The solvatochromic comparison method. 4. Dilution studies - The

The Journal of Physical Chemistry, Vol. 82, No. 23, 1978 2477

Solvatochromic Comparison Method (2) S. E. Khalafalla and L. A. Haas, J . Catal., 24, 115 (1972). (3) C. L. Llu and I. G. Dalh Lana, Paper presented at the Canadian Sulphur Symposium, Calgary, Alberta, 1974. (4) R. W. Glass and R. A. Ross, Can. J. Chem., 50, 2537 (1972). (5) Y. W e t , M. Francois, and M. Debrun, J. Chim. fhys., 71, 666 (1974). (6) M. P. Rosynek and F. L. Strey, J. Catal., 41, 312 (1976).

(7) M. P. Rosynek, W. D. Smith, andJ. W. Hlghtower, J . Catal., 23, 204 (1971). ( 8 ) P. C. Flynn and S.E. Wanke, Can. J. Chem. Eng., 53, 636 (1975). (9) T. T. Chuang and I. G. Dalla Lana, J. Chem. Soc., Faraday Trans. 1 , 68, 773 (1972). (10) A. E. Hirschler, J. Catal., 2, 428 (1963).

The Solvatochromic Comparison Method. 4. Dilution Studies’ Mortimer J. Kamlet, Eleanore 0. Kayser,la Mary Elizabeth Jones,la Josi! LUISAbboud,lb John W. Eastes,2c and R. W. Taft*2b Naval Surface Weapons Center, White Oak Laboratory, Silver Spring, Maryland 209 10; the Department of Chemistry, University of Californla, Irvine, California 927 17; the Engineer Topographic Laboratory, Fort Belvoir, Virginia 22060 (Received January 27, 1978; Revised Manuscript Received June 19, 1978) Publication costs assisted by the Naval Surface Weapons Center

Solvatochromic comparisons of UV-visible spectral data for 4-nitroaniline (1) and N,N-diethyl-4-nitroaniline (2) are carried out in progressively more dilute mixtures of HBA (hydrogen bond acceptor) solvents in non-HBA cosolvents. VF50values of Me2S0 (volume fractions of MezSO in the cosolvents at which the hydrogen bonded complex with 1 is approximately half dissociated) are shown to increase in magnitude with increasing polarity of the cosolvent. A linear relationship between log VFbOvalues for the Me2SO:1:Me2S0complex and formation constants reported earlier for the 4-fluorophen01:Me~SOhydrogen bonded complex in the same cosolvents is demonstrated. Self-associatingalcohol solvents show nontypical dilution behavior. Dissociation of the (ROH), complexes simultaneously with the l:(ROH), complexes causes -AAv( 1-2) values (enhanced bathochromic shifts attributable to hydrogen bonding) to fall off more rapidly on dilution into non-HBA cosolvents. A comparison of dilution behavior provides strong evidence that the (MeOH), self-association equilibrium constant is significantly higher than that for (t-BuOH),.

In the present paper we describe an extension of the solvatochromic comparison method to spectroscopic behavior in mixed solvents. We describe a facile new method for estimating the strengths of complexes between HBD (hydrogen bond donor)3 indicator solutes and HBA (hydrogen bond a ~ c e p t o r solvents, )~ and we report results which provide insight into the chemistry of these complexes as the HBA solvents are diluted into non-hydrogen-bonding cosolvents. We also present some interesting findings regarding the relative strengths of aliphatic alcohol self-association complexes and the effects of self-association on alcohol basicities. We recently described the use of the solvatochromic comparison method in the analysis of UV-visible spectral data for 4-nitroaniline (1) and N,N-diethyl-4-nitroaniline (2) in 44 pure solvent^.^*^ It had been shown earlier6 that in HBD solvents sp2-hybridized4-nitroaniline derivatives behave as hydrogen bond acceptors a t the nitro oxygens, but not a t the amine nitrogem2* These effects were assumed t o be similar for 1 and 2, thus approximately cancelling out in the solvent shift comparisons between 1 and 2. In HBA solvents, 1, but not 2, can act as a hydrogen bond donor at the amine site [forming two hydrogen bonds to the HBA bases whose spectral effects are in the ratio 1/(0.93 i 0.13)];718hence, differential solvatochromic behavior between 1 and 2 was considered to reflect the spectral effects of hydrogen bonding to HBA solvents by the -NH2 protons of 1. Details of that earlier solvatochromic comparison in the unitary solvents, which are germane to the present study in mixed solvents, are as follows: (a) First it was shown that ~ ( 1values ) ~ were ~ ~nicely linear with corresponding ~ ( 2 ) , ~values , in nine non-HBA

solvents; the least-squares regression equation is ~ ( 1= )1.035~(2),,, ~ ~ + 2.64 kK (1) with r (the correlation coefficient) = 0.989 and SD (the standard deviation) = 0.16 kK. This correlation indicates that solvent polarity-polarizability effects on the spectra of 1 and 2 are closely comparable. (b) Next, it was shown that in HBA solvents urnax positions for 1 were displaced bathochromically relative to positions predicted from corresponding ~ ( 2values ) ~ ~ ~ and eq 1. These enhanced solvatochromic displacements, represented by - A A V ( ~ - ~ ) ~ , ~ ~were N , ’ considered to result from lowering of the transition energies for the [H2+N=C(1) C(4)=NO2-1 electronic transition as a consequence of type-B hydrogen bondslo formed by the amine protons of 1 to HBA solvents. The lowering of the transition energy (bathochromic shift) results from stronger type-B hydrogen bonding in the electronic excited

-

oJN\\o

type-B hydrogen bonding by I to HBA solvents

state than in the ground state. Magnitudes of the enhanced solvatochromic shifts, calculated from - A A v ( ~ - ~ ) ~ , H=~ N ~ ( 1lcalcd ) ~-- V(l),ax”bsd ~

(2) range from 0.38 kK for the very weak HBA base solvent,

This article not subject to US. Copyrlght. Publlshed 1978 .by the American Chemical Society

The Journal of Physical Chemistry, Vol. 82, No. 23, 1978

2470

Kamlet et al.

TABLE I: Solvatochromic Comparison of Spectral Data (in kK) for 4-Nitroaniline (1)and N,N-Diethyl-4-nitroaniline (2) in Dimethyl Sulfoxide plus Non Hydrogen Bond Acceptor Cosolventsa -.-I

cosolvent CC1,

m

YO1 70

Me,SO 100 80 60 40 20 10

8 6 4 2 1 0.8 0.6 0.4 0.2 0.1 0

’(‘I ’(2) 24.33 24.45 24.60 24.84 25.16 25.58 25.71 25.84 26.04 26.32 26.49 26.63 26.56 26.60 26.63 26.70 26.70

cosolvent C,H,Cl -AAv

’(‘I

colsolvent ClCH, CH,Cl

cosolvent CH,Cl,

’(‘I

-AAv

’(‘1

calcd obsd (1-2) . . ’(2) ~. calcd obsd (1-2) ’(21 calcd obsd (1-2) ’(2) calcd obsd (1-2) 27.82 25.74 2.08 24.33 27.82 25.74 2.08 24.33 27.82 25.74 2.08 24.33 27.82 25.74 2.08 27.95 25.84 2.11 24.45 27.95 25.81 2.14 24.45 27.95 25.87 2.08 24.51 28.01 25.87 2.14 28.10 25.91 2.19 24.54 28.04 25.87 2.17 24.57 28.07 26.01 2.06 24.63 28.13 26.04 2.09 28.35 26.04 2.31 24.72 28.23 26.04 2.19 24.72 28.23 26.18 2.05 24.72 28.23 26.25 1.98 28.68 26.28 2.40 24.91 28.42 26.25 2.17 24.81 28.32 26.49 1.83 24.84 28.35 26.63 1.72 29.11 26.63 2.49 25.06 28.58 26.56 2.02 24.88 28.39 26.88 1.51 24.88 28.39 27.10 1.29 29.25 26.77 2.48 25.09 28.61 26.67 1.94 24.91 28.42 27.03 1.39 24.91 28.42 27.29 1.13 29.38 26.95 2.43 25.19 28.71 26.85 1.86 24.94 28.45 27.21 1.24 24.91 28.42 27.51 0.91 29.59 27.25 2.34 25.22 28.74 27.06 L 6 8 24.97 28.48 27.47 1.01 24.94 28.45 27.70 0.75 29.88 27.89 1.99 25.32 28.85 27.62 1.23 24.97 28.48 27.78 0.70 24.97 28.48 28.01 0.47 1.57 25.35 28.88 27.97 0.91 24.97 28.48 28.01 0.47 24.97 28.48 28.29 0.19 30.06 28.49 30.10 28.65 1.45 25.38 28.90 28.05 0.85 24.97 28.48 28.09 0.39 24.97 28.48 28.33 0.15 30.13 28.90 1.23 25.38 28.90 28.21 0.69 24.97 28.48 28.17 0.31 24.97 28.48 28.37 0.11 30.17 29.24 0.93 25.38 28.90 28.37 0.53 24.97 28.48 28.21 0.27 24.97 28.48 28.45 0.03 30.20 29.90 0.30 26.38 28.90 28.45 0.45 24.97 28.48 28.29 0.19 24.97 28.48 28.49 -0.01 30.27 30.12 0.15 25.38 28.90 28.65 0.25 24.97 28.48 28.29 0.19 24.97 28.48 28.53 -0.05 30.27 30.40 -0.13 25.38 28.90 28.82 0.08 25.00 28.51 28.37 0.14 24.97 28.48 28.57 -0.11

a Correlation equation in non-HBA solvents: ~ ( 1 = 1.035~(2),, ) ~ ~ 1kK= em-’.

trichloroacetone to 2.80 kK for the strong HBA base solvent, hexamethylphosphoramide. (c) Finally, the - A A v ( ~ - ~ ) ~ , H terms ,~ were employed in the construction of (and are linearly related to) a p scale of solvent HBA ba~icities.~Jl In the present paper we consider how ~(1)- and u(2),, and the -AAu(l-2) hydrogen bonding terms are influenced as HBA solvents are diluted into non-HBA cosolvents, with consequent dissociation of the hydrogen bonded complexes. Dimethyl sulfoxide has been chosen as a representative good HBA solvent and carbon tetrachloride as a non-HBA cosolvent. Observed values of the spectral maxima of 1 and 2 in the binary solvent mixtures are assembled in Table I and plotted against volume fraction (VF) MezSO in C C 4 (on a logarithmic scale) in Figure 1. Also assembled in Table I and plotted in Figure 1 are values of ~ ( 1 ) ~as~well as - A A U ( ~ - ~ ) terms,g ~ + ~ ~ determined ~ for the mixed solvents through eq 2, exactly as done previously4 for the pure HBA solvents. It may be seen in the lower family of curves in Figure 1that, while both quantities show near monotonic variations with log VF(Me2SO/CC14), there is a significant difference between the shapes of the plots for observed and calculated ~ ( 1 ) [the ~ - latter being essentially parallel with the plot for 42) obsd]. As might be expected, the spreads values, corresponding between ~ ( 1 and ) ~u( l)eq ~ 2Ca1Cd ~ ~ to the -AAu(1-2) terms, are large at the higher VF’s of MezSO and decrease toward zero as solvent compositions approach 100% CC14. T h e Cybotactic Polarity Increment (CPI). The upper plot in Figure 1 shows one general feature of the solvent dilution behavior which was not expected but which, upon reflection, seems a necessary consequence of the chemistry of such complexes. It is seen that, before the fall-off in magnitude of the -AAu(1-2) term as the MezSO becomes progressively more dilute in CC4, there is first an increase to a maximum [from a value of 2.08 kK in neat MezSO to 2.49 kK at VF(Me2SO/CCl4)= 0.101. We believe that two or three concurrent phenomena may account for this on initial dilution 0.41-kK bathochromic effect on v( l)mm of the MezS0.12 (a) From the formation constant estimate in the Appendix, it is likely that 90+% of the 4-nitroaniline amine protons are hydrogen bonded to Me2S0 molecules in both

t 2.64

-

il

o*Lx-..

kK. The appropriate SI unit in nanometers.

-_ _ _ _ --_-

\

cybotoctic polarity increment (CPII

‘\

‘\,,

2’10

a

\

? I

I

0.5

02

l

l

I

l

l

x.-

30

29

28

Y

*.

27

P 26

25

10

0 1 005

002 001 0005 0.002 0001

VOLUME FRACTION D M S O I N CC14

Flgure 1. Variation of ~ ( l u(l), )~ lcalod, *~(2), & *~, , and -AAv(1-2) with solvent composition in Me2SO/CCI4.

neat MezSO and 10% Me,SO/CC14, but that the l-Me2S0 complex is “tighter” (i.e., stronger, shorter hydrogen bonds) in the less polar solvent mixture. The “tighter” complex would be expected to produce the greater solvatochromic effect.13 We had originally considered this rationale to be consistent with our observation, discussed below, that hydrogen bonded complexes between 1 and MezSO break up on dilution at increasingly higher volume fractions of MezSO as the cosolvent is changed from C C 4 to chlorobenzene to ethylene dichloride to methylene chloride. (b) However, a referee has suggested the possibility that the 4-nitroaniline is more completely hydrogen bonded (not only more “tightly” hydrogen bonded) in the 10% Me2SO/CC14solution than in the neat MezSO solvent. We


Solvatochromic Comparison Method

TABLE 11: Representative Values of the Cybotactic Polarity Increment (CPI) for HBD Indicator Solutes in Mixed Solvents HBD solute 4-nitroaniline 4-nitroaniline 4-nitroaniline 4-nitroaniline 4-nitroaniline 4-nitroaniline N-ethyl-4-nitroanilineC 3-nitroanilined N-ethyl-3-nitroanilined 3,5-dinitr~aniline~ 2-nitroanilined 4-aminobenzo henoned 4-nitrophenol 4-nitrophenold

B

VF 0.10 0.40 0.80 0.90 0.40 0.40 0.06 0.60 0.10 0.20 0.09 0.20 0.020 0.040

composition for maximum CPI value solvent 71 *a cosolvent Me, SO 1,000 CCl" MeiSO 1.000 C,H;Cl Me, SO 1.000 CH,Cl, Me,SO 1.000 C2H4C12 CCl, THF 0.576 CCl, MeOH 0.586 CCl, Me, SO 1.000 cc1, Me, SO 1.000 cc1, Me, SO 1.000 1.000 CCl, Me, SO CCl, Me, SO 1.000 Me,SO 1.000 CCl, cc1, Me, SO 1.000 Me, SO C*H4C12 1.000

71 *a

0.294 0.709 0.802 0.807 0.294 0.294 0.294 0.294 0.294 0.294 0.294 0.294 0.294 0.807

CPI, kK 0.41. 0.53b 0.11' 0.06 0.01 0.05 0.16 0.61 0.08 0.29 0.28 0.16 0.20 0.38 0.16

Reference 8. Results of two independent dilution studies using different spectrophotometers. a Reference 17. vatochromic comparisons and dilution studies on these compounds will be reported in detail in future papers.

Sol-

had at first discounted such an explanation, since it would require that the equilibrium Me2S0 + 1:Me2S0

KZ

Me2SO:l:Me2S0

be shifted to the right with a tenfold decrease in MezSO concentration. On reflection, however, the possibility appeared more plausible, since it would follow if the strongly solvent dependent K2 increased by more than a factor of 10 as the MezSO concentration underwent the tenfold decrease. From the variation of VFw with solvent polarity, discussed below, this is not too unlikely and, on balance, we consider that this effect may also contribute to the increase in -AAv( 1-2) between neat Me2S0 and VF = 0.10. (c) The third effect relates to the assumption, necessary to the solvatochromic comparison method, that solvent polarity effects on the spectra of 1 and 2 are similar when +' , 10 0.4 01 004 001 0004 O( both compounds are dissolved in the same solvent. For V O L U M E F R A C T I O N DMSO I N C O S O L V E N T this assumption to apply rigorously, the solvent compoI LOGARITHMIC S C A L E ) sitions in the cybotactic environments of both compounds Flgure 2. Solvatochrornic dilutions of l:Me,SO in several non-HBA would need to be the same,14J5a requirement which is cosolvents. unlikely to be satisfied when HBD and non-HBD solutes are compared in HBA/non-HBA solvent mixtures. In the to represent solvent sorting beyond that caused by polarity present instance, 4-nitroaniline solute molecules, by virtue effects (which are assumed to be similar for 1 and 2). of their hydrogen bonds, attract from the solvent mixture Representative cybotactic polarity increments for a (and retain in their immediate solvent shells) greater number of HBD indicators in mixed solvents are reported numbers of the more polar MezSO solvent molecules than in Table II.I6 It is seen that the phenomenon is rather do N,N-diethyl-4-nitroaniline solute molecules. That is, general and that, as required by the rationale, the CPI is although their macro environments are similar, the micro larger the greater the difference in polarity (n* values)17 environments of 1 and 2 differ significantly from these of the HBA solvent and non-HBA cosolvent. It is macro environments and from each other. somewhat surprising to us, however, that the CPI values Thus, in mixed solvents, in addition to influencing the in Me2SO/CC14 for N-ethyl-3-nitroaniline and N-ethylsolvatochromic shift through its direct bonding effects (displacing electron density through u, r,and hydrogen 4-nitroaniline (one hydrogen bond each to HBA solvents) bonds), the solvent association phenomenon influences the are higher than corresponding values for 3- and 4-nitrospectrum by virtue of an augmented polarity effect in the aniline (two hydrogen bonds e a ~ h ) . ~ " J ~ cybotactic region. We shall refer to this augmentation of Effect of Polarity of Non-HBA Cosolvent. Included also -AAv in mixed solvents as the cybotactic polarity inin Table I are positions of spectral maxima of 4-nitroaniline crement (CPI). and its N,N-diethyl derivative in a series of mixtures of It should not be inferred from the above that we conMe2S0 with non-HBA cosolvents of varying polarity, as sider solvent-sorting effects to be nil when non-HBD well as ~ ( 1lCa'Cd ) ~ results ~ and -AAz~(l-2)~," terms solutes are dissolved in mixed solvents. N,N-Diethylcalculated through eq 2. The cosolvents are C h 4 , a* = 4-nitroaniline, by virtue of purely dipolar attractions, also 0.294; C6H6C1, n* = 0.709; C2H4C12,K* = 0.807; and has a cybotactic environment which is enriched in more CH2C12,a* = 0.802 (K* values are measures of solvent ) ~ ~ polarity ~ polar solvent molecules. If we assume that ~ ( 2varies on a scale ranging from 0.000 for cyclohexane to linearly with composition in the cybotactic environment, 1.000 for Me2S0).17 the results in Table I indicate that the average solvent shell Plots of -AAv( 1-2) against logarithmic VF(Me2SO/ of 2 contains 50% Me2S0 when the gross composition non-HBA cosolvent) are shown in Figure 2. It is seen that comprises 10% Me2SO/CC14. The CPI term is intended the complex of 4-nitroaniline with Me2S0 dissociates a t

2480

Kamlet et al.


TABLE 111: Solvatochromic Comparison of Spectral Data (in kK)for 4-Nitroaniline (1) and N,N-Diethyl4-nitroaniline (2 ) in Hydrogen Bond Acceptor Solvents Plus CCI, Cosolvent solvent THF vol % HBA solvent

v(2)

100 80 60 40 20 10 8 6 4 2 1 0.8 0.6 0.4 0.2 0.1 0

25.67 25.77 25.92 26.04 26.32 26.49 26.52 26.56 26.60 26.67 26.67 26.67 26.67 26.70 26.70 26.70 26.70

411 obsd 29.21 27.66 29.31 27.74 29.46 27.86 29.69 28.16 29.88 28.57 30.06 29.11 30.08 29.20 30.12 29.37 30.17 29.63 30.24 29.94 30.24 30.21 30.24 30.21 30.24 30.26 30.27 30.30 30.27 30.35 30.27 30.35 30.40 30.27

calcd

solvent MeOH

-

411 obsd 28.65 26.88 28.68 26.88 28.74 26.88 26.95 28.87 29.24 27.36 29.56 27.93 28.17 29.74 29.81 28.45 29.03 29.91 29.72 30.06 30.13 30.08 30.30 30.15 30.17 30.33 30.37 30.20 30.40 30.24 30.40 30.27 30.40 30.27

-AAv

(1-2)

v(2)

1.55 1.57 1.60 1.53 1.31 0.95 0.88 0.75 0.54 0.30 0.03 0.03 -0.02 -0.03 -0.08 -0.08 -0.13

25.13 25.16 25.22 25.35 25.71 26.01 26.18 26.25 26.35 26.49 26.56 26.58 26.60 26.63 26.67 26.70 26.70

calcd

lower MezSO concentrations the lower the polarity of the solvent medium. This result corresponds to previously well-recognized behavior,18-20namely, that formation constants of hydrogen bonded complexes tend to decrease in solvents of increasing polarity. To provide a quantitative framework for these findings, and to fit them into a context of meaningful linear free energy relationships, we have adopted the following method of treating the results. We define VFbOas the volume fraction of MezSO (or other HBA solvent) at which the -AAu term is half of the maximum value observed in the solvatochromic dilution (the maximum -AAu being that in the neat HBA solvent plus the CPI). The VF, is nearly, but not exactly, the fractional composition at which the hydrogen bonded complex is half dissociated; hence, the lower the VF50, the greater the value of Kf may be presumed to be in the cosolvent (in the Appendix, we describe a method for estimating Kf from VFm, and relate the latter quantity to VFsOCor, which more closely approximates the half-dissociation point). The points on the dilution plots in Figure 2 corresponding to VFsoare indicated by arrows, and are as follows for the l:MezSO complex in the cosolvents studied: CC14, 0.0062; C6H5C1,0.015, CzH4C12, 0.043; and CH2C12,0.073. The above non-HBA cosolvents were chosen for the present comparison because Taft and co-workers18 had reported formation constants for the hydrogen bonded complex of 4-fluorophenol (4-FP) with Me2S0 in these same media. The log Kf[4-FP:MezSO] values are in CC4, 2.53; in C6H5C1,2.20; in CZH4Clz,1.65; and in CH2C12, 1.44. A plot of -log VFs0(1:Me2SO)vs. log Kf[4-FP:Me2SO] is shown in Figure 3. It is seen that the regression is nicely linear. The correlation equation is -log VF50 = 0.950 log Kf - 0.22

(3)

with r = 0.997 and SD = 0.04 log unit. In the context of the linear solvation energy relationships demonstrated in our other solvatochromic comparison studies, the above result lends confidence that VF, values and the shapes of the solvatochromic dilution plots do indeed reflect meaningful properties of the HBD solute, the HBA solvent, and the non-hydrogen bonding cosolvent. In this and future papers we shall use dilution studies in combination with solvatochromic comparisons as mutually supportive methods of assessing relative HBA basicities and/or HBD acidities of a variety of solvents and indicator solutes.

solvent t-BuOH -AAv

(1-2) 1.77 1.80 1.86 1.93 1.89 1.63 1.57 1.36 0.88 0.34 0.05 -0.15 -0.18 -0.17 -0.16 -0.13 -0.13

1.5

v(2) 25.58 25.77 25.97 26.21 26.39 26.49 26.53 26.57 26.60 26.63 26.67 26.67 26.67 26.70 26.70 26.70 26.70

41) calcd obsd 29.12 26.46 26.85 29.31 29.52 27.36 28.05 29.77 29.95 28.69 29.24 30.06 29.37 30.09 29.56 30.14 29.76 30.17 30.20 29.99 30.24 30.17 30.21 30.24 30.24 30.26 30.30 30.27 30.30 30.27 30.35 30.27 30.40 30.27

2.0

-AAv

(1-2) 2.66 2.46 2.16 1.72 1.26 0.82 0.73 0.58 0.41 0.21 0.07 0.03 -0.02 -0.03 -0.03 -0.08 -0.13

2.5

LOG K f , 4 - F - C g H q - O H t DMSO

Flgure 3. Correlation of VF, values for 4-nitroaniline:Me2S0with K , values for 4-fluorophenol:Me2S0 in corresponding solvents.

Comparison of Dilution Behavior of HBA Base and Amphiprotic Solvents. Interesting and unexpectedly useful information was adduced when we compared dilution behavior of HBA base solvents in non-HBA cosolvents with the behavior of aliphatic alcohol solvents which can act as both HBA bases and HBD acids. As representative examples in the former category we will discuss Me2S0,p = 0.749, and tetrahydrofuran, = 0.556; and in the latter category, 2-methyl-2-propanol (t-BuOH), p = 1.014, a = 0.436, and methanol, = 0.615, a = 0,990 ( p terms are measures of HBA basicity on a scale which ranges from 0,000 for non-HBA solvents to 1.056 for hexamethylpho~phoramide;~~~ CY terms are measures of HBD acidity on a scale ranging from 0.000 for non-HBD solvents to 1.068 for water"). CC14 is the non-HBA cosolvent in all instances, and as indicator solutes we again use the 1-2 pair. The alcohols are both known to form type-A hydrogen bonds to the nitro oxygens but not to the amine nitrogens of 1 and 26v22 and, as has been mentioned, the effects are assumed to be about equal for 1 and 2, and cancel out in the solvatochromic comparison. Hence, insofar as their differential interactions with 1 and 2 are concerned, methanol and t-BuOH may be regarded mainly as hydrogen bond acceptors. The additional spectral data and -AAu results for these


The Journal of Physical Chemistry, Vol. 82,

No. 23,

1978

2481

TABLE IV: Dilution Effects on Apparent HBA Basicities of Aliphatic Alcohols

- AA V association constant with 10%in phenol” K,, CC1, dm3 mol-’ 1.63 11.0 1.51 9.5 1.36 7.5 0.82 10.2

(‘-2)B+~,~q

kK

solvent MeOH EtOH i-PrOH t-BuOH

neat solvent 1.77 2.16 2.58 2.66

Concentrations 0.1-0.7 M in CC1,; 298 “C; ref 31.

VOLUME FRACTION HBA SOLVENT IN CC14

Figure 4. Solvatochromic dilutions of complexes of 1 with HBA and amphiprotic solvents; cosolvent CCI4.

mixtures are assembled in Table 111, and -AAv( 1-2) values are plotted against VF(HBA solvent/CC14) in Figure 4. Considering first the results for T H F and Me2S0,Figure 4 shows solvatochromic dilution curves about as might be expected from a priori considerations. The dilution curve for T H F begins at a lower -AAu value in the neat solvent, shows a smaller cybotactic polarity increment a t a higher volume fraction, and falls off very much more rapidly, remaining well within the Me2S0 “dilution envelope” a t all volume fractions. VF50values of 0.066 (corresponding to about 0.81 M) for THF, and 0.0062 (corresponding to 0.088 M) for Me2S0 suggest about a 9-10-fold higher formation constant for Me2SO:1:Me2S0as compared with THF:l:THF. For comparison, Kf values in CC14 for the and 4-fl~orophenol~~ complexes with Me2S0 are both about a factor of 18 higher than for corresponding T H F complexes. Neat methanol has an HBA basicity ( p value) intermediate between T H F and Me2S0. Accordingly the methanol dilution plot in Figure 4 starts at an intermediate -AAu and remains between the T H F and Me2S0 curves at all volume fractions. Indeed from VF = 1.0 to about 0.08, the MeOH curve maintains about the same relative distances from the T H F and Me2S0 curves, behaving like a straightforward HBA base in this range of solvent compositions. At VF’s below about 0.06, however, the MeOH/CCl, mixtures begin to act nontypically, the -AAu values falling off more rapidly than for either the THF/ CC14 or Me2SO/CC14 mixtures. The VFbOvalue is 0.044 (corresponding to about 1.1M MeOH in CC14)whereas a linear relationship between log VF,, and p would require VFbO= 0.017 for methanol.25 With t-BuOH the characteristic pattern of HBA solvent dilution behavior breaks down completely. Since the t-BuOH P value is the highest of the solvents considered, its -AAu(l-2) value at VF = 1.0 is also highest [it should be recalled that the -AAu( 1-2) terms in the neat solvents are proportional to P value^],^ but immediately as dilution begins the -AAu value plummets. There is not even an inflection to mark the existance of a cybotactic polarity increment, and the curve rapidly crosses the traces for Me2S0, MeOH, and THF, the latter a t VF = ca. 0.2. Half-dissociation of the 1:t-BuOH complex is near VF = 0.24, this value being about a factor of 5.5 higher than that for methanol and about a factor of 3.6 higher than that

for THF. Thus, t-BuOH which is the strongest HBA base when the neat solvents are compared, appears to be the weakest hydrogen bond acceptor of the four solvents at concentrations below 20 vol % in CC1& Effects of Self-Association on Alcohol HBA Basicities. The methanol and t-BuOH results in Figure 4 are readily rationalized as follows: (a) Alcohols are amphiprotic self-associating solvents, able to act simultaneously as hydrogen bond donors and acceptors. (b) An ROH molecule, associated through its hydroxyl hydrogen to another ROH molecule, should have increased electron density on the oxygen of that hydroxyl group. Effectively, this should lead to alcohol dimers and polymers being significantly stronger HBA bases than corresponding alcohol monomers. Such enhanced HBA basicity has been demonstrated for (H20), relative to H 2 0 by Gordon.26 H u y s k e n ~has ~ ~also recently shown that, when an amphiprotic molecule acts simultaneously as hydrogen bond acceptor and donor a t the same site, both the donor and acceptor strengths are enhanced substantially relative to the same species acting only as acceptor or only as donor. (c) Steric considerations should also lead to dimers and polymers of methanol being more strongly self-associated than corresponding self-association complexes of t-BuOH. (d) In addition to showing the spectral effects of dissociation of the alcohol:4-nitroaniline complexes as the alcohols are diluted into CC14, the curves in Figure 4 also show the HBA-weakening effects of dissociation of the various (ROH), complexes. These plots appear to us to indicate that significant dissociation of (MeOH), does not begin until VF(MeOH/CCl,) is below about 0.10, but that dissociation of (t-BuOH), begins almost immediately as non-HBA cosolvent is added. The higher - A b ( 1-2)B-H N value (the shift attributable to hydrogen bonding) for t-buOH when the neat solvents are compared is a result of the higher intrinsic HBA basicity of t-BuOH polymer compared with methanol polymer. That methanol shows the higher -AAu(l-2) when the solvents are compared a t VF = 0.10 (corresponding to 2.5 M for MeOH and 1.1M for t-BuOH) derives primarily from the fact that at these concentrations we are comparing methanol, mainly polymer, with tBuOH, mainly monomer.28 The results and the rationale are completely consistent with findings by earlier workers. D U ~ Ohas C ~recently ~ shown that the self-association constant for methanol tetramer in CC14, KI4,is significantly larger (by a factor of about 11)than the KI4for t-BuOH. Ens and Murray30 have reported that for (ROH),, n = 3.5-3.8, association constants are in the order methanol > ethanol >> t-PenOH. Fragmentary - A b ( 1-2)B,H2N results for ethanol and 2-propanol conform as expected with the methanol-tBuOH trends (Table IV). For the neat solvents, the enhanced solvatochromic shifts are in the inductive order: t-BuOH > i-PrOH > EtOH > MeOH; for the same sol-

2402

Kat++


vents at VF(ROH/CCl,) = 0.10, the order is exactly reversed (the methanol-ethanol-2-propanol progression in the latter case may be due to concentration effects).28 It is a necessary consequence of the above rationale that, when concentrations become sufficiently low that all alcohols are essentially completely dissociated to monomers, we should again observe the inductive basicity order. A tendency in this direction is seen in the findings of Davis, Deucher, and I b b i t ~ o nwho , ~ ~ have measured the association constants of alcohols with phenol in CCll at ROH concentrations of 0.1 to 0.7 M (results included in Table IV). These workers observed a mixed ordering of HBA basicities: MeOH > t-BuOH > EtOH > i-PrOH, indicating that methanol is still partially self-associated at these concentrations. In terms of inductive effects, one would expect ethers to be both more basic and better HBA's than alcohols; however, most indicator and titration studies (including also our own earlier ~ o r k ) represent , ~ ~ ~ the ~ ~ROH compounds as being more basic and better HBA's than the corresponding ROR derivatives. Also, insofar as relative alcohol basicities are concerned, attempts a t general understandings of hydrogen bond donor and acceptor and proton transfer effects have been complicated by seemingly conflicting reports of inductive, antiinductive, and mixed basicity orderings for (CH,)&OH, (CHJ2CHOH, CH3CH20H, and CH30H.32*33Indeed, Arnett32 has argued convincingly that any attempt to establish a single ordering for the oxygen bases is likely to be doomed from the start. Our present findings constitute further evidence consistent with this observation. We do believe, however, that many seemingly contradictory observations may be resolved if comparisons of non-self-associating with self-associating HBA bases, or of the latter with one another, are carried out under conditions where extents of self-association are comparable. Thus, for example, comparing the neat solvents, we found p = 0.949 for 2-propanol, mainly polymer, vs. 0.488 for diisopropyl ether,'^, whereas Davis and c o - w ~ r k e r s , ~ ~ working with 0.1-0.7 M solutions, reported the association with phenol, KO= 7.5 dm3 mol-l for 2-propanol, mainly monomer, vs. 8.2 for diisopropyl ether. The latter result is, of course, more consonant with conventional wisdom regarding inductive effects of alkyl groups on solution phenomenology.

Acknowledgment. The work by M.J.K. and E.G.K. was carried out under Independent Research Test IR-144 of the Naval Surface Weapons Center. We are indebted to Drs. H. G. Adolph, W. H. Gilligan, and R. R. Minesinger (deceased) for useful discussions. Appendix Estimation of Formation Constants for the Hydrogen Bonded Complexes of N-Ethyl-4-nitroaniline and 4Nitroaniline with MezSO in CClk In part 5 of this series8 (published out of turn),l we reported the following solvatochromic dilution behavior for N-ethyl-4-nitroaniline (3) relative to 2 in MeZSO/CCl4:The maximum enhanced solvatochromic shift + CPI, -AAv(3-2),bsdmax,amounted to 1.44 kK and occurred at VF = 0.060, corresponding to ca. 0.85 M MezSO in CC14. The VFbOvalue was 0.0057, corresponding to ca. 0.080 M Me2S0 in CClk From these results and the dilution data in Table I11 of part 5 and Table I of the present paper, we can use a method of successive approximations to estimate the formation constants for the 3:MezS0, l:MezSO,and Me2SO:1:Me2S0 complexes in C C 4 . We first estimate the formation constant for 3:Me2S0.

et al.

In the first approximation, we assume that the VFN value corresponds to half-dissociation. Concentrations of complexed and uncomplexed 3 being equal

12.5 L mol-l (A-1) From this Kf, we estimate the ratio of complexed to uncomplexed 3 a t the maximum in the solvatochromic dilution plot to be (3:Me2SO)/(3) = (Kprox)(Me2SO)= (12.5)(0.85) = 10.6 (A-2) We next estimate -AAV(~-~),,,~", the hypothetical value of the enhanced solvatochromic shift + CPI if the Nethyl-4-nitroaniline were 100% complexed at the maximum in the dilution plot: 11.6 -AAv(~-~),,,= ~ ~-[-Ab(3-2),bsdmax] ~ = 10.6 (1.094)(1.44) = 1.58 kK (A-3) From this value and the dilution data in Table I11 of part 5: we estimate a new VF value corresponding more closely to half-dissociation, VFSOCor = 0.0060, which is equivalent to (Me2SO) = 0.085 M. Substituting this concentration into eq A-1, we obtain in the second approximation

K,""' = 1/(0.085) = 11.8 L mol-I

(A-4)

Carrying the procedure through another cycle of approximations does not materially change the result. We next consider the singly and doubly hydrogen bonded complexes of 1 with Me2S0. The -AAV(~-~),,+~" value is 2.49 kK and occurs at VF = 0.10, corresponding to (Me2SO) = 1.41 M. The VFbOvalue is 0.062, corresponding to (Me2SO) = 0.087 M. For the singly hydrogen bonded complex of 1 with Me2S0 K1 = (1:Me2SO)/[(1)(Me2S0)] (A-5) and for the doubly hydrogen bonded complex (Me2SO:1:Me2SO) K2 = (1:Me2SO)(Me2SO)

A t half-dissociation, (1) = (Me2SO:1:Me2SO),so that KlK2 = 1/(Me2S0)2

(A4

Again, in the first approximation, we assume that VFwobd corresponds to half-dissociation, from which it follows that

K1K2approx= 1/(0.087)2 = 132 L2 moF2

(A-9)

In part 5,8 we estimated that the first hydrogen bond by 1 to Me2S0 is about 1.1times as strong as the 3:Me2S0 hydrogen bond. Taking into account the statistical factor for the two protons and the K f value from eq A-4, we arrive at (A-10) K1 = 2(1.1)(11.8) = 26 L mol-'

K2aPProX = 132/26 = 5.1 L mol-l

(A-11)

From KZaPProx we next estimate the ratio of doubly hydrogen bonded to singly hydrogen bonded 4-nitroaniline at VF = 0.10.



(Me2SO:l:Me2SO) = (KzaPProX)(Me2SO) = (5,1)(1,41)= (MePSO:l) 7.2 (A-12) In part 5, we also offered evidence that the ratio of the spectral effects of the first and second hydrogen bonds by 1 to HBA solvents was ca. 1.1. From this it follows that -AAv( l-2)c,,malL

-AAv(l-2),i,sdmax

-

(8.2)(2.1) = 1.062 (7-2)(2.1) i1.1

-AA~(l-2),0,max = (1.062)(2.49) = 2.64 kK

(11)

(A-13) (A-14)

From the dilution data in Table I, we now estimate that the corrected volume fraction at half-dissociation, VFSOCor = 0.068, corresponding to (Me2SO)= 0.096 M. Substituted into eq A-8, this gives us

K1K2C0r = 108.5 L2 mol-2

(10)

(12) (13)

( 14)

(A-15)

and again taking Kl = 26 L mol-I (from eq A-lo), we obtain

(15) (16)

K2", = 4.2 L mol-l

(17)

(A-16)

As before, carrying the procedure through another cycle of approximations does not materially change the result. In a future paper we shall show that these estimates agree surprisingly well with formation constants determined by alterative methods.

References and Notes (1) Part 3. T. Yokoyama, R. W. TaR, and M. J. Kamlet, J. Am. Chem. Soc., 98, 3233 (1976). The present paper Is publlshed out-of-turn because R was declded to obtain additional experimental information relating the VFsovalues to formatlon constants. Further papers in the series are: part 5, footnote 8; part 6, footnote 17; part 7, M. E. Jones, R. W. Taft, and M. J. Kamlet, J . Am. Chem. SOC.,99, 8452 (1977). (2) (a) Naval Surface Weapons Center; (b) University of California: (c) Engineer Topographic Laboratory; (d) Visiting Sclentlst, UC/Irvlne, 1976-1977. (3) There has been some confusion In the hydrogen bonding literature as to whether the terms donor and acceptor refer to the proton or the electron pair. In the present paper, HBD (hydrogen bond donor) and HBA (hydrogen bond acceptor) refer to donation and acceptance of the proton. (4) M. J. Kamlet and R. W. Taft, J. Am. Chem. Soc., 98, 377 (1976). (5) M. J. Kamlet. R. R. Mlneslnaer. - . and W. H. Gilllaan. J . Am. Chem. Soc.. 94. 4744 (1972). (6) M. J.'Kamlet, E. G.Kayser, J. W. Eastes, and W. H. Gilligan, J. Am. Chem. Soc., 95, 5210 (1973). (7) R. R. Minesinger, E. G. Kayser, and M. J. Kamlet, J . Org. Chem., 38, 1342 (1971). (8) Part 5. R. R. Minesinger, M. E. Jones, R. W. Taft, and M. J. Kamlet, J. Org. Chem., 42, 1929 (1977). (9) Earller nomenclature becomes confuslng and cumbersome when several types of hydrogen bonding wlth concommitant spectral effects occur simultaneously. For this reason, we have introduced a new nomenclature system as follows: AAv denotes a hypsochromic effect .

I

-

(18) (19) (20)

(21) (22)

(23) (24) (25) (26) (27) (28)

(29) (30) (31) (32) (33)

or reduced bathochromic shift; -AAv denotes an enhanced bathochromlc shlft or effect; the numbers in parentheses (1-2) indicate that the enhanced or reduced effect is for compound 1 relative to compound 2; the superscript B indlcates that the effect is attributed to type-B hydrogen bonding;" and the subscript -H2N indicates that the bonding is by amine protons to solvent. In type-B hydrogen bonding, the solute acts as proton donor, the solvent as acceptor; the converse Is the case In type-A hydrogen bonding; M. J. Kamlet, R. R. Minesinger, E. G. Kayser, M. H. Aldridge, and J. W. Eastes, J . Org. Chem., 36, 3852 (1971). M. J. Kamlet, J. L. Abboud, M. E. Jones, and R. W. Taft, J . Chem. Soc., Perkin Trans. 2 , in press. Actually there is a hypsochromic displacement of ~ ( 1on) going ~ ~ from 100 to 10% Me,SO, but this blue shift is at least 8 nm less than expected. It follows from the linear relationships between -AAv terms,pK,, values, fi terms, and limiting F NMR shifts discussed in part 1 that the "tighter" the complex, the greater should be the magnitude of the enhanced bathochromic effect for near 100% associated indicator solutes. The cybotactlc region is the volume around a solute molecule in which the ordering of the solvent molecules has been influenced by the solute. J. R. Partington, "An Advanced Treatise in Physical Chemistry", Vol. 5, Longmans, Green & Co., London, 1951, pp 390ff. B. R. Knauer and J. J. Napier, J. Am. Chem. Soc., 98, 4395 (1976). The solvatochromlc dilution studies involving the HBD indicator solutes other than 1 will be reported in detail in future papers. Part 6. M. J. Kamlet, J. L. Abboud, and R. W. Taft, J. Am. Chem. Soc., 99, 6027 (1977). L. Joris, J. Mitsky, and R. W. Taft, J . Am. Chem. Soc., 94, 3438 (1972). M. D. Joesten and L. J. Schaad, "Hydrogen Bonding", Marcel Dekker, New York, N.Y., 1974, pp 293ff. M. Nakano, N. I. Nakano, and T. Higuchi, J, Phys. Chem., 71,3954 (1967); K. M. C. Davis, J. A. Deucher, and D. A. Ibbitson, J. Chem. Soc., Perkin Trans. 2 , 793 (1975); F. M. Slasenski, J. M. Tustin, F. J. Sweeney, A. M. Armstrong, Q.A. Ahmed, and J. P. Lorand, J , Org. Chem., 41, 2693 (1976). R. W. Taft and M. J. Kamlet, J. Am. Chem. Soc., 98, 2886 (1976). The possibility of the alcohols acting as hydrogen bond donors and the nitroanilines as hydrogen bond acceptors at the amine nitrogens has been essentially eliminated in the case of sp2-hybridized 4nitroaniline derivatives." T. Gramstad and J. Sandstrom, Spectrochim. Acta, Part A , 25, 31 (1969). D. Gurka and R. W. Taft, J . Am. Chem. Soc., 91, 4794 (1969). A hear relationship between log VFsoand /3 wlll be demonstrated for non-self-associated solvents in a future paper. J. E. Gordon, J . Am. Chem. Soc., 94, 650 (1972). P. L. Huyskens, J. Am. Chem. Soc., 99, 2579 (1977). The greater molar concentration of methanol than 2-methyl-2-propanol at similar VF values also contrlbutes to the effect. A comparison at roughly equal molarities, Le., t-BuOH at VF = 0.23 vs. MeOH at VF = 0.10, shows ca. 51% dissociation of 1:t-BuOH vs. ca. 14% dissoclation of 1:MeOH. C. Duboc, Spectrochim. Acta, Part A , 30, 431, 440 (1974). A. Ens and F. E. Murray, Can. J. Chem., 35, 170 (1957). K. M. C. Davis, J. A. Deucher, and D. A. Ibbitson, J . Chem. Soc., Perkin Trans. 2 , 793 (1975). E. M. Arnett, Prog. Phys. Org. Chem., 1, 233 (1963). See, for example, L. S.Levitt and B. W. Levitt, J . Phys. Chem., 74, 1812 (1970); P. Ballinger and F. A. Long, J. Am. Chem. Soc., 82, 795 (1960); E. M. Arnett and J. N. Anderson, ibid.,85, 1542 (1963); L. S.Guss and I. M. Kolthoff, ibM., 62, 1494 (1940); N. C. Deon and J. 0. Turner, J. Org. Chem., 31, 1969 (1966); C. E. Newell and A. M. Eastham, Can. J . Chem., 39, 1752 (1961); W. Gerrard and E. D. Macklin, Chem. Rev., 59, 1105 (1959).

The solvatochromic comparison method. 4. Dilution studies - The

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