Correlation of Aromatic Substituent Effects with Gas-Sol id Chromatographic Retention David J. Brookman and Donald T. Sawyer Department of Chemistry, Uniaersity of California, Riuerside, Calif. 92502 RECENT STUDIES of salt-modified aluminas as adsorbents in gas-solid chromatography (GSC) have allowed the evaluation of sorbate-sorbent interactions ( I ) . These interactions are separable into nonspecific and specific contributions, depending upon characteristics of the sorbate. However, a major limitation has been that oxygen-containing compounds are not eluted from salt-modified alumina columns. The use of porous silica beads as adsorbents in GSC has been reported recently (2), but considerable peak asymmetry occurs when high-molecular weight hydrocarbons are eluted. Such asymmetry indicates a nonlinear adsorption isotherm similar to those observed with unmodified alumina adsorbents. The improvement of the latter by salt modification has led to an investigation of the gas chromatographic behavior of Na2S04-deactivated porous silica beads. EXPERIMENTAL
A weighed portion of oven-dried porous silica spheres with a surface area of 50 m2/gram (Porasil C, Waters Associates, Framingham, Mass.) was added to an aqueous solution of reagent grade Na2S04. Enough salt was used to give a coating of 10% by weight of NasSOa on the Porasil C. The mixture was stirred with a pointed glass rod, the water was partially evaporated on a steam bath (until a thick slurry resulted), and the remainder of the water was allowed to evaporate at room temperature. The resulting coated adsorbent was passed through a 100-mesh sieve and used to pack a 0.125-inch (0.d.) X 3-foot stainless steel column. Prior to packing, the column was rinsed with polar and nonpolar solvents. The column was activated at 300 “C for 2 hours before retention data were taken. Sorbate samples were prepared by adding a small amount of the pure liquid to a septum-closed vial which contained nitrogen. A small amount of pentane was added to each sample vial to serve as an internal standard. Samples for injection were removed from these vials by means of a gastight syringe. A modified Barber-Colman Series 5000 gas chromatograph was used for the gas chromatographic measurements. The ratio of apparent retention volumes (referred to pentane) was calculated by taking the ratio of the retention time of the compound under consideration to that of pentane, retention times being measured from injection point. This ratio, S, was converted to the apparent retention volume, VR’, by multiplication by the apparent retention volume of pentane. The latter was determined by the relation VR’ = tn’ X (TCITA)X j X Fa
where tR’ is the retention time, Tc and TA are the temperatures (OK) of the column and the flowmeter, respectively, j is the James-Martin compressibility correction factor, and F, is the volume flow rate at T A . The apparent retention volumes were converted to capacity factors, k I , by the relation
2D 1.5-7
LO--
0.5--3 0 ..I 0
0.0--
-0.5I
0
I
100
200 Tsp
0
1°C
Figure 1. Logarithms of capacity factors, k’, for a series of hydrocarbons us. their boiling points using a 10% wt/wt Na,SO,-modified Porasil C (100-120 mesh) column at 200 “C k’ = V R / V ~with , V R the corrected retention volume and v d the dead volume of the column. For a given temperature, A( AG”)r = 2.3 RT d(1og k ’ ) T , Carrier gas; He, 20 ml/min
where V,‘ is the system dead volume, determined by injection of methane onto an initial base line of methane and Vd is the free gas space of the column determined by comparison of V d ’with the value for a known column volume ( I ) . RESULTS AND DISCUSSION
Figure 1 illustrates the variation of the logarithm of the capacity factor, k ’ , for the investigated compounds as a function of their boiling points. On the basis of Trouton’s rule, the enthalpies of vaporization for nonpolar compounds are directly proportional to their boiling points. A similar but less direct relation exists for the enthalpy of adsorption because of its approximate proportionality to the enthalpy of vaporization. Under the experimental conditions of gas chromatography the enthalpy is the main part of the free energy of adsorption (3). Thus, the latter generally is proportional to the boiling point of the nonpolar compound.
(1) D. J. Brookman and D. T. Sawyer, ANAL.CHEM.,40, 106
(1968). (2) C. L. Guillemin, M. LePage, R. Beau, and A. J. de Vries, ibid., 39, 941 (1967).
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ANALYTICAL CHEMISTRY
(3) H. Purnell, “Gas Chromatography,” Chap. 10, John Wiley & Sons, New York, N. Y., 1962.
Table I. Relative Retentions of Substituted Benzenes and Their Relation to Pi-Electron Density and Substituent Constants 10% wt/wt Na2S04 on 100-120 mesh Porasil C, 3 feet X 0.125 inch 0. d. Temperature, 200 “C; carrier gas, He, 20 ml/min Column; Subst. Data from Figure 2 ( R ) Data from Figure 1 (TBP) constants Substituent A log k’ A (-AGO), cal A log k’ A (-AGO), cal (51, UP A. -F - CI
-Br -1
-0CH3 B. -CHI -CFa -NO* -CsH:,
-0.065 -0.302 -0,403 -0.510 +O. 287 0.000 -0,090 +O. 395 -0.155
- 141 - 653 - 873 -1105 $621 0
- 195 +856 - 336
In addition, for gas-solid chromatography the logarithm of the corrected retention volume and hence the logarithm of k’ is linearly related to the free energy of adsorption. Therefore, a proportional relationship between log k’ and the boiling points of nonpolar compounds is expected. This is confirmed by the smooth curves for the n-paraffin series as well as the methyl benzenes (Figure 1). The latter line does not coincide with the paraffin line because of the presence of specific interactions causing the free energy of adsorption to deviate nonlinearly from the enthalpy of vaporization ( I ) . The substituted aromatic compounds deviate from the aromatic line for the same reason. The separation between the aromatic and aliphatic lines appears to be due to the pi-electron interaction of the aromatic system with the surface. Because cyclohexene falls approximately one third of the vertical distance between the paraffin line and the aromatic line, the implication is that displacement from the aliphatic line is due solely to differences in the pi-electron specific interaction. A similar relationship is illustrated by Figure 2 which gives the variation of the logarithm of the capacity factor, k’, with the room-temperature molar refraction, R, for several of the same compounds. The straight lines that are obtained for methyl-substituted benzenes and normal paraffins imply that for these essentially nonpolar compound groups the free energy of adsorption is a linear function of the polarizability. Again, the separation between the two lines appears to be due to differences in degrees of specific interaction. The vertical distance between compounds on the logarithmic plots (and between compounds and the aromatic line) corresponds to their difference in standard free energy of adsorption, A(-AGO), at the column temperature, T. This conclusion is based on the Cremer-Muller relation ( 4 ) -AGO = RTln V,
- RTln A
+B = RTln k ’
- RTln ( A / V d )+ B
where R is the gas constant, A is the surface area of the sorbent material in the column, and B is an integration constant. The vertical distance between the n-paraffin line and the line for the methylated benzenes corresponds to the free energy of adsorption, -AGO, due to the specific interaction of the three pi bonds; this is approximately 1100 calories based on the average of the data of Figures 1 and 2. The vertical deviation of the substituted benzenes from the line for the methylated benzenes indicates the effect of the (4) E. Cremer and R. Muller, 2.Elecfrochem.,55,66 (1951).
- 37 - 178
-0,017 -0.082 -0.093 -0.191 +0.618
+0.06 +0.23 +0.23 +0.28 -0.27
-201 -414 1340
+
0.000 -0.121 +1.300
0
+2810
...
L1
25
-0.17
+o. 55
-262
+0.78 01
+o.
I
I
I
3s
45
55
MOLAR REFRACTION, cm3/more
Figure 2. Logarithms of capacity factors, k‘, for a series of hydrocarbons us. their molar refractions using a 10% wt/wt Na,SO,-modified Porasil C (100-120 mesh) column at 200 “C substituent upon the interaction by the pi-electron system with the adsorbent. Table I summarizes the differences of log k’ for the substituted benzenes (from Figures 1 and 2) and the corresponding differences in the standard free energies of adsorption, A(-AGO). The Hammett para sigma constants, u p ,for the substituents also are tabulated (5). A reasonable correlation between retention and substituent constants is evident in Section A of Table I, which implies that resonance has a major influence on the pi-electron density of the aromatic ring. For those compounds without the possibility of pi-electron shifts through resonance with the substituent group, the substituent effect is much smaller than implied by the r P value. This is illustrated by toluene and trifluorotoluene (Section B, Table I and Figures 1 and 2). Hence, the substituent effects observed by gas-solid chromatography appear to be due to pi-electron withdrawal from (or transfer to) the aromatic ring. Purely inductive effects evidently have little effect on retention. For this reason, reten(5) E. S . Gould, “Mechanism and Structure in Organic Chemistry,” Holt, Rinehart and Winston, New York, N. Y., 1959, p 221. VOL. 40, NO. 8, JULY 1968
1369
tion measurements with modified adsorbents should provide a means to separate resonance effects from inductive effects. The data in Figures 1 and 2 illustrate several other interesting points. In the case of biphenyl, only one ring appears to be able to interact as a pi-electron system with the adsorbent because its retention is approximately that expected from its up value. Previous studies indicate that the two aryl rings are not coplanar (67); this could account for the second aryl ring not interacting with the sorbent as a pi system. In contrast, nitrobenzene has a much higher retention than the substituent constant would predict. Evidently the additional pi system of the nitro group is in a position to interact with (6) 0. Bataiansen, Acta Chem. Scand., 3, 408 (1949). (7) F. J. Adrain, J. Clzem. Phys., 28, 608 (1958).
the surface. Naphthalene shows a decreased retention relative to the aromatic line even though the number of pi-electrons has increased. This may be a steric effect limiting the approach of the pi system to the adsorbent surface, or it may be because naphthalene has a smaller pi-electron density than benzene. Research in progress on other adsorbents and modifying materials may provide the means for measuring additional specific functional effects for organic molecules. RECEIVED for review February 15, 1968. Accepted May 6, 1968. Work supported by the US. Atomic Energy Commission under Contract No. AT-(1 1-1)-34, Project No. 45, and by an Environmental Sciences Predoctoral Fellowship (US. Public Health Service)to D.J.B.
Conversion of Water to Carbon Dioxide with N,N’-Ca rbonyIdiim idazole James C. Warf Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif,
THE VIGOROUS
REACTION of N,N’-carbonyldiimidazole with water to form carbon dioxide and imidazole has been known for some time ( I ) . In some current studies involving incorporation of oxygen-18 into microorganisms, an improved means of analysis for oxygen-18 was developed. Combustion of the microorganisms to carbon dioxide and water left the oxygen-18 divided between the two compounds ; conversion of all the oxygen-18 to carbon dioxide was desired for mass spectrometric analysis. The use of N,N’ - carbonyldiimidazole seemed promising for this purpose, despite the fact that attempts to employ this reaction analytically were not previously successful ( 2 ) owing to the formation of a protective coating of imidazole on the solid reagent.
EXPERIMENTAL
Reagents. The N,N’-carbonyldiimidazole was purchased from the J. T. Baker Chemical Co. The solvents, White Label quality from Eastman Organic Chemicals, were dried with Drierite and distilled at atmospheric pressure; the first 10% of the distillates was rejected. Oxygen-18 water was supplied by the Oak Ridge National Laboratories. USE OF SOLIDN,N’-CARBONYLDIIMIDAZOLE. Small columns containing 100 mg of the powdered reagent became inactivated and converted only a part of the water to carbon dioxide, as mentioned above. Tests showed, however, that a few milligrams of water, stored several hours in the presence of an excess of N,N’-carbonyldiimidazole, were completely converted to carbon dioxide. No residual water vapor could be detected mass spectrometrically. A method employing a solution of the reagent was expected to be rapid and to make the conversion practical. SOLVENTS FOR N,N’-CARBONYLDIIMIDAZOLE. A Solvent for the reagent was needed which had negligible vapor pressure at room temperature and which had appreciable solubility for water (so as to promote reaction). The first solvent tested (1) H. A. Staab, Angew. Clzem., 68, 754 (1956). (2) H. Malissa and E. Pell, “Microchemical Techniques,” N. D. Cheronis, Ed., Interscience, New York, 1961, p 378.
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ANALYTICAL CHEMISTRY
was diglyme [bis (Zmethoxyethyl) ether], but even without water, the mass spectrum showed many peaks, including those corresponding to mass numbers 44 and 46, thus making diglyme useless as a solvent. Other solvents tested were Carbitol acetate [2-(2-ethoxyethoxy)-ethyl acetate], quinoline, isoquinoline, and adiponitrile. None of these, when employed with N,N’-carbonyldiimidazole, gave extraneous mass spectrometric peaks, and all were suitable for the purpose. But only adiponitrile (bp 295 “C) seemed to form solutions which did not become discolored on storage. When the procedure given was followed, using 2 or 3 p1 of water, only carbon dioxide peaks were visible in the mass spectrometric tracing, with no water or other peaks. Apparatus and Procedure. The HzO or Hz0-C02 mixture from a combustion tube was carried by a stream of helium through a microbubbler containing 1.0 ml of a 10% solution of N,N’-carbonyldiimidazole in adiponitrile. The microbubbler was of 9-mm tubing and 10 cm long, and was packed with small glass beads. Vitron O-ring connections were employed. The COZ was finally trapped out in a liquid nitrogen bath, and the trap evacuated. A fresh portion of the adiponitrile solution was used for each run. The apparatus was preflushed with helium before use. A Consolidated Electrodynamics Corp. Model 103C instrument was employed in the mass spectrometric analyses. RESULTS
Analysis of Natural Water. Three analyses of ordinary water for by the procedure outlined gave the following results: 0.202, 0.200, and 0.204 atomic % I s 0 . The accepted natural abundance is 0.204 atomic % (3). In the calculations, correction was made for the l8Ocontributed to theCI601 8 0 by the N,N’-carbonyldiimidazole. Analysis of Enriched Water. A sample of Oak Ridge HZ1*0, specified as 8.6 atomic % l8Owas also analyzed in triplicate. This gave 8.36,8.60, and 8.43 atomic % I8O. (3) D. Samuel, “Oxygenases,” 0. Hayaishi, Ed., Academic Press, New York, 1962, p 32.
