,i” rather than by H atom scavenging. At 0.1 M phosphate and pH 7 the pseudo-first-order rate constant for reaction 1 is (7.7 X 10*)(0.05) == 3.9 X 10s sec”1. For compounds such as oxalate, glycine, and the pyrimidines appreciable scavenging of H atoms occurs only at concentrations where the pseudo-firstorder rate constant for reaction of eaq” with these compounds is >10* sec'"1 so that it is necessary to make a very significant correction for this latter reaction. Unless indicated otherwise in Table I the corrections for this effect were less than 10%. Kinetic Experiments in the Absence of Phosphate. Because of the above complication it was decided to attempt measurements in neutral solutions containing no phosphate and to use the small residual yield of H atoms in the competition. In these experiments it was necessary to scavenge the electrons and remove them as a source of H atoms and as a complication in the terminating reactions. N20 was used for this purpose. The initial experiments with N20-saturated solutions (0.02 M N20) gave an H atom signal which corresponded to a yield of 0.5. Since the reported rate constants for reaction of H atoms with N20 are 104105 Mm·' see”1 a drop in signal as high as ~20% might occur as the result of H atom scavenging by the K20. To examine this possibility a solution saturated with 10% XT20 in Xa (i.e.; 2 mili in N20) was irradiated and the signal found to increase to a value corresponding to a yield of 0.64. This value is, within experimental error, in agreement, with the generally accepted residual II atom yield of 0.6.8 At pH 1 addition of 2 mM of N20 caused no significant decrease in the H atom signal while saturation with N20 resulted in ~30% deThe XXO had been shown mass spectrometricrease. to contain cally