NOTES
1314
Based on spectrophotometric measurements, Simons2 reported a value of 0.05 M-’ for the equilibrium constant
K
=
0.03(1
[A~~I/[Ml[Al
and values of the extinction coefficients a t 3650 A for complexed and free anthracene of 3100 and 820 M-1 cm-l, respectively. We have made a few preliminary measurements which are in general agreement with these values. If these values are correct and the complex is nonfluorescent, the constant corresponding to static quenching is 0.2 M-l, which is much smaller than the experimental value of 67 M-l. The present observation, that maleic anhydride quenches the fluorescent state much more efficiently than the triplet state, supports Simon’s view that the triplet state of anthracene is not involved in this photochemical reaction. Indeed, the opposite assumption, that only the triplet state leads to the formation of the adduct, results in the erroneous prediction that the quantum yield should have a maximum value a t a relatively low concentration of maleic anhydride. If the present values for the quenching constants and for the (oxygen-dependent) mean lifetime of the triplet state are used, the quantum yield would have a maximum value for 0.02 M maleic anhydride. Accordingly, we shall assume that the triplet state is not detectably involved in the reaction. The remaining plausible reaction steps may be written as follows, where MA represents the weak complex and AM the DielsAlder adduct.
XI
+A
1
MA* 7 M
MA
,L”]5-1
7
-e,AM
+A
AM
/ir
M
+ A* 7?tl+ A
+ 3100 X 820 ) = 0.34 0.05 X 0.5
More likely, both steps 4 and 7 contribute to the chemical reaction. Unfortunately, it is not possible to compare this prediction t o the experimental data, since the actual qua.ntum yields a t different concentrations of maleic anhydride were not reported (cf. Figure 3, ref 2).
Carbon-13 Chemical Shifts of Carboxyl Species from Acetic, Benzoic, and Mesitoic Acids in Sulfuric Acid and Oleum Solutions’ by Daniel D. Traficante The Frank J . Seiler Research Laboratory, Oflee of Aerospace Research, U. S. A i r Force Academy, Colorado
and Gary E. llaciel The Department of Chemistry, University of California, Davis, California (Received October 11, 1966)
The nature of species present in solutions of carboxylic acids in sulfuric acid and oleum media has been the subject of a considerable amount of Several experimental methods including cryoscopy and ultraviolet, infrared, and nuclear magnetic resonance spectroscopy have been employed, and it has been concluded that the protonated carboxyl species RC(OH)2+ and the “acylium ion” RCOf are the important cations in these strongly acidic media. Den0 and coworkers4 have reported the results of an extensive proton magnetic resonance study of several carboxylic acids including acetic and mesitoic (2,4,6-trimethyl~~
For intuitive and not very convincing reasons, Simons2 assumed that step 4 is negligible compared to step 3. Tentatively accepting this postulate and using a quantum yield of 0.03 for a maleic anhydride concentration of 0.5 ill (cf. Figure 2, ref l), the bimolecular IC7) of 1.1 X 1Olo M-’ sec-1 quenching constant (IC, indicates that only about 3% of the quenching encounters result in the formation of the stable adduct. The opposite assumption, that only light absorbed by the complex, MA, is photochemically active, leads to the value
+
T h e Journal of Physical Chemistry
(1) Reproduction in whole or in part is permitted for any purpose by the U. S. Government. (2) R. J. Gillespie and J. A. Leisten, Quait. Rer. (London), 8 , 40 (1954). (3) W. M. Schuhert, J. Donohue, and J. D. Gardner, J . Am. Chem. SOC., 76, 9 (1954). (4) N. C. Deno, C. 1’. Pitman, and hI. J. Wisotsky, ibid., 86, 4370 (1964). (5) R. Stewart and K. Yates, ibid., 82, 4059 (1960). (6) G. E. Maciel and D. D. Traficante, J . Phys. Chem., 69, 1030 (1965). (7) D. Cook, “Friedel-Crafts and Related Reactions,” Vol. I. G. A. Olah, Ed., Interscience Publishers, Inc., New York, N. Y., 1963, Chapter IX.
NOTES
1315
benzoic) acids over the range of sulfuric acid strengths nmr spectra were obtained at 35' at a frequency of for which the transformations RCOzH --t RC(OH)z+ --t 15.085 Mc/sec, using dispersion mode, rapid passage RCO+ were observed. In a recent paper6 we reported conditions as previously described. 15,16 The shifts results of a natural abundance C13 magnetic resonance reported here may be considered internally self-constudy of the protonation of acetic and benzoic acids sistent to about h0.2 ppm. and their ethyl esters in concentrated sulfuric acid. Results and Discussion While the results conclusively identified the carbonyl Carbon-13 chemical shifts of the carboxyl carbon oxygen atom as the position of protonation, the relaatom of acetic, benzoic, and mesitoic acids in sulfuric tively high solution concentrations (mole ratio of acid media of various compositions were determined solute:HzS04was 1:6) necessary for observation of a and are collected in Tables I, 11, and 111. The solusignal markedly influences the effective acidity of the tions represented by these data were each about 0.5% medium.8 Furthermore, using this technique we were carboxylic acid by weight, corresponding to less than unable to observe signals due to acylium ions. A pre1 mole % of solute for each solution. I n a few cases, vious report of the C13 chemical shift of the CHQ3solutions with about four times this solute concentraO+SbFG- complex, determined by the INDOR (internuclear double resonance) method, has a ~ p e a r e d , ~ tion were studied and additional signals of low intensity were observed in some instances. but there appears to be an appreciable mutual inconAcetic Acid. It is of interest to view the C13 chemisistency in the reported data. Because of the theocal shift data of Table I in terms of the pertinent conretical interest in the C13 chemical shifts of acylium clusions of Den0 and co-workers4 based on proton magions and the need for further characterization of the netic resonance measurements. I n agreement with carboxylic species present in H20-HzSO4-SO3 media, previous cryoscopic studies, they concluded that acetic we have measured the C13 chemical shifts of C13acid exists predominantly in the protonated form in carboxyl-labeled acetic, benzoic, and mesitoic acids 100% HzS04 and that in 85% HZSO4-15%SO3it exists in dilute solutions in such media with compositions half in the form of the acylium ion and half as the procovering the ranges of major interest. tonated carbonyl species. Their results indicate that Experimental Section the major shift of the equilibrium Jifuteriuls. Trifluoroacetic acid was an Eastman CHsC(OH)z+ 803 $ CH3CO+ H2S04 (1) White Label sample. Triethyl orthoacetate (bp 58" occurs between 87% &SO4-13% SO3 and 83% H~SOC(23 mm)) and diethyl sulfate (bp 81" (3 mm)) were re17% SO3. Their plot of the weight-average shift" vs. distilled Eastman Practical materials. Acetyl fluoride per cent SO3shows a sharp incline between 10 and 20y0 was prepared by the reaction of sodium fluoride with SO3, and its apparently zero slope between 20% SO3 acetyl chloride according to a modification of the and 65% SO3 implies the presence of only acylium ion method of Tullock and Coffman'O using dimethylin oleum solutions in this composite range. Our data formamide as the solvent. The product boiled at 19.8'. I are only partially consistent with these conin Table The carbon-13-enriched (57 atom % (213) acetic clusions. The -65.3-ppm shift in 100% H2S04 is and benzoic acids were used directly as obtained from assigned to the species CH3C(OH)z+ in agreement with Volk Radiochemical Co. Ethyl benzoate was prepared by the general procedure of Clinton and Laskow(8) N. C. Deno, N. Friedman, and J. Mockers, J . Am. Chem. Soc., skill and was purified by collection from a gas chroma8 6 , 5676 (1964). tography column.lZ The infrared spectrum of the (9) G. A. Olah, UT.S. Tolgyesi, S. J. Kuhn, 31. E. AIoffatt, I. J. Bastien, and E. B. Baker, ibid., 8 5 , 1328 (1963). collected ester was identical with that obtained from (10) C. W. Tullock and D. D. Coffman, J . Org. Chem., 25, 2016 the corresponding nonlabeled ester. The mesitoic (1960). acid carboxyl C13 was prepared by the method of (11) R. 0. Clinton and S. C. Laskowski, J . Am. Chem. SOC.,70, 3135 (1948). Schmid and Banholzer13 for the analogous C14 com(12) An 8 ft X 0.75 in. silicone gum rubber (methyl) column G E pound and was recrystallized from aqueous ethanol, SE-30 at 70' was employed. mp 150-152' (litf3mp151-153'). (13) H. Schmid and K. Banholaer, Helv. Chim. Acta, 37, 1706 The sulfuric acid solutions were prepared in a glove (1954). box filled with dry nitrogen, by mixing the appropriate (14) Determined by titration with standardized sodium hydroxide. amounts of 68.52% oleum14 (technical grade) and (15) P. C. Lauterbur, J . Am. Chem. Soc., 83, 1838 (1961). (16) G. E. Maciel and D. D. Traficante, ibid., 88, 220 (1966). 94.32% sulfuric acid14 (reagent grade), both from Baker (17) J. A. Pople, W. G. Schneider, and H. J. Bernstein, "Highand Adamson. Resolution Nuclear Magnetic Resonance," McGraw-Hill Book Co., Cl3 Magnetic Resonance Measurements. The Cl3 Inc., New York, N. Y . , 1947, Chapter io.
+
+
Volume 70, Number 4
April 1966
1316
NOTES
Table I: C13 Chemical Shifts of Acetic Acid Carboxyl C L 3in Sulfuric Acid Solutions, Ppm with Respect to Benzene
----
Expt
no."
0-100-0
1 2 3 4 5 6 7
-65.3 -65.1
8
-65.4
70H ~ 0 - 7H&Oa-To ~ SOa---0-85-15 0-80-20
-----_____-
I _ _
0-95-5
0-90-10 --+
'
0-75-25
* NS
NS
0-32-68
- --
-2 3 . 5
NSb NS * NS
-
7
0-60-40
---+
NS NS NSc
-23.6 -23 6
-23.4d
4
a Arrows indicate the sequence of operations on each sample. The absence of an arrow indicates that the datum was obtained on a solution freshly prepared a t the stated sulfuric acid concentration. No signal observed. No signal was observed even when the amount of acetic acid employed was increased by a factor of 4. I n a sample containing four times the normal amount of "acetic acid" another weak signal a t - 32 was observed.
Table 11:
C13
Chemical Shifts of Benzoic Acid Carboxyl CL3in Sulfuric Acid Solutions Ppm with Respect to Benzene
-54.3 -54.2
10 11 12 13 14
+ - 51.8
- -54.5
-54.4
-
-51.7 -26.2
-52.6b -26.3'
4
' Arrows indicate the sequence of operations on each sample. The absence of an arrow indicates that the datum was obtained on a solution freshly prepared a t the stated sulfuric acid concentration. normal amount, a weak signal a t - 11 ppm was also observed.
I n a sample containing four times the
Weak, broad signal.
~~
Table 111: Expt
C13
Chemical Shifts of Mesitoic Acid Carboxyl C13 in Sulfuric Acid Solutions, Ppm with Respect to Benzene 70~ ~ 0 H - ~ 7 s~oso8-----------------~-~~
_ _ _ I -
n0.O
15-85-0
15 16 17 18 19 20 21
- 57.3b
9-91-0 t -
-59.2'+
5-95-0
-60.0
3-97-0
xs
NS
0-100-0
0-90-0
0-65-35
0-32-68
- 33 .2d
-32.4
-32.2
-32.3
-57.9 -+
-32.2
-59.5
a Arrows indicate the sequence of operations on each sample. The absence of an arrow indicates that the datum was obtained on a solution freshly prepared at the stated sulfuric acid concentration. A weak signal at 3.0 ppm was also observed. A weak signal A weak signal a t -26.7 ppm was also detected. a t -54.2 ppm was also observed.
our earlier papere and with the interpretation of Den0 and co-workers.4 Also, the failure to observe a signal in the range of SO, concentrations from 10 to 20% could be considered consistent with a possible broadening effect of the weight-average signal of the equilibrating system represented by eq 1.l' However, the failure to observe signals in the solutions which T h e Journal of Physical Chemistry
contain 25% excess so3would require a different explanation since, in this range of oleum compositions, the species present according to previous conclusions4 would be presumably the same as that giving the sharp signal corresponding to a -23.5-ppm shift a t 40 and 68% excess SO3. Furthermore, it seems unlikely that this high-field shift could be due to the CHI-
1317
NOTES
CO+ ion since it is about 41 to 54 ppm higher than one would estimate from the data of Olah and COworker^.^ These investigators reported the C13 shift of the complex CH3C130+SbFe-in anhydrous H F from an INDOR experiment at -30.9 ppm relative to CF3C1302Hor -45.4 ppm relative to CH3C130F. From our data on these reference compounds these correspond to -64.7 and -77.8 ppm, respectively, with respect to benzene. While the acylium ion in an H2S04-S03medium need not give an identical shift to the complex studied by Olah and co-workers, a 41 to 54 ppni difference seems unlikely, and we conclude that the species observed with solutions in 40 and 68% so3 is not the acylium ion. The inability to observe a signal at 5, 10, 15, 20, and 25% SO3 may be due to rapid equilibria between interconvertible species. On the basis of the data of Den0 and co-workers4 we are forced to conclude also that whatever the identity of this “high-field” species may be, it must have a proton chemical shift almost identical with that of the acylium ion. It is also worth noting that a comparison of experiments 1, 2, 6, and 8 indicates that the equilibria are largely reversible, a t least to the extent indicated by the intensities of the primary resonance peaks. Benzoic Acad. Aside from cryoscopic2 and C13 magnetic resonance data6 which demonstrated the existence of the protonated carboxyl species of benzoic acid in 5% H20-95% HzS04and 100% HzS04solutions, the benzoic acid species in H2S04-S03systems have been rather poorly characterized. However, the data in Table I1 can be understood by analogy with results obtained with the other two acids. Thus, the peak at -54.3 ppm which occurs in all the solutions studied with less than 6% so3 is attributed to the protonated carboxyl species. The resonance at about -51.5 ppm which is observed in solutions with oleum compositions ranging from 90% Hzs04-10% so3 to 65% Hzso4-35% SO3 is attributed to the acylium ion, and the broad character and position of the signal in the 94% HzS04-6% SO3 solution is attributed to the effect of rapid equilibrium between these two species. The broadness of the signal obtained with the solution in 65% HzSO4-35% SO3 may be due to a broadening effect of an appreciable concentration of a species with which the acylium ion is in rapid, reversible equilibrium and to which the value -26.2 ppm is assigned for the solution in 32(% HzS0,--68% SO3. Presumably the latter species is analogous to the acetic acid species responsible for the -23.5 shift in the solutions with highest SO3 concentrations. As in the acetic acid case, comparison of the results from experiments 10 and 14 indicated the predominant reversibility of these systems.
Mesitoic Acid. The shifts reported in Table 111 are readily interpreted in terms of data previously obtained by proton nmr and ultraviolet spectroscopic methods3v4 Both types of study have shown that mesi toic acid exists predominantly as the protonated carboxyl species in about 91% H2SOd-9% HzO, as the acylium ion in 100% H2S04and as a one-to-one equilibrium mixture of these two species in 97y0 H2S04. Accordingly, we assign the value -59.3 ppm to the protonated carboxyl species and -32.2 ppm to the acylium ion and note that the latter appears to be the predominant species for all of the oleum compositions studied. We attribute our inability to detect a signal in the solution in 3% &0-97% H2S04to the broadening effect of the rapid equilibrium between acylium ion and the protonated carboxyl species. The Unknown, “High-Field Species.’’ The peaks occuring at -23.5 and -26.2 ppm in solutions of acetic and benzoic acids, respectively, in the oleum solutions with highest SO3 concentrations apparently betray similar species which heretofore have gone unnoticed or at least unreported. From the present results one notes that their formation is reversible to a t least a major degree. Their relative algebraic proximity suggested that perhaps they might be due to a common species; however, an experiment in which both acids were dissolved in 32% H2S04-68% SO3 gave two lines, a t -26.3 and -23.1 ppm, rather than the single line one might have expected from this hypothesis. Further work will be required to characterize these new species adequately. Acknowledgment. The authors gratefully acknowledge helpful discussions with Professor A. T. Bottini and the assistance of Mr. Paul Ellis with some of the experiments.
The Reaction of Oxygen Atoms with Iodine’ by D. I. Walton and L. F. Phillips Chemistry Department, University o f Canterbury, Christchurch, N e w Zealand (Receized November I,1965)
In a previous study of the reaction of iodine wit’h atomic nitrogen2 it was observed that a solid film, having the physical properties expected for 1 2 0 6 , ~~
(1) This work was supported by Grant AF-AFOSR-264-65 from the
U. S. Air Force Office of Scientific Research. (2) C . G. Freeman and L. F. Phillips, J . Phys. Chem., 68, 362 (1964).
Volume 70, Number 4
A p r i l 1966
