NOTES
1812 solvent^,'^-^^ and the outer-inner sphere equilibrium has been checked quantitativelya6 Some complications arise a t high temperature, where a sharp line broadening is observed in the presence of various ligands, which cannot be explained in terms of outer-sphere complexation. However, if the temperature dependence of AH,,t and equilibrium constants are taken into account, the ligand dependent line broadening can be handled in the same way.?ts Complexation of Fea+by C1- and SCN- ions has been recently investigated by Levanon, et aZ.ls The curves of esr absorption intensities as a function of ligand concentration are quite similar to those observed for Cr3+F- and Cr3+--S02- systems, thus suggesting the presence of ion-pair species. The hexaaquo ion itself displays a relatively short relaxation time which results in a broad spectrum about 1100 G wide, almost independent of ligand concentration. This suggests that the line broadening mechanism in Fe3+ complexes is much more sensitive t o random distortion of the cubic field from collision with neighboring particles rather than from static distortion due to outer-sphere coordinated ligands, However, too little work has been done on this subject to derive quantitative conclusions. (15) R. G. Hayes and R. J. Myers, J . Chem. Phys., 40, 877 (1964). (16) B. B. Garrett and L. 0. Morgan, ibid., 44, 890 (1966). (17) H. Levanon and 2, Luz, ibid., 49, 2031 (1968). (18) H. Levanon, 2. Lus, and G. Stein, in press.
Evaluation of the Basic Ionization Constants
of Water and Alcohols from Their Ionization Potentials
by L. S. Levitt and Barbara W. Levitt Chemistry Department, School of Science, University of Texas at El Paso, El Paso, Texas (Received September 10, 1969)
The presence of electron-releasing alkyl substituents on the oxygen of an alcohol molecule obviously increases the electron density at the oxygen atom and this results in a corresponding decrease in the magnitude of the ionization energy for removal of an electron from the oxygen lone pair.
R ~ 5 H R$H
+ e-
The ionization potential, E I , of course, corresponds approximately to the energy of the highest occupied molecular orbital. It is apparent that the increase in electron density should result also in a concomitant increase in the basicity at the oxygen atom The Journal of Physical Chemistry
ROH
+ H+
A
+
ROHz
The ~ K B Hvalues of various N-heterocycles have already been correlated with the ionization potentials calculated theoretically by a self-consistent molecular field method.' With respect to the aliphatic alcohols, Ballinger and Long2 have determined the acid ionization constants by a conductivity procedure in aqueous solution for H20 e ROH30+. The bathe reaction ROH sicity constants P K B Hfor ~ the alcohols are defined for the reaction ROH H30+ e ROF12+ HzO, and are, of course, identical with the hydrolysis constants of the conjugate acids (ROHz+) of the alcohols. Until recently, the ~ K Bt H values for the alcohols were known only to an approximate order of magnitude. The PKBHt values were believed by Arnett3 to fall within the range -2 to -4. A more recent study4 of CH30H in aqueous H2S04using Raman spectroscopy yielded a value of -2.2 for the ~ K B Hof- CH30Hz+, but the method was judged unreliable for EtOH, i-PrOH, and t-BuOH by the experimenter^.^ Subsequently, two investigations of the basicity of alcohols, determined by solvent extraction5 and by their solubilities6 in varying concentrations of aqueous HzS04, were carried out. The following pKBH + values were arrived a t : 31eOH6 (-2.5); n-BuOH5l6 (-2.3); sec-BuOH5s6 (-2.2); and t-BuOH6.6 (-2.6). It was concluded by Arnett and -4nderson5 that the effect of a change in structure of the R group on the basicity of the alcohol is probably too small to be resolved by these experimental methods. Water, as pointed out by Arnett,3is the most important solvent in chemistry, and yet surprisingly little is known about its behavior as a base, particularly insofar as definitive quantitative data are concerned. For example, the PKBHt of H 3 0 + has been estimated variously as -1.8,7 -2.35,s -3.43,9 -5.9,'O and -6.66.l' A careful and detailed study of the base strengths of aliphatic alcohols was carried out by Gerrard and Ilacklen,'* who measured the solubility of gaseous HC1 in
+ +
+
+
(1) K. Nakajima and B. Pullman, J . Chim. Phys. Physicochim. Bid., 55, 793 (1958). (2) P. Ballinger and F. A. Long, J . Amer. Chem. Soc., 82, 795 (1960). (3) E. M ,Arnett, Progr. Phys. Org. Chem., 1, 223 (1963). (4) N. C. Deno and M. J. Wisotsky, J . Amer. Chem. Soc., 85, 1735 (1963). (5) E. M. Arnett and J. N . Anderson, ibid., 8 5 , 1542 (1963). (6) N. C. Deno and J. 0. Turner, J . Org. Chem., 31, 1969 (1966). (7) J. K. Br$nsted and W.F. K. Wynne-Jones, Trans. Faraday Soc., 25, 59 (1929). (8) H . Lemaire and H. J. Lucas, J . Amer. Chem. Soc., 73, 5198 (1951); T. L. Smith and J. H. Elliott, ibid., 75, 3568 (1953). (9) L. P. Hammett and A. J. Deyrup, ibid., 54, 4239 (1932). (10) W.Smith, Jr., Thesis, Harvard University, 1960. (11) N. C. Deno and R. W. Taft, Jr., J . Amer. Chem. Soc., 76, 244 (1954). (12) W.Gerrard and E. D. Macklen, J . A p p l . Chem., 9 , 85 (1959); 9 , 89 (1959); Chem. Res., 59, 1105 (1959).
1813
NOTES the various pure alcohol solvents. The question, however, may legitimately be raised (and indeed has been) whether the solubility of HC1 in the alcohol a t a fixed temperature really represents the base strength of the alcohol, or rather does it represent a combination, in varying proportions, of the base strength, the dipoledipole interactions between solute and solvent molecules, hydrogen-bonding interactions between solute and solvent molecules, polarizability effects, and extraneous ionic field eff ects.13 Conductance studiesI4 on these systems have shown that the interaction is predominantly one of proton transfer since the solutions of HC1 in the alcohols produce considerable conductivity, the A, in the case of HC1 in methanol approaching that of HC1 in water. I n their studies of the basicity of various aliphatic and alicyclic alcohols, Gerrard and ?tIacklen12 found, in general, that the alcohols with electron-releasing R groups absorb larger quantities of gaseous HC1 than those which have electron-attracting R groups, This behavior was interpreted, qualitatively, in terms of the relative electron densities at the oxygen atom. For example, the magnitude of the solubility of HC1 a t lo”, (in moles of HCl/mole of ROH) , was found to be in the following order: (CH,),COH >> CH30H >> C1,CCHZOH. It would appear, therefore, that the HC1 solubility data are actually representative of the basicity of alcohols, and might well be related to the magnitude of the EI’s of the alcohol. Such, indeed, we find to be the case. Table I gives the solubility datal2 CS) and the ionization energiesI6 ( E I ) for the corresponding
Table I : Solubility of Anhydrous Hydrogen Chloride in Various Alcohols and t’he Ionization Potentials of the Alcohols
1.20
-
0.80
-
0.40 -
10.0
10.5
11.0 11.5 E I , eV.
12.0
12.5
Figure 1 . A plot of the “relative basicity parameters”, p, of the alcohols us. their ionization potentials, E I .
alcohol or water. I n order to interpolate water into the series of alcohols, we have calculated the solubility of gaseous HC1 in HzO a t 10” from data in the literature.lB Also included in Table I are the “relative basicity parameters,” p, referred to water as the standard, where
s,
ROH
HOH MeOH EtOH n-PrOH n-BuOH i-PrOH t-BuOH c-CGHIIOH C~HECHZOH ClCHzCHzOH ClaCCHzOH FaCCHzOH
HCl soly a t loo, (mol of HC1/ mol of ROH)
0.380” 0.857 0 950 0.956 I
0.964
1.030 1 115 I . 030 G.812 0.550
0.087 0.060
8, relative basicity parameter
Er , ionization potential, eV
0
12.59
1.26
10.85 10.50
1.50 1.52 1.53 1.71
10.20
10. 17b 10.16
1.93
9.92O
1.71 1.11
. . .d
0.45 -0.77 -0.84
. . .d
. . .d
. . .d . . .d
a Calculated from solubility data in ref 16. b Average of values given in ref 15 and in W. Reed, “Ion Production by Electron Impact,” 1962; see C. Noller, “Chemistry of Organic Compounds,” W. B. Saunders Co., Philadelphia, Pa., 1965, Value not given in ref 15. See W. Reed and C. Nolp 995. 1er.b a Er values not available.
The data are presented graphically in Figure 1 where the “relative basicity parameters,” p, are plotted as a function of EI. It is seen that an excellent correlation exists between the p values and E I . Note that water has been included as a special case of the simplest alcohol, and it is interesting to observe that it fits right into the correlation. The ionization energies are actually free energy changes,17 which are therefore proportional to log KiOn. (13) F.Franks and D. J. G. Ives, Quart. Rev., 20, 1 (1966). (14) G. J. Janz and S. S. Danyluk, Chem. Reu., 60,209 (1969); T. Shedlovsky and R. 1,. Kay, J . Phys. Chem. 60, 151 (1956); A. M. El-Aggan, D . C. Bradley, and W. Wardlaw, J . Chem. SOC.,2092 (1958). (15) K. Watanabe, J . Chem. Phys., 26, 542 (1957); Dept. of the Army, Report No. 5B-99-01-004,ORD TB2-0001-00R-1624(1959). (16) N.A. Lange, Ed., “Handbook of Chemistry,” Handbook Publishers, Sandusky, Ohio, 1944, p 1246. (17) P. R. Wells, Chem. Rez.., 63, 171 (1963). Volume 74, Number 8
April It?, 1970
NOTES
1814 The linear relationship of Figure 1 makes it apparent that the P values are proportional to EI, and therefore, we must have
-3.8
I
I
I
I
I
I
-3.0
If, then, we have an absolute value for any one of the K's of the alcohols or water, we are in a position to establish a quantitative scale of K's for the entire series of compounds. As indicated above, the values for the alcohols are not known with any reasonable degree of certainty, and the values for water are widely disparate. We are prompted, therefore, to select arbitrarily one of the values for H 3 0 + upon which to construct the scale. The value decided upon is that of Hammett and for pKBHt of H30+, since this was D e y r ~ p ,-3.43 ~ obtained by what appears to be a valid experimental procedure and it is, incidentally, the median (and nearly the mean) value of the five quoted above. From eq 2 it is seen that the P K B Hof~ the alcohol can be calculated from
+P
PKBHt(R0H) = PKBH+(HOH)
= -3.43
fP
(3) Table I1 gives the new values for PKBH"for various alcohols as calculated from eq 3.
+ for Various Alcohols Table 11: Calculated ~ K B HValues
Alcohol
HOH MeOH
EtOH n-PrOH n-BuOH i-PrOH
t-BuOH C-CGH~~OII CeHbCHtOH ClCHzCHzOH ClaCCH20H FsCCH20H a
-3.43Q -2.17 -1.93 -1.91 -1.90 -1.72 -1.49 -1.72 -2.32 -2.98 -4.20 -4.27
-3.44 -2.17 -1.92 -1.70 -1.68 -1.67 -1.49
..*
% -2.2
- 1.4 10.0
10.5
11.0
11.5 EI, eV.
12.0
12.5
Figure 2. A plot of the calculated ~ K B Hvalues + for various alcohols us. the ionization potentials, EI, of the alcohols.
The slope of the line, a, is found to be -0.72718 and therefore
~ K B H=+ +5.73 - 0.72731 (5) The p K m t values calculated from eq 5 are also included in Table 11. Calculated values are not shown for the last five alcohols listed because their corresponding ionization potentials either have not been experimentally determined or they do not represent removal of an electron from an oxygen lone pair (e.y., C6H;CH20H). Using eq 5 we can, however, calculate the ionization energy from the ~ K B H +The . values obtained are as follows: c-CGH1lOH, 10.2 eV; C6H&H20H, 11.1 eV; C1CH2CH20H, 12.0 eV; C13CCH20H,13.6 eV; F,CCH20H, 13.8 eV. In the future, should a different and better value for the P K B H - of H30+be obtained, this relationship is still valid and the only change would be a shift in the line of Figure 2 up or down, correspondingly. (18) ilctually, a least-squares treatment gives the slope as -0.691 with a correlation coefficient of 0.990. However, chemical intuition requires, in this case, placing greater emphasis on the series H , Me, Et, i-Pr, t-Bu and lesser weight on n-Pr and n-Bu.
... ...
Standard.
Pure Nuclear Quadrupole Resonance
I n Figure 2 the new P K B H values ~ for the alcohols are plotted vs. the corresponding EI values from Table I. The excellent correlation would appear t o justify the assumptions and the procedures adopted in this analysis. It is interesting to note that our ~ K B Hvalue + for CHBOHz+,-2.17 is in close agreement with the Raman experimental4 value, -2.2, but it does not agree well with the solubility value516 of -2.5. The equation for the straight line of Figure 2 relating the ~ K B Hof+ROH2+ to the ionization potential (in eV) is given by PKsHt = 4-5.73 f aEI T h e Journal of Physical Chemistry
(4)
in Hexaohlorostannates of Hydrated Divalent: Cations
by Jack D. Graybea1,l Ruth J. ilIcKownj2 and Shen D. Ing'& Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, and the Department of Chemistry, Virginia Polytechnic Institute, Blacksburg, Virginia 24061 (Received September 22, 196.5)
The pure nqr frequencies of KzSnCla, (NH&3nCls and RbSnCl8 at 23' have been reported to be a t 15.065
