J. Phys. Chem. 1983, 87, 4995-4998
indirect way affect the EFG fluctuation at the sodium ion. In view of the complexity of the system, further attempt for explanation of the origin of this 23NaTIvs. L trend in the mesophase must await further experiments. In the case of the isotropic melt there are several possible explanations for the trend of the 23Na T,with the molecular length L. Provided (6Vk2)idoes not differ among the compounds studied as we have already proposed, then TIand consequently 72 both follow the expected sequence of molecular length for all the isotropic molten salts studied. A t a given temperature, for compounds with a shorter L , a more rapid EFG fluctuation at the Na+ ion is obtained. Should T: reflect dominantly ion-pair reorientational motion, this relationship between rCiand L is also reasonable. Smaller ion pairs are able to reorient at a greater rate. Should the correlation time be dominated
4995
by the Na+ ion translational motion, it is also plausible that the Na+ ion diffusion is hindered by the anion and requires some movement of the anion prior to a diffusive jump. Thus, the 7: for the sodium ion in the isotropic melt may be related to L via the motional characteristics of the anion. We are well aware of the fact that our discussion is only qualitative and many new experiments have to be carried out to put these qualitative statements on a firm theoretical basis. Acknowledgment. This research was supported by the Air Force Office of Scientific Research under Grant AFOSR 81-0010. Registry No. Sodium n-butyrate,156-54-7;sodium n-valerate, 6106-41-8;sodium n-hexanoate,10051-44-2;sodium n-heptanoate, 10051-45-3;sodium, 7440-23-5.
Thermochemistry of Solid Naphthalene Anion Salts and Their Interaction with Water Gerald R. Stevenson;
Charles R. Wiedrlch, Steven S. Zlgler,
Department of Chemistry, Illinois State University, Normal, Illlnois 6 176 I
Luis Echegoyen, and Rene Maldonado Department of Chemistry, University of Puerto Rico, Rio Piedras, Puerto Rico 0093 1 (Received: November 22, 1982: In Final Form: January 19, 1983)
Calorimetric techniques have been used to determined that the solid sodium salt of the naphthalene anion radical lies at about the same energy level as does sodium metal and naphthalene, while the tetrahydrofuran (THF) solvated anion radical lies 13 kcal/mol lower in energy. The solid potassium naphthalenide lies about 9.6 kcal/mol lower than solid potassium metal and naphthalene. The calorimetric results have been placed into a thermochemical cycle to yield the crystal lattice energy and the heat of solvation of the sodium salt. ESR has been used to monitor the decomposition of the solid anion radical salt into the sodium-naphthalene mixture.
Solvated hydrocarbon anion radicals and dianions have been known to be rapidly protonated (without exception) upon the addition of protic solvents, such as water or alcohols, for over 30 years (Birch reduction).' In fact, there exist a vast number of reports including those that elucidate the kinetics and mechanisms involved in these reactions.2 However, in a preliminary publication we reported evidence that the solid sodium salt of the naphthalene anion radical can cleave water to yield hydrogen gas as opposed to 100%protonation of the anion r a d i ~ a l . ~ This conclusion was based upon the observation of evolution of hydrogen gas upon the addition of water to the anion radical salt (Na'NP-.) in the absence of solvent. During the same year Szwarc and co-workers4 refuted these conclusions. Their argument was based, in part, upon their contention that (1)the tetrahydrofuran (THF) solvated naphthalene anion radical salt retains its integrity and stability only as long as the T H F vapor pressure exceeds the pertinent saturation pressure; (2) the application
of an open vacuum to the system leads to desolvation, and the nonsolvated salt spontaneously decomposes into naphthalene and metallic sodium. In support of this, they prepared sodium naphthalene anion radical solutions in THF and were able to sublime all of the naphthalene from the reaction vessel after removal of the THF. In short, they contend that the reduction of naphthalene by sodium metal will not take place without the large exothermic solvation of the Na+,NP-. ion pairs by a suitable solvent such as THF.4 In contrast to Szwarc's conclusions, Dewald et al. have recently shown that hydrogen gas does result when sodium naphthalenide is exposed to water or ethanol in liquid a m m ~ n i a .Dewald's ~ results in liquid ammonia are similar to ours in the solid state, namely, that the anion radical of naphthalene can react with water via two different pathways
+ 2Hz0 2NP-. + 2Hz0 2NP;
-
-+
(1) Birch, A. J. J. Chem. SOC. 1944, 430.
(2) Citation of a few examples: (a) Bank,
s.;Bockrath, B. J. Am.
Chem. SOC. 1972,94,6076. (b) Minnic, E. R.; Long, L. D.; Ceraso, J. M.; Dye, J. L. Ibid. 1973,95,1061. (c) Garst, J. F.; Pacifici, J. A. Ibid. 1976, 97, 1802. (3) Stevenson, G. R.; Valentin, J.; Meverden, C.; Echegoyen, L.; Maldonado, R. J. Am. Chem. SOC.1978, 100, 353. (4) Wang,H. C.; Levin, G.; Szwarc, M. J. Am. Chem. SOC. 1978,100, 3969. 0022-3654/83/2087-4995$01.50/0
+ Hz + 20HNPHz + N P + 20H2NP
(1)
(2)
It is our intention here to present evidence that subOur Original that the 'lid 'Odium naphthalene anion radical is stable in the absence of solvent ( 5 ) Dewald, R. R.; Boll, H.; Willett, C. G. Tetrahedron Lett. 1980,
1593.
0 1983 American Chemical Society
4996
Stevenson et ai.
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Scheme I NP(g + NaOH(,) + 1/2HZ(p) + Na+NP-.(,) + HzOg Na(s) + H20(1) NaOH(aq) + '/2Hz(g)
AH" = + 4 4 . 5 kcal/mol AH" = - 4 4 . 1 kcal/mol
+
Na(,)
+ NP(,)
+
AH" =
Na+NP-.(,)
+ 0 . 4 kcal/mol
AH' = - 1 . 2 kcal/mol
due to the large crystal lattice energy of Na+NP-.(,lid, and that this solid anion radical may indeed cleave water to yield H2(g). Further, the crystal lattice energy of Na+NP--(solid) has been measured and a complete energy diagram for this salt generated. Experimental Section The anion radical salts were generated by reduction of the naphthalene on a freshly distilled alkali metal mirror in THF. After complete dissolution of the deficient amount of alkali metal, the anion radical solution was passed to another bulb on the evacuated apparatus. The THF was then stripped off under high vacuum. The solid anion radical salta were kept under high vacuum for several hours to ensure complete removal of the THF, after which they were transferred into thin-walled glass bulbs and sealed off from the apparatus. This procedure has been previously described.6 To ensure that no THF was left incorporated into the crystal lattice of the dry salts, we added DzO to the salts and submitted the solution to NMR analysis. No THF could be detected in the D20 after 1 mL of DzO was added to 1 mmol of salt. If each mmol of salt was associated with 1 mmol of THF there would have been 72 mg of THF in the D20. Since OUT WMHz NMR spectrometer can detect 0.5 mg of THF in 1 mL of DzO,there is necessarily less than 0.01 mmol of THF per mmol of salt. "he glass bulbs were placed into a modified cell of a Parr solution calorimeter and the bulbs broken under 100 mL of deoxygenated water in the calorimeter.' The change in the temperature of the calorimeter was due to the reaction between the contents of the bulb and the water. After the reaction, the contents of the calorimeter were titrated with standardized HC1 to obtain the amount of salt and/or sodium metal in the bulb. In separate experiments, degassed water was added to samples of the salts in a vacuum system fitted with a gas buret and a Toepler pump so thay any liberated hydrogen gas could be measured. Results and Discussion A sample of the naphthalene anion radical salt in the solid phase (Na+NP-)was submitted to ESR analysis, and it yielded a single isotropic line at g = 2.0045 indicative of the solid anion radical (Figure 1). The apparatus containing the Na+NP-. was then connected to a vacuum system with a U tube, which was immersed in liquid nitrogen, between the vacuum line and the apparatus. After several hours the U tube contained some naphthalene, and the remaining salt yielded an ESR signal showing some anisotropy (Figure lb). The anisotropy is due to the presence of a second line with a g value of about 2.0042. (6) Stevenson, G . R.; Valentin, J.; Williams, E.; Caldwell, G.; Alegria, A. E. J. Am. Chem. SOC.1979,101, 515. (7) Stevenson, G. R.; Williams, E.; Caldwell, G. J. Am. Chem. SOC. 1979, 101, 520.
b
d
Figure 1. ESR spectra for the solid sodium naphthalene system: (a) ESR spectrum of the solid anion radical salt; (b) ESR spectrum after some of the naphthalene has been sublimed from the sample (note the appearence of the broad sodium meati peak): (c-d) spectra as more naphthaleneis progressively sublimed from the solid salt metal mixture; (e-f) naphthalene has been readded to the mixture prior to the recording of these spectra.
This second line is due to the free sodium metal. As the sample is exposed to vacuum, naphthalene continues to escape from the salt and is captured in the U tube. Simultaneously, the ESR signal changes to one indicative of a higher and higher concentration of Na metal relative to the concentration of salt in the solid mixture (Figure 1, c and d). The samples of the solid sodium naphthalene salt which were sealed into the evacuated glass bulbs and submitted to the calorimetric study must have been contaminated with sodium metal. However, the enthalpy of reaction of the mixture in the bulbs with water did not vary with the Na+NP--/Na ratio (0-10). This can only mean that the enthalpy of reaction of the solid salt with water (eq 1 and 2) is very close to that for the enthalpy of reaction of sodium metal with water (-44.1 kcal/mol). Indeed, a plot of the change in the temperature of the calorimeter vs. the mmol of salt plus Na metal did yield a straight line, and the enthalpy of reaction is -44.5 f 1.4 kcal/mol (Figure 2). This value is well within experimental error of the enthalpy of reaction of sodium with water. The enthalpy of formation of Na+NP-. from sodium metal and naphthalene can now be calculated. If the reaction of the anion radical with water proceeds via eq 1, this enthalpy is calculated with Scheme I. However, if the reaction proceeds as depicted in eq 2, then Scheme I1 must be utilized to obtain this heat of formation. Both the enthalpy derived from Scheme I and that from Scheme I1 for the heat of formation of the solid anion radical salt are within experimental error of zero. This means that the energy difference between the anion
The Journal of Physical Chemistry, Vol, 87, No. 24, 7983
Thermochemistry of Solld Naphthalene Anlon Salts
4997
TABLE I : Enthalpies of Reaction in kcal/mol AH" for A = reaction
NP
anthracenea
ref
A(s) Na(s) + e-,) Na,) + + A(g) + e-,)
+44.5 + 44.5 -44.1 -1.6 - 17.4 - 25.9 -118.4 + 3.5
+41.1 b -44.1 -8.5 -23.5 - 25.9 - 118.4 + 12.7
d d e
+ Na+(g)
- 159.4
- 166.6
+ 1/2A s) + NaOH(,
Na+A-,) + H20(1) 4s) + + rfaOH(,a 3a+A-Ys) + ZO(1) Na(s) + HzO(1) NaOH(aq) + '/2H2(9) '/& + '/2H,(g) '/AH,(S) 1/,AH,
w,,)
)
-+
+
+
+
A,) N%) Na',) A-.,)
+
A-.(,)
-+
--f
--f
Na*A-.(,)
C
f g h
a The data for anthracene, which does not yield H, when the anion radical reacts with water was taken from Stevenson, N o reaction. Gunn, S. R. J. Phys. Chem. 1967, 7 1 , G. R.; Wiedrich, C. R.; Clark, G. J. Phys. C h e m . 1981,8 5 , 374. Cox, J. D. ; Pilcher, G. "Thermochemistry of Organic and Organometallic Compounds"; Academic Press: London, 1386. Becker, R. S.; Chen, Lotz, W. J. Opt. SOC.A m . 1967,5 7 , 873. 1970. e Hicks, W. T. J. Chem. Phys. 1963,38, 1873. The final error is estimated from a propagation o f all errors t o be about c 3 kcalimol. E. J. J. Chem. Phys. 1966,4 5 , 2403.
/
e
samples of the salt that are almost free of metallic sodium, based upon the ESR spectrum, do yield significant quantities of hydrogen gas when reacted with degassed water. From many such determinations, it appears that 40 f 15% of the reaction proceeds via eq 1 rather than eq 2. The addition of tert-butyl alcohol to the solid salt also yields hydrogen, but in smaller quantities. This reaction only gives about 20% of the theoretical amount of H2gas. This result further supports our contention that the anion radical is reacting with the proton donor to give H2 If the only source of H2 were metallic sodium, then the amount of H2 generated would not be a function of the proton donor. The potassium salt of naphthalene racts less exothermically with water than does the sodium salt (Figure 1) K+NP-.(,) + H20(1) NP(,) + KOH,,,) + NPH,,,) AHo = -39.1 f 1.1 kcal/mol (4)
-
Utilizing a thermochemical cycle analogous to that shown in Table I, but with potassium replacing sodium, we found the crystal lattice energy for K'NP-.,,) to be 144.9 f 3.0 kcal/mol and the heat of formation of the salt from solid naphthalene and solid potassium metal is exothermic by 9.6 kcal/mol K(,) + NP,,) K+NP-.(,, AHo = -9.6 f 3.0 kcal/mol (5)
-
1.0
2.0
m o l e s of s a l t
Flgure 2. Plot of the change in the temperature of the calorimeter (in O C ) vs. the mmol of solld Na'NP-. plus scdlum metal (open circles) and vs. mmol of K'NP-. (solid points).
radical salt and a mixture of sodium and naphthalene is very close to zero Na(,) + NP,,) Na+NP-.(,) AHo = 0 (3)
-
It is important to note that this is thermodynamically correct regardless of the products that are produced from the reaction of the anion radical with water. Further, this result is not dependent upon the Na/Na+NP-. ratio in the glass bulbs, since the salt and metallic sodium have the same enthalpy of reaction with water. From 1 mmol of Na+NP-., 0.5 mmol of hydrogen gas would be expected if the reaction proceeds as described in eq 2. The 0.5 mmol of H2 would generate about 0.5 of pressure in the 10-mL bulb of the gas buret that is connected to the Toepler pump. The actual amount of H2 evolved from the reaction of the salt with water is only about 40% of this amount, but some of this may come from the presence of sodium metal in the salt. However,
Since the potassium salt is more stable than the potassium and solid naphthalene one would not expect the evolution of hydrogen from the reaction of K+NP-.(,) with water. This was found to be the case. However, it should be noted that the work described here for the potassium salt does not necessarily support the case for H2evolution from H 2 0and Na+NP-.(,), since the exothermic nature of reaction 5 would suggest that potassium metal would not be present in the potassium salt. Thus, if all of the H2 comes from an alkali metal source, none would be expected from K+NP-.(,),as the thermodynamics would predict that all of the potassium would be bound to the naphthalene. When sealed ESR tubes of the Na'NP-. are heated to 100 O C for 24 h and the ESR spectra recorded again, decomposition of the salt back to sodium and naphthalene is observed. Thus, the salt may be metastable.
Conclusions A summarization of the energy states for the naphthalene and anthracene anion radicals is shown in Figure 3. Since the magnitudes of the crystal lattice energies are close to those of the solvation energies, it is expected that the chemistry of the solid anion radical systems be very
4988
The Journal of Physical Chemistty, Vol. 87, No. 24, 1983
I
!.2
I
Figure 3. Energy diagrams for the sodium naphthalene and sodium anthracene systems. The energy differences are in kcallmol. Note that the energy difference between the soli salt and the polyacene plus sodium met1 is very small for the naphthalene system compared with that for the anthracene system. The heat of solution of Na'NP-. (13.1 kcal/mol) was obtained from the difference in the crystal lattice energy (159.4) minus the solvation enthalpy ( 1 7 2 4 . '
different from the gas-phase chemistry but similar to that in the solution phase. Indeed, the solid sodium salt of the anthracene anion radical reacts with water to yield the normal Birch reduction products (9,lO-dihydroanthracene). This appears to be the case for all anion radical systems that are more stable in a thermodynamic sense than alkali metal and the solid polyacene. However, it can be seen from Figure 3 that the solid sodium naphthalenide lies at about the same energy level as does sodium metal and naphthalene, while the THF solvated anion radical lies 13 kcal/mol lower in energy. Thus, a different behavior can be expected from the solid and THF solvated systems. This is the case, as Na+NP-.(,)and Na+,NP-,,,, react with water, at least in part, to yield hydrogen gas.
Stevenson et al.
The previously published contention4that the sodium naphthalene anion radical will not remain intact after the removal of the THF was based, in part, upon the observation that the addition of diethyl ether (DEE) to the THF solutions results in a transfer of an electron from the anion radical back to the sodium cation to give sodium metal and solvated naphthalene. To assume that the nonsolvated salt is unstable based upon this observation may cause error, since part of the driving force for the electron transfer back to the sodium cation upon the addition of DEE is the solvation of the neutral naphthalene. This free energy of solvation of naphthalene may be negative enough to make the salt loose its integrity in DEE but remain intact in the solid phase. Remember, the enthalpy of reaction of sodium metal with solid naphthalene to give Na+NP--(,)is almost zero, so even a small stabilizing interaction with the naphthalene is enough to shift the reaction toward the metal and polyacene. Of course the solvation of the anion radical is much greater than that of the naphthalene, but we are comparing the heat of dissociation of the solid salt and the DEE solvated salt. The removal of naphthalene from the solid salt by continual evacuation is expected, since the salt exists in equilibrium with the surface of the sodium metal and naphthalene in the solid phase. However, this is probably not a true thermodynamic equilibrium, since the naphthalene can only react on the surface of the sodium metal to form the anion radical. By simply distilling sodium under vacuum onto naphthalene the color of the sodium naphthalene anion radical can be observed on the surface of the sodium mirror. One final point should be mentioned: entropy effects would also favor the reaction depicted in eq l over that depicted in eq 2. This is, of course, a thermodynamic statement and does not consider elementary reactions (or mechanism) of the overall reaction. Acknowledgment. We thank the donors of the Petrolium Research Fund, administered by the American Chemical Society, for support of this work. Registry No. Naphthalene radical anion sodium salt, 348112-7; potassium naphthalenide, 4216-48-2; sodium, 7440-23-5; potassium, 7440-09-7; naphthalene, 91-20-3; water, 7732-18-5; ethanol, 64-17-5; ammonia, 7664-41-7; sodium hydroxide, 131073-2; tert-butyl alcohol, 75-65-0;anthracene, 120-12-7;anthracene radical anion sodium salt, 4216-47-1.
