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Intermolecular Association Eugene E. Schrier
State University of
N e w York Binghamton
And Molar Polarization Physical chemistry experiment
The dipole moment of a compound in solution is often determined1 by extrapolating the molar polarization of the solute, PZM, to zero solute concentration. This procedure leaves the impression that there is nothing to be gained from examining with solute concentration. In the variation in PZM fact, interesting information can be derived from these trends. For most polar solutes, Pexshows a small decrease with increasing solute concentration due to dipoledipole interactions. In certain cases, however, the changes of PzM with solute concentration are very large with either a positive or negative slope. These variations are attributed to intermolecular interaction involving hydrogen bonds. Solution of amides in nonpolar solvents display this type of behavior. In the experiment to be described, the concentration dependence of PZM is obtained for the amides, 2-oxohexamethyleneimine (caprolactam), I, and N-methylacetamide, 11, in benzene solution:
The data are interpreted in terms of the contrasting nature of the modes of intermolecular association of these compounds. While caprolactam and N-methylacetamide have the same functional group, they exhibit different stereochemical configurations, the lactam, I, and the amide, 11, possessing cis and trans coniigura tions, respectively, in solution. In the cis configuration, the oxygen and hydrogen are on the same side of the central C-N bond while the opposite is trne for the trans compound, as depicted in the structures given above. This difference of configuration allows only ring dimerization in the case of the lactam while Nmethylacetamide associates, through hydrogen bonding, to chain oligomers containing several monomer units. These differing modes of noncovalent association for compounds containing the same functional group are indicative of effects ~roducedbv variations in st&eochemical configuration. The dekonstration, by polarization measurements, of these differences in the 'SROEMAKER, DAVIDP., AND GARLAND, CARLW., "Experiments in Physical Chemistry," McGraw-Hill Book Company, he., New York, 1962, pp. 27583.
behavior of the lactam and amide is the purpose of this experiment. Configurational isomerism is possible in these compounds because the amide group possesses partial double bond character with contributions from structures such as
Free rotation around the C-N bond is thereby restricted, allowing the existence of cis and trans isomers. LaPlanche and RogersZ recently demonstrated that N-alkyl substituted amides exist entirely in the trans configuration when the carbonyl substituent is larger than hydrogen. Miznshima and co-workersareached a similar conclusion from dielectric measurements. On the other hand, the small ring lactams have been assigned the cis configuration by the use of a combination of polarization and spectroscopic measurements.' Mizushima et aLa showed that, for the trans amide, N-methyl acetamide, in carbon tetrachloride solution, the molar polarization of the solute, PZM, increased sharply with increasing concentration of solute. Here,
where a = measured dielectric constant, X = mole fraction, M = molecular weight, p = density, and the subscripts, s, 1, and 2 refer to the solution, solvent, and solute, respectively. For lactams with a ring size of less than nine members, Huisgen and Walz'observed that the molar polarization of the solute decreased with increasing concentration. The results of these investigations were interpreted in the following way. The cis lactam was considered to form primarily cyclic dimers by hydrogen bonding,
LAPLANCHE, L. A,, AND ROGERS, M. T.,J. Am. Chem. Soc., 86. 337 119641. . . hl1rus~1rr.8, S., KT nr.., J . :lm. (.%em. Sac., 72, .180 1050). ' HII n c E s , R., A N D W.\LZ,H., Chem. Her., 89,2616, IYX,. Volume 43, Number 5, May 1966
in which there would be a reduction of the net moment of the dimer from that expected for two monomer units by partial cancellation of the moments of the monomer units. I n the case of the trans amide, because of the steric arrangement of the monomer, extended chain oligomers were assumed to form H
leading to a higher resulting moment for the oligomer over the sum of the moments for the free monomers. Huisgen and Walz4 supposed that the enhanced moment of the oligomer was due to increased participation of the polar form (B) of the amide group (see above) in the ground state of the monomer when hydrogen bonds were formed. The dipole moment of the monomer unit was assumed, therefore, to be increased when it participated in oligomer formation. These larger dipoles might be aligned parallel to each other in a relatively rigid structure, leading to the moment of the oligomer in excess of that given by the sum of the contributing monomer units. Huisgen and Walz4 also observed that the concentra, tion dependence of the molar polarization .changed from a negative to a positive slope as the lactam ring size increased past nine members, implying that the configuration changed from cis to trans a t that point. The molar polarization of the solute, extrapolated to infinite dilution, and the resulting dipole moments show little trend with ring size. Therefore, these inferences regarding the configuration of the amide group could not have been made from a conventional treatment of the data, but required interpretation of the coucentration dependence of the molar polarization.
The experiment, based on these observations, may be carried out in one four-hour laboratory period using a moderately sensitive measuring instrument. We have employed the Dipolemeter DM-01, distributed in this country by the Kahl Scientific Company, El Cajon, California. The instrument utilizes the heterodyne beat principle and is capable of sensitivity in the parts per ten thousand range. Eastman N-methylacetamide and 2-oxohexamethylenimine (caprolactam), which are available a t low cost, were used without further purification. The experimental procedure is similar to that described by Shoemaker and Garland' except that the instrument used cannot be calibrated against air. Instead, a calibration curve of capacitance reading versus dielectric constant was prepared by measuring the capacitance of analytical grade benzene, ethyl ether, and chlorobenzene at 25.0°C; these readings were then plotted against values of the dielectric constant for these compounds (as given in the "Handbook of Physics and Chemi~try").~The amide and lactam were dissolved in benzene to make approximately 0.25 M solutions of each compound. The capacities of these stock solutions and of two-, four-, and tenfold dilutions of each stock solution were measured a t 25.0°C. Using these measurements and
Figure 2. The molar polarization of the solute, coproloctmm, in benzene solution as a function of rolvk mole fraction a t 25.0°C.
the calibration curve, the dielectric constants of these solutions could be calculated. The densities of the solutions and that of benzene were measured using a Christian Becker density balance which gives densities to the fourth decimal place. Mole fractions were calculated using molar concentrations and densities in the equation :
3m 0 m5
o.s,o MOLE FRACTIOH
Figure 1. The molar polarlrdion of the solute, N-methylocetomide, in benzene rolvtion as a funcllon of solute mole fraction 01 25.0°C.
where Xz = mole fraction of the solute, cz = molar concentration of the solute, MI and MI = molecular weight of solvent and solute, respectively, and p. = density of the solution. Alternatively, the solutions may be made up by weight. The molar polarization
journal o f Chemical Education
6 "Handbook of Chemistry and Physics," 45th edition, The Chemical Rubber Co., Cleveland, Ohio, 1964, pp. E-3&E-33.
of the solute a t each concentration was calculated using equation (1). The data are shown in Figures 1 and 2. The trends described ahove are clearly seen in these plots; the molar polarization of caprolactam decreases with increasing solute concentration, and the opposite behavior is observed for N-methylacetamide. To run the experiment in our physical chemistry laboratory, we give the above experimental instructions to our students and refresh their memories regarding the nature of arnides and the possible steric configura tions of the amide group. They run the experiment and calculate the data. We ask them to give an inter-
pretation of the trends observed in the data on the basis of the configurations of the amides used and the possibility of molecular association in solution. After some thought on their part and a bit of leading by us, a fair proportion have come up with an interpretation similar to the one given ahove. The research flavor of the problem and the simplicity of the approach make the experiment a popular one. Acknowledgment
We are grateful to Dr. B. E. Norcross for a useful suggestion regarding this experiment.