KOTES
1930
that even such direct computations could be made to enough precision to be useful. Steps a and b would provide a practical approach to gas imperfection in the presence of a solid phase and would not require condensed phase cluster expansions. This latter approach is similar to treatments of the first few virial coefficients in adsorption on solids.la Acknowledgments.-The author wishes to thank Professor I. Prigogine and Professor A. Bellemans for their hospitality during his stay a t the Free University of Brussels and for many helpful discussions on the above subject. (13) M. P. Freeman and G. D. Halsey, J. Phys. Chem., 69, 181 (1955); 62, 729 (1958).
LI. P. Freeman, zbid.,
T H E RELATIVE BASE STRENGTHS OF DIMETHYL ETHER AND DIETHYL ETHER1 BY CHARLES H.
I r A X DYKIG' AND
ATAXG. MACDIARMlD3
John Harrison Laboratory of Chemistry. University of Pennsylaania, Philadelphia 4, Pennsylvania
Receiaed March 2% 2969
Although it is well est'ablished that the methyl group has electron releasing characteristics, previous investigations on the relative base strengths of dimethyl and diethyl ethers indicate that dimethyl ether acts as a stronger Lewis base when either boron trifluoride or diborane is used as reference a ~ i d . ~It, ~has been suggested that this apparent anomaly is due to the greater steric requirements of the ethyl, as compared to the methyl, group when interacting with the relatively large boron acids. Studies involving the relative extents of protolysis of the ethers in sulfuric acid solution were not sufficiently sensitive to distinguish between their relative base strengthse6 I n the present study, we have measured the relat'ive base strengths by a method which would considerably reduce contributions from steric factors. Experimental The relative base strengths of dimethyl and diethyl ether were determined by means of an infrared hydrogen bonding study. High vacuum techniques were used in all possible manipulations. Infrared absorption spectra were recorded in the 3100-3800-cm.-1 region using a Model 421 Perkin-Elmer double beam grating spectrophotometer. The reproducibility of the absorption maxima was =t1 em. -l for the free OH stretching frequency and + 3 cm.-' for the hydrogen-bonded OH stretching frequency. Matched quartz cells (10 mm.) each fitted with ground glass joint connections t o a side arm and a stopcook, which could be attached to a vacuum system, were employed. Phenol was purified by sublimation in DCCCUO,and methanol was dried over barium oxide. Reagent grade carbon tetrachloride, dried over phosphorus pentoxide, was employed as solvent. (1) This report is based on portions of a thesis t o be submitted by C. H. Van Dyke to the Graduate School of the University of Pennsylvania in partial fulfillment of the requirements for the degree of Doctor of Philosophy. This study is in part a contribution from the Laboratory for Research on the Structure of Matter, University of Pennsylvania, supported b y the Advanced Research Projects Agency, Office of the Seoretary of Defense. Reproduction in whole or in part is permitted for a n y purpose of the United States Government. (2) DuPont Teaching Fellow. (3) Alfred P. Sloan Fellow. (4) H. C. Brown and R. M. Adams, J . Am. Chem. Sac., 64, 2557 (1942); A. W. Laubengayer and G. R. Finlay, ibid., 65, 884 (1943); D. E. McLaughlin a n d M. Tamres, ibid., 82, 6618 (1960); H. E. Wirth and P. I. Slick, J . P h y s . Chem., 66, 2277 (1962). (5) B. Rioe, J. A. Livasy, and G. W. Schaeffer, J. Am. Chem. Soc., 77,2750 (1955). (6) E. M. Arnett and C. Y . Wu, ibid., 82, 4999 (1960).
Vol. 67
The ethers were carefully purified in the vacuum system and were distilled into the side arm of the cell assembly containing the solvent. The mixture was poured into the cell by tilting. Absorption effects due to the ether were eliminated by using a solution of the ether in the cell in the reference beam, the concentration of which was exactly equal to that of the ether in the solution containing the alcohol. The reliability of the experimenal technique employed was first checked by measuring AV values for two compounds previously studied using phenol as reference acid: (CZH&O,found 279 cm. -1, reported7 282 em. -1; [(CH&SiI20, found 172 em.-', reported? 169 crn.3.
Results and Discussion I n the present investigation, it has been found that when Lewis acids with lower steric requirements than boron trifluoride or diborane, such as phenol or methanol, are employed, the relative base strength of diethyl ether is in agreement with that predicted from electronic theories of organic chemistry and is greater than that of dimethyl ether. The interaction of the hydroxyl hydrogen atom in either methanol or phenol with the oxygen of the ether would appear to be more favorable sterically than the analogous interaction of the initially planar BF3 molecule.* The mechanism of reaction of BzH6 with an ether appears to be more complex and probably involves the initial formation of BHB units which could also possibly have a planar configuration. The extent of interaction between the ethers and the reference Lewis acids, methanol or phenol, was determined by observing the difference (Av in cm.-l) between the free OH and the hydrogen-bonded OH stretching frequencies in the infrared spectrum of a solution containing both the ether and the alcohol. A larger value of Av was observed for diethyl ether, regardless of whether phenol or methanol was employed. It has been well established that under similar experimental conditions, the ether which shows a greater value of Av with a given reference alcohol is the stronger ba~e.~.~ The results obtained with dimethyl and diethyl ethers are given in Table I. TABLE I FREQUENCY DIFFERENCESBETWEEN FREE AND HYDROQENBONDED OH STRETCHING FREQUENCIES -Methanol-------Ether Alcohol concn., concn., M IM
Ether
(CHa)&
i i
(CzH&O
0.25 .25 .25 .25
----
OH
M
Phenol----Alcohol concn., M
0,0061 .0091
130 128
0.33 .071 .25
0.0025 ,0025 ,0075
251 252 252
.0061 .0091
145 144
.33 .071 .25
.0025 .0025 .0075
279 276 279
Av
-
Ether concn.,
Av
-
OH
It is of interest to note that the Av values are independent of the concentration of both the ethers and the alcohols within experimental error in the concentration range investigated. I n fact, the value reported'" for phenol in pure diethyl ether, A V = 276 cm.-', is essentially identical with the values given above. (7) R. West, L. S.Whatley, and K. J. Lake, ibid., 83,761 (1961). (8) H. A. LBvy and L. 0. Brockway, ibzd., 69, 2085 (1937). (9) L. L. 8. Whatley, Ph.D. Thesis, University of Wisconsin, 1962; M. HorPk, V. BaZant, a n d V. Chvelovskl, Collection Czech. Chem. Commun., 26, 2822 (1960); S. Searles and M. Tamres, J . Am.. Chem. Sac., 79, 3704 (1951); W. Gordy a n d 8. C. Stanford, J. Chem. Phys., 8 , 170 (1940); 9 , 204 (1941); W. Gordy, zbzd., 7, 93 (1938); 9, 215 (1941); G. C. Pimentel and A . L. McClellan, "The Hydrogen Bond," W. H. Freeman and Co., San Francisco, Calif., 1960. (10) G . M. Barrow, J . Phys. Chem., 69, 1129 (1955).
