H-Bonding of Amine with Phenols
The Journal of Physical Chemistry, Vol. 83, No. 79, 1979 2455
used a SE-30 Silicone oil column or an Apiezon L column. Gas chromatography-mass spectrometric studies were carried out with a DP-1 mass spectrometer system (Du Pont Instruments). Spectroscopy. UV-visible spectra were recorded with a Cary-219 instrument. Emission spectra were obtained with an Aminco-Bowman spectroflurimeter.
based on the original value from Algar, B. E.; Stevens, B. J . Phys. Chem. 1970, 74,3029-3034. (6) The mechanism for the formation of phenyl benzoate is unclear; however, irradiation of authentic samples of benzil in oxygen-saturated benzene leaves no doubt that this is the major product of the reaction. (7) A related molecule, 2,3-diphenyl-p-dioxene, is known to react with singlet oxygen, ultimately leading to carbonyl-containing products: Schaap, A. P.; Thayer, A. L.; Blossey, E. C.; Neckers, D. C. J. Am. Chem. SOC.1975, 97, 3741-3745;Srinivasan, V. S.; Podolski, D.; Westrick, N. J.; Neckers, D. C. J. Am. Chem. SOC. 1978, 100,
Acknowledgment. Thanks are due to Dr. J. Brummer for valuable suggestions.
6513-6515. (8) Scaiano, J. C. J. Am. Chem. SOC.1977, 99, 1494-1498. (9) Schwerzed, R. E.; Caldweli, R. A. J . Am. Chem. SOC.1973, 95, 1382-1389;Webs, D. D. J. Photochem. 1976/77, 6 , 301-304; Formosinho, S.J. Mol. Photochem. 1976, 7, 13-39. (10) Watkins, A. R. Chem. Phys. Lett. 1974, 29, 526-528;Gijzeman,
References and Notes (1) (a) Radiation Laboratory, University of Notre Dame. (b) Department of Chemistry, Indian Institute of Technology, Kanpur, India. (2) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-1990from the Notre Dame Radiation Laboratory. (3) Lahiri, S.;Dabral, V.; Bhat, V.; Jemmis, E. D.; George, M. V. Proc. Indian Acad. Sci. Sect. A 1977, 86, 1-14. (4) For related references on the technique see: Small, Jr., R. D.; Scaiano, J. C. J . Phys. Chem. 1977, 81, 828-832,2126-2131;1978, 83,
2662-2664. (5) Brummer, J.; Wilkinson, F. J. Phys. Chem. Ref. Data, to be published:
0. L. J.; Kaufman, F.; Porter, G. J. Chem. SOC.,Faraday Trans. 11973, 69, 727-737. (11) Caldwell, R. A.; Pac, C . Chem. Phys. Lett. 1979, 64,303-306. (12) Burnburry, D. L.; Wang, C. T. Can. J. Chem. 1968, 46,1437-1479. (13) Burnbuny, D. L.; Chuang, T. T. Can. J. Chem. 1969, 47, 2045-2055. (14) Mladelung, W.; Oberwegner, M. E. Annales 1936, 526, 245. (15) Beck, T. G.; Barton, D. H. R.; Britten-Kelly, M. R.; Guziec, Jr., F. S. J. Chem. SOC., Chem. Commun. 1975, 539. (16) Patterson, L. K.; Scaiano, J. C., to be published. (17) Small, Jr., R. D.; Scaiano, J. C. J. Am. Chem. Sac. 1978, 100,
45 12-45 19.
Hydrogen Bonding Interactions of Aliphatic Amines with Ortho-Substituted Phenols Louis Farah, George Giles, Donna Wilson, Agnes Ohno,+ and Ronald M. Scott" Department of Chemistty, Eastern Michigan University, Ypsilanti, Michigan 48 197 (Received February 22, 1979)
Hydrogen bond formation between aliphatic amines and ]phenolsis not inhibited by a single ortho substitution, but is prevented when both ortho positions are substituted. Tertiary aliphatic amines are somewhat hindered sterically in forming hydrogen bonds when compared to primary and secondary aliphatic amines.
Introduction Spectrophotometric analysis has been used to study hydrogen bonding of a number of phen01s.l-l~ In one of these studies,13 the hydrogen bonding of p-chlorophenol with a variety of amines was studied in cyclohexane solution. It was found that a linear relationship exists between the log of the equilibrium constant for the formation of the hydrogen bond and the strength as Bronsted bases of primary and secondary aliphatic amines as represented by their aqueous pK,. Tertiary aliphatic amines also display a linear relationship between log K and aqueous pK,, but one whose slope indicates that hydrogen bond formation is less favored. Comparative studies were performed with N-ethylmorpholine and triethylenediamine, tertiary amines in which the nitrogen is a member of a ring so that alkyl groups are held back from interfering sterically with the phenol. N-Ethylmorpholine is more reactive in hydrogen bonding to phenol than its aqueous pK, would predict for a tertiary amine, and triethylenediamine was comparable to primary and secondary amines in reactivity. Steric hinderance was therefore described as the cause of the low equilibrium constants associated with the normal aliphatic tertiary amines. The data reported here are a continuation of the investigation of phenol-amine hydrogen bonding. The experiments run with p-chlorophenol are repeated with three more phenols: o-cresol, o-sec-butylphenol, and 'Department of Biological Chemistry, University of Michigan, Ann Arbor, Mich. 48106. 0022-3654/79/2083-2455$01 .OO/O
-
p-cresol. These phenols were selected because they are very similar in acid strength (pK, 10.2) but differ in substitution a t the ortho position. By comparison of results the steric effect of ortho substitution could be assessed.
Experimental Section Sublimation was used to purify o-cresol, p-cresol, 2,6dimethylphenol, and triethylenediamine. The remainder of the reagents and cyclohexane were purified by distillation. Phenol and amine stock solutions were prepared by weighing both solute and solvent. Solutions were prepared by pipetting aliquots into volumetric flasks, and weighing after e,ach addition. The order of addition was cyclohexane, phenol stock, amine stock, and cyclohexane to the mark. In each study the phenol concentration was held constant and amine concentration was varied. Absorption spectra were obtained with a Beckman Model DK-2A spectrophotometer with matched 1-cm silica cells. The temperature of both sample and reference cells was controlled by circulating water from a Lauda K-2/R constant temperature bath through the thermospacers of a brass cell holder similar to that described by Coggeshall and Lang.ll The spectra of the solutions were recorded from 240 to 315 nm. Each run consisted of at least five different temperatures ranging from 15 to 55 "C. All calculations of equilibrium and thermodynamic parameters were performed on the DEC 10 computer of Eastern Michigan University. Programs in Fortran IV
0 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 79, 7979
Farah et al.
TABLE I: Thermodynamic Parameters of Amine Hydrogen Bond Formation with p-Cresol p-cresol concn, M
amine
AH", kcal/mol
x 10-4
isopropylamine
2.7
n-propylamine
2.3 X
n-but ylamine
2.8
x 10-4
tert-butylamine
2.3
x 10-4
morpholine
2.5 x 10-4
N-ethylmorpholine
2.3
x
triethylamine
4.6
x 10-4
-8.25 -8.19 -8.67 -9.00 -8.09 -7.77 -7.73 -7.68 -5.90 -5.93 -6.77 -6.32 -7.24 -7.1
10-4
i
0.48
* 0.62
0.90 f 0.34 1.44 2.14 0.24 i 0.17 ?I 0.37 i: 0.6 1.33 + 1.28 0.44 0.06 i:
* * *
AS", cal/deg
-2.675 i. 0.014 -2.68 i: 0.03 -2.58 i. 0.024 -2.57 i. 0.01 -2.41 i 0.45 -2.42 i. 0.11 -2.72 i: 0.028 - 2 . 7 1 i 0.01 -2.39 0.027 -2.39 i. 0.03 -2.00 i- 0.11 -2.05 i. 0.07 -2.42 i. 0.067 -2.44 i: 0.003
- 18.3' - 18.5 - 20.5 -21.7 -19.1 - 15.0 -16.8 -16.7 -11.4 -11.9 - 16.0 - 14.4 - 16.2 - 15.7
*
*
*
A G o ,lrcal/mol
*
a The top values are the averages of the data collected with standard deviations. The lower value is the result of a single calculation with deviations calculated as shown in Figure 1.
2.0
i
/
1.0
I 7
H.
a
9
IO
II
pKa of Amine
1.2
V I
Flgure 2. Hydrogen bonding of o-cresol to amines. The log of the equilibrium constant at 25 O C for hydrogen bond formation is plotted vs. the aqueous pK, of the amines studied. The upper line contains values for primary and secondary amines, while the lower line represents values for tertiary amines. The amines are (A) n-propylamine, (6) isopropylamine, (C) n-butylamine, (D) terf-butylamine, (E) diethylamlne, (F) dibutylamine, (G) morpholine, (H) Nethylmorpholine, (J) triethylamine, (K) tripropylamine, (L) tributylamine.
1
I
I
I
I
3. I
3.2
3.3
3.4
3.5
IIT
Figure 1. A plot of log Kvs. 1/Tfor the formation of a hydrogen bond between p-cresol and triethylamine. Data for this plot were obtained spectrophotometricallyat 290.6 nm. From the slope of the line a value for AHo of -7.12 kcal/mol is obtained with a standard error of slope of f0.067 kcallmol. The value for A G O is obtained from the value of log K a t 25 O C on the best least-squares fit of a straight line to the experimental points. The value calculated is -2.44 kcal/mol with a standard error of Ygiven %of f0.0036 kcallmol. ASo is calculated as (AH" - AG")/T, given here a value of -15.7 calldeg.
convert data in terms of weight and density into molarities, then by using molarities and absorbance readings calculate equilibrium constants by the method of Rose and Drago.14 Calculations were made at several wavelengths, corrections were made for overlapping absorbance bands, and the constants so obtained were tested for their statistical validity by the method of C h a u ~ e n e t , applying '~ this test twice to the data for each series of amine additions. Those constants remaining were averaged. Thermodynamic parameters were obtained from average log K values at various temperatures. Values of AGO were based on the value of log K at 25 "C on a line representing
the least-squares fit of the data to a log K vs. 1 / T plot (Figure 1). Values for AGO were based on the value of log K at 25 O C on a line representing the least-squares fit of the data to a log K vs. 1 / T plot. Values for AH" were obtained from the slope of this line, and ASo values were calculated.
'Results Solutions of 2,6-dimethylphenol and 2,6-di-sec-butylphenol in cyclohexane were mixed with a wide range of amine concentrations. There was no significant shift of peaks in the phenol spectrum at any amine concentration. The values for standard enthalpy, free energy, and entropy change are presented in Table I for p-cresol for each of the amines studied. To illustrate typical results this table presents both values from the averaging of several wavelengths and sample values from a single wavelength showing probable error based on the fit of the points to a straight line of the plot used to estimate AH" (1/T vs. log K ) . For compactness average values only for the data obtained with o-cresol and o-sec-butylphenol are presented in Table 11. Plots of the log of the ratio of hydrogen bonded to non-hydrogen bonded substituted phenols vs. log of the various amine concentrations are linear and have slopes
H-Bonding of Amine with Phenols
The Journal of Physical Chemistty, Vol. 83, No. 19, 1979 2457
TABLE 11: Thermodynamic Parameters of Amine Hydrogen Bond Formation with o-Cresol and o-sec-Butylphenola o-cresol
o-sec-butylphenol _ I _ -
amine
A H " , kcal/mol
n-propylamine isopropylamine n-butylamine N-ethylmorpholine tri-n-butylamine tri-n-propylamine morpholine piperazine tert-butylamine diethylamine triethylamine di-n-butylamine
A G" , kcal/mol
- 2.74
-8.15 - 6.8 -6.8 i 0.36 -6.59 + 0.11 -5.99 i 0.06 -6.97 0.17
*
-9.3 -6.8 -6.9
- 2.4b -2.5gb -2.04 -1.59 i -1.52 i -2.31 i
* 0.01 0.01 0.01 0.02
-2.4b -2.65b -2.25b - 2.56b
AS",
AS",
cal/deg
A H " , kcal/mol
-18.2 -14
-8.01 0.06 -6.21 0.86 -8.49 i 1.08 -6.47 ~t 1.64 -6.0 t 1.60 -6.92 2 0.89 -7.35 2 0.54 -7.65 ~t 0.58 -7.71 i 1.59 -7.50 0.27 - 8 . 4 1 i 0.32 -7.21 2.10
-16.0 -16.8 -15.0 -15.6
- 23 -14 -14
* *
* *
a Values reported are numerical averages with standard deviation of all values obtained. from a single determination.
A G O ,
-2.62 -2.70 -2.74 -2.03 --1.49 -1.69 -2.37 -2.85 -2.60 -2.72 -2.27 -2.55
kcal/mol
cal/deg
0.006 0.05 i 0.03 0.04 i 0.06 i 0.03 f 0.02 i 0.01 i: 0.01 i. 0.01 i 0.03 i 0.03
-18.1 - 11.8 -19.3 - 14.9 -15.1 -17.6 -16.7 -16.1 - 17.2 - 16.0 -20.7 - 15.6
i.
*
*
* These data represent reports
I 7
8
9 IO PKa of Amine
II
Figure 3. Hydrogen bonding of p-cresol to amines. The data for p-cresol are plotted in the same fashion and with the same letter code as for o-cresol in Figure 2.
of one. Plots of log K vs. amine aqueous pK are presented for o-cresol (Figure 2), p-cresol (Figure 3), and o-sec-butylphenol (Figure 4). Figure 5 shows a typical set of spectra for one of the determinations.
Discussion The results of these studies are of interest with respect to three steric effects. There are, first, the contrast of the relative reactivities of tertiary amines contrasted with primary or secondary amines, second, the reactivity of amines toward di-ortho-substituted phenols, and third, the reactivity of amines toward mono-ortho-substituted phenols. Only alkyl amines were used in this study because aromatic amines absorb light in the same region of the spectrum as do phenols. The plots of the ratio of hydrogen bonded to non-hydrogen bonded phenols vs. the log of amine concentration establish, by the universally obtained slope of one, that the phenols and amines are reacting in a 1:l ratio. The reactivity of tertiary amines relative to primary or secondary amines is seen in Figures 2-4. These plots exactly parallel the results obtained with p-chl~rophenol,'~ establishing that the relative reluctance of acyclic tertiary aliphatic amines to hydrogen bond with phenols is a general characteristic. Qualitative observations of 2,6-disubstituted phenols, specifically 2,6-dimethylphenol and 2,6-di-sec-butylphenol, revealed no shift in absorption peak upon addition of sizable amounts of pure amine to cyclohexane solutions
7
a
9 pK,
IO
Ii
of Amine
Figure 4. Hydrogen bonding of o-sec-butylphenol to amines. The data for o-sec-butylphenol are plotted in the same fashion and with the same letter code as for o-cresol in Figure 2. Point M is the value for piperazine.
Figure 5. A typical spectrum for determination of the hydrogen bonding equilibrium constant. The absorption curves superimposed here include pure o-cresol (lowest curve in series) and o-cresol with a series of additions of n-butylamine, the absorbance increasing with increasing amine concentration. Absorbances were read at several wavelengths, each set being calculated separately.
of the phenols. This is interpreted to mean that substitutions as small as methyl groups on both phenol ortho
2458
Peterson et al.
The Journal of Physical Chemistry, Vol, 83, No. 19, 1979
position block hydrogen bonding with amines. Phenols with alkyl substituents at both the 2 and 6 positions were divided by Stillson et a1.16 into two groups: hindered phenols and cryptophenols. The hindered phenols were described as insoluble in water and aqueous alkali, as sparingly soluble in alcoholic alkali, as unreactive with either aqueous alcoholic Fe3+ or with sodium in anhydrous petroleum ether or diethyl ether, and an unable to esterify with acetic acid or benzoic acid by conventional ester forming reactions. These hindered phenols have large groups such as tert-butyl or tert-amyl in the ortho position. Partly hindered cryptophenols were described as being water insoluble and sparingly soluble in aqueous alkali, but as displaying phenol-type reactivity otherwise. Coggeshall17reported that, at high concentration, hindered phenols formed only very weak hydrogen bonds with one another, while crytophenols were intermediate in bond forming ability between hindered and unhindered phenols as evidenced by the shift in the hydroxyl infrared band a t 2.7 pm. It would appear that amines are more readily blocked in their approach to the phenol hydroxyl to form a hydrogen bond than are reactants in any of the above reactions. Even o-methyl groups prevent phenol-amine hydrogen bond formation. By contrast o-methyl groups have only a small effect on phenol-phenol hydrogen bond formation. In assessing the importance of a single ortho substitution, we find that comparison of the results, amine by amine, for the interaction of phenols of similar acid strength and no ortho substituent @-cresol), a small ortho substituent (0-cresol), and a more bulky ortho substituent (0-sec-butylphenol) reveals no significant difference in reactivity as measured by the equilibrium constant for hydrogen bond formation (Tables I and 11). Presumably,
the amine approaches the phenol at an angle that causes it to occupy the same position as would even a very small ortho substituent, but that this causes a problem only when both of the positions are substituted. Finally, these data provide repeated confirmation of the relative difficulty experienced by tertiary amines in hydrogen bonding to a phenol.
Summary The formation of hydrogen bonds between mono- and di-ortho-substituted phenols and aliphatic amines is studied. The results support the proposal that tertiary aliphatic amines are sterically hindered in this reaction. The presence of two ortho groups on the phenol prevents any hydrogen bond formation with amines, while a single ortho group has no effect on such a reaction. References and Notes (1) R. A. Morton and A. L. Stubbs, J . Chem. Soc., 1347 (1940). (2) A. Burawoy and I. Markowitsch-Burawoy,J. Chem. Soc., 36 (1936). (3) A. Burawoy and J. T. Chamberlain, J. Chem. Soc., 2310 (1952). (4) A. Burawoy and J. T. Chamberlain, J . Chem. Soc., 3734 (1952). (5) S.Nagakura and H. Bada, J . Am. Chem. Soc., 74, 5693 (1952). (6) S.Nagakura, J. Am. Chem. Soc., 76, 3070 (1954). (7) L. Bellon, C. R . Acad. Scl., 254, 3346 (1962).
(8) R. A. Hudson, R. M. Scott, and S. N. Vinogradov, Spectmhlm. Acta, Part A , 26, 337 (1970). (9) L. Bellon, Trav. Inst. Sci. Cherifien Ser. Sci. Phys., No. 6 (1960). (10) M. Bonnet and A. Julg, J . Chem. Phys., 59, 723 (1962). (11) N. S.Coggeshall and E. M. Lang, J . Am. Chem. Soc., 70, 3263
(1948). (12) S.Nagakura and M. Gouterman, J. Chem. Phys., 26, 881 (1957). (13) M. Lin and R. M. Scott, J . Phys. Chem., 76, 587 (1972). (14) N. J. Rose and R. S. Drago, J. Am. Chem. Soc., 81,6138 (1959). (15) M. D. Young, “Statistical Treatment of Experimental Data”, McGraw-Hill, New York, 1962. (16) G. M. Stillson, D. W. Sawyer, and C. K. Hunt, J . Am. Chem. Soc., 67, 303 (1945). (17) N. D. Coggeshall, J . Am. Chem. Soc., 69, 1620 (1947).
A Radioisotope Labeling Technique for Vapor Density Measurements of Volatile Inorganic Speciest E. J. Peterson,* J. A. Calrd,t Jan P. Hessler, H. R. Hoekstra, and C. W. Willlams Chemistry Division, Argonne National Laboratory, Argonne, flllnois 60439 (Received April 27, 1979) Publication costs assisted by Argonne National Laboratory
A new method for complexed metal ion vapor density measurement involving labeling the metal ions of interest with a radioactive isotope is described. The isotope chosen in the present work is unstable and leads to emission of a characteristic y ray. Thus the y-counting rate was related to the number density of complexed metal ions in the vapor phase. This technique is applicable to the study of any volatile inorganic species, but in the present study has been used to measure vapor densities of complex species in the TbCl3-A1Cl8system by using tracer 160Tb.
Introduction The study of high temperature vapors has practical application in a variety of metallurgical processes, in the understanding of unique properties of vapor deposited single crystals and in synthetic procedures which utilize vapor species as reactants.l Generation of volatile metal t Work done under the auspices of the Office of Basic Energy Sciences of the Department of Energy. Bechtel National, Inc., San Francisco, CA 94119.
*
0022-3654/79/2083-2458$01 .OO/O
containing compounds can be accomplished in an intermediate temperature regime by addition of a complexing agent (e.g., group IIIB metal halides) to a metal halide system thus forming volatile complex species.2 The recent suggestion of Krupke3 that LnC13(A1C13), vapor complexes (where Ln = lanthanide) be considered as possible energy storage media for high power optical gain systems has encouraged examination of the chemical and optical properties of these s y ~ t e m s . ~In - ~ analyzing the optical properties it is necessary to understand the nature and
0 1979 American Chemical Society
