NOTES
1596
oxidation-reduction equilibrium between ferro- and ferricytochrome c or quinone and hydroquinone.2 Is it possible to find a similar reaction between quinone and hydroquinone in solid state? As we have already reportedjs molecular hydrogen adsorbs dissociatively on a surface of a solid alkali metal-aromatic complex. In this paper, we show that hydrogen adsorbed on a quinone-sodium complex in the solid phase reduces the quinone to the hydroquinone. The p-benzoquinone-sodium complex was used in this study and was prepared as follows. Sodium metal was deposited onto the surface of a quartz plate in vacuo and then p-benzoquinone, which was purified with repeated recrystallizations and sublimations, was brought into contact with the sodium film by means of vacuum sublimation. Further, the dark green complex film between p-benzoquinone and sodium was subjected to heat treatment a t 353'K for 24 hr and was evacuated completely to remove excess quinone. The purified hydrogen gas was admitted into the vessel containing the p-benzoquinone-sodium complex up to a pressure of about 100 Torr (at room temperature). When the vessel was maintained a t 353'K overnight, white crystals were isolated from the complex. The white crystals, dissolved in alcohol solution, were identified clearly as p-hydroquinone from its absorption spectra in the ultraviolet region. This reaction may be illustrated as absorb
2H2
I 2Na -+
2Na*
over
complex
+
2[Ht][H']
111
Q
-1 . . . . . . . I " -
I
200
1
JOO
t
1
eco w a w m mp
,400
I
a00
mm
.
mo
Figure 1. The absorption spectra of the p-benzoquinone-sodium complexes.
white crystals, identified as p-hydroquinone, were isolated as described above. The spectrum of a p-hydroquinone solid film is also drawn in Figure 1 as curve 1.
Acknowledgment. We gratefully acknowledge an important suggestion with Dr. Y. Harada and express our thanks to the Kawakami Foundation for financial support. (2) I. Yamazaki and T. Ohnishi, Biochim. Biophys. Acta, 112, 469 (1966). (3) T. Kondow, H. Inokuchi, and N. Wakayama, J . Chem. Phys., 4 3 , 3766 (1965);H.Inokuchi, N. Wakayama, T. Knodow, and Y . Mori, ibid., 46, 837 (1967). (4) N. K. Bridge and G. Porter, Proc. Roy. SOC.(London), A244, 259,276 (1958). ( 6 ) J. H. Baxendale and H . R . Hardy, Trans. Faraday Soc., 49, 1433 (1953). (6) Y. Harada and H. Inokuchi, Mol. Phys., 8 , 265 (1964).
2e
Step 1 is postulated to be the addition of electrons from alkali metal to the quinone producing the corresponding dianion radical 11. The addition of the hydrogen is assumed to be a secondary process forming the hydroquinone, The role of H- in reaction 2 is not clear. The observation of the spectra in the reaction processes in the solid phase supports these hypotheses. Curve 2 in Figure 1 shows the absorption spectrum of the p-benzoquinone-sodium complex in solid film; the peaks assigned originate from a semiquinone-type ~ompound.~-6When hydrogen gas was admitted into the reaction vessel, these peaks disappeared and only a peak around 295 mp was found, as shown in curve 3; this peak is assumed to be that of p-hydroquinone, but its assignment is rather uncertain because the absorption peaks of p-hydroquinone and a semiquinone-type compound exist in the same wavelength region. When the reaction vessel was heated a t 353'K overnight, The Journal of Phusical Chemistrg
Nuclear Magnetic Resonance Study of Hydrogen Bonding in Ethanol by W. L. Chandler and R. H. Dinius Department of Chemistry, Auburn Un$versttg, Auburn, Alabama (Received October 10, 1 9 6 8 )
Van Ness and coworkers' have recently reported the results of a rather thorough investigation of the infrared spectra of ethanol-n-heptane and ethanol-toluene midtures in the frequency range of the hydroxyl-stretching mode. These authors point out that the literature (1) H. C. Van Ness, J. Van Winkle, H. H. Richtol, and H. B. Hollinger, J. Phys. Chem., 7 1 , 1483 (1967).
NOTES
1597
reporting investigations of alcohol-carbon tetrachloride solutions is quite large but alcohol-hydrocarbon solutions have been neglected. This had also occurred to us, especially in view of the fact that a large amount of the reported work had been designed to investigate hydrogen bonding in alcohols. If carbon tetrachloride is used as a diluent to study hydrogen bonding in alcohols, it might be anticipated that additional alcohol-carbon tetrachloride interactions would be introduced. We wish to report the results of an investigation of ethanol-cyclohexane solutions by nmr spectroscopy. The chemical shift of the hydroxyl proton over the concentration range of 0.004-1 .O mole fraction of ethanol and a t 20, 30, and 40" has been measured. The instrument used was a Varian BOIL nuclear magnetic resonance spectrometer with provisions for maintaining the probe at &lo. The ethanol used was dehydrated according to Vogel's2 procedure and distilled from sodium carbonate; this last step ensured that hydroxyl proton exchange was very slow and consequently the hydroxyl proton resonance was always a well-defined t r i ~ l e t . ~The cyclohexane was reagent grade from J. T. Baker Chemical Co. The boiling point was 81" a t 1 atm and contained no trace impurities t o catalyze the hydroxyl proton exchange of ethanol. Solutions were prepared by direct weighing of the components. Details of all procedures are set forth el~ewhere.~ We have repeated the experiments carried out by Becker, Liddel, and Shoolery6 with the following exceptions: (1) proton exchange has been reduced to a 1201
0.2 0.4 *
I
1
1
1.0
0.8
0.6 0
I
,
I
I
- 4.00 0 3.95 I-
x
p 3.90 d
2 3.85
8 A
3.80
///
Figure 2. Plot of the log of the limiting slope of chemical shift of the hydroxyl proton of ethanol with respect to mole fraction of ethanol us. the reciprocal of the absolute temperature.
very low rate; (2) carbon tetrachloride has been replaced by cyclohexane as the diluent; and (3) temperature has been controlled and varied. Davis, Pitaer, and Raoa have also repeated the work of Becker, e2 al., incorporating item 3. The same interpretation of the concentration and temperature dependence of the chemical shift of the hydroxyl proton has been used as in the above papers. That is, the existence of an equilibrium system of monomers, dimers, and polymers due to hydrogen bonding between ROH units accounts for the observed changes in chemical shift of the hydroxyl proton. In Figure 1 the chemical shift between the hydroxyl proton and methyl proton resonance us. the mole fraction of ethanol at 20, 30, and 40' is plotted. In these plots the chemical shift has been extrapolated approximately 0.004 mole fraction unit to the zero mole fraction intercept. It is interesting to note that this extrapolated intercept value agrees very well with the chemical shift value for the monomeric ethanol determined in the gas phase.' On the basis of the model proposed for ethanol the variation in chemical shift difference between the hydroxyl proton and methyl protons is due to a change in the resonance frequency of the hydroxyl proton. The observed chemical shift can then be represented by the relation
-200'
'
I
where S,,
B-3OoC C- 4OoC '
'
'
'
'
'
'
3.45
3.30
I I T PKj' X IO'
SO A- 2OoC
-
3.75 3.15
sd,
=
x m s m
+ 2XdSd +
P x p s p
and Sp are the characteristic chemical
(2) I. Vogel, "Practical Organic Chemistry," 3rd ed, John Wiley & Sons, Inc., New York. N. Y., 1956, p 167.
I
0.01 0.02 0.03 0.04 0.05 MOLE FRACTION ETHANOL
Figure 1 . Plot of the chemical shift (Hertz) of the hydroxyl proton of ethanol referenced t o the methyl group protons vs. mole fraction of ethanol; diluent is cyclohexane.
(3) J. M. Bruce and P. Knowles, Proc. Chem. Soc., 294 (1964). (4) W. L. Chandler, M.S. Thesis, Auburn University, Auburn, Ala., 1968. (5) E. D. Becker. U. Liddel, and J. N. Shoolery, J. Mol. Spectrosc. 2, 1 (1958). (6) J. C. Davis, Jr., K. 9. Pitzer. and 0. N . R. Rao, J. Phys. Chem., 64, 1744 (1960). (7) J. A. Pople, W. G. Schneider. and H. J. Bernstein, "HighResolution Nuclear Magnetic Resonance," McGraw-Hill Book Co., Inc., New York, N. Y., 1959,p 410. Volume W , Number 6 M a y 1069
NOTES
1598
shifts of ethanol monomers, dimers, and polymers, respectively. X,, xd, and X, are mole fractions of ethanol in the monomer, dimer, and polymer form, respectively. P represents the number of monomer units per polymer. Van Ness, et al., have shown that in dilute ethanolic solutions the predominant species are monomers and dimers. Then following Huggins, Pimentel, and Shoolery8the equilibrium constant for monomer-dimer formation in dilute solution can be expressed in mole fraction units as Kd = Xd/Xm2and at infinite dilution If 8 d (dXo/dX)(x=o) = 2KdAd where Ad = Ed is similar to the characteristic chemical shift of many other species, then it can be expected to be relatively independent of temperature. Following Davis, et al., we may write
s,.
In Figure 2, log (dXo/dX)(x=o)vs. ( l / T ) is plotted from the chemical shift-concentration curves a t 20, 30, and 40". The slope of this plot yields a AH value for dimerization of -5.04 kcal/mol. Van Ness reported a value of -5.2 kcal/mol from infrared data and using heptane as the diluent. The AH of dimerization for ethanol using carbon tetrachloride as a dilutent and applying infrared and nmr techniques has been reported as -7.29 and -7.6 kcal/mo1,6 respectively, for the two techniques. (8) C . &I. Huggins, G . 0.Pimentel, and J. M. Shoolery, J . Phys. Chom., 60, 1311 (1956). (9) U. Liddel and E. D. Becker, Spectrochdm. A c t a , 10, 70 (1957).
Reaction of O( "p>
with 2-Methyl-2-penteneat
Low Temperatures and Its Implication for the Transition State by R. Klein and M. D. Scheer National B u r e a u of Standards, Washington, D . C. (Received October 14, 1 9 6 8 )
20834
Recently, we have proposed a new transition state for the reaction between oxygen atoms and olefinic hydrocarbons on the basis of the experimental results of the O(aP) reaction with cis- and trans-2-butene.' The formation of this transition state requires that the 0 atom attack be in the plane of the olefinic structure and that there be an interaction between 0 and the nearest-neighbor hydrogens bonded to the olefinic The Journal of Physical Chemistry
carbons. The transition state in the case of reaction with trans-Bbutene, for example, is represented as
I The major carbonyl product from this reaction at 77°K is 2-butanone-approximately tenfold that of isobutyraldehyde-and therefore the ease of migration of H relative to the CH, group is demonstrated. An ambiguity unresolved by these studies is whether the H interacting with the 0 is completely or only partially immobilized with respect to migration. In further studies the carbonyl products of the 0 atom reaction with 2-methyl-%butene were found to be neopentanal and 3-methyl-2-butanone in a 4 to 1 ratio a t 90°K. This did not distinguish insofar as the formation of 3 methyl-2 butanone was concerned between the two possibilities [refer to (2)] of (a) a partially inhibited
(b)
II H-atom migration, the H interacting with the 0 in the transition state, or (b) an attack of 0 on the olefinic bond from the side of the median plane opposite the C-H bond and hence easy H migration. Furthermore, addition to the 2 carbon of 2-methyl-%butene with migration of a methyl group gives the same result as addition to the 3 carbon with migration of H. The ambiguity can be effectively resolved through observations on the 0 (3P) reaction with 2-methyl-2-pentene. After passage through the transition state, the final products are epoxides or carbonyl compounds. To form the carbonyl compounds, one of the two groups bonded to the carbon to which the 0 finally becomes attached must migrate to the neighboring carbon of the double-bond pair. The scheme for 2-methyl-2-pentene, applying the new transition state concept, is shown in Scheme I. It is to be noted that when the oxygen becomes localized in a carbonyl group after passage through the transition state, no doubt exists as to whether the 0 atom was bonded to (a) or (b) . The uniqueness of the products assures the conclusion. A localization of 0 on (a) can result only in (111) or (IV). Thus dimethyl butyraldehyde arises from ethyl migration and 2-methyl-3-pentanone from H migration. The addition a t (b) gives only 3-methyl-2-pentanone. (1) M.D. Scheer and R . Klein, J . P h y s . Chem., 73, 507 (1969).
