432
Vol. 75
HERBERT E. UNGNADE
iiiore effect on the rate than does the term containiiig the entropy of activation. The m-substituents however produce a real effect on activation energy, although ever1 here the charlgcs ill log PZ nlay he significant. Since it car1 thus be show11 that elltropy of activation terms are not all the same and C;ITI in fact account for an important part of the
[COUTRIBUTIOh' FROM T H E C H E M I S T R Y
rate changes, it is not fruitful t o discuss the other two alternative explanations which are based 011 modifications of the potential energy effects alone. j\ssignmetit of effects to potciitial energy ternis a h i e is especially utijustified in cases of this sort where the variatiorls in rate are small. HOLTSTOV,
TEXAS
DEPARTMENT, NEW
hfEXIC0
HIGHLANDS IT\IVERSITT
1
The Effect of Solvents on the Absorption Spectra of Aromatic Compounds BY HERBERT E. UNGNADE' RECEIVED MAY19, 1962 Solvent shifts have been determined for benzene derivatives with electron-donor and electron-acceptor substituents. I t is proposed that the direction of the shift., of the high-intensity bands depend5 on the solvent-solute interaction
Solvents can affect the fine structure of absorption curves as well as the intensities and wave lengths of the maxima. From the standpoint of fine structure solution spectra of aromatic conipounds in paraffin solvents most nearly resemble those in the vapor state. With increasing solvent polarity the spectra suffer a loss in vibrational fine structure which may perhaps serve as a measure of the extent of the solvent interaction. The absorption spectrum of benzene is only very slightly affected by solvents. Aromatic hydrocarbons more complex than benzene undergo definite shifts of their absorption curves when the polarity of the solvent is changed. Still larger solvent shifts are observed in substituted benzenes with polar substituents. The direction of the solvent shifts is not always the same for all absorption bands of the same compound and i t is therefore necessary to classify the bands. Above 200 mp most benzene derivatives possess two types of bands which are distitlguishable by their relative abs~rbancies.~The high intensity bands, also called K-bands, prinlary bands4 or E-bands,j are usually displaced to longer wave lengths with increasing polarity of the solvent. The low intensity bands are known as secondary bands4 or B-bands3 and are shifted toward higher frequencies when the dielectric constant of the solvent is increased. In mono-substituted benzene derivatives with electron-acceptor substituents such as COOH, COCH3, CHO and NO? the high intensity bands become identical with the red-shift bands of McConnell.6 When measurements are omitted in solvents where association occurs (e.g., benzoic acid dimer in hydrocarbons, chloroform, etc.),7the shifts are progressive from hydrocarbon solvents to water and, for a given solvent pair, roughly proportional to the shift of the corresponding benzene band due to the particular substituent (Table 1) (1j Chemistry Department, Purdue University (2) N D Coggeshall and E M Lang, Txrs JOURNAL, 70,3283 (1948) (3) A Burawoy. Dtsc Faraday Soc , 9, 70 (1950). J Chrm Soc , 1180 (1939); 2310 (1932) (4) L Doub and J & Vandenbelt, I THISJOURNAL, 68, 2714 (1947) (5) K Bowden and E A Brdude. J Chem Soc , 1068 (1932) (6) H hlcConnel1, J Chem Phys , 20, 700 (1952) 17) H E Ungnade and K Lamb, THISJOURNAL, 74, 3789 (1952)
TABLE I PRIMARY BATDSOF BENZENE DERIVATIVES WITH ELECTROX A4CCEPTOR S U A S T I T U E S T S Xmaa
Compound
(Hz0)
Xmax
(EtOH)
Ma
I hh
Benzoic acid 23OC 2289 26.5 $2 Acetophenoned 245.5' 241" 42.0 +4 5 Benzaldehyde 249. F 243h 48.0 +H . 5 Nitrobenzene' 288.5" 260i 65.0 +8.5 " Ct'ave length shift of the benzene band at 203.5 m p due hmax (H20) - Xmax ( EtOH). Refto the substituent .< erence (4). I n hexane, , ,A 235 mpe and XH?O - XHC 10.5 mp. G. Scheibe, F. Backenkohler and A. Rosenberg, B e y . , 59, 2617 (1926). In isoCictane Xmax 252 mp and X H ~ O - XHC f 16.5 mp (M'. G. Brown and H. Reagan, THIS JOURNAL, 69, 1032 (1947)). Reference ( 7 ) . * R . A . Morton and A . L. Stubbs, J . Chon. SOC.,1349(1940). ; H. E. I-ngnade, i'. Kerr and E. Youse, S c k n r ~ 113, , 601 (1951).
+
'
These findings are in accord with the conclusions reached by Doub and Vandenbelt,4 that the absorption bands in mono-substituted benzene derivatives are displaced benzene bands and that the displacements are due to electronic interaction of the group with the nucleus. Polar solvents, such as water, would tend to stabilize polar quinoid resonance forms of the types __
+
O __ = C ' " \
+
O z X / O '0 ~
The stabilization of the arrangement of the solvent niolecules in the excited states would be greater in molecules with strong interaction between substituent and nucleus. Literature data for monosubstituted benzene derivatives with electron donor groups (OH,s (J\,fe,4,." SH,1 0 , I I SR, 11-13 &TH2,4,9-11.11 N~J&I1C,IX3 (8) J. M. Vandenbelt, C. Henrich and S. I,. Bash, Scieirre, 114, 570 (1931). (9) R. A Morton and A. I-. Stubbs, J . Chem. Soc., 1348 (1940). (10) !A.' 11930).
W. Robertson and F. A. hfatsen, T H I SJOURNAL, 72, 52.51
(11) R. A. Friedel and M.Orchin, "Ultraviolet Spectra of Aromatic Compounds." John Wiley and Sons, Inc., New York, N. Y., 1951. (12) 13. A. Fehnel and M. Carmack, Tnrs JOURNAL, 71, 91 (1949). (14) C. C. Price and J. J . Hycock, i b i d . , T 4 , 1944 (1952). (14) (a) R. A. Motton and A. McGookin, 1.Chem. SOC., 901 (1934); (b) W. D. Kumler and L. A. Strait, THISJ O U R N A L , 61, 2351 (1943); (c) 1,andolt-Bdmstein, "Zahlenwerte und Funktionen," Vol. I , part 3, Springer, &rlitl, 1951. (1.5) H. 3.Klevens and J . R.Platt, THISJOURNAL, 71, 1714 11948)
Jan. 20, 1953
EFFECTOF SOLVENTS ON ABSORPTION SPECTRA OF AROMATIC COMPOUNDS
433
TABLE I1 PRIMARY AND SECONDARY BANDSOF BBNZLCNE DERIVATIVES WITH ELECTRON DOSORSUBSTITUENTS" Compounds
Amax'
log
Watw, t
A
n(a*
108
Alcoho) h a t
log
A mal
t
log
t
Amax
C yclohexaqe log 8 A mnx
loge
217' 3 . 8 0 269' 3 . 1 7 220 3 . 8 5 270 3 . 2 4 222 3.86 271 3.33 220" 3 . 9 4 270 3.17 225" 4 . 0 1 271 3.31 226" 3.05 273 3.31 228 4.02 ..a ,. 242 4.12 . .." .. (238)d 4 . 1 2 (274)d 2 . 9 2 244 3 . 9 4 2%" 3.11 251 4 . 1 1 298 3 . 2 8 251 4.16 298 3.37 227 3 . 9 3 254" 2 . 8 2 227 4 . 0 6 256 2.82 228.5e 4.12 258 2.83 The author is indebted to J. McDowell, S. Lee and J. Mundzak for some of the measurements. ?).
be of the same order of magnitude as the intrinsic constant found for the association of the same metal with serum albumin. The present paper verifies this conclusion for cadmium. A similar verification for zinc has been made by Gurd and G~odman.~ Experimental The reaction was studied potentiometrically by the method of Bjerrum.6 A Beckman model G PH meter with external electrodes was uqed, and the procedure employed ( 5 ) J. Rjerrum. “hletal Ammine Forniati(in P €{adze and Son Cnf)enhngen I W l
111
kqiieou. k ) l i i t i m
