Nov., 1960
SPECTRA OF COMPLEXES BETWEEN KETONES AND CALCIUM ~IOSTMORILLONITE
D,O than in HzO. Glutamic acid has a second ionization and the slightly greater difference between p K in HzO and DzO may be due to a contribution from this second ionization. The difference of 0.5 pK unit is equal to the difference in pK for the ionization of acetic acid in light and A the acetic heavy water. Although the ~ K for acid ionization is 4.7 rather than -2, the proton transfer mechanism is the same for each of the amino acids as for the carboxylic acid. This is in agreement with the conclusion mentioned above that the most important single criterion in determining the difference in pK in HzO and DzO
1655
systems is the nature of the group to which the proton is attached. Unfortunately, this A p K is so close to 0.5 for all equilibria where valid measurements exist, that a systematic study of the differences which can be attributed to the specific bonds affected seems remote. When deuterium is substituted for hydrogen in an acid base interaction, the change in equilibrium constant measures a difference in the relative binding power of two bases for hydrogen and deuterium. There appears to be no simple way to determine how much of this difference between the two bases should be attributed to either base.
ISFRARED STUDIES OF SOME COMPLEXES BETWEEN KETONES AND CALCIUM M09TMORILLONITE. CLAY-ORGANIC STUDIES. PART IIIl BY
LOWELL G. TENSMEYER,~ REINHARD
w.HOFFMANN AND G. lv. BRINDLEY
Contribution N o . 59-87 f r o m the College of Mineral Industries, Dept. of Ceramic Technology, The Pennsylvania State University, University Park, Pennsylvania Received April 20,1960
Infrared spectra of one- and two-layer complexes of 2,5-hexanedione and 2,5,8-nonanetrione with calcium montmorillonite have been obtained by differential techniques. The samples were prepared by evaporation of clay suspensions with and without ketones on AgC1-windows and glass plates. Infrared absorption spectra and X-ray diffraction patterns were taken of the resulting films. Infrared spectra were obtained also of solutions of the ketones of CC1, and CS2, of liquid 2,5-hexanedione and of solid 2,5,S-nonanetrione1 the latter presented to the beam by several techni ues. The following vibrational assignments are made for the unadsorbed ketones: 3005 cm.-' CHI stretching a to C=8; 2959 cm.-l CH2 asymmetric stretching; 2912 cm.-l CH2 symmetric stretching; 1724 cm.-' C=O stretching; 1412 cm.-l CH2 deformation CY to C=O; 1401 cm.-l CH2deformation coupled to C-C; 1365 cm.-l CH3 deformation LY to C=O; 1360 cm.-l CH2 and CH, deformation a to C=O. Upon adsorption, significant changes occur in the carbonyl stretching frequency and the methyl and methylene deformation frequencies. Spectra of the one-layer complexes of 2,5-hexanedione and 2,5,&nonanetrione are quite similar to that of solid 2,5,&nonanetrione, whereas the two-layer complexes show less similarity. These data are interpreted in terms of a highly ordered one-layer complex and a decrease in order upon introduction of a second layer.
1. Introduction
This investigation forms part of a program for studying the adsorption of organic molecules on clay minerals. A previous paper' gave results for the adsorption of neutral organic molecules from aqueous solutions on calcium montmorillonite. With a view to obtaining more detailed information on the state of the adsorbed molecules this study has been made of the infrared absorption spectra of two organic-clay complexes, chosen from among the organic materials examined in the previous work. The compounds selected were 2,Ei-hexanedione and 2,5,8-nonanetrione. Structurally similar to each other, they are adsorbed to differing extents on montmorillonite from aqueous solutions. The most interesting parts of their spectra, the C=O and C-H vibrations, have frequencies not obscured by the vibrations of the montmorillonite lattice. The interaction between clay mineral surface and adsorbed organic molecules would be expected to produce frequency and intensity changes in the spectrum of the adsorbed molecule. Intensity changes may also arise from different orientations (1) Part 11. R. W. Hoffmann and G . W. Brindley, ".4dsorption of Non-ionic Aliphatic Molecules from Aqueous Solutiona on Montrnorillonite," snbmitted to Geochzm. Cosmochzm Acta. (2) Temporary Research Fellow a t the Pennsylpania State Univerm t ) during Summer 1959, now a t Llnde Company, Indianapolis, Ind.
of the organic molecules on the clay surface and from different orientations of the clay particles with respect to the infrared beam. To interpret the spectra of the adsorbed molecules profitably and also to detect unadsorbed ketone in a clay-organic complex, one must have available the frequencies, frequency assignments and the extinction coefficients for the unadsorbed compound, at least for the spectral regions not obscured by infrared absorption of the clay. Strictly considered, only the frequencies of a gas at low pressure can be considered unperturbed, especially for molecules containing highly polar bonds. The spectrum of a dilute solution of a compound in a non-polar solvent approaches that of the gas, and dilute solution spectra are used as references in the present work, as well as the spectra of liquid 2,5-hexanedione and of solid 2,5,8-nonanetrione. 2. Experimental I. Materials.-( a) 2,BHexanedione was obtained from Aldrich Chemicals Co. and was redistilled before use; b.p. 82.2-82.7' (17 mm.). (b) 2,5,8-Nonanetrione was prepared in this laboratory as white plate-like and prismatic crystals, m.p. 55'. (5Methylfurfury1)-acetone was prepared according to Alder and Schmidt.3 The reaction was by no means spontaneous (3) K. Alder and
(1943).
C. H. Schmidt, Ber. deut. chem. Ges., 16B, 183
1656
L. G. TENSMEYER, R. W. HOFFMANN AND G. W. BRINDLEY
and the yield obtained (38%) waa leas than that quoted (65%). Both effects may be due to the higher hydroquinone content of the methyl vinyl ketone starting material. Attempts to saponify 5-methylfurfury1)-acetone with HCl/CHaOH according to der resulted in black tars which gave only poor yields of 2,5,8-nonanetrione. Saponification similar to that described by Benson4 for 2,5-dimethylfurane ave satisfactory results. Forty-seven and one-half g. of 5-methylfurfuryl)-acetone, 30 ml. of HeO, 25 ml. of acetic acid, and 1.5 ml. of 10% H&Oc were refluxed for 36 hours. After adding 1.65 g. of CHsCOONa the solvent was distilled off in vacuo. The dried residue waa digested with 200 ml. of ether and subsequently extracted in a Soxhlet apparatus with ether. From the combined ether solutions the product waa Rfecipitated by the same volume of petroleum ether and tered off after standing overnight in a refrigerator. Including a second fraction from the mother liquor the yield of crude product was 29.5 g. (55%). Two recrystallizations from ether-petroleum ether with Norite produced 18.5 g. (c) The preparation of the clay suspension containing 30 mg. of Ca montmorillonite per ml. has been described elsewhere.’ (d) All solvents used were spectral grade. II. S ectra.-All spectra reported were obtained on a Perkin-%mer Model 21 Infrared Spectrophotometer, using a resolution setting of 925. Mechanical and spectral slit widths for this setting a t absorbed frequencies are presented in Table I. The wave length scale waa expanded to 50 cm./p when necessary for accurate wave length determinations. The instrument, using a NaCl prism, was calibrated with polystyrene film. and gaseous H20, NH3 and Cot using freuencies listed in the Perkin-Elmer Instruction Manual. Zalibration wave lengths were reproducible to within 0.004 p. The wave length drive was run very slowly, often in the range 1-2 p / h r . (a) Spectra of Solutions.-Spectra were measured using a 0.103 mm. NaCl cell against a solvent-filled 0.106 mm. NaCl cell. The solutions had the concentrations: 2,5hexanedione: 1.305 M in CSt, 0.2898, 0.1967, 0.1179 and 0.0143 M in CCl4; 2,5,&nonanetrione: 0.440 M in cS2, 0.3366,0.1757 and 0.0780 M in CCL. (b) Spectrum of Liquid Z,S-Hexanedione.-The optical absorption of the carbonyl group is so intense that, for the pure liquid, a cell of 2 p or less would be required to determine extinction coefficients. As this is impracticable, a qualitative spectrum was obtained by pressing the liquid between two NaC1-plates. (c) Spectra of Solid 2,5,8-nonanetrione.-The spectra of solid 2,5,8-nonanetrione were obtained by three methods: 2,5,&Nonanetrione was mixed with Harshaw KBr and “ground” for a definite period of 15,25 or 120 seconds on a Wig-LBug amalgamator, then evacuated and pressed to 24 tons/cm.z into a stainless steel disk having an oval opening of 1.18 cm.*. The disc was notched for positioning in the spectrometer.6 I n the second method, flat flakes of 2,5,&nonanetrione were placed on a pure KBr pellet, then covered with KBr powder and pressed again in the die, leaving the “layered” 2,5,8-nonanetrione firmly fixed between KBr pellets, but perhaps with some crystal distortions. I n the third method, CC14 solutions were allowed to evaporate on KBr discs, presenting a “precipitated” group of small crystals to the infrared beam. (d) Spectra of the Clay-Organic Complexes.-One- and two-layer complexes of the ketones with calcium montmorillonite were prepared by mixing 2 ml. of Ca montmorillonite suspension (30 mg./ml.) and 0.5 ml. of an appropriate concentrated ketone solution in water. One ml. each of the resulting suspension was dried on a glass slide (for X-ray measurement) and on a silver chloride infrared window. Teflon washers each having an open area of 3.75 cm.2 held the evaporating suspension and fixed the shape of the resultant film. Both samples were dried a t temperatures below 20’. To protect together over PzO~ the sample from atmospheric moisture, the film was covered by a NaCl window separated from the AgCl plate by a Teflon spacer. The whole system was held tightly in a lucite sample holder. For the reference beam a similar assembly waa prepared containing a film of the same amount of pure Ca montmorillonite without any organic material.
lu
f
(4) G. Benson, Org. Synthesis, XVI, 26 (1936).
(5) H.T. Grendon and H. I,. LovcI1, Anal. Chem.. 88, 300 (1960).
VOl. 64
III. Calculations of Extinction Coefficients. (a) Dissolved Ketones.-Following Jones and Sandorfy,’ ap arent molecular extinction coefficients d a ) were obtainec? from the relationship A,
=
log
(g)
=
CZ
where A , is the observed absorbance, Tuand T the intensity of incident and transmitted li ht, respectively, a t frequency Y, C the concentration of sofute in moles per liter, and I the cell length in cm. The dimensions of e,(*) are therefore loacm.2per mole. In the calculation of E”(*) only absorbance values between 0.15 and 0.80 were used. When several values in this range were obtained for a particular frequency, the average deviation in e, waa less than 2%, w t h the exception of the carbonyl peak a t 1721 cm.-l (5.809 p) which is discussed below. The apparent molar extinction coefficient is a function of the epectral slit width, and therefore of the resolution setting. The resolution program used in this research, 925, gives slit widths narrower than are usually used in solid and liquid studies, but still not so narrow that the e(*) values can be taken aa emax(8) values.’ Quantitative reproducibility suffers, however, with narrower slit spacings, and the setting 925 waa chosen as a compromise between reproducibility and an approach to emax(*). (b) Solid 2,5,8-Nonanetrione.-It has been shown8 that under certain conditions the infrared absorption of a finely divided (
