NOTES
1560
cance can be attached to the 25 kcal. difference in actiConclusions vation energies except to say that it seems consistent I t is possible to reconcile most low- and high-temperawith the experimental results of Robertson, et U E . ~ ~ ture data in the literature on acetylene pyrolysis by Beyond this, our ignorance of the details of the postua chain mechanism in which the main chain-ending lat?edsteps IV and V is almost total. reaction has a stronger temperature dependence than A final remark should be made on the report by does one of the chain-carrying reactions. This leads Skinner and Sokoloski" that the rate constant for forto long chains a t low temperatures and essentially mation of vinylacetylene below about 1500°K. lies nonchain behavior a t high temperatures. It seems above the low-temperature extension of their results for very probable that the first step in the mechanism is conversion to all products at temperatures above 1500" excitation of CzHzto its lowest-lying triplet state, and K. (points shown in Fig. 4). If the result is real, it that one of the chain steps is a spin-exchange reaction. presents a severe complication. We would suggest In systems possessing high surface-to-volume ratios, tentatively that their low yields of vinylacetylene may e.g., in heavily sooting systems, a first-order heterogehave led to analytical errors, as indicated to some exneous decomposition reaction, the products of which tent by their difficulty in obtaining a mass balance on appear to be principally carbon and CH4, is expected reactants plus products. to dominate the kinetics.
NOTES
Deuterium Isotope Effect in the Ionization of Substituted Succinic Acids in Water and
in Deuterium Oxide by Paul K. Glasoe Wittenberg University, Springfield, Ohio
and Lennart Eberson University of L u n d , L u n d , Sweden (Received M a y 27, 1868)
The fact that certain dicarboxylic acids have unusually high ratios of the first to the second ionization constant has been attributed to intramolecular hydrogen bonding in the monoanion.112 This explanation for the very high value of the ratio for maleic acid to that for fumaric acid (20,000 to 23) is substantiated by evidence that the monopotassium salt exists in a hydrogen-bonded condition in aqueous solutions. Eberson4 has shown that the KI/& ratio for highly substituted succinic acids is very high if the acid is in the racemic form and that the monopotassium salt of the racemic acid shows evidence of intramolecular hyT h e Journal of Physical Chemistry
drogen bonding both by infrared6 and by proton magnetic resonance.6 It is well-established that the ionization of a deuterio acid is less than that of the corresponding hydrogen acid and that the difference is dependent to some extent on the strength of the hydrogen acid, becoming slightly larger as the acid gets weaker. For many gcids, the values of ~ K D ~ K are H around 0.5 and fall in the vicinity of a straight line having a slope of 0.02 . ~ and Dahlgrens in the plot of ApK us. ~ K H Long measured ~ K and H ~ K for D maleic and fumaric acids and found that ApK1 for maleic acid was abnormally high, 0.62, and that ApKz was abnormally low, 0.38. Fumaric acid, as well as the monoesters of both acids, had what can be considered as "normal" values of ApK. They suggested that this high value of ApKI for (1) L. Hunter, Chem. I n d . (London), 155 (1953). (2) D. H. McDaniel and H. C. Brown, h'cience, 118, 370 (1953). (3) R. E. Dodd, R. E. Miller, and W. F. K. Wynne-Jones, J . Chem. Soc., 2790 (1961). (4) L. Eberson, Acta Chem. Scand., 13, 211 (1959). (5) L. Eberson, ibid., 13, 224 (1959). (6) L. Eberson and S. Forsen, J . P h y s . Chem., 64, 767 (1960). (7) R. P. Bell, "The Proton in Chemistry," Cornell University Press,
Ithaca, N. Y., 1969, p. 189. (8) G. Dahlgren, Jr., and F. A. Long, J . A m . Chem. Soc., 82, 1303 (1960).
NOTES
1561
Table I : Ionization Constants for Substituted Succinic Acids and Their Monoesters in Water and in Deuterium Oxide hid
Succinic acids Tetramethyl Tetraethyl me-a, a’-Di-t-butyl Monomethyl esters Tetramethyl Tetraethyl mc-a,a’-Di-t-butyl
pKiH
:3.56 4= 0 . 0 2 3 . 3 9 rt 0 . 0 2 2 . 2 0 f 0 01
4 . 2 0 rt 0 . 0 2 4 . 2 9 i 0.02 2 . 7 7 rt 0.01
4 . 9 8 f 0.01 5.34 f0.02 5 . 5 7 f 0.05
$5.48f 0.01 5 . 8 3 rt 0.02 6 . 0 8 f 0.03
pIGH
Succinic acids Tetramethyl Tetraethyl ~ a e - aol’-E)i-t-butyl ,
pKiD
7 . 4 1 f 0.02
8 . 0 6 rt 0.06 10.25rt0.05
maleic acid might be due to the difference in the strength of deuterium bonds compared to hydrogen bonds in the monoanion, the deuterium bonds being weaker. In view of the fact that the succinic a,cids investigated by Eberson show high K1/Kz ratios and shorn evidence for hydrogen bonding in the acid salt, it was of interest to see whether these acids would show an abnormal deuterium isotope effect similar to maleic acid. To examine this point, the ionization constants of some substituted succinic acids and their monoesters were measured in water and in deuterium oxide.
Experimental Materials. The succinic acids and the monomethyl esters were prepared by Eberson aEi described previously.9 Deuterium oxide was obtained from Liquid Carbonic Corp. as greater than 99.5Oj, DzO. Solutions of the deuterio acids were made by dissolving the solid hydrogen acid in deuterium oxide. Solutions of alkali deuterium oxide were made by diluting concentrated carbonate-free alkali hydroxide with deuterium oxide. Since the concentration of the diluted solutions was around 0.02 M , the deuterium concentration was not changed by more than 0.1%. Titrations. pH titrations were carried out on small volumes, usually around 10 ml., of the acid solution by adding standard alkali from a platinum tip microburet. The pH was measured with a Beckman Model GS pH meter equipped with a Beckman No. 39167 glass electrode and a No. 39168 silver-silver chloride reference electrode. The titrations were conducted a t 25 i 0.05’ in an atmosphere of nitrogen. The glass electrode system was standardized against water buffers; usually 0.0500 M potassium acid phthalate (pH 4.00) was used. In titrations with deuterium oxide solu-
pKzH
- pKiH
ApKi
3.85 4.67 7.95
0.64 0.90 0.57 0.50 0.49
0.51 ApKz
pKzD
0.34 0.44 0.67
7.75 f 0.02 8 . 5 0 i.0.05 10.92 f 0.05
tions values of pD were obtained from the reading of the pH meter, which had been standardized in water buffers, by adding 0.40 to the pH meter reading according to the method given by Glasoe and Long.’O Values for pK1 and for pKa were calculated a t several points on the titration curve above and below the points of half-neutralization. The following formulas were used
- 1%
+ C H +- log.
CHAN
UA
where pH is defined as - log(cH +y&), CA-2, CHA-, and CH,A are the added molar concentrations of anions and free acid, respectively, and y* was calculated from -log yh = 0.509&‘(1 di)where p is the ionic strength of the solution. A program was set up on an IBRl 1401 computer which made it possible to make a large number of calculations all along the titration curve. No additional electrolyte was added to increase the ionic strength. In no case was the ionic strength greater than 0.01 in solutions for which pK1 was calculated or greater than 0.02 in pKz calculations. The second pK for ala’-di-2-butylsuccinic acid is so large that only one break is obtained in the titration curve. To determine pKz a definite amount of the acid was dissolved in twice the molar quantity of standard alkali and this solution was titrated with standard acid. A summary of the results is given in Table I.
+
(9) L. Eberson, Acta Chem. Scand., 13, 40 (1959). (10) P. K. Glasoe and F. A. Long, J . P h y s . Chem., 64, 188 (1960).
V o l u m e 68, hrumber 6
J u n e , 1964
1662
Discussion Tetramethylsuccinic and tetraethylsuccinic acids H and in this reshow an abnormal ~ K -D~ K difference spect are like maleic acid. The value of ApK for tetraethylsuccinic acid is the highest reported for any acid. However, the di-t-butylsuccinic acid, which has a very high K1/K2 ratio, does not show a significant abnormal deuterium effect. The monomethyl esters show what can be considered as normal acid strengths and the deuterium-hydrogen difference is the normal value of around 0.50 pK unit. The weakening of the acid with increasing size of the alkyl substituents is the normal inductive effect. The very low value for pK1 for the di-t-butylsuccinic acid, compared to the high value for the monoester, suggests that there may be strong hydrogen bonding in the monoanion of this acid. However, the deuterium effect is relatively small. In this respect, this acid resembles y-resorcylic acid which, according to Long and hlacDougall,ll also has a low pK,, probably due to strong hydrogen bonding, but has a ApK of 0.56, near to the “normal” value for carboxylic acids. On the other hand, salicylic acid, which is probably less strongly internally hydrogen bonded, shows an abnormal ApK of 0.75. If these results are compared to those for the succinic acids, it appears that the abnormal deuterium effect shows up in moderately strong internal hydrogen-bonded acids but not in acids in which this bonding is very strong. There is no readily apparent explanation for these results. Acknowledgment. The work on the measurement of the ionization constants mas carried out under a grant from the Research Corporation. ( 1 1 ) A.C.bIacDougallandF.A.Long,J . P h y s . Chem.,66,429 ( 1 9 6 2 ) .
Deuterium Isotope Effect in the Ionization of Substituted Malonic Acids in Water and i n Deuterium Oxide by Paul K. Glasoe and James R. Hutchison Wittenberg C n i w r s i t y , Springfield, Ohio (Receiaed M a y 27, 1963)
In view of the abnormal deuterium isotope effect obtained in the ionization of maleic acid1 and in the ionization of certain substituted succinic acids2 having high K 1 / K zratios, it was considered of interest to study The Journal of Physical Chemistry
SOTES
a series of alkyl-substituted malonic acids. These acids show a marked increase in the ratio of K1 to Kz when both malonic hydrogens are replaced by alkyl groups. The ratio for malonic acid is about 700 and that for diethylmalonic acid is 121,000. If the KlIK2 ratio and the abnormal deuterium isotope effect in ionization are related, the highly alkylated malonic acids, especially from diethylmalonic up, should show pronounced deuterium effects. To examine this point, the ionization constants for a series of malonic acids were measured in mater and in deuterium oxide. To get a measure of the effect of the alkyl substituents on the strength of the acid without the complication of electrostatic effects or hydrogen bonding between the oxygen of the monoanion and the second acid hydrogen, the ionization constants of some monoesters were also measured in the two solvents. The malonic acids and the monoesters were prepared from the diethyl esters. The monoesters were obtained as sirupy liquids which were analyzed by titration and showed no detectable amount of the diacid. Appropriate amounts of a standard solution of a particular malonic acid were mixed with standard base and the pH was measured with a glass electrode on a Beckman Model GS pH meter at 25’. To this solution was added a measured amount of the solvent to decrease the ionic strength and the pH was measured again. Usually five different concentrations were used. The same procedure was followed with water and with deuterium oxide, using solutions of comparable concentrations to produce nearly the same change in the ionic strength of the five solutions. The values for pD were obtained from the readings of the pH meter which had been standardized with water buffers by the addition of 0.40. The pK values for the monoesters were obtained from pH titration curves of the monoesters with potassium hydroxide or potassium deuterium oxide in mater or in deuterium oxide. In these titrations the solutions had ionic strengths less than 0.01. The values for pKl’ and pKz’ were calculated in the same manner as described for the succinic acids. The calculated values of pK’ were then plotted against 4 ~The . value of pK obtained by extrapolation of the straight line to zero concentration is the value given in Table I. The results are summarized in Table I. A value of ~ K D ~ K Hlisted , as ApK, was computed from the zero ionic strength pK for the malonic acids and from ( 1 ) G. Dahlgren, Jr., and F. A. Long, J . Am. Chem. Soc., 82, 303 (1960). ( 2 ) P. K. Glasoe and L. Eberson, J . P h y s . Chem., 6 8 , 1560 (1964).
