benzil yields two well defined waves of approximately equal height. The half-wave potentials are 0.772 and approximately 1.57 V us. ACE. Under identical conditions, benzoin yields a wave identical in shape to that of the second benzil wave and with identical half-wave potential, indicating that benzoin is the product of reduction for the first wave. When benzil is reduced at a potential on top of the first wave, concentrations of up to 0.02M yielded a n-value of 2.04 electrons (16). It is interesting to note that the “depletion effect” discussed by Reynolds and Shalgosky (17) evidently did not interfere with the determinations of n-values as in each case the value obtained was within a few per cent of an integral number. Using conditions identical to those with benzil, solutions of diethyl benzoylphosphonate (18) have been electrolyzed for lengthy periods. A value of 2.03 electrons was found for the (16) A similar reduction in an aqueous system at low pH values has been reported. See Robert H. Philp, Jr., Robert L. Flurry, and R. A. Day, Jr., J . Electrochem. SOC.,111,328 (1964). (17) G.F. Reynolds and H. I. Shalgosky, Anal. Chim. Acra, 10, 386 (1954). (18) K. D.Berlin and H. A. Taylor, J . Am. Chem. SOC.,86, 3862 (1964).
reduction, and sufficient amounts of products were formed to permit analysis by GLC. The reduction was found to yield diethyl a-hydroxybenzylphosphonate as the only major product. In the absence of a proton source, fractional electron values are obtained, and benzoin is found to be the product. Further work on esters of this nature will be discussed in a forthcoming report. CONCLUSIONS
The apparatus described herein has been found useful for determining accurate coulometric values for various metal ions and organic materials. Sufficient reductions of two organic compounds at the DME have been accomplished so that product analysis was performed with accuracy by GLC. Various organic oxidations or reductions could be determined in a like manner. In addition, inorganic complexes could be electrolyzed and the mechanisms examined by coulometric determinations and product analysis. RECEIVED for review February 1, 1968. Accepted March 8, 1968. Work partially supported by the Public Health Service, G M 10367-06.
Determination of Complex Constants by Gas Chromatography H. Schneckol Organisch-chemisches Institut, Universitat Mainz, Germany
DISTRIBUTION MEASUREMENTS to determine complex formation constants were first used 30 years ago. In 1938, Winstein and Lucas determined equilibrium constants for olefin-Ag+ complexes by distribution between two liquid phases (CCla and aqueous salt solutions of constant ionic strength) (I). Recently, Muhs and Weiss applied gas chromatography (GC) for similar investigations by using AgNO3 loaded liquid phases (2). This very elegant and fast method suffers from the disadvantage that the plots of partition coefficient H us. AgN03 concentration are not linear because of a salting-out effect at high AgN03-concentration; an empirical approach had to be developed in order to evaluate slope and intersection with the abscissa required for calculation of the equilibrium constant K1, according to the Equation (2)
where KL is the partition coefficient of the uncomplexed solute in 4 M LiN03 solution. Interest in both procedures has focussed on the determination of olefin-AgN03 complexes ( I , 2). In the investigation of the polymerizability of unsaturated nitriles by AgN08 (3), we tried to apply the latter method to find out whether complex formation between salt and monomer took place. It was felt, however, that by maintaining constant ionic strength of the liquid phase, the salting-out effect might be eliminated, 1 Present address, Dunlop Research Centre, Sheridan Park, Ontario, Canada.
Table I. Equilibrium Complex Constants Kl and Partition Coefficients of Some Unsaturated Compounds with Ag+ at 61 “C Compound KL KI(I/mole) Acrylonitrile 85 0.703 a-Chloro acrylonitrile 51 0.283 Methacrylonitrile 65 0.774 Crotononitrile 165 0.940 Methyl methacrylate 51 0.0
thus improving the technique. LiN03 was used to maintain = 4. It is evident from Figure 1 that both partition coefficient and complex constant can be determined directly by the improved technique from the straight lines obtained. The values calculated are seen in Table I. The linear shape of all lines indicates that a) there is no interaction between LiN03 and the solute, b) the salting-out effect of both nitrates (Ag and Li) is at least similar. The complex constants were also determined by the older method ( I ) in aqueous lMAgNOa/NaNOs-benzene at 25 “C. In all cases, the equilibrium constants were larger by a factor of 5 compared to those of Table I. This increase might be due to the different ionic environment and to the lower temperature. The formation of weak complexes between AgNO3 and benzene ( 4 ) might, however, complicate the interI.(
(1) S. Winstein and H. J. Lucas, J. Amer. Chem. SOC.,60, 836
(1938). (2) M. A. Muhs and F. T. Weiss, ibid., 84,4697 (1962). (3) H.Schnecko, Makromolekulare Chem. 111, 146 (1968).
(4) L. J. Andrews and R. M. Keefer, J. Amer. Chem. SOC.,71,
3644 (1949). VOL. 40, NO. 8, JULY 1968
0
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Table 11. Columns Used to Determine Complex Constants Concentration in glycol, molesfl &Nos LiNOs 0 0 0 4.0 1.0 3.0 2.5 1.5 4.0 0
No 0
I I1 I11
IV 5
Density, 60 OC P , &ma 1.09 1.22 1.31 1.47 1.61
Liquid volume VL,m l a
6.53 6.17 5.71 5.37 4.58
Rate of flow, corr. Fc, ml/min5 115 102 127 102 118
For abbreviations, cf. (8).
pretation of the results, although basically, agreement between both methods has been reported (5). A third method of determining these complex constants by GC at one salt concentration has not been approached here because it require sadditional determination of vapor pressures of solutes (5, 6). Clearly, complex constants for the nitriles are smaller than for many unsaturated hydrocarbons (I, 2). This can be explained by the fact that the electron-donating power of the olefinic double bond is reduced by CN-substitution. For methyl methacrylate IClis essentially nil, although the steric effect of the methyl group appears to be insignificant because crotononitrile and methacrylonitrile have higher values than acrylonitrile. On the other hand, C1-substitution of the tertiary H results in decreased complex formation, caused by the electronic (-1)-effect prevailing over the mesomeric (+M)-effect. There may also be a contribution of the CN-group to the complex formation with Ag+ as polyacrylonitrile can be easily complexed with Ag+ (7) and inorganic cyanides are well known to form Ag’ complexes. On the other hand, one might expect larger KIvalues and smaller effects due to substitution on the double bond if the CN-group contributed significantly to the complex formation. Determination of complex constants on saturated nitriles to clarify these questions will be carried out. EXPERIMENTAL,
The chromatograms were obtained at 61 “C on a PerkinElmer Model 116E gas chromatograph with thermal conductivity detector, He carrier gas and 1.6 atm initial pressure (pi = 1215 mm). The column fillings (I/&. o.d., 2m) were prepared from firebrick (0.4-0.5 mm) with 4M salt solutions in a wt ratio of 60 :40. Table I1 shows the characteristics of the columns. Ten-microliter samples of the neat unsaturated compounds were injected and the retention times determined. The range of error was zk2Z (3 determinations for each point of Figure 1). Although the stability of the column at 60 “C will be limited, there was no change observed during this study in the values obtained. The partition
( 5 ) E. Gil-Av and J. Herling, J. Phys. Chem., 66, 1208 (1962). (6) J. H. Purnell, in “Gas Chromatography, 1966,” A. B. Littlewood, Ed., Elsevier, New York, Co., 1967. (7) H. Schnecko, Chimia, 19, 113 (1965).
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ANALYTICAL CHEMISTRY
0
1
2
Figure 1. Partition coefficient H VS. AgNOs-concentration of columns used in GC Curve 1. Acrylonitrile Curve 2. Methacrylonitrile Curve 3. a-Chloro acrylonitrile Curve 4. Crotononitrile Curve 5 . Methyl methacrylate coefficient H was calculated according to Johnson and Stross (2, 8). RECEIVED for review February 7, 1968. Accepted March 29, 1968. (8) H.W.Johnson and F. H. Stross, ANAL.CHEM., 30.1586 (1959).
