Hydrogen Bonding and Complex Formation in Solutions of t-Butyl Hydroperoxide' Cheves Walling and LaDonne Heaton2 Contribution.from the Department of Chemistr v, Columbia Universit v, New Y o r k 2 7 , New Y o r k Received August, id64 Dilute solutions of t-butyl hydroperoxide in CC14 show a sharp -OH stretching band at 3554 cm.-' which is replaced bjs a lower frequency band at higher concentrations and on addition of aromatic solvents. The change is interpreted as indicating a cyclic hydrogen-bonded dimer and the formation of hydrogen-bonded complexes with the aromatics. Association constantsf o r these complexes have been calculated. Deuterated hydroperoxide, with an - O D band at 2630 cm.-', behaves similarly. Its dimerization constant is smaller, but it forms stronger complexes with aromatic solvents. Partition experiments of hydroperoxide between 4 M styrene-carbon tetrachloride and water give results consistent with the above equilibria. Other aromatic solvent systems show additional afinity f o r the hydroperoxide dimer not discernible f r o m infrared measurements.
Table I. Stretching Frequencies and Band Widths for ROO-H in Various Solvents Solvent
cm.-l
Width; cm.-'
CCI, Hexane Cyclohexane m-CsH,CIz O-CsHaClz C&jC1 C6Hs Ce"C(CH3)a CsHjCH=CHz CeHsOCH3
3554 3561 3556 3530 3530 3523 3509 3 504 3500 3480, 3418
30 29 29 46 46 53 58 64 66 73, 114
h a x r
a
Band width at half-height.
tudes of the observed shifts. Qualitatively the shifts parallel the expected order of basicities of the aromatic solvents, and anisole yields a two-peak spectrum, Although a number of workers have suggested that perhaps indicating association with both the aromatic hydrogen bonding plays a n important role in the rering and the ether oxygen. Benzotrifluoride also shows actions of hydroperoxides,3-6 and qualitative spectral evidence for such bonding has been r e p ~ r t e d , ~ , ' , ~a double peak. If it is assumed that we are observing equilibrium with n o quantitative study of such association in a variety of a hydrogen-bonded complex between T B H P and the solvents has been available. Our own work on taromatic molecule acting as a base butyl hydroperoxide (TBHP) decompositiong required such data in order t o estimate changes in hydroperP+B=PB (1) oxide activity with solvent and concentration, and this paper reports our results, obtained by infrared spectrosas has been found in other systems,'" and governed by copy and measurement of distribution constants. an equilibrium constant Infrared Spectra of t-Butyl Hydroperoxide. The K = [PBI/[PI[Bl (2) hydroxyl stretching band of T B H P which appears a t 3554 cm.-' in dilute solution in CCla shows significant K may be determined from the decrease in absorbshifts to lower frequencies at higher concentrations or ance, A , a t 3554 crn.-l, since in aromatic solvents, a phenomenon generally associated with the formation of hydrogen bonds.'O [PI = [PI"A/AO (3) Thus, as shown in Figure 1, addition of benzene to a whence 0.2 M solution of T B H P in CC14 leads to a decrease in absorbance a t 3554 cm.-' and appearance of a lower frequency shoulder. In pure benzene the 3554-cm.-' peak disappears and is replaced by a broader, more intense peak at 3509 cm.-'. Other aromatic solvents (zero subscripts indicating initial absorbance and total show qualitatively similar results, summarized in concentrations). Measurements on systems containing Table I. Measurements at 70" produce little change in 0.02 M T B H P and 0.3-1.5 M aromatic base in CCl, the 3554-cm.-' band and small decreases in the magniyield consistent values of K a t both 30 and 70' (Table 11). Although K values vary little between solvents, ( 1 ) Taken from the Ph.D. Thesis of L. Heaton, Columbia University, 1963. Support of this work by grants from the National Science Foundation is gratefully acknowledged. (2) American Cyanamid Co. Fellow, 1961- 1962. (3) L. Bateman and H. Hughes, J . Chem. SOC.,4594 (1952). 14) A . V . Toholskq and L. R. Matlack, J . Polymer Sci., 55, 49 (1961). ( 5 ) H. E. De La Mare, J . Org. Chem., 25, 2114 (1960). (6) J . 0. Edwards, "Peroxide Reaction Mechanisms," Interscience Publishers, Inc., New York, N. Y . , 1962, pp. 70, 86 ff. ( 7 ) 0 . D. Shreve, et al., A n d . Chem., 23, 282 (1951). (8) R . \Vest and R. H. Bane>. J . Phj's. Chem., 64, 822 (1960). ( 9 ) C. Walling and L. Heaton, J . A m . Chem. SOC.,87, 38 (1965). (LO) G . C . Pimentel and A . L . McClellan, "The Hydrogen Bond," W . H . Freeman and Co., San Francisco, Calif., 1960.
48
Journal of the American Chemical Society
87.1
Table 11. Complex Formation by TBHP in CCh Solutions -AH, kcal / mole
-AS,
015 0 15
2 39 158 0 74
10 5 Y O 6 2
1 90
5 95
18 4
K, I /mole Complex
70'
30"
TBHP-styrene TBHP-benzene TBHP-o-dichlorobenzene TBHP-chlorobenzene TBHP dimer
0 17 011 0 13 0 10 0 60
0 27
January 5, I965
eu
c
B
I I
i
l
l
I
I
I
I
I
I
l
l
l
l
AH and AS both become increasingly negative in the order o-dichlorobenzene < benzene < styrene, paralleling the infrared shifts observed and increasing basicity. The changes in spectra observed with changing T B H P concentration in CCI, are shown in Figure 2. As concentration increases the 3 5 5 4 - c m . ~ ' peak is replaced by a broad band at about 3400 cm.-l, which we believe is associated with a cyclic hydrogen bonded dimer perhaps together with higher polymers. The equi-
I
3700
;O-O,
H
\
R
librium constant for its formation Kz
=
(5)
[PP]/[P]*
was calculated by the method of Liddel and Becker," which assumes a series of successive polymers, present in low enough concentration (in dilute solution) so that [PI G [PIo. Under these conditions E =
l
l
3500
I
3300
Figure 2 . Effect of concentration on the absorption of free and hydrogen-bonded ROO-H in CC14.
H
' 0 -0"
PP
w,
Frequency, cm.-*.
R\
2P
I
I
~ o ( l- 2K?[P]o - 3K3[P]02 - . . . )
(6)
where E is a measured molar extinction coefficient at 3554 cm.-' and eo that for monomeric TBHP. If E is plotted vs. [PIo, the intercept yields eo, and the slope a t [PIo = 0 then yields K2. Plots a t 70 and 30" (and also data for deuterated hydroperoxide discussed below) are shown in Figure 3. They indicate K2 = 0.60 I./mole a t 70" and 1.90 at 30", from which A H = -6 kcal./mole and AS = - 18.4 cal./deg. Both values seem consistent with the cyclic dimeric structure proposed. Infrared Specfra of Deuterated t-Butyl Hydroperoxide ( T B D P ) . Deuteration moves the absorption maxima of T B H P in dilute CC1, t o 2630 cm.-', and changes in solvent and concentration produce shifts qualitatively similar t o those just described. l 2 Equilibrium constants and thermodynamic quantities were determined as above, and are summarized in Table 111. Comparison of the T B H P and T B D P results are in(11) U. Liddel and E. D . Becker, Spectrochim. Acta, 10, 70 (1957). (12) For all species v(TBHP)!v(TBDP) g 1.35, consistent with observations on many other compounds with -OH and -OD bonds.
Walling, Heatori
teresting in that deuteration decreases the tendency to form cyclic dimers, but increases complex formation with aromatic bases. Without going into possible explanations (discussed in references cited below), Table 111. Complex Formation by TBDP in
Corn p 1ex TBDP-styrene TBDP-benzene T B D P dimer
K , l./mole 30" 70" 0.50 0.25 0.64
cc14Solutions -AH, kcal./ mole
-AS,
e.u.
0.84
2.7
...
...
, .
9
1.65
4.9
15
we may note that similar results have been observed in other systems. Thus, for example, deuteration increases the bonding of phenol t o various bases13 and of C H F 3 to t e t r a h y d r ~ f u r a n . ' ~O n the other hand, it decreases the dimerization of phenolI3 and trifluoroacetic acid. l 5 Partition Experiments. Although infrared studies may indicate the nature of association phenomena, partition techniques provide a more unequivocal method of determining changes in solute activity with concentration and solvent. In addition, comparison of the two techniques shows whether additional interactions are involved not indicated by spectral changes. (13) G.R. Plourde, Dissertation Abstr., 22, 1400 (1962). (14) A. L. Allred and C. J. Cresbvell, J . A m . Chem. Soc., 84, 3966 (1962). (15) M. D. Taylor and M. B. Templeman, ibid., 78, 2950 (1956).
Complex Formation in t-Butyl Hydroperoxide Solurions
49
60
I
by adding an additional term involving association of peroxide dimer with solvent. Whether such terms have any direct physical significance or simply represent empirical corrections which take into account u n specified interactions between solvent and solute molecules cannot be answered, since infrared spectra of complexed peroxide at high peroxide concentrations in aromatic solvents are too broad and diffuse to indicate changes in the species present. The empirical "complexes" and their association constants are listed in Table I V . It will be noted that all (except the simple
T=70°C.
L
40
20
tI
0
I
I
l
1
1
1
1
1
80 -
1
1
1
Table IV.
Equilibrium Constants for Formation of Peroxide-Solvent Complexes Based on Distribution Data
wO0C.
Compkxa P- M P-P-M P-P-S P-P-M
2oE 0
I
P2-MZ P2-M2
l
l
l
l
l
l
l
l
l
l
Temp., Solvent mixture
Peroxide
"C.
Kb
4 M stlrene in CCId Bulk styrene 6 M benzene4 hl styrene 4 7 hlo-dichlorobenzene-4 M styrene 4 M styrene in CCl, Bulk styrene
TBHP TBHP TBHP
34 30 5 32 5
0 25 0 27 0 14
TBHP
27 5
0 32
TBDP TBDP
24 25
1 3 I O
P = peroxide, M = styrene monomer, and S = solvent. P2-M2 denotes a hypothetical complex containing two molecules each of peroxide and styrene. Equilibrium constants are based on concentrations in mole:l.
l
Distribution Data for TBHP and TBDP in Water-Solvent Mixtures
Table V.
When a hydroperoxide is partitioned between water (in which it presumably exists in a monomeric form strongly hydrogen bonded t o water) and a n organic solvent in which it may exist as monomer and various associated species, its distribution is determined by a distribution coefficient Kd such that (7) where [PIw and [PIs represent hydroperoxide concentrations in water and organic phase, respectively, a n d a is the fraction of peroxide existing in associated form. I n the presence of aromatic bases, association involves both dimer formation and association with the aromatic base and is governed by the equilibria 2 and 5. Substituting these expressions into eq. 7 yields the relation
Peroxide
Solvent
TBHP
3 . 9 4 M styrene in
34
TBHP
8 . 4 7 M styrene
30.5
TBHP
3 . 9 6 M styrene6 . 0 0 M benzene
32.5
TBHP
4 . 0 3 M styrene4 . 7 1 M o-dichlorobenzene 5 . 0 3 M styrene3 , 6 9 M o-dichlorobenzene 3.01 M styrene5 75 M o-dichlorobenzene 4 . 0 3 M styrene in CCI?
27.5
TBHP TBHP TBDP
This expression was checked by determining the distribution of T B H P at a series of concentrations a t 34" between water and 3.94 M styrene in CCI,. Substituting the value of K 2 obtained by interpolation from our infrared measurements (1.67 1 'mole) gave Kd = 2.51 and K = 0.25 I.,!mole, in good agreement with our infrared value of 0.26. Measurements i n a number of other solvent systems gave less satisfactory agreement with eq. 8, the peroxide in general showing too high a solubility in the organic phase, particularly at higher concentrations. Interestingly, in every case the data could be fitted 50
Journal of the American Chemical Society l 87.1
TBDP
cc1,
8 . 6 6 M styrene
Temp., 'C.
27.5
[PI),, mole/l.
[PIS, mole/l.
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
2321 1900 1426 0954 2297 1841 1396 0937 2437 1903 1423 1406 0921 2534 2049 1541 1997 1521
2132 1625 1217 0803 2202 1728 1265 0831 1910 1552 1194 1174 0791 2010 1570 1173 1622 1193
27.5
0 1959 0 1542
0 1660 0 1172
24
0 0 0 0 0
0 0 0 0 0
25
2974 2496 1888 2620 2144
2117 1635 1218 2166 1690
peroxide-styrene complex) involve two peroxide molecules, so that their perturbing effect on peroxide activity becomes very small at high dilution. This in turn is consistent with our findingsg that the interpretation of the homolytic decomposition of T B H P is fairly straightforward in dilute solution, but must be-
/ January 5, I965
come increasingly complex as concentrations are increased. Experimental
Materials were the same as those described previouslyg except that styrene was used without removal of a trace of phenolic inhibitor which did not interfere with any of the physical measurements. Infrurrd spectra were taken with a Perkin-Elmer Model 421 grating spectrophotometer, using 0.1- or 1 .O-mm. NaCl cells. Measurements at room temperature were at constant temperature within 1 ’ determined by a thermometer attached to the cell. Measurements at 7 0 ” were made by enclosing the cell in a box with NaCl windows heated by circulating hot air, k0.5’. The spectrum from 3800-3500 (-OH) or 2900-2600 cm.-l (-OD) was scanned three times for each sample. During the time required for measurement there was no change attributed to evaporation or decomposition, and samples measured at 30°, heated t o 70’, cooled,
and remeasured gave identical spectra. At room temperature slit widths were 135 (3554 cm.-l) and 308 p (2630). Since the thermostating arrangement prevented considerable light from reaching the cell, 70” measurements were made with maximum source intensity and slit widths of 210 and 599 p . Complex formation with aromatic bases was determined in CCI, solutions 0.3-1.5 M in aromatic and 0.02 M in T B H P (or 0.03 M TBDP), at which concentration dimer formation was negligible. Distribution Experiments. Known concentrations of T B H P in solvent (previously saturated with water) were shaken with an equal volume of w a t x (previously saturated with solvent) in a small separatory funnel, and the phases allowed to separate. An aliquot of the organic layer was removed and titrated for peroxide, and the concentration in the water layer calculated by the difference. Experiments with T B D P were carried out in the same way using 99.5 DzO. Results from which distribution constants were calculated are listed in Table V.
Optical Rotatory Dispersion Studies. XCVII. Anomalous Rotatory Dispersion and Circular Dichroism Curves Associated with Thionamides. Application to Stereochemical Studies of Carboxylic Acids * Joseph V. Burakevich3 and Carl Djerassi
Contribution f r o m the Department of Chemistry of Stanford University, Stanford, California. Received August 5, 1964 In the past, a number of “chromophoric” derivatives containing the thione moiety (C=S) have been successfully employed f o r stereochemical correlations. However, these derivatives all had the structural feature that the thione was separated f r o m the asymmetric center b y one or two atoms. In the thionamide derivative (-C(=S)N R R ’ ) , prepared b y reaction of phosphorus pentasuIfide with an amide, the thione is adjacent to the asymmetric center. This favorable situation produces more reliable and pronounced Cotton efects when a thionamide is used as a “chromophoric” derivative f o r carboxylic acids than the formerly employed acylthiourea derivative (-C(=O)NHC(=S)NR‘2). In general, a correlation can be made between the absolute conjguration ( R or S ) of the asymmetric center closest to the chromophore and the sign of the Cotton effect associated with the derivative’s low extinction ultraviolet absorption maximum near 330 mp. The operation of free rotation in thionamides is demonstrated b y circular dichroism measurements over the range - I92 t o f 168’.
The stereochemistry of many optically active co mpounds which are transparent in the 2 10-700-mp region such as alcohols, amines, and carboxylic acids has been successfully investigated through optical rotatory dispersion measurements of Cotton effects associated with “chromophoric” derivatives. These derivatives contain the required low intensity and conveniently measurable absorption bands, and give rise to anomalous optical rotatory dispersion and circular dichroism curves. Dithiocarbamates (-NHC(=S)SR’), xanthates (-0C(=S)SR’), and acylthioureas (-C(=O)NHC(=S)NR’2) thus serve as useful “chromophoric” derivatives for amino acids, alcohols, and carboxylic acids, respectively. The three sulfur derivatives all possess a low intensity ultraviolet absorption maximum at 330-350 m p which most likely arises from transitions in which the thione plays the principal role; the isolated thione is known to have a low intensity band at about 500 mp.j Since it is this grouping which is responsible for the long wave
(1) Paper X C V I : C. Djerassi, Proc. Chem. SOC.,314 (1964). (2) Supported by Grants No. GM-06840 and CA-07195 from the National Institutes of Health of the U. S. Public Health Service. (3) Taken from part I1 of the Ph.D. Thesis of J. V . B . , Stanford Uni-
(4) For leading references to these and many other “chromophoric” derivatives, see Table I in ref. 1. (5) See C. Djerassi and D. Herbst, J . Org. Chem., 26, 4675 (1961): R . Mayer, G. Hiller, M . Nitzschke, and J . Jentzsch, A n g e w . Chem. [tzlertt. Ed. E n g l . , 2, 370 (1963)
versity, 1964.
Burakevich, Djerassi
I
Curves of Thionamides
51
