The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
994
M. K. Chantooni and I. M. Kolthoff
different thermal effects in the two media. The above argument based on circumstantial evidence is of limited significance. However, it seems that for a reasonable set of the parameters the temperature can be raised just sufficiently to be consistent with the observed effect of illumination. Whether the local heating is a unique mechanism or not, however, must be judged on further accumulation of experimental findings such as presented here and fundamental thermal data on amorphous solids.
I
:\ 3
-1
'-2
-
L -
-
3 1 33 ~ 2 11 I O 9 12 1 1 IO 9 12 11 -log t(sec)
10
-
3
Acknowledgment. The author thanks Professors J. Sohma and K. Yoshihara and Dr. Y. Nosaka and Mr. T. Kat0 for their helpful discussions.
1
9 12 11 10 9
References and Notes
Figure 5. Calculated temperatures vs. time and radius. The instance of absorption of photons by the ion radical is taken as t = 0. The parameter r is the distance from the center of the ion radical. The ~ , cal deg-' g-', and values for p , s, and yare taken as 1.0g ~ m - 0.3 2 X lo-**cm3, respectively, and bv is set equal to 2 eV.
T. Shida and S.Iwata, J. Phys. Chem., 75, 2591 (1971). T. Shida and S. Iwata, J. Chem. Phys., 56, 2858 (1972). T. Shida and S. Iwata, J. Am. Chem. Soc., 95, 3473 (1973). T. Shida, S.Iwata, and M. Imamura, J. Phys. Chem., 78,741 (1974). T. Shida, T. Kato, and Y. Nosaka, J . Phys. Chem., 81,1095 (1977). H. E. Armstrong, Berichte, 7, 404 (1874). P. Balk, S.de Bruijn, and G. J. Hoijtink, Red. Trav. Chim., Pays-Bas, 76, 907 (1957). S. L. Hager and J. E. Willard, J . Chem. Phys., 63, 942 (1975). D. Shooter and J. E. Willard, J . Phys. Chem., 76, 3167 (1972). M. A. Neiss and J. E. Willard, J . Phys. Chem., 79, 783 (1975). H. Suzuki, "Electronic Absorption Spectra and Geometry of Organic Molecules", Academic Press, New York, N.Y., 1967. S. Arai, A. Kira, and M. Irnamura, J . Phys. Chem., 81, 110 (1977). A. Ishitani and S. Nagakura, Mol. Pbys., 12, 1 (1967). H. Suzukl, K. Koyano, and T. Shida, unpublished data. J. A. Dean, Ed., "Lange's Handbook of Chemistry", 11th ed, McGraw-Hill, New York, N.Y., 1973. G. R. Fleming, J. M. Morris, and G. W. Robinson, Chem. Phys., 17,
sample while for K 2 5 X the surroundings of the ions will be insufficiently warmed. It may be interesting to note that the temperature distribution around the ions is fairly uniform by the time appropriate for molecular rotation. It is also to be noted that the temperature depends rather sensitively on the conductivity (at r = 0, T i s proportional to K - ~ / ' so that the difference in K by a factor of 10 produces a difference in T by a factor of -30). Therefore, if the thermal conductivity of cracked samples is depressed by heterogeneity due to partial crystallization and cracks there is a chance for absorbed photons t o induce significantly
91 (1976).
Proton Solvation in the Lower Aliphatic Alcohols with Emphasis on Isopropyl and ferf -Butyl Alcohols M. K. Chantooni, Jr., and I. M. Kolthoff" School of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received December 8, 1977) Publication costs assisted by the National Science Foundation
The basic strength of alcohols increases in the order primary < secondary,tertiary. The logarithms of the transfer activity coefficients of the solvated proton, log MyS(H+)com between methyl alcohol (M) and normal butyl, normal hexyl, isopropyl, and tert-butyl alcohols, corrected for the Born effect, are reported to be -0.9, -0.9, -1.9, and -2.0, respectively. The relatively much larger basicity of tert-butyl and isopropyl alcohol as compared to that of methyl alcohol is attributed to a large extent to the fact that the monomer content in the normal alcohols is considerably smaller than that in the corresponding secondary and tertiary alcohols. As a solute, water in tert-butyl alcohol is a considerably stronger base than is methyl alcohol, the hydration constant of the solvated proton in tert-butyl alcohol being equal to 12.5 and the methanolation constant 0.5. The Walden product of the proton in isopropyl alcohol is 0.279, in tert-butyl alcohol 0.224, as compared to 0.795 in methyl alcohol. It is concluded that there is no "proton jump" mechanism in the former two alcohols.
Introduction Relative solvation energies of the proton in various lower alcohols are of interest for several reasons. Among these are, the effect of solvent on (a) Bransted acid-base equilibria, including the basic strength of the solvent, (b) protonation of water and methyl alcohol by the solvated proton, and (c) lyonium-catalyzed reactions, such as formation of esters. For a quantitative study of these effects the transfer activity coefficient of the proton, "yS(Hf), is essential, M being methyl alcohol, the reference 0022-3654/78/2082-0994$0 1 .OO/O
solvent, and S being another alcohol. A positive value of log MyS(H+)denotes a more negative free energy of solvation of the proton in M than in s,i.e., a stronger basicity of M than of S. In this study the alcohols chosen are the isomeric propyl and butyl alcohols and n-hexyl alcohol (n-Hex). Sterically, isopropyl and tert-butyl alcohols (t-BuOH) differ considerably from the normal alcohols and they have been made the focus of the present paper and of acid-base studies to be reported in subsequent papers. For com-
0 1978 American
Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 9. 1978 995
Proton Solvation in the Lower Aliphatic Alcohols
TABLE I: Absorbance Indices of Picrate and Dichloropicrate in Alcohols Alcohol
'410
'450
0.44"
Tetrabutylammonium 6 . 9 , x 103 4.5, x 10' 6.3, x 103 8.4, X 10' 6.2, x 103 4 . 1 , X 10' 3.8, X 10'
i-PrOH t-BuOH n-Hex Watera t-BuOH n-Hex
0,
3.2, X 10' 2.40 X 10'
Picrate 2.3, x 103 3.4, x 10' 2.1, x 10' 2.2 x 103
0460
1.5, X l o 3 0.9" x 10' 0.99 x 10'
041"
0.54,
X
10'
Tetraethylammonium Dichloropicrate 2.2, x 10' 1.4, X 10' 0.72, x 103 1 . 6 , x 10' 0.88, X 10' 0.42, x 103
a Tetramethylammonium picrate.
parison, n-Hex, which is practically isodielectric with t-BuOH, served as the primary alcohol analogue. For the evaluation of the assumption that Mys(TAB+)= MyS(BPh,-),proposed by Popovych,' was used, TAB+ being triisoamyl-n-butylammonium. This involved determining the solubility products of TABBPh4,TABPi (Pi = picrate), solubilities of picric acid, and pKd(HPi) in the various alcohols. Reliable literature values of pKd(HPi) are available in MeOH, i-PrOH, n-PrOH, n-BuOH, sec-BuOH, and i-BuOH. For i-PrOH, t-BuOH, and n-Hex they were estimated spectrophotometrically and conductometrically in the present work. The glass electrode was found to be reliable for paH measurements in the latter three alcohols. It was calibrated in solutions of CF,SO,H, picric and dichloropicric (HPiC12) acids alone, and in mixtures with their tetraalkylammonium salts. This necessitated the determination of the dissociation constants of these acids and their tetraalkylammonium salts. Finally, the hydration and methanolation constants of the proton in t-BuOH were estimated conductometrically and potentiometrically in solutions of dichloropicric acid.
Experimental Section Chemicals, Solvents. n-Propyl, n-butyl, and iso-, sec-, and tert-butyl alcohols were Eastman Kodak White Label, n-hexyl alcohol was Eastman Kodak Yellow Label, and isopropyl and isobutyl alcohols were Fischer products. All the alcohols were shaken with calcium hydride ( 2 g/L), decanted, and distilled a t atmospheric pressure in a 1-m column packed with glass helices. This procedure was repeated and the alcohol finally distilled by itself. Boiling points corrected to 760 mmHg were within 1 "C of the literature values. Water contents found by Karl Fischer titration ranged from 0.05 to 0.170. Specific conductivities of the pure alcohols in this study and those in a compilation by Riddick and Burger' are as follows: n-PrOH 1.80 X 0.92 X 10-8;2i-PrOH 0.38 X 5.8 X nB ~ O H5.45 x 10-9, 1.1 x 10-9;2~ - B ~ O1.10 H x 10-8, 1.6 x 10'8;2sec-BuOH 2.5 X primary. A knowledge of the aggregation of the alcohol molecules in the liquid state provides further insight into the protonation of the alcohols. Huyskens et al.,31on the basis of vapor pressure studies, report that the monomer content of the alcohols studied is low; the mole fraction of monomer in t-BuOH being only 0.032 and decreasing in the order tertiary > secondary > primary. Thus, the order of basicity, tertiary, secondary > primary alcohols may be attributed to the same order of monomer content of the alcohols. From the temperature dependence of dielectric constant and the Kirkwood correlation factor, g, based on a linear model of hydrogen bonded chains, Dannhauser and Bahe32have estimated AG, AH, and A S of 0-.H-0 hydrogen bonding in the various types of alcohols up to and including the butyl alcohols. They find an average value of MASAG of -0.4 log K unit for the primary alcohols and -o.73, -1.4, and -2.2 for i-PrOH, sec-BuOH, and t-BuOH, respectively. These values change in the same order going from primary to secondary to tertiary alcohols as log M YS EWcorr does. Medium Effect on the Hydronium Ion. Further insight into the solvation of H30+ in the various alcohols is provided by formulating Kf(H30+)in terms of transfer activity coefficients, viz: A s log Kf(H30') = log
rS(H30')
log MrS(H') - log MyS(H20)
--
(12)
In eq 12 H+ refers to the alcohol solvated proton. Using potentiometric values of Kf(H30+)in Table IV or in the literature33and values of log MyS(H+)in Table VI, column 5 , resulting values of log MyS(H30+) - log "yS(H20)are as
1000
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
follows: -0.65, -1.7, -1.0, -1.3, and +4.6, S being EtOH, i-PrOH, n-BuOH, t-BuOH, and acetonitrile, respectively. Availability of Henry's law constants of water in these alcohols would permit evaluation of log MyS(H20)and hence log MyS(H30+).It appears that H30+a t low water content (
